Introduction to Plant Development

• Development → process that builds an organism

  • What has changed?

  • Analyzes the process by which the change has come about

Approaches to the study of plant development

• Analysis of the molecular genetic mechanisms that underlie developmental processes

• Characterization of the biochemical reactions that carry out development

• Investigations of the structures of cells and how these structures help bring about developmental changes

• Investigations of the integrated functions of tissues and organ systems

Genetics of Control of Plant Development

• Zygote → single origin of all the cells in plant body

• Alll cells in plants contain the same genetic material

• Differential gene expression → expression of different genes by cells with the same genome

• Differences between cell types are not due to the presence of different genes but due to the expression of different genes

Regulation of Gene Expression

• Regulation of gene expression can occur at any of the steps of protein synthesis

• Levels of control

  • Transcriptional level → during formation of primary transcript

  • Processing level → at the stage of splicing

  • Transport of mRNA from nucleus to cytoplasm

  • Translational level

  • Protein modification/transport

Characteristics of Plant Development

1.  Continuous Development

• Reiterative process

  • Ex: Apical meristems repeat the same developmental patterns to produce an extending root or an extending series of nodes and internodes

• Indeterminate → open ended patterns of development

  • Ex: shoot apical meristem

• Determinate → patterns restricted in time and space

  • Ex: leaf development because leaf meristem activity stops after the leaf is produced

• Developmental patterns may switch from indeterminate to determinate, vice versa

  • Ex: vegetative SAM becomes determinate if it becomes a floral meristem

  • Ex: determinate plant organs may give rise to indeterminate adventitious root or shoot buds

2.  Plastic Development

• Development can be adjusted according to the prevailing environmental conditions 

• Important for sedentary organisms a.k.a plants

Cabomba caroliniana

• Their underwater leaves are feathery → protects them from damage by lessening their resistance to moving water

• Their surface leaves are pads that aid in flotation

• Both leaves are genetically identical, but different environments result in the turning on or off of different geens during development

3.  Regeneration and Totipotency

• Totipotency → ability to become any organ or cell and can give rise to a complete plant

• Callus → amorphous mass of cells which can then reorganize and differentiate like the cells of meristems

• Regeneration may or may not involve callus formation

  • In the case callus formation is not needed, missing tissues are directly replaced by highly organized cell proliferation and differentiation

A comparison of plant and animal development

1.  Post-Embryonic vs. Embryonic developemnt

• Animal development is almost synonymous with embryogenesis

  • Ex: most adult organs are formed during embryogenesis

  • Animal body plans are predetermined by embryonic development

• Angiosperm embryogenesis is concerned with establishment of the meristems so major organs and tissue systems are not yet found in mature embryo

  • Major organs and systems are only formed after seed germination → post-embryonic

  • Plants can adapt their body plan to environment changes → great plasticity

2.  Nature of Cell commitment for differentiation

• Plants can retain totipotency

  • Ex: mesophyll cells can be made to re-differentate into tracheary elements

• Animals are irreversibly committed to a developmental pathway

  • Ex: neurons cannot be induced to become muscle cells

3.  Cell movement and planes of cell division

• Animal cells are motile and cell migration is important → gastrulation

• Developmental fate of plant cells determined by its position in the plant body

  • Anticlinal division → cell plates perpendicular to surface → expansion of surface

  • Periclinal division → cell plates parallel to surface → protrusion from surface

• Ex: epidermis → anticlinal division → expands as a single layer covering entire surface

• Ex: pericycle → periclinal division → outgrowth of lateral roots in primary root

4.  Variety of plant organs and cell types

• Higher animals have a greater variety of organs and cell types than higher plants

• Embryos of eudicots have 4 organs

  • Plumules → embryonic axis above the cotyledons containing epicotyle

  • Cotyledons

  • Hypocotyls → embryonic axis below the cotyledons but above radicle

  • Radicle → Embryonic root

• Mature plants have 3 vegetative organs

  • Stems

  • Roots

  • Leaves

• Flowers have 4 organs

  • Sepals

  • Petals

  • Stamens

  • Pistils

• 40 cell types in plants compared to hundreds of cell types in higher animals

• Intercellular signaling network between plant cells is less complicated

5.  Control of Cell Fate

• In multicellular organisms, cell fates are influenced by activities of neighboring cells

• Regulation of cell fate dependent on ability of cells to transduce intercellular and extracellular information into changes in gene activity

• Both plants and animals adopted the transcriptional cascade as principal mechanism for cell fate determination

6.  How plant and animals police cell fates

• Animal cells require survivial factors from neighboring cells 

• Programmed cell death → happens in the absence of survival factors + safeguards that is disabled in cancer cells

• Plants cells have the capacity for apoptosis but it is not invoked by isolation

• A plant cell displaced out of its normal position will just sqitch to a fate appropriate to its new position

7.  Consequences of Autotrophy vs Heterotrophy

• In plants, assimilation of energy and of nutrients and water are achieved at separate sites (leaves and root hairs respectively)

  • Ex: more leaves → more sunlight and CO₂ absorbed

  • Ex: more roots → more water and mineral uptake

• In animals, energy and nutrients come from food 

  • Ex: development of new organs will not improve feeding ability 

  • Ex: changes in environmental conditions cause a change in animal behavior

• Thus, different morphology between plants and animals

Model organisms in Plant development

• Model organism → short generation time, characterized genome, similar to a member of a particular group

• Ex: Arabidopsis thaliana → mustard family

What makes Arabidopsis thaliana a good model organism?

• Small Size

• Short lifecycle → 6-8 weeks from germination to production of new crop seeds

• Self-fertilizing → bisexual flowers and can self-pollinate so recessive mutations quickly become homozygous and mutatnt phenotype observed faster

• High seed Production 

• Small genome

• Ideal for genetic characterization of mutants

Use of mutants in the study of plant development

• Most mutations result in a loss of function of a gene

• Ex: MALE STERILITY or MS1 gene → no stamen development → thus this gene is required for stamen development

Nomenclature for genes identified by mutation

• Full descriptive names of the wild-type gene should be capitalized and italicized (i.e. ALPHABETICA)

• Full descriptive names of the mutant should be in lower case and italized (i.e. alphabetica)

• Mutant gene symbols should be written in lower case letters and italicized (i.e. abc) while wild type gene symbols should be capitalized and italicized (i.e. ABC)

• Protein products of genes should be written in capital letters without italics (ABC)

• Ex: 

  • Wildtype gene: SHORT INTEGUMENt 1 (SIN1)

  • Mutant gene: short integument 1 (sin1)

  • Protein product: SIN1

Plant Developmental Processes

Development vs. Growth

• Development is the progression from earlier to later stages in maturation

  • Process whereby tissues, organs, and whole plants are produced

  • Involves growth, morphogenesis, and differentiation

• Growth is the irreversible change in size of cells and plant organs due to both cell division and cell enlargement

  • Enlargement requires a change in elasticity of the cell walls together with increase in size and water content of vacuole

  • Determinate growth when an organ or part or whole organism reaches a certain size and then stops growing

  • Indeterminate growth when cells continue to divide indefinitely (i.e. plants)

• Cell division is the number of cells increasing due to mitosis

  • Symmetry, rate, and orientation greatly affects cell fate

• Cell enlargement is the size of a cell increasing due to an increase in the volume of its protoplasm

Differentiation, dedifferentiation, and redifferentiation

• Differentiation is the process in which generalized cells specialize into the morphologically and physiologically different cells

  • It is a function of which particular genes are either expressed or repressed thus guided by gene expression

  • In plants, it involves changes in size, biochemistry, structure, and function of cells

  • Ex: meristematic cells of procambium and vascular cambium differentiate into xylem elements

  • Cells undergoing differentiation may undergo major structural changes

  • Ex: differentiation of tracheary elements involves losing the protoplasm and developing a secondary cell wall

• Dedifferentiation is the reversal of cell development in plants so that the differentiation that had occurred previously is lost and the cell becomes more generalized in structure

  • Living differentiated cells that have lost the capacity to divide can regain capacity of division

  • Ex: in the formation of meristems, both interfascicular cambium and cork cambium develop from fully differentiated cells

• Redifferentiation is the differentiation after dedifferentiation takes place either forming the same mature cell type or an entirely different one

  • Ex: callus tissue produced from a mature leaf explant can be induced to differentiate into roots or shoots

Pattern Formation

• Process whereby organisms create spatially-ordered and reproducible structures

• Organisms must noy only generate different cell types, they must also ensure that the different cells are correctly arranged in time and space

Organized cell growth and cell architecture

• Preferential elongation of cells along certain axes is a major determinant of form

• Orientation of cell growth is in the plane perpendicular to the orientation of the cellulose microfibrils in cell wall

• Elongation favored when CMFs are oriented transversely to the direction of growth 

• Elongation limited when CMFs are oriented in oblique or longitudinal direction

Control of cell proliferation

• Placement and activity of meristems are major determinants of shape and form

• Lateral meristems account for increases in girth

• Apical meristems account for elongation of plant axis

External cues

• Initial arrangement of cells may use external signals in order to create a spatial pattern

• Phototropism/gravitropism

Assymetric cell divisions

• Early plant embryo results in unequally sized daughter cells

• Creates polarity and permit unequal distribution of cellular material

• Ex: first division of the angiosperm zygote results in a smaller apical cell and a larger basal cell

Lateral inhibition

• Interactions between neighboring cells that prevent both from adopting the same fate 

• Ex: during stomata formation, precursor cells inihibit the development of other precursor cells close to them, ensuring spaces in between

Programmed cell death

• Tissue can be sculpted by the selective death of some cells

• Ex: formation of holes in Monstera leaves

Plant cell division - finding the sweet spot for cell plate insertion

• Cytokinesis in cells of flowering plants is achieved through the construction of a new cell wall form the inside out

• Cell plate is initiated between daughter nuclei after mitosis and expands centrifugally to form a new cell wall sandwiched between new plasma membranes

• Plant cytokinesis proceeds through a microtubule-dependent mechanisms

  • Involves mitotic spindle, pre-prophase band and phragmoplast

• Pre-prophase band (PBB) is a ring like microtubule and actin structure formed around the pre-mitotic nucleus

  • During mitosis, actin dissociates from the PPB and the accumulation of a yet unknown factor marks division site

  • Found in the G1 phase of cell cycle and disappears when M phase starts

• Mitotic spindle gives rise to Phragmoplast that expands toward the cell cortex

  • Composed of two opposing disks of parallel microtubules and actin filaments

  • + ends near the equatorial plane and - ends near the poles

• Cortical division zone (CDZ) is the division plane in plant cells that is outlined by the PBB in response to selection cues

• Phragmoplast guides the vesicles containing cell wall material toward the division plane for fusion (brown arrows)

• Cell plate and phragmoplast expand toward the CDZ in a centrifugal direction (outward pointing red arrows)

• Subsequent membrane trafficking and fusion at its edge leads to cell plate formation initiated through the fusion of golgi-derived vesicles

• Phragmoplast guides the vesicles to the cell plate containing polysaccharides, proteins, and membranes

Plant Cell Wall

• Meristematic cells are usually isodiametric and then differentiate by developing distinct forms to acquire specific functions

• Dynamic structures that act as an exoskeleton by participating in the establishment and maintenance of cell shape and by protecting the cell content

• Primary cell walls are made of glucan-based cellulose microfibrils embedded in a highly hydrated matrix composed of 

  • pectins, 

  • hemicelluloses, 

  • structural proteins, and 

  • proteoglycans

• Also extensible to allow cell expansion which is driven by strong intracellular turgor pressure

• Auxin is a stimulator of cell elongation, as it increases cell wall extensibility and regulates cell wall properties by initiating wall loosening

• Secondary cell walls are usually present in specialized, non-growing cells 

Cellulose microfibrils 

• Embedded in components such as non-cellulosic polysaccharides and structural proteins

• Determine the direction of cell expansion

• Cellulose synthesis takes place beneath the cell wall at the plasma membrane via cellulose synthases

• Cellulose microfibril patterning is mediated via cortical microtubules and cellulose synthases

• Microtubules as raild road tracks thus controlling microfibril organizationg by guiding cellulose synthase complexes

Interaction between Autonomous and microtubule guidance systems controls cellulose synthase trajectories

• Cellulose synthase trajectories remained the same when microtubules were disrupted

• Cellulose synthase complexes can interact with trails left by other complexes → autonomous mechanism

• This mechanism can be overridden by the microtubule guidance system

• Dual guidance model

  • Autonomous system with interactions between cellulose synthases and microfibrils maintain aligned cellulose synthase trajectories

  • Microtubule guidance system allowing alignments to be steered by environmental and developmental cues

Hemicelluloses and Pectins

• Hemicellulose xyloglucans are found mainly in primary cell walls and participate in cell wall extension during cell elongation

  • influence wall extensability and stiffness

• Pectins are important in the regulation of wall properties by controlling wall porosity and hydration causing wall swelling and thus influences wall thickness

  • Adjust wall extensibility by influencing the alignment of cellulose microfibrils

  • Form the middle lamella → adhesive compartment between two adjacent cell walls

  • Composed of highly heterogenous polysaccharides

    • Homogalacturonan

    • Rhamnogalacturonan

    • Rhamnogalacturonan II

    • Xylogalacturonan

Structural proteins

• Expansins are cell wall loosening proteins and enhances wall expansion in acidic pH

• Extensins are required for cell wall assembly

• Arabinogalactan proteins AGPs play a role in plant protection known to specifically control pollen tube growth but also regulated overall plant development

Role of Auxin in Wall extension

• Controls plant growth and development by promoting cell division (proliferation), growth (expansion, elongation), and differentiation

• Auxin or indole-3-acetic acid (IAA)

  • Responsible for cell wall loosening and cell expansion via modifications of cell wall composition

  • Causes pectin polymerization

  • Increases pectin viscosity and xyloglucan depolymerization

  • Activates the expression of cell wall related genes

  • Stimulates the synthesis of proton pumps leading to apoplast acidification

  • Acts on the cytoskeleton through Rho GTPases and promotes trafficking of vesicles containing new cell wall material

• Recall

  • [1] auxin activates H+ -ATPases 

  • [2] apoplast acidification

  • [3] wall loosening proteins like expansins become active 

  • [4] causes wall enlargement

  • [5] changes trigger the cell to activate calcium channels to pump calcium into the wall 

  • [6] pH increases

  • [7] growth cessation

Auxin Signaling and Gene Expression

• Auxin acts through the transport inhibitor resistance 1 or Auxin signalling F-box (TIR1) nuclear auxin receptor family

• F-box proteins target other proteins for degradation via ubiquitin degradation pathway

• TIR1 proteins bind to auxin → auxin acts as a glue → proteins bind to their targets

• Auxin/Indole-3-Acetic Acid (AUX/IAA) is a target of TIRs which represses auxin-mediated transcription through the interactions with Auxin Response Factors (ARFs) in the absence of auxin

• Binding of TIR promotes degradation of AUX/IAAs

• Auxin is absent

  • ARFs bind to Aux/IAAs (repressor)

  • Aux/IAA suppresses the ability of ARFs to enhance gene transcription

  • Binding of Aux/IAA to ARFs brings Aux/IAA into contact with promoters of auxin regulated genes where Aux/IAA repress the expression of these genes by recruiting other factors to make modifications to the DNA structure

• Auxin is preset

  • Binding of auxin to TIR1/AFBs allows them to bind to Aux/IAAs → Aux/IAAs are marked for degradation through proteosomal activity → frees ARF proteins → activate or repress genes at whose promoters they are bound

• TIR1 → protein receptor, ARFs → transcription regulator, Aux/IAA → inhibitor of ARFs

Auxin and Cell wall related genes

• Auxin treatment results in the upregulation of key genes related to cell wall components

• Reported genes are not necessarily related to cell elongation (wall expansion) and could be linked to different auxin-driven processes such as cell division, growth or differentiation

Movement of materials through apoplast

• Interconnected cell walls of plant form the apoplast 

• Apoplast acts as a barrier and a channel for intercellular signals

• They allow movement of small negative charged molecules

• Apoplastic route is continuous with the lumen of xylem vessels (except for endodermis cell walls which are suberized)

• Molecules secreted into the cell wall may be transported via the xylem providing a unidirectional route (root to shoot) for long range developmental signals

Movement of materials through plasmodesmata

• Plasmodesmata connect adjacent cells

• Each plasmodesmata is lined by a membrane continuous with the plasma membrane of the connected cells

• Symplast formed by the connected protoplasm of cells throughout the plant

• Symplast includes the cytoplasm of the phloem elements and provides another route besides the xylem for long range bidirectional transport

• Symplastic communication includes

  • Passive diffusion for smaller molecules

  • Active transport for large nucleic acids and proteins 

• Some parts act so well connected they act as a syncytium

Review: Angiosperm Life Cycle

Angiosperm Life Cycle

• Dominated by the spore generating sporophyte stage rather than sexual gametophyte stage

• Produce two types of spores: male and female gametes

• Megaspore produced in a carpel (ovary + ovules)

• Each ovule contains a megasporangium where megaspores are produced 

• Megasporocyte within each megasporangium is a megaspore mother cell

• Megasporophyte produces 4 megaspores where 1 survives and develops into an embryo sac consisting of an egg and other cells

• Microspores originate from anthers at the tips of stamens

• Within an anther are microsporangium containing microsporocyte

• Microsporocyes produce microspores via meiosis 

• Miscrospore develops into a pollen grain which contains a tube cell and a generative cell

• Once the pollen reaches the stigma, the upper part of the carpel, tube cell becomes a pollen tube that extends to the ovule containing the embryo sac

• Generative cell divides to form 2 sperm which are released together in an act of double fertilization 

• One sperm fertilizes the egg and one fertilizes the central cell

• Fertilized egg develops into a zygote then into an embryo

• Fertilized central cell becomes an endosperm 

• Endosperm and embryo are packed into a seed

Sporophyte and Gametophyte generation

• A haploid generation alternates with a diploid generation where diploid is the sporophyte and haploid is the gametophyte 

• Both sporophyte and gametophyte are always multicellular

• Sporophyte (2n) generation produces unicellular spores (n) via meiosis

• Spores (n) germinate and divide via mitosis to produce multicellular gametophytes (n)

• Gametophyte produces gametes egg (n) and sperm (n) via mitosis

• Two haploid gametes join to form a diploid zygote

• Zygote (2n) divides to form multicellular sporophyte (2n)

Angiosperm megasporogenesis and Megagametogenesis 

• Megasporogenesis

  • Megasporocyte (2n) in the nucellus of the megasporangium undergoes meiosis to produce 4 haploid megaspores (n) where in only 1 survives

• Megagametogenesis forms the female gamete

  • Surviving megaspore divides by mitosis 3 times without cytokinesis to produce a huge cell with 8 nuclei

  • Multinucleate structure is divided to form the 7 celled embryo sac → female gametophyte

• Cells of embryo sac

  • 1 central cell (2n) → later fuses with sperm to form triploid endosperm

  • 3 antipodal cells (n) → positioned at the ends of the embryo sac opposite the micropyle

  • 1 egg cell (n) → positioned closest to the micropyle becomes the egg cell

  • 2 synergid cells (n) → positioned at either side of the egg that help attract and guide the pollen tube for successful fertilization (egg apparatus)

Angiosperm Microsporogenesis and Microgametogenesis

• Microsporogenesis

  • Anther of the stamen contains four microsporangia or pollen sac

  • Each pollen sac contains diploid microsporocytes (2n) that divide via meiosis to produce haploid microspores (n)

  • Each microsporocyte gives rise to four microspores that later develop into pollen grains

• Pollen sac has a layer of cells, tapetum, that provides nutrition to microspores and contributes to pollen wall

• Each pollen grain has two coverings

  • Exine → thicker outerlayer that contains sporopollenin a complex waterproofing substance supplied by tapetal cells

  • Intine → inner latyer

• Microsporogenesis

  • Microsporocyte (2n) divides via meiosis to produce 4 microspores (n) that initially occur in tetrads

  • Released from tetrads to form pollen grains

  • Haploid nuclei of pollen grain divides via mitosis

  • Mature pollen grains containing two cells the generative cell that is contained within the tube cell 

  • Generative cell divides via mitosis to produce 2 sperm nuclei

• Sperm delivery

  • When pollen grain lands on stigma, it absorbs water and germinates to produce a pollen tube which is an extension of the cytoplasm of the tube cell

    • No cell division as pollen tube elongates 

    • Pollen tube grows via tip growth

  • Pollen tube elongates through style the generative cell divides by mitosis to produce 2 sperm nuclei 

  • Tube nucleus leads ahead of two sperms as tube grows towards micropyle in response to chemical attractants produced by synergids

  • Arrival of pollen tube = death of one synergid = serving as a passage way into embryo sac

Meiosis in Angiosperms

• Microsporogenesis and megasporogenesis are the two processes in plants that involve meiosis

Stamen and Pollen Development

• Section of a flower bud not a mature flower

• 6 developing stamens surrounding the developing ovaries

Stamen structure

• Concerned with male sexual reproduction (anther is where microsporogenesis occurs)

• Consists of the anther and filament

  • In gumamela, the filaments are fused to form the staminal tube

• Androecium refers to all the 6 stamens collectively

• Anther is a two-lobed organ with 2 locules in each lobe

  • Each locule has a microsporangium or pollen sac 

  • Anther is a group of 4 microsporangia

• Outermost layer of cells is epidermis

• Connective is the tissue found in between two lobes (joins the two lobes of the anther)

• Vascular strand (with xylem and phloem tissue) is embedded in the connective 

• Endothecium is inner to epidermis with radially elongated cells

  • Endothecium cells develop fibrous thickening made of cellulose with pectin and lignin which helps in dehiscence of anther

• Stomium are thin-walled cells in between the cells of the endothecium

  • Joins adjacent anther walls 

  • Serves as final breakage site (opening site) for dehiscence

• Middle layers is next to endothecium which become compressed and obliterated in mature anther

  • There are species without a middle layer 

• Tapetum circumscribes the locule and nearest to developing microspores

• Cicular cell cluster can only be found in tobacco and other solanaceous plants

  • Composed of idioblast cells that accumulate calcium oxalate crystals

  • Cells undergo apoptosis before dehiscence

  • Degeneration of ccc and connective allows 2 locules of each theca to become confluent to form a unified chamber so pollen grains can be released from stomium region

  • Degeneration of ccc breaks the septum between the two pollen sacs to make them continuous so when the stomium opens the spores from both sacs are released together

Anther development

• All the structures of the flower develop from the floral meristem

• Anther primordium is seen as an oval structure in xs → mass of undifferentiated and homogenous meristematic cells

  • Consists of 3 germ layers

    • L1 → gives rise to outermost epidermis and stomium cell cluster

    • L2 → hypodermis and gives rise to archesporial cells

    • L3 → gives rise to connective tissue, vascular bundle, and circular cell cluster

• Vascular tissue begins to form in the middle region

• Tapetum has two origins for those species with more than one layer

  • Outer layer from L2

  • Inner layer from L3

• Derivatives of L2, the hypodermis

  • Archesporial cells formed from the periclinal division of L2 layer

  • Archesporial cells have large, prominent nuclei and dense cytoplasm

  • Periclinal division of archesporial cells form 2 cell layers

    • Inner layer → consists of primary sporogenous cells that become microsporocytes

    • Outer layer → consists of primary parietal cells that become secondary parietal cells that divide to become non-reproductive anther wall layers (endothecium, middle layer, and tapetum)

  • All wall layers except epidermis from L2 cells

• Derivatives of primary parietal cells

  • Divide tangentially (periclinal) to give rise to two layers of secondary parietal cells

  • Basic → both secondary parietal cell layers divide to yield 2 middle layers

  • Dicotyledonous → only outer secondary parietal cell layer divides to yield endothecium and single middle layer

  • Monocotyledonous → only inner secondary parietal cell layer divides to yield tapetum and single middle layer

  • Reduced → secondary parietal cells do not divide and develop into endothecium and tapetum respectively

• Sporangium initiation is restricted to the four corners of the developing anther

• Refer to days of flower opening where 0 represents day 0 

• All the days after day 0 is marked positive meaning it has been a few days since the flower opened and same for the negative numbers

• The numbers correpond to a specific event

• Example: stage 7 corresponds to day 0 which is whe nthe flower will open

Summary of stages of flower development in Arabidopsis

Stages in the development of the anther

• Position dependent developmental pathway

  • Rare divisions enable L2 cells to penetrate L1 causing these cells to follow an epidermal pathway

  • Cells within each germ layer are conditionally specified and that the ultimate fate of these cells depends upon their position

• Two phases of stamen development

  • Phase I → sporogenic cells engage in microsporogenesis while nonsporogenic cells form epidermis, tapetum, etc…

    • Growth, histodifferentiation, meiosis 

  • Phase II → anther enlarges and filament elongates, pollen grains form, dehisce, and released 

    • Tissue degeneration, dehiscence, pollen release

Schematic view of development of anther layers and microsporogenesis in Arabidopsis

Tapetum

• Usually only a single layer that circumscribes the locule

• Special features

  • ER-golgi complex

  • Numerous secretory vesicles toward side facing locule

  • Small vacuoles containing lipophilic substances

  • Loosening of cellulose structure or total dissolution of cells in certain areas which permit transfer of materials into anther locule

  • Exhibit endoreduplication and polyploidy → though cells are functionally differentiated and nondividing, they retain their mitotic potential resulting to multinucleate cells (no cytokinesis)

• Attain max development when microspores are in tetrad stage

• After tetrad stage, tapetal cells degenerate

• Two types of tapetum based on morphological changes during degeneration involving the remodelling of protoplasm and migration into locule

  • Amoeboid or periplasmodial tapetum → protoplasts lacking cell walls enlarge and fuse with one another and move into locule to surround developing pollen grains

  • Glandular or secretory tapetum → protoplast is stationary after cell walls lyse. Protoplasts break down and are resorbed

• Substances produced by tapetum

  • Callase → digests the tetrad wall (made of callose) to release individual microspores

  • Pollenkitt → compounds imparting stickiness to the pollen grains usually composed of lipoidal compounds, carotenoids, and flavonoids

  • Tryphine → mixture of hydrophilic substances derived from tapetal cell debris that serves as the main encrustation of the pollen wall and aids in the adhesion of pollen grains to stigma

Anther dehiscence involves programmed destruction of specific cell types

• Phase II in anther development is marked by a change from growth to degeneration of supporting tissue then dehiscence

• Dehiscence begins after tetrad formation

• Involves the stomium, endothecium, and circular cell cluster

• Series of events:

  • Formation of fibrous band thickenings on endothecial cell wall

  • Degeneration of circular cell cluster and merging of the two pollen sacs in each theca into a single locule

  • Breakdown of the tapetum and connective

  • Rupture of anther at stomium and pollen release

• Dehiscence requires gene activation of RNases, proteases, and cellulases

• Onset of dehiscence results in transcriptional activation of genes that are inactive during Phase I of anther development

Pollen Development

• 3 stages

  • Microsporogenesis → differentiation of the sporogenous cells and meiosis

    • Occurs in microsporangia

    • Microsporocytes occur as a large mass 

    • Callose separates microsporocytes from diploid tissue because will later on have haploid cells

• Post-meiotic development of microspores

• Microspore mitosis

• Sporogenous cells undergo 2-3 mitosis divisions to form microsporocytes within locules

• Microsporocytes connected with each other and with tapetum via plasmodesmata

• As microsporocytes enter meiosis, the plasmodesmatal bridges break and connections between microsporocytes are replaced with wide cytoplasmic channels

• Microsporocytes are surrounded by primary cell wall composed of cellulose, hemicellulose, and pectin

• Microsporocytes have a secondary cell wall composed of callose that acts as a protective seal 

• They undergo meiosis to form tetrads of microspores while encased in callose wall

• As microspore matures, primary cell wall and callose layer degrade to release microspores into locules

QRT mutants

• QUARTET (QRT) genes are required for pollen separation

• qrt mutants have four products of microsporogenesis fused and the pollen grains released as tetrads

• QRT1, QRT2, QRT3 genes are required for proper degradation of dividing microsporocyte primary cell walls and subsequent separation of microspores resulting in the release of single pollen grains

• qrt1 mutant degrades the callose layer but the primary sporocyte cell wall remains partially intact following meiosis

• qrt2 mutant retains patchy callose around the microspore

• qrt3 mutant fails to degrade the pectic polysaccharides of the walls which mechanically constrains the developing pollen grains leading to fusion of developing walls

Microgametogenesis: Pollen Mitosis I and II

• Events that lead to development of microspores into microgametophytes 

  • Expansion of microspores with formation of large vauole

  • Displacement of microspore nucleus to an eccentric position against microspore wall

  • Nucleus undergoes pollen mitosis I resulting in formation of 2 unequal cells

    • Large vegetative cell with haploid nucleus

    • Small generative cell with haploid nucleus that lacks mitochondria and chloroplasts because cytoplasm was partitioned unequally (basis of maternal inheritance of chloroplast and mitochondrial genomes)

  • Generative cell detaches from the pollen grain wall and engulfed by vegetative cell forming a cell within a cell structure

  • Generative cell undergoes pollen mitosis II to form two sperm cells enclosed within the vegetative cell cytoplasm either before pollen is shed or within the pollen tube

    • Before pollen is shed → tricellular/trinucleate pollen

    • Within pollen tube → bicellular/binucleate pollen

• Asymmetrical cell division is dependent on microtubules 

The pollen wall

• Outer exine → sporopollenin found to be resistant to both physical and chemical decay

  • Does not develop over certain regions which define the positions of the pollen apertures or germ pores

  • Pollen aperture show wide variation

  • Exine sculpturing important in attachment to insect pollinators and adhesion to stigma

• Inner intine → pectocellulosic (pectin + cellulose)

  • May also consist of hemicellulose and callose

Pollen wall synthesis

• Exine → developed through contribution of microsporocyte cytoplasm and tapetum which play an important role in producing sporopollenin

  • Under sporophytic control

• Intine → under the control of the microspore cytoplasm and involves gametophytic gene expression from microspore nucleus

Hormonal regulation of pollen development and dehiscence

• Gibberellins → associated with early filament elongation and tapetum development

• Jasmonic acid → linked to later stages of pollen maturation, filament extension, and anther dehiscence 

  • Acts by controlling water transport in anther

• Auxin → important role in early and late pollen development by regulating entry into the cell cycle, controlling dehiscence, and controlling stamen filament growth

  • Principal source is local synthesis 

  • yuc2yuc6 auxin biosynthesis double mutant has no pollen grain formation, no stamen elongation, and male flowers are sterile

  • Auxin receptor mutants exhibited premature anther development with early endothecium thickening, premature pollen mitotic divisions, stomium splitting, dehiscence

Mutants in stamen and pollen development

• Three broad categories of male sterility in angiosperms 

  • Structural male sterility due to morphological anomalies in stamens

  • Functional male sterility where viable pollen is produced but it is incapable of affecting fertilization

  • Sporogenous male sterility where pollen development is interrupted anywhere from the pre-meiotic formation of microsporocytes to the second mitotic division

• Male sterile mutants provide a bsis for hybrid seed production

• Possible: male sterile but female fertile → cannot self fertilize but can be fertilized using pollen from related plants to produce hybrids

• Male sterility may result from mitochondrial mutations or nuclear mutations

• Genes that cause male sterility

  • Maize mutant antherless (at) has normal filaments but lacks anthers

  • Gibberellic acid0deficient maize mutants d2, d3, d5 form small anthers and fail to produce pollen

  • Tomato stamenless-2 mutants have short stamens and nonfunctional microspores

  • Arabidopsis male sterile (ms1) mutants do not produce viable pollen, but are otherwise phenotypically normal. Degeneration of pollen occurs soon after microspore release from the tetrads, at which time the tapetum also appears abnormally vacuolated

Ovule Development and Embryo Sac Formation

Structure of the Ovule

• Carpel → female reproductive organ

• Pistil or gynoecium → collective term for all the carpels of a flower

  • Central most whorl of a flower

• Carpel vs pistil

  • If there is only 1 carpel, that carpel serves as the pistil

  • Separated multiple carpes are equivalent to 3 pistils

  • Multiple carpels that are fused equals 1 pistil 

• Functional parts of the carpel at anthesis

  • Ovary → basally located 

  • Ovules → found in the ovary

  • Stigma → terminally located where pollen grains land and germinate

  • Style → connects the stigma and ovary

• Functional parts of the ovary

  • Placenta → specialized ridge in the ovary where ovules are attached

  • Locule → space where ovules are found which can hold just one or many ovules

• Funiculus → stalk where the mature ovule is raised to attach to the placenta

• Nucellus → mass of homogenous cells in the mature ovule where megasporogenesis occurs (considered as the megasporangium)

• Integuments → one or two multilayered covering outer to the nucleus 

• Micropyle → small opening along the integuments at the free end of the ovule

• Chalaza → poorly defined region at the opposite pole of the ovule where the nucellus, integumants, and funiculus meet

• Degree of curvature of the ovule adds to the variation in its external morphology

• Differential growth rates result in the curving or bending of ovules

Ovule Determination and Development

• Ovule primordium is initiated by periclinal divisions from the sub-epidermal or hypodermal tissue of the placenta 

  • L1 → outermost layer

  • L2 → sub-epidermal (or hypodermal) layer

    • Single hypodermal cell enlarges to become the archesporial cell

    • Archesporial cell divides periclinally and gives rise to an outer parietal cell and inner sporogenous cell

    • Sporogenous cell functions as megasporocyte

    • Primary parietal layer divides anticlinally and periclinally to form parietal tissue

    • In some species, archesporial cells and primary parietal cells do not divide anymore.

  • L3 → innermost cells

• Relatively homogenous mass of cells of primordium will be organized into 2 different regions

  • Funiculus

  • Chalaza

  • Nucellus

• Arabidopsis → archesporial cell developing in the hypodermis becomes the megasporocyte (no periclinal division)

• Ovule is polyclonal → derived from more than one cell layer

• Megasporocyte derived from L2

• Integuments arise from the tissue surrounding the nucellus (i.e. chalaza)

• Dicots and monocots have 2 integuments

  • Inner integument from L1

  • Outer integument from L1 and L2

• Before embryo sac matures, nucellus degenerates in many species, leaving the embryo sac in direct contact with the inner integument cell layers that may differentiate into endothelium

• Endothelium → similar to tapetum of anther and may function in the production and secretion of substances for the developing reproductive cells

• During enlargement, ovule bends so the micropyle lies close to the placenta along which the pollen tubes grow

Megasporogenesis

• Where megasporocyte (2n) undergoes meiosis I to form a dyad and then meiosis II to form 4 haploid megaspores arranged in linear tetrad (separated by cell walls)

• Megaspore nearest the chalaza remains functional out of the tetrad and other 3 degenerates

• Functional megaspore produces female gametophyte via mitosis

Megagametogenesis (no cytokinesis) → Monosporic or polygonum type

• Megaspore grows and gets nutrients from nucellus

• Megaspore nucleus divides mitotically to form 2 nuclei

• Each nuclei moves towards the opposite pole and once again divide twice mitotically 

• Each pole now has 4 nuclei (8 in total)

• Embryo sac is not coenocytic during development

• Callose shell surounds the entire embryo sac during development

• 1 nuclei at each pole migrates towards center becoming the polar nuclei of central cell

• 3 remaining nuclei at each pole is surrounded by cytoplasm and membranes to form cells via cytokinesis

• Movement of nuclei due to remnants of spindle fibers

• 3 cells towards micropyle → egg-apparatus 

  • Larger cell → Egg cell

  • 2 smaller cells → synergids

• 3 cells towards the chalaza → antipodal cells

• 8 nucleated and 7-celled structure is the female gametophyte or mature embro sac

• Polar nuclei may or may not fuse before fertilization

  • If they fuse → polar nuclei form a secondary nucleus (2n) 

  • If they dont fuse → becomes a single cell with 2 nuclei (n+n)

• Filiform apparatus → finger like processes produced from the outer wall of the synergids 

  • help synergids absorb food from the nucellus and transfer to the embryo sac

  • May secrete chemicals which attract the growing pollen tube

Characteristics of the Cells of the Embryo Sac

Egg cell (n)

• Highly vacuolated

• Amount of cytoplasm is limited and is spread as a thin layer surrounding the vacuole

• Cytoplasm with little ER, limited number of plastids, mitochondria, and dictyosomes, but high number of ribosomes that are randomly distributed

• Cell wall does not extend over the entire cell

• Strongly polarized where micropylar end has a large vauole and chalazal end has most of the cytoplasm

Antipodals (n)

• Transient existence

• Minimal cytoplasmic organelles

• May have nuclear abnormalities like endoreplication

Polar nuclei (n)

• Metabolically active

• Extensive ER, numerous plastics, mitochondria, dictyosomes, and polysomes

• Large quantities of starch, proteins and lipids

Synergids (n)

• Limited life span, degenerate after fertilization

• Probably involved in nutrition of the egg cell

• Has extensive wall ingrowth at micropylar region called filiform apparatus 

• Produce chemicals that attract pollen tube

Variations in gametophyte development

• Deviations from monosporic megagametophyte development

  • Number of megaspores or megaspore nuclei that participate in the formation of the embryo sac

  • Total number of divisions that take place during the formation of the megaspore and gametophyte

  • Number and arrangement of the nuclei and their ploidy level in the mature embryo sac

Monosporic trimitotic embryo sac

• Meiosis of megaspore mother cell (2n in nucellus produces 4 megaspores (n)

• 3 undergo apoptosis 

• All 8 nuclei are genetically identical → products of mitosis of the megaspore nucleus

• Polygonum type

Bisporic bimitotic development

• Results from failure of cytokinesis after meiosis II

• 2 binucleate cells are produced after megasporogenesis

• Bisporis bimitotic embryo sacs → allium-type 

• Micropylar binucleate cell is suppressed while chalazal binucleate cell undergoes development

• 2 nuclei in functional megaspore contain different genetic combinations due to being products of meiosis thus the nuclei of mature embryo sac will not all be genetically identical

• Only 2 mitosis divisions are involved in the formation of mature embryo sac

Tetrasporic bimitotic development

• Associated with suppression of cytokinesis after both meiosis I and II

• Four-nucleate megaspore

• Produces a chimeric embryo sac after mitosis of 4 genetically different nuclei

• 2 mitotic divisions of the 4 nuclei = 16 nucleate embryo sac

Tetrasporic bimitotic ‘Fritillaria-type’

• Where three somatic spores of megaspore tetrad fuse to form a triploid nucleus

• Egg cell and synergids are haploid and antipodal cells are triploid

• One polar nucleus is haploid and the other is triploid

Gene regulation of ovule formation

• ANT transcription factor → clear role in ovule primordia formation

  • Expressed in the placenta and in the integuments of the developing ovules

  • ant mutant plants → ovules do not develop integuments and megasporogenesis is blocked at the tetrad stage → female sterility

  • ant-9 mutant → number of ovules per carpel is reduced by more than half in respect to the wild type

• HUELLENLOS (HLL) → encodes a mitochondrial ribosomal protein

  • hll mutants → ovule do not develop integuments

  • hll-1 and hll-3 → reduction of 10% in number of ovules and display smaller gynoecia

• Double mutant hll ant → more severe at the level of primordia outgrowth

• short integument 2 (sin2) mutants 

  • Arrest in cell division in both ovule integuments

  • Shorter pistils bearing less ovules than wild type

• Double mutant sin2 ant-5 → same with ant-5 single mutant 

• ANT is epistatic to SIN2 with respect to ovule development

• sin2 hll-1 double mutant → stronger effect on ovule development than their single mutants

• ANT plays a master role, SIN2 and HLL contribute to ovule primordia formation

Role of hormones in ovule primordium formation

• Auxin 

  • Responsible for the correct apical basal patterning of the gynoecium 

  • Auxin gradient hypothesis supports 

    • high levels of auxin in gynoecium apical regions control stigma and style formation

    • Medium levels direct ovary formation 

    • Low levels for the gynophores at the gynoecium base

  • yucca1 yucca4 (yuc1 yuc4) and weak ethylene insensitive8 tryptophan aminotransferase related2 (wei8 tar2) double mutants → severe gynoecium defects lead to a pistil with a reduction or complete absence of ovules → complete sterility

• Cytokinins → activate ovule primordia formation

• Brassinosteroids → involved in the control of the initiation and formation of reproductive organs

  • BR-deficient and BR-insensitive mutants → smaller and less seeds

  • BR-enhanced → more seeds 

  • Play a role in ovule initiation

Pollination and Double Fertilization

Pollination in Angiosperms

• Pollination → transfer of pollen grains from anther to the stigma of the same or different flower of the same species

• Self pollination → same flower or stigma of another flower in the same plant

  • Autogamy → transfer of pollen from anther to stigma of same flower 

    • Possible when flower is bisexual and male and female parts mature at the same time

    • No need for external agents of pollination

  • Geitonogamy → transfer of pollen from anther to the stigma of another flower of same plant

    • Unisexual or bisexual flower

    • All flowers of same plant are genetically identical so still considered self pollination

    • Ecologically considered cross pollination

    • Needs external agents of pollination

  • Geitonogamy genetically similar to autogamy and functionally involves a pollinating agent in cross-pollination

• Cross pollination → transfer to another plant

  • Xenogamy

  • Requires external agents

  • Only type that brings genetically different pollen grains to the stigma

  • Biotic or abiotic external agencies are okay

Wet vs Dry stigma

• Wet stigma → consists of a loose aggregate of secretory cells that produce a fluid rich in glycoproteins, mucilages, and nutrients for pollen germination

• Dry stigmas → do not secrete large quantities but are are highly specialized tissue that allow pollen germination

• Pollen growth involves formation of the pollen tube that emerges from the pollen pore which grows down through the stigma and into the style carrying sperm to egg

Open vs Closed styles

• Open styles → central cavity whose inner epidermal surface is coated with mucopolysaccharides, lipoproteins, and glycoproteins

  • Serves as a nutrient medium for pollen tube 

  • Play a role in directing growth of pollen tube 

  • Continuous stylar canal lined with secretory epidermis

  • Epidermal layer of secretory cells lining a canal with extracellular matrix

• Closed styles → cells are embedded in an extracellular matrix similar to the inner surface of the open style 

  • Pollen tubes growth through the extracellular matrix, deriving both guidance and nutrition from matrix molecules

  • Continuous strand of transmitting tissue inside pistil

  • Presence of substantial intercellular spaces filled with extracellular matrix

  • Elongated cells connected via plasmodesmata 

Pollen germination

• Does not involve cell division

• Pollen tubes are extensions of the tube cell

• Tip growth

• Cell wall of pollen tube has callose 

• Callose → synthesized by golgi apparatus and transported to the extreme tip by golgi derived vesicles

  • Fusion of vesicles with plasma membrane expands cell membrane of elongating tube while contents of the vesicle expand the wall

  • Membrane of these vesicles coated with myosin

  • Vesicles are transported to the tip via actin filaments

• Total cytoplasmic volume does not increase as pollen tube grows

• Bulk of cytoplasm is in close proximity to growting tip and continues to move with tip 

• Distal vacuole expands as the tube elongates which pushes cytoplasm towards tip

• Elongating cell forms periodic callose cross walls or callose plugs at distal region of cytoplasm that seals off newer portions of the tube

  • Only terminal portion of  tube has living cytoplasm

• Cytoskeleton of pollen tube continually transports organelles generative nucleus and vegetative nucleus to growing tip

Pollen tube guidance

• Chemcal attractant released by ovule prior to fertilization is developmentally regulated only occurring when the flower reaches a certain stage

  • Underaged ovules cannot attract pollen tubes because the structures that produce the chemical attractants are not yet there

  • Proposed source of chemical attractant is the synergids

• This stage coincides with synergid development

• Guidance signals are species specific where ovule signals of a certain plant species will not attract pollen tubes from another plant species

• Only one pollen tube can gain access to each micropyle and other approaching tubes turn sharply away once a tube has entered

• Chemical repellant serves as a block to polyspermy

Double fertilization

• Pollen tube enters one of the synergids at the base of embryo sac

• Inside cytoplasm of synergid, pollen tube ruptures to release the tube nucleus and 2 sperm

• 1 sperm fuses with egg → zygote 

• 1 sperm fuses with 2 polar nuclei → triploid endosperm

Embryogenesis and Endosperm Development

Embryogenesis

• Establishes the axis of the plant, with RAM and SAM at opposite ends, and the basic pattern of tissues within axis

• Accompanied by the growth and development of the endosperm 

• Organs only formed after seed germination → post embryonic 

Angiosperm Embryogenesis

• Develops at the micropylar end of the embryo sac where zygote is situated

• Zygote divides to form embryo via mitosis

• Division of zygote only begins when ample endosperm has been formed

Eudicot embryogenesis

• Egg cell → polarized structure with 

  • large central vacuole asymmetrically positioned micropylar end of the egg

  • Nucleaus and cytoplasm opposite end

• Zygote undergoes unequal transverse division to form 2 cells

  • Basal cell → larger cell at micropylar end that inherits the vacuole

  • Apical cell → smaller cell at chalaza that inherits most of cytoplasm

• Basal cell divides transversely to form the suspensor 

  • Suspensor → attaches the embryo to the embryo sac

  • Hypophysis → refers to the uppermost cell of the suspensor

  • Descendants of hypophysis forms the quiescent center or primary root meristem and columella initial

• Apical cell becomes the embryo proper → GLOBULAR STAGE

  • 2-celled spherical embryo → longitudinal division

  • Quadrant (4-celled) → 2-celled embryo divides longitudinally

  • Octant (8-celled) → 4-celled embryo divides transversely

  • Dermatogen stage (16-celled) → each of 8 cells produce a surface layer of 8 cells to cover itself

    • Surface layer → becomes protodern

    • First sign of tissue differentiation

  • 32-celled and 64-celled embryo → protoderm and internal cells continue to divide 

  • Radial symmetry is established

• Triangular stage → transition between late globular and early heart stage

• Rapid cell divisions occur leading to formation of 2 cotyledon primordia → HEART STAGE 

  • RAM and SAM are established in the embryo

  • Procambium can be distinguished in late heart stage

  • Bilateral symmetry is established → axial polarity

• Elongation of the embryo axis → TORPEDO STAGE

  • Hypocotyle and radicle recognized

  • Vascular tissue differentiation within begins

  • Suspensor deteriorates 

  • Cotyledon → functions in food storage, food absorption, and/or photosynthesis

• Cotyledons fold over assuming the WALKING STICK STAGE

• MATURE EMBRYO

  • Radicle → embryonic root

  • Embryo is dormant

  • Seed is ready for dispersal

Endosperm

• Triploid tissue formed when a sperm fertilized 2 polar nuclei

• Some eudicots → endosperm divides and fill portion of mature seed

• Function → stores nutrients

• Non-endospermic eudicots → endosperm is digested and nutrients moved to 2 cotyledons

Suspensor

• Structure formed by the larger basal cell after division of zygote

• Range from single to massive collection of cells

• Can contain tiers of multi-nucleated cells forming a syncytium

• Have basal cells at micropylar end → site of max metabolic activity

• Can be polyploid and/or undergo endoreduplication

• Legumes → presence of giant polytene chromosomes

• Suspensor pushes embryo proper into endosperm cavity and connects embryo proper to surrounding maternal and endosperm tissues

• Serves as conduit for nutrients and growth regulators 

• Have structures that enhance ability to transfer molecules

  • Cell-wall ingrowths

  • Haustorial outgrowths

  • Numerous plasmodesmata

• Lacks a cuticle layer 

• Organelles present: mitochondria, ER, specialized plastids

• In early development stages, suspensor cells have higher RNA and protein synthesis levels than embryo proper

• Hormones present: GA, auxin, cytokinin, abscisic acid

• Programmed cell death upon entering maturation 

Chief events of embryogenesis

• Establishment of the precursors/initials for dermal, ground, and vascular tissues

  • Differentiated in a radial pattern

  • By globular stage (sometimes during octant stage)

• Establishment of apical-basal polarity 

  • By transition from globular to heart stage

• Establishment of RAM and SAM

  • Heart stage

Monocot Embryogenesis

• More complex than eudicots

• Early embryo development is similar 

Proembryo stage

• First cell division is asymmetrical (in various planes)

  • Apical cell → divides faster to become embryo

  • Basal cell

Globular stage

• Suspensor is not a single or double row of cells and is less differentiated

• Late globular → outer epidermal layer is evident + group of cells on one side of proembryo divides faster to produce embryo axis

Scutellar stage

• Remnant of cotyledon can be seen

• Scutellum → single modified cotyledon that acts as a conductive tissue between endosperm and embryo axis

Coleoptilar stage

• Embryo axis differentiates into plumule and radicle

• Coleoptile → specialized tissue protecting the shoot

• Coleorhiza → specialized tissue protecting the root

Plant embryos form from regions that develop autonomously

• 8cell stage has four regions with different developmental fates

• 4 regulatory genes affect aspects of the apical-basal pattern 

• Mutations in these genes result in deletion of specific embryonic regions 

Endosperm Development

• Seed development initiated by double fertilization

  • Fertilization of haploid egg cell → diploid embryo

  • Fertilization of diploid central cell → triploid endosperm

• Function

  • Nourish and support embryo by delivering nutrients acquired from mother plant

  • Protects embryo from mechanical injury

• Fates

  • Consumed by developing embryo before maturation

  • Persist in mature seed and used up during seed germination

• Gymnosperm endosperm is haploid and formed before fertilization

• Angiosperm endosperm is triploid and formed after fertilization

• Absent in : Orchidaceae, Podostemaceae, and Trapaceae

• Primary endosperm nucleus (PEN) → where endosperm develops from as a result of triple fusion

  • Triploid (fusion of one male gamete with 2 polar nuclei)

• Free nuclear proliferation without cytokinesis (syncytial.coenocytic phase) → cellularization phase initiated in a region surrounding embryo → outer to inner region of endosperm

• Arabidopsis, endosperm cellularization during early heart stage

• Endosperm cellularization failure → embryo arrest and seed abortion

• Early stages of seed germination

  • When seed dormancy is broken, embryo starts to produce GA

  • GA triggers aleurone cells within the seed to start releasing amylase

  • Amylase will hydrolyze starch in endosperm into maltose

  • Cotyledons absorb the maltose from the endosperm and give it to the embryo

Types of Endosperm

• Cellular endosperm (advanced)

  • PEN division → cell wall formation

  • First division = 2 equal sized cells: chalazal and mycropylar cells

  • Subsequent divisions followed by cell wall formation

  • Thus, endosperm is cellular from the beginning

• Nuclear Endosperm (primitive)

  • Most common in angiosperms

  • PEN division = many free nuclei → Coenocytic stage

  • Division not accompanied by wall formation

  • Free nuclei arrange towards periphery of cytoplasm → wall formation starts from periphery towards center 

  • Cell plate formation centripetally 

  • Arabidopsis and Capsella 

  • Liquid endosperm of coconut

• Helobial endosperm (advanced)

  • Intermediate between cellular and nuclear types

  • PEN division → large micropylar cell and small chalazal cell

  • Nucleaus of micropylar divides freely without cell wall formation and cell wall forms from periphery to inward

  • Nuclaeus of chalazal cell remains undivided or divides for few times (basal apparatus)

  • Helobial endosperm

WEEK 1

• All the cells in the plant body will have the exact same copy of the DNA of the genes. Different gene expression is the reason for the different cell types

• Plants: Formed after germination. Dependent on apical meristems. Indeterminate growth. 

  • Plants are sedentary, instead they alter its development and morphology to help them survive. 

  • Cell division in plants are concentrated in the meristems. In animals, it happens everywhere

  • Indeterminate: shoot and root

  • Determinate: flower meristem, 

  • Developmental plasticity → effect of environment to the development of plants 

    • Unlike animals. By the time they get to their environment, they are already fully formed

  • Totipotent → ability to become any cell type in the body of that organism

    • Zygote can give rise to any other structure later on → naturally totipotent

    • Even if it is a mature cell type it can be induced to become totipotent. This is not always natural so it needs to be induced

    • Callus 

    • Animals → Cells of the blastula

    • Youngest cells of the meristems are still naturally totipotent

  • Pluripotent → lesser ability because fates are determined, slightly determined fates

    • Protoderm cannot give rise to a xylem and phloem but it can develop into any epidermis cell type

    • Animals → cells of the gastrula

  • Embryogenesis

    • Embryo formation in animals, all organs have been formed already

      • Development during embryogenesis

    • Plant embryogenesis is just one small portion of the entire plant life.

      • Only meristems are established

      • No organs in plant embryogenesis

      • Development happens post embryonic

  • Plant development has no cell migration

    • Anticlinal → perpendicular, all in one row, for wide organs

    • Periclinal → parallel, all in one column, for elongated organs

    • Depends on where cell plate is located

• Model organisms

  • Arabidopsis → eudicots

  • Zea mays → monocot

  • Tobacco 

  • Rice 

  • Characteristics 

    • Short life cycle to look at progeny 

    • With high seed production = more offspring = more replicates

    • Self-fertilization = to look at homozygosity and heterozygosity of gene 

WEEK 1

• Development vs growth

  • Development → differentiation, maturation

  • Growth → increase in number or size of cells

• Differentiation → normal process,

  • Meristem cell becomes cell of stomata then guard cell

  • Proplastids → chloroplast

• Dedifferentiation → mature to immature 

  • Phloem cell → procambium 

  • Mature leaf → callus

  • Chloroplast → proplastid

• Rediffirentiation → Mature cell to nother mature cell type

  • Chloroplast → chromoplast, vice versa

  • Callus → shoot or root

  • Do they need to dedifferentiate to differentiate? 

  • Can occur directly

  • Mesophyll cells → tracheary element without reversion to undifferentiated state

• Pattern formation

  • Asymmetric cell division 

    • Apical → embryo proper, transversely and longitudinally

    • Basal → suspensor, longitudinal

  • Lateral inhibition → prevents cells beside it from becoming the same cell type as them

  • Programmed cell death → holes in leaves

• Plane of cell division very important → determines plant morphology

  • Preprophase band and pragmoplast → both composed of microtubules and actin but they both appear in different stages

  • Preprophase occurs prior to actual mitosis, during interphase specifically G1 phase 

  • Preprophase disappears and leaves behind a signal telling the phragmoplast where to form

  • Phragmoplast appears during telophase of mitosis

  • Phragmoplast tells the golgi derived vesicles where to go 

  • Cell plate formation form the inside going out starting from the middle spreading outward 

  • CDZ is part of cytoplasm where phragmoplast is formed → just a region 

  • Centrifugally

• Microtubules are the tracks of the train

  • Cellulose microfibrils deposited the same way the microtubules are laid 

  • Cellulose microfibrils also provide guidance 

  • Dual guidance model by microtubules and existing cellulose microfibrils 

• Auxin

  • Auxin is the hormone

  • Presence of auxin 

    • TIR1 will be able to mark Aux/IAA for degradation 

    • ARFs will be free to induce changes in transcription 

    • Change in transcription happens 

  • Absence 

    • TIRI is not able to mark Aux/IAA for degradation 

    • AFFs are not free to induce changes in transcription 

• Communication

  • Apoplast → cell walls

  • Symplast → plasmodesmata

Week 2 

• Plant life cycle

  • Gametophyte generation → haploid cells that function to produce gametes via mitosis 

    • Haploid and multicellular 

    • Different genetic composition compared to sporophyte due to meiosis

    • Fern → prothallus 

  • Spore formation via meiosis (haploid and unicellular)

    • Megasporogenesis

    • Microsporogenesis 

    • Fern → sporogenesis only

    • Sexual reproduction → genetic variation

• Pistil and stamen structures all part of sporophyte 

• Embryo sac and pollen grain are gametophyte 

• Stamen → within anther → in pollen sacs (microsporangium) → meiosis for microspore formation

• Dehiscence → anther opened and pollen grains are released 

• Microspores are in tetrads → released from tetrads become pollen grains but whether they are not they are mature we are not sure 

  • Mature → two or more cells inside

    • Vegetative + generative = bicellular

    • 2 sperm cells + vegetative = tricellular

  • Not mature → only one cell

• Microsporocytes are not in tetrads 

  • Pollen grains are smaller than microsporocytes

  • Microsporocytes are larger 

  • Pollen grain not completely round

  • Microsporocyte are very round

  • Microsporocytes have walls that connect them so that they undergo meiosis at the same time

• L3 → connective and vascular + inner tapetum

• LI → outer → epidermis and stomium

• L2 → middle → primary parietal (outer), sporogenous cells (inner)

  • Hypodermis

  • Middle wall layer, tapetum, pollen mother cells, endothecium → all diploid

  • Microsporocytes → haploid

• Degeneration → complete deterioration of the structure

• Senescence → related to aging, meaning the structure aged and stops to divide completely

• Stomium is the point where anthers dehisce 

• CCC → inner to stomium composed of large cells sometimes with crystals that degenerate and connect the two pollen sacs 

Week 3

• Pistil → stigma, style ovary

• Compound pistil = fusion of carpels (typically fusion in the ovary like lily)

• Many separate carpels = 1:1 ratio

• All pistils are gynoecium 

• Ovule contains the cells that become the embryo sac later on

• Nucellus surrounds the embryo sac → megasporangium 

• Megaspore mother cell via meiosis = megaspore = 3 degenerate, 1 functional

• 3rd mitosis then cytokinesis 

• Synergids accept the pollen tube

• Monosporic

  • All resulting cells are geentically identical

• Bisporic, bimitotic

  • 1 binucleate cell → 2 different haploid nuclei divide → cells of embryo sac have different genetics 

• Tetrasporic, bimitotic

  • 1 tetranucleate cell → 4 different haploid nuclei divide → cells of embryo sac have different genetics 

• Lily 

  • Nuclei fusion → 3n 

  • Antipodal are 3n

  • Synergids are n

  • Polar nuclei 1 is n

  • Polar nuclei 2 is 3n

  • Endosperm is 5n (4n + haploid sperm)

• Pollination

  • Resulting offspring will never be the same as the parent plant even if self pollination

  • Never be a clone because of meiosis

  • Clone only for asexual reproduction

  • Spore formation = sexual reproduction = no clones

Embryogenesis 

• Shoot and root apical meristem establishment = embryogenesis

• Monocot → coleoptile, coleorhiza, scutellum 

  • Suspensor is multiseriate 

  • Scutellum doesnt have a storage function, it is an absorptive structure

  • Coleorhiza protects the RAM

  • Coleoptile protects the SAM

  • 1 cotyledon 

  • Most of the seed is full of endosperm 

• Eudicot

  • Dermatogen stage → 16 cell stage is where tissue differentiation can be observed 

    • Outer 8 become protoderm

  • Hypophysis is the only cell that becomes part of the RAM 

  • Heart stage → cotyledon primordia

  • Torpedo stage → elongation of cotyledon

  • Walking stick stage → cotyledons fold over to fit inside the seed

  • Mature 

• Go (GURKE) For (FACKEL) More (MONOPTEROS) Gold (GNOM)

•  Endosperm development → all become cellular at the end

  • Nuclear → outermost to innermost cytokinesis

  • Helobial → Chalazal cell does not divide, upper cell like nuclear