Plant Tissues and Development

Features unique to plant cells

• rigid cell wall - support and protection

• vacuole: cell turgor, holds water and solutes and pigments in certain cell types

• more extensive range of plastids and microbodies

• no intermediate filaments

How does the plasma membrane of a plant differ from others?

• lower levels of cholesterol

• high levels of sterols

• contains galactolipids in cholesterol membrane so phosphates can be used for other essential cellular processes

Comparison of Plant vesicle transport

• is never static

• secretory pathways mediated by COPII coated vesicles

• endocycotic pathways mediated by clatharin coated and COPI coated vesicles

UNIQUE TO PLANTS

• more transport of sterols to PM and glycoproteins processed in plant specific enzymatic reactions

• sterols make high proportion in leaf epidermal cells and seed coat cells

• root cap cell has 20x amount of vesicle pits

Comparison of Plant endomembrane system

• forms close associations with different organelles

• certain proteins are specific to each type of association

• rough and smooth ER

• produce microbodies e.g peroxisomes and oil bodies

• UNIQUE TO PLANT CELLS

  • ER shared between cells

  • transvascular strands

  • predominate polygonal network structure elongated plant cell types

  • cisternae

  • protein PM associations

Microbodies

small specialised organelles with no DNA or ribosomes

• semi autonomous

• simple PM leaflets

• carry out specific and specialised enzymatic reactions

Cisternae

more flattened and very prevalent in young plant cells

Comparison of Plant plastids

• have leaflets

• have their own DNA and ribosomes

• semiautonomous

• mitochondria

• have complex double membranes made of galactolipids inherited from prokaryotic endosymbiotic ancestor

• moved around cytosol via actin filaments

UNIQUE

• chloroplasts

• more types of plastids

Maturation of Specialised Plastids in different cells

1. Proplastids - membrane is rudimentary and internal grana are missing

2. Etioplasts - internal lattice of rudimentary membranes which will develop into grana

3. Leucoplasts - contain no pigment

• different plastid types change in response to cues and changes in plant development during life cycle

Plants compared to Bacteria and Viruses

• different makeup of peptidoglycan cell walls in Bacteria and protein coats in viruses

Features of the primary cell wall

• thin and flexible

• formed first

• cellulose microfibrils

• hemicellulose

• pectins

• proteins

• acts as a barrier

Features of secondary cell wall

• thicker and more rigid

• formed second and after cell growth

• cellulose, lignin and less pectin

Plants compared to funghi

• funghi used to be classified as plants

• funghi have chitin based walls and are multinucleated

Do plants have intercellular junctions

• sessile lifestyle

• have partially permeable primary cell walls stuck together with a gel like middle lamella for flexibility

• specialised tissues have additional impermeable secondary cell walls for rigidity

• only one type: osmotic control and communication: plasmodesmata

Plasmodesmata

• desmotubules link to ER and contribute to biomechanical sensing and signalling

• can have a complex architecture to maintain cell wall rigidity

Function of cells in hierarchical organisation

housekeeping functions

can be specialised

Function of tissues in hierarchical organisation

• uniform looking group of cells that carry out the same function

Function of organs in hierarchical organisation

different tissues made up of groups of different cell types

Dermal Cells/Tissues

3 types of vegetative cell and tissues

1. ground cells of cortex tissue

2. vascular cells of vascular bundles

3. dermal cells of dermal tissue

Dermal Cells

• may form several protective layers

can be associated with:

• epidermis cells: single layer, often has a cuticle to prevent water loss

• guard cells: control water loss and affect gaseous exchange

• trichomes: on leaves and stems, physical barrier and secretory function

Types of ground cells

1. Parenchyma

2. Collenchyma

3. Sclerenchyma

Parenchyma cells

• thin and flexible primary cell walls

• grouped in strands and help support young parts of plant cell

• in roots: have colourless plastids to store starch

• can differentiate

Collenchyma cells

• differentiated parenchyma cells

• grouped in strands and help support young parts of plant shoot

• thicker and uneven cell walls

• no secondary walls

Sclerenchyma Cells

• differentiated parenchyma cells which die once secondary walls are alid down

• Sclereids: short and irregular shape, thick lignified secondary walls

  • found in seet coats

• Fibers: long and slender and arranged in threads

Water conducting Xylem cells

• tracheids and vessel elements

Tracheids

• common to most plants

• long thin tapered ends

• promote lateral water movements

Vessel elements

• tall trees

• more lignified vascular tissue

• perforated end walls

• promotes upwards movement

Sieve tube elemts

• alive at functional maturity

• no organelles

• porous and has end sieve plates

• can conduct nutrients

• non conducting companion cell has organelles that serve both cell

Developmental Biology

• the process of growth and differentiation that shape the body plan

Plant life cycle

• has two phases

• vegetative phase: dominant form is the sporophyte

• reproductive phase: dominant form is the gametophyte

Vegetative growth

• indeterminate: allows for continual growth

• determinate: stops growing after reaching a certain size - gradual loss of meristematic tissue

• leaves, thorns, trichomes

Reproductive growth

determinate: flowers

When does no growth occur?

• dormant periods

• severe prolonged stresses

Location of meristematic tissue

• apical

  • roots (RAMs)

  • stems (SAMs)

• axillary

  • for the morphogenesis of new stems and leaves

• lateral meristems

  • pericycle of roots: lateral root formation

  • cambiums: secondary growth (girth increase)

Morphogenesis

• development of vegetative organs or reproductive organs

• less complex ftissue and organ arrangement

  • sessile

  • ability to respond flexibly to environmental cues

  • less resources: growth in plants is slowed

• adapted life cycles: to avoid stresses:

  • annuals: grow, produce reproductive organs, go to seed, due in one season

  • billenials: need two years to complete life cycle e.g turnips

  • perennials: grow year on year, reproductive phase may vary but usually happens once a year

Plant development meristems

• stem/shoot apical meristems (SAMs)

  • vegetative and reproductive

• root apical meristems (RAMs)

  • vegetative

• lateral:

  • axillary buds

  • roots

• cambiums

  • secondary growth

  • almost unlimited plasticity because of sessile lifestyle

Pluripotency

stem or meristematic cells that can develop into several cell types

Totipotency

stem or meristematic cells that can develop into any other cell type

Pluri and totipotency in plants

• much easier to grow a whole new plant with all organs

• this is due to more developmetnal genes e.g TFs and sRNAs

SAM Structure

• a shoot apical meristem is a dome shaped mass of dividing totipotent cells at shoot tip

• gives rise to pluripotent cells of the primary growth meristems

  • protoderm

  • ground meristem

  • procambium

• leaves develop from leaf primordia along the sides of apical meristem

Gradient Signalling in SAMs

• vegetative phase

  • mediate primary growth and morphogenesis: maintains balance

  • mitosis, differentiation, elongation

• usually indeterminate: SAM is maintained

• multiple signals control patterning

Reproductive phase transition in primary growth

• triggered by specific cues

  • multiple contributory

  • redundant

  • some inhibitory

  • some cause activation

• control floral patterning

  • sepals, petals, stamen, carpel

Cell division and elongation away from the SAM: Signalling profiles

• determine the fate of cells and to produce and ordered body plan of tissues which make up organs

• important to maintain apical dominance (position of OC-red) for continued primary growth

• brought about by auxin-cytokinin integration which affects expression of specific genes (transcription factors, small RNA molucules) transcribed in the OC, initials, elongating cells, fully differentiated cells

• groves rise to primary growth and new vegetative cells and tissues

• development of conserved shoot architecture for a plant species

• core of totipotent cells in the OC maintained by

  • WUS expression and cytokinins

  • CLV-3 and auxin repression

Phyllotaxis and the SAM: repetitive patterns of tissue created

• genetically determined by the autonomous developmental programme of each species

• most common: stem, leaf, bud

• will respond flexibly to environmental cues e.g changing the number/arrangement of leaves

• fractal patterning: each bud resembles the overall shape and structure

• angling determined by the exact nature of gradient signalling of molecules and transcription factors

Tissue Meristem Tissue

• arise from the RAM

• divide further to form the 3 common cells

• fully develop into dermal, ground tissue and vascular tissue in maturation zone

Cell division and elongation away from the RAM

• Quiescent centre - triggered by autonomous development, water availability, gravitropism

• brought about by auxin-cytokin interactions which affects expression of specific genes transcribed differentially in the QC, initals, elongating cells, fully differentiated cells

• cell division in primary meristems increases cell number and the potential for growth

• however cell elongation accounts for the actual volume increase in plant size

• gives rise to primary growth and new vegetative cells and tissues

Cell division in plant cells (cytokinesis)

• entry into mitosis is controlled by coordinated intrinsic and extrinsic signalling cues

• plane of cell division can: longitudinal, radial or both

• formation of a band of microtubles called the preprophase band, give rise to phragomoplasts then cell plates which become new cell wall

• preprophase band: microtubules array determines the future plane of division

  • usually forms perpendicular to apical-basal polarity

  • the biggest microtubule arrays are seen during phragmosome formation

• extrinsic signalling: seasonal changes, water availability, nutrient availability, temp

Plant mitosis and the plane of cell division

• the site of the phragmoplast during telophase which will develop into the new cross plant wall during cytokinesis

• division is usually symmetrical and longitudinal

• phragmoplast usually made of microtubules and F actins, membrane vesicles that fuse to form new plasma membrane from which primary cell walls will then be laid down

  • e.g production of cellulose from cellulose synthases and tubulovesicular netowrk of the endomembrane

• the plus ends of microtubules are associated with proteins attached to the membrane vescibles and are quite dense to allow for cell plate formation in a ring structure around the cortex of the cell

• occurs in the apical meristems and then in primary meristems of the 3 tissues

Asymmetrical cell division and microtubule change

• synergic cells recognise molecules on the pollen tube of the same or compatible species, before degrading to allow access to the egg cell and the central cell

Asymmetrical cell divisions

• asymmetrical cell divisions also play a role in establishing polarity - a critical step in morphogenesis in embryonic tissue patterning

• the first division of a plant zygote is usually symmetrical and it initiates morphogenic polarization into zones of the organism in the 1 cell stage into the shoot system and the root system

Polarity in Cell Divisions

• after the first asymmetrical division of the zygote into an apical cell and a basal cells, structural and biochemical differences occur in cells at the cpial and basal ends of the plant

• differences are driven by the localised expression of development genes such as the WOX family of transcription factors via complex negative and positive feedback loops between neighbouring cells to ensure finely-tuned control

Plant Elongation in the elongation zone

• plant cells grow rapidly at a lower energy cost by intake and storage of vacuoles

• only 10% of the cytoplasm needs to be synthesized in mature cells

• plant elongation is primarily upwards or downwards in the elongation zone, along the apical-basal axis

• the orientation of cellulose microfibrils in the cell walls restrict the direction, direction of cell expansion

Plant cell differentiation after elongation

• pericycle and cambium meristems are predominantly pluripotent

• they are circular and continous throughout the plant, tapered at either end

• tissues in this zone don’t divide or grow anymore

  • the cell fates, forms and functions of the tissues in these two organs

How are the dermal, ground and vascular tissue cells physiologically different after differentiation

• dermal: absorptive and protective

• ground: diffusion of water and key nutrients, most metabolically active cells

• vascular: distinct physiological features that facilitate their roles in transport

Leaf development

• determinate growth

• axillary totipotent meristem and primary pluripotent meristems ebcome terminally differentiated

• deciduos trees: leaving senesce, turn red then fall off

• leaves develop from primordia: undergo cell division, cell elongation, and cell differentiated longitudinally and radially to form a flat blade

Cell division and leaves

• mutant leaves also grow slower than wild type leaves but overall maize leaf is long and narrow

• therefore direction of cell division, which is usually related to microtubule arrangement and stabilised anchoring to the internal leaflet of the PM, does not solely control spacial control of the leaf shape

• it may be that not all components of MT regulation in terms of anchoring vs rapid treadmilling and dynamic instability as they relate to both types of cell division in leaves is not yet fully described

Asymmetrical Cell Division and Stomata

• asymmetrical division gives rise to cells with different fates depending on distribution of cytoplasm

• this is important for the formation of specialised cells such as guard cells