Plants’ Hierarchical Organization
- Cell: fundamental unit of life
- Tissue: group of cells consisting of 1+ cell types that perform a specialized function
- Organ: consists of several type of tissues that carry out particular functions
Vascular Plant Organs
Vascular Plants: plants that can transport things up and down
Root system: underground
- Roots rely on sugar produced by photosynthesis in the shoot system (cannot make sugar by themselves)
- ==Root==: organ with important functions
- anchoring the plant
- absorbing minerals and water
- storing carbs (excess made in photosynthesis
- because plants aren’t always guaranteed to have sunlight
- ==Lateral roots==: roots that emerge from the sides
- Help anchor
- Spread, help absorb nutrients
- ==Primary root:== first root to emerge from the seed
- ==Taproot System==: one main root and branches
- ==Taproot==: develops from primary root & anchors plant in soil
- ==Fibrous root system:== lots of small roots (no need for strong anchor)
- ==Adventitious roots==: side roots that arise from the stem & give rise to branching lateral roots
- ==Root hairs==: extensions of epidermal cells forming near root tip
- Increase surface area, more absorption
- ==Mycorrhizal associations:== roots have fungus living in it that help them transform something they cannot use into something they can
- ex/ Break down unusable nitrogen (symbiotic relationship)
Types of Roots
- ^^Prop roots:^^ more anchorage
- ^^Storage roots:^^ store carbs
- ^^Pneumatophores^^: some can do gas exchange (similar to leaves)
- ^^“Strangling” aerial roots:^^ add further anchorage
- ^^Buttress roots^^: very tall and thin roots above ground, typically in forests and jungles (spread far and wide)
Shoot system: above ground
- Shoots rely on water and minerals absorbed by the root system
- ==Stem==: plant organ consisting of nodes & internodes
- ==Nodes==: points at which leaves are attached
- ==Internodes==: space between nodes
- ==Apical bud==: causes elongation of a young shoot
- where plant grows taller
- ==Axillary bud:== structure that can form a lateral branch, thorn, or flower
- not main bud for height, additional buds for leaves, flowers, etc.
==Leaf==: main photosynthetic organ
- Intercept light, exchange gases, dissipate heat, and defend the plant from herbivores and pathogens
- ex/ chemical coat prevents fungus from attacking plant
- Consist of blade and petiole
- ^^Blade^^: flat part
- ^^Petiole^^: stalk that joins leaf to node of stem
- Chloroplasts in leaf
- light shines & photosynthesis occurs
- Why are leaves flat?
- To maximize surface area and sun exposure
- ==Monocot==: 1 thing comes out of seed
- Parallel veins
- Vascular bundles are scattered throughout the ground tissue
- ==Eudicot==: 2+ things come out of seed
- Branching veins
- Vascular tissue consists of vascular bundles arranged in a ring
- ==Vein==: the vascular tissue of leaves
- How do we categorize plants?
- Determine if monocot or eudicot
- What do the flowers look like?
- What do the leaves look like?
- ==Simple leaf==: 1 leaf
- ==Compound leaf:== has several individual ^^leaflets^^
Evolutionary Adaptations of Leaves
- ^^Tendrils^^: help anchor it above the ground
- ^^Spines^^: defend
- ^^Storage leaves^^: store carbs
- ^^Reproductive leaves:^^ leaves can reproduce
Dermal, Vascular, and Ground Tissues
- Tissue system: interconnected leaf, stem, and root
- Dermal tissue system
- Nonwoody plants
- Dermal tissue system: protection of the plant, consists of epidermis
- ==Cuticle==: waxy coating that helps prevent water loss from the epidermis (too hot from the sun)
- No periderm
- Woody plants
- ==Periderm==: outer layer protective tissues that replace epidermis in older regions of stems and roots
- They have both periderm & epidermis
- ==Guard cells==: epidermal cells that facilitate gas exchange in shoots
- too hot→ close
- prevent water loss
- cold→ open
- ==Trichomes==: outgrowths of the shoot epidermis that can reduce water loss and reflect light, also defend against insects
- Vascular tissue system: facilitates transport of materials through the plant & provides mechanical support
- ==Xylem==: conducts water & dissolved minerals upwards from roots to shoots
- ==Phloem==: transports sugars from leaves (where they are made) to storage/sites of growth (down)
- ==Stele==: everything except the leaves
- In angiosperms, stele of root is central solid ==vascular cylinder== (in the middle)
- ==Angiosperms==: plants that make flowers
- Ground tissue system: tissues that are neither dermal nor vascular
- Cells specialized for storage, photosynthesis, support, and transport (everything else)
- ==Pith==: ground tissue inside vascular tissue
- ==Cortex==: ground tissue outside vascular tissue
- Stem cells are not differentiated
- Structure changes depending on function
Different Meristems Generate New Cells for Primary & Secondary Growth
- Indeterminate growth: plant grows until death
- Determinate growth: stop growing after certain age/size
- Meristems: unspecialized tissues made of dividing cells
- how plants grow continuously
- ==Apical Meristems:== located at tips of roots and shoots (up and down)
- ^^Primary growth^^: growing vertically
- ==Lateral Meristems==: grow sides/in diameter/in width (^^secondary growth)^^
- ==Vascular Cambium==: adds layers of vascular tissue called secondary xylem (wood) and secondary phloem
- ==Cork Cambium:== replaces epidermis with periderm (thicker and tougher)
- ==Initials==: stem cells (new cells) that remain in meristem and produce more cells
- unspecialized
- the farther from meristem, they get specialized
Primary Meristems
- ==Protoderm==: produces dermal tissue
- ==Ground meristem==: produces ground tissue
- ==Procambium==: produces vascular tissue
Classification of Angiosperms
- ==Annuals==: complete life cycle in <=1 yr
- ==Biennials==: require 2 growing seasons
- ==Perennials==: live for many years
- “Life cycle”- moment of sprouting→ reproduction
- ==Root Cap==: collection of cells that protects root apical meristem as root pushes through soil
Primary Growth of Shoots
- ==Shoot apical meristem==: dome-shaped mass of dividing cells at shoot tip
- leaves of apical bud protect meristem
- Axillary buds develop from ==meristematic cells== left at the bases of ==lead primordia== (collection of cells that were told to become leaves)
- Kept dormant by chemical communication from apical bud (==apical dominance)==
- If axillary bud + apical bud closer→ more inhibited
- Released if shoot tip removed or shaded
Leaf Growth and Anatomy
- Leaves develop from lead primordia along sides of shoot apical meristem
- Epidermis (outer dermal tissue) in leaves interrupted by ==stomata== (pores that allow CO2 and O2 exchange between air and photosynthetic cells)
- Major evaporative loss of water, so located on bottom of leaf
- Each stomatal pore has 2 guard cells to regulate opening and closing
- ==Mesophyll==: where photosynthesis occurs; ground tissue of leaf
- Between upper and lower epidermis
- 2 layers:
- ^^Palisade mesophyll:^^ upper part of leaf
- ^^Spongy mesophyll:^^ lower part of leaf (loose for gas exchange)
- Veins are leaf’s vascular bundles; skeleton of leaf
- Each vein enclosed by protective ==bundle sheath== (xylem & phloem)
- Why?
- If xylem and phloem damaged, no transport can occur
Secondary Growth
- Growth in thickness produced by lateral meristems
- Occurs in ==gymnosperms== (don’t make flowers) & many eudicots; RARE in monocots
- Occurs in stems and roots of woody plants; RARELY in leaves
- Consists of growth of tissues produced by vascular cambium and cork cambium
Growth, Morphogenesis, and Cell Differentiation Produce the Plant Body
- Developmental plasticity: effect of environment on development
- Plant can change form depending on environmental triggers
- ==Morphogenesis==: form body shapes
- Growth is irreversible
- Cell differentiation
- Certain genes become accessible/inaccessible for the lifetime of the cell
Model
- Arabidopsis
- Why is it a model?
- Small size, short generation time, abundant seed production, small genome size
- Appearance changes depending on environment (genes on/off)
- Easily transformed with transgenes (genes from different organism)
Cell Division
- Increases # of cells & potential for growth
- Cell elongation: process responsible for increase in plant size
- ==Prophase==: nucleus dissolves
- ==Metaphase==: chromosomes line up in middle
- ==Anaphase==: chromosomes separated by microtubules
- ==Microtubules==: pull apart chromosomes; become concentrated into ring called ^^preprophase^^ band that predicts future plane of cell division
- ==Telophase==: cell dividing
- ==Cytokinesis==: pinching off middle part
- Transverse cell division: left and right
- Longitudinal cell division: up and down
- Symmetry of cell division & distribution of cytoplasm affects cell fate
- Concentration of chemicals→ different genes turn on
- Axis of division is predetermined
- ==Polarity==: condition of having structural/chemical differences at opposite ends of organism
- Asymmetrical cell divisions
- Uneven distribution of molecules→ determine what is the root and what is the shoot
- Plant cells grow fast by intake and storage of water in large central vacuole
- Cellulose microfibrils in cell wall restrict direction of cell elongation (prevent from lysing)
Morphogenesis & Pattern Formation
- Pattern formation: development of specific structures in specific locations
- ==Lineage-based mechanisms==: cell fate determined early in development and passed on to daughter cells (animals)
- ==Position-based mechanisms:== cell fate determined by final position (plants)
- Homeotic (Hox) Genes: affect # and placement of appendages in embryos
- MADS-box genes: position & # of plant organs
- KNOTTED-1: development of lead morphology (3d shape)
- Different cells have different proteins
- Different structure→ different function
- Gene activation/inactivation depends on cell-to-cell communication (hormones and signals)
- ==Phase changes:== developmental stages
- Juvenile phase→ adult vegetative stage→ adult reproductive stage
- Occur within shoot apical meristem
Genetic Control of Flowering
- ==Flower Formation:== phase change from vegetative growth→ reproductive growth
- Triggered by environmental cues & internal signals
- ex/ sudden increase in water
- Associated with switching on of ==floral meristem identity genes==
- takes energy to flower
- Order of primordium’s emergence determines its fate:
- ==Sepal==: supports flower after blooming
- ==Petal==: attract pollinators
- ==Stamen==: male reproductive part; make pollen
- ==Carpel==: female reproductive part
- 3 classes of floral organ identity genes
- ABC Hypothesis: how floral organ identity genes direct the formation of the four types of floral organs
- ==A==→ sepal
- ==A+B==→ petal
- ==B+C==→ stamen
- ==C==→ carpel