Vascular Plant Structure, Growth, and Development

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