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