Plant Bio Exam 2 BS idk even know if this will be on the exam

How does cell differentiation occur as cells leave the Shoot Apical Meristem (SAM)?

• Cells differentiate as they move from the Central Zone into the Peripheral Zone and Rib Zone.

• This transition is driven by physical location, cellular layer history (L1/L2/L3), transcription factors, and hormonal gradients.

What are the origins and fates of the three primary meristems derived from the SAM?

1. Protoderm (Epidermis): Arises strictly from the L1 layer.

2. Ground Meristem (Cortex & Pith): Arises primarily from L2 and L3 layers pushing into peripheral/rib zones.

3. Procambium (Xylem & Phloem): Forms deeper within L3, specifically at localized sites in the peripheral zone where leaf primordia develop.

What are the two models for the origin of Axillary Meristems (AMs)?

1. Detached Meristem Model: A few pluripotent cells are left behind in the leaf axil to form a new niche.

2. De Novo Induction Model: Positional signals induce the dedifferentiation of cells in the leaf axil to form a new niche.

• Note: Recent data suggests STM (Shoot Meristemless)-expressing cells are required for AM initiation.

How is the Vascular Cambium formed in stems?

It arises from two sources:

1. Fascicular (Dascular) Cambium: Portions arising from within the vascular bundles.

2. Interfascicular Cambium: Arises from dedifferentiated parenchyma cells located between the vascular bundles.

What are Lenticels?

• They are porous tissues within the periderm.

• They contain numerous intercellular spaces that allow for gas exchange between the internal tissues and the atmosphere.

What constitutes Secondary Growth in roots and stems?

1. Secondary Vascular Tissues: Formation of secondary xylem and phloem from the vascular cambium.

2. Periderm (Cork): Formation of protective bark from the cork cambium.

How does Apical Dominance work?

• Controlled primarily by Auxin.

• Shoot tips produce auxin, which moves basipetally (downward) down the stem.

• This auxin flow suppresses the outgrowth of lateral (axillary) buds.

• Evidence: Decapitation (removing the tip) leads to side bud outgrowth; applying exogenous auxin to the stump inhibits this outgrowth.

How do chemical inhibitors of Polar Auxin Transport (PAT) affect branching?

• Chemicals like TIBA or NPA block auxin transport.

• This induces lateral bud outgrowth by disrupting the auxin gradient that maintains apical dominance.

What genetic findings explain branching in Petunia vs. Pea?

• Petunia: Genes from the dad, rms, and max series control branching (orthologs exist in other species).

• Pea: Branching phenotypes were shown to be independent of auxin and cytokinin pathways.

How did domestication affect branching in Wheat and Rice?

• Selected for multiple tillers.

• This distributes grain production evenly and allows for relatively simultaneous maturation.

How did domestication affect branching in some other Cereals?

• Selected for strong apical dominance.

• This suppresses side branching and concentrates seed production on a single, large terminal head.

What is the genetic basis for the difference between Teosinte and Cultivated Maize?

• Teosinte: Bears elongated lateral branches at most nodes.

• Cultivated Maize: Lacks tillers/side branches.

• Key Mutation: A gain-of-function mutation in the gene tb1 (teosinte branched1).

• Effect: tb1 encodes a transcription factor that inhibits side branching, altering source-sink relationships and increasing yield.

What is the evolutionary timeline of plant structures?

• ~425 Million Years Ago: Earliest land plants appeared.

• ~360–400 Million Years Ago (Devonian Period): Leaves evolved.

• ~130 Million Years Ago (Cretaceous Period): Flowers evolved.

What is a Phytomer?

The basic structural unit of the shoot system, consisting of:

1. A leaf

2. A node

3. An internode

4. An axillary meristem

How is Leaf Initiation determined?

• It originates from the Shoot Apical Meristem (SAM).

• Initiation is determined in part by auxin distribution (accumulation of auxin at specific sites triggers primordium formation).

How are Boundaries maintained in leaf development?

• SAM vs. Leaf Primordium: Boundaries are maintained by mutually antagonistic gene regulators.

• Upper vs. Lower Leaf Surface: Polarity is established and maintained by mutually antagonistic transcription factors.

How does a meristematic cell become a leaf primordium cell?

The cell must first acquire a new identity as “leaf” rather than remaining a “meristem” cell. This involves a shift in gene expression profiles.

What controls Leaf Polarity and Blade Outgrowth?

Mutually antagonistic transcription factors are expressed in specific domains to establish polarity (adaxial/abaxial) and drive the outgrowth of the leaf blade.

What is the genetic difference between Simple and Compound leaves?

• Simple Leaves: Single, undivided blade.

• Compound Leaves: Divided into multiple leaflets.

• Control: Specific transcription factors regulate the extent of blade expansion and segmentation to determine complexity.

What are the five “Take Home Messages” regarding leaves?

1. Leaves originate from the Shoot Apical Meristem (SAM).

2. Leaf initiation is determined in part by auxin distribution.

3. Boundaries (SAM/Leaf and Upper/Lower surface) are maintained by mutually antagonistic gene regulators.

4. Cellular differentiation is regulated by gene expression and modified by developmental/environmental signals.

5. Leaves can be modified for water conservation, defense, and nutrient acquisition.

What are the two main components of Leaf Growth?

1. Cell Proliferation: Cell division.

2. Cell Expansion: Increase in cell size.

Describe the pattern of Cell Proliferation in leaf development.

• Early Stage: Proliferation occurs across the entire primordium.

• Later Stage: Proliferation is restricted to subsets of cells called meristemoids (e.g., in grasses like maize).

Describe the pattern of Cell Expansion in leaf development.

• Follows and partially overlaps with division phases.

• Initial: Primordium expands isodiametrically (equally in all directions).

• Later: Growth becomes uni- or bi-directional (e.g., linear growth in maize leaves).

What are Meristemoids?

Subsets of cells in the leaf that retain the ability to divide and differentiate into specific cell types (like stomata or trichomes) based on developmental and environmental cues.

How is Stomatal Development regulated?

• Guard cell differentiation is regulated by transcription factors, proteins, and environmental factors.

What are Trichomes?

Hair-like structures on the leaf surface.

• Mutants:

  • Underproducing mutants: Fewer trichomes.

  • Overproducing mutants: More trichomes.

  • Spacing mutants: Defects in the regular spacing of trichomes.

What happens to leaves in Autumn (in deciduous plants)?

• Chlorophyll (green pigment) is degraded.

• Nitrogen-rich nutrients are reabsorbed into the plant.

• Leaves are dropped (abscised).

Why are leaves considered Renewable Resources?

• They are often short-lived organs.

• They can be replaced annually or if damaged, allowing the plant to maintain photosynthetic capacity.

What are the four main types of Plant Life Cycles based on duration and flowering?

1. Annuals: Survive one growing season.

   • Summer annuals: Germinate in spring, flower in late spring/summer.

   • Winter annuals: Germinate in fall, overwinter, flower in early spring.

2. Biennials: Require two years to complete their life cycle (e.g., spinach, parsley). They do not flower in the first season.

3. Perennials: Live for more than two years (e.g., prairie grasses, trees). Some grow for many years before flowering.Monocarpic vs. Polycarpic:

   • Monocarpic: Flower once, set seed, and then die (e.g., many weeds).

   • Polycarpic: Flower and produce seed many times during their life cycle (e.g., trees). They must maintain vegetative meristems.

What is the significance of the decision to flower?

It marks the transition from vegetative growth (leaves/stems) to reproductive growth (flowers). This is a key life cycle decision where the plant shifts resources from growth to reproduction.

What is the structure that supports the flower organs?

The Receptacle, which is attached to the stem via the Peduncle.

What are the four whorls of a Modern Flower (from outermost to innermost)?

1. Sepals: Leaf-like; form the outermost whorl (collectively called the calyx).

2. Petals: Second whorl; often colorful to attract pollinators (collectively called the corolla).

3. Stamens: Male reproductive parts.

   • Filament: Stalk.

   • Anther: Produces pollen.

4. Pistil (Carpel): Female reproductive part.

   • Stigma: Where pollen lands.

   • Style: Stalk leading to the ovary.

   • Ovary: Encloses ovules; develops into fruit.

What is the Perianth?

The non-reproductive part of the flower, consisting of the Sepals (outer) and Petals (inner).

How do Basal Angiosperms differ from Eudicots in floral organ development?

• Basal Angiosperms: Have less distinct floral organs and show gradients of ABC gene expression with fading borders.

• Eudicots: Have distinct organs with sharp boundaries of gene expression.

• Evolutionary Insight: The classical ABC system likely evolved from a “fuzzy” system with gradual transitions by sharpening gene expression domains.

What is the ABC Model of flower development?

A model explaining how three classes of genes (A, B, and C) determine floral organ identity in overlapping whorls:

• Class A: Determines Sepals (Whorl 1).

• Class A + B: Determines Petals (Whorl 2).

• Class B + C: Determines Stamens (Whorl 3).

• Class C: Determines Carpels (Whorl 4).

• Note: Class A and Class C genes are mutually antagonistic.

What connects Flower Development to Fruit Development?

• After fertilization, the Ovary (part of the pistil/carpel) matures into the Fruit.

• The ovules inside the ovary develop into Seeds.

• Other floral parts (sepals, petals, stamens) typically wither and fall off.

Summarize the transition from Vegetative to Reproductive Meristems.

• Vegetative meristems produce leaves and stems.

• Under specific environmental and genetic cues, the Shoot Apical Meristem (SAM) transitions into a Floral Meristem.

• The floral meristem produces floral organs (sepals, petals, stamens, carpels) arranged in whorls on the receptacle.

• In polycarpic plants, this transition often occurs in specific seasonal windows, while vegetative meristems are maintained for future growth.