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Chapter 32. Plant Growth and Development. AP Biology Spring 2011. Chapter 32.1. Overview of Plant Development. Seed Germination. Germination : the resumption of growth after a time of arrested development . Environmental Factors Influence Seed Germination.

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chapter 32

Chapter 32

Plant Growth and Development

AP Biology

Spring 2011

chapter 32 1

Chapter 32.1

Overview of Plant Development

seed germination
Seed Germination
  • Germination: the resumption of growth after a time of arrested development
environmental factors influence seed germination
Environmental Factors Influence Seed Germination
  • Seasonal Rains: provide water amounts necessary to swell and rupture the seed coat
    • Water activates enzymes necessary to hydrolyze the stored starch
  • Starches are converted to sugars
    • Provides the energy for the meristems to initiate cell division
    • Oxygen is required, reaches embryo and aerobic respiration provides ATP needed for growth
environmental factors influence seed germination5
Environmental Factors Influence Seed Germination
  • Repeated cell divisions produce a seedling with a primary root
    • When the primary root breaks through the seed coat germination is complete
  • Seed dormancy and germination is climate specific
    • Occurs only when conditions are favorable for the seedling to survive
patterns of early growth
Patterns of Early Growth
  • Growth: an increase in the number, size, and volume of cells
  • Development: the emergence of specialized, morphologically different body parts
  • Patterns of germination, growth, and development have a heritable basis dictated by a plant’s genes
patterns of early growth7
Patterns of Early Growth
  • Early cell divisions may result in unequal distribution of cytoplasm
    • Cytoplasmic differences trigger variable gene expression, which may result in variations in hormone synthesis
    • Even though all cells have the same genes, it is the selective expression of those genes that results in cell differentiation
patterns of early growth8
Patterns of Early Growth
  • Plant growth and development starts with the selective transcription and translation of genes
  • Ex. Page 543 Fig. 32.3 and 32.4
    • Pattern of growth and development of corn (monocot) and bean plant (dicot)
chapter 32 2

Chapter 32.2

Plant Hormones and Other Signaling Molecules

major types of plant hormones
Major Types of Plant Hormones
  • Plant hormones have central roles in the coordination of plant growth and development
giberellins
Giberellins
  • Acidic compounds synthesized in seeds and young shoot tissues
  • Promote stem elongation, germination and starch hydrolysis
  • Help induce flowering in some plants
auxins
Auxins
  • Produced at apical meristems of roots and shoots, coleoptiles in monocots
  • Influence cell division and elongation either positively or negatively depending on the tissue
  • Cause leaves to grown in patterns, stems to bend toward light, roots to grow down
  • Auxins at shoot tips prevent lateral bud growth- apical dominance
  • Help prevent abscission where leaves,

flowers, or fruits drop from plant

    • Abscission: dropping of leaves,

flowers, fruits

cytokinins
Cytokinins
  • Stimulate cell division in root and shoot meristems, where they are most abundant
  • Can release lateral buds from apical dominance and can stop leaves from aging prematurely
  • Used commercially to prolong the life of stored vegetables and cut flowers
ethylene a gas
Ethylene (a gas)
  • Can promote or inhibit cell growth so that tissues expand in the most suitable directions
  • Induces fruit ripening
  • Concentrations high when plant is stressed
    • Ex. Autumn or end of life cycle
    • Induces abscission of leaves and fruits, and sometimes death of whole plant
abscisic acid aba
Abscisic Acid (ABA)
  • Inhibits cell growth
    • When growing season ends, ABA overrides gibberellins, auxins, and cytokinins; causes photosynthetic products to be diverted from leaves to seeds
  • Helps prevent water loss (by promoting stomata closure)
    • When plant is water stressed, root cells produce more ABA which xylem move to leaves
  • Promotes seed and bud dormancy
other signaling molecules
Other Signaling Molecules
  • Brassinosteroids: help promote cell division and elongation
    • Stems stay short in their absence
  • Jasmonates: help other hormones control seed germination, root growth, and tissue defense responses to pathogens
  • FT protein: part of a signaling pathway that induces flower formation
other signaling molecules19
Other Signaling Molecules
  • Salicylic Acid: interacts with nitric oxide in respose to attacks from pathogens
  • Nitric Oxide: functions in plant defense response
  • Systemin: peptide that forms when insects attack plant tissues; travels throughout the plant turning on genes for substances that interfere with the insect’s digestion
commercial uses
Commercial Uses
  • Many synthetic and natural plant hormones are used commercially
  • Ethylene: makes fruits ripen quickly
  • Gibberellin: promotes larger fruits
  • Synthetic Auxins: spayed on unpollinated flowers to produce seedles fruits
  • Synthetic Auxin 2,4-D: used as herbicides
    • Accelerates the growth of eudicot weeds to a point that the plant cannot sustain it and the weeds die
chapter 32 3

Chapter 32.3

Mechanisms of Plant Hormone Action

signal transduction
Signal Transduction
  • Plants have pathways of cell communication

Cell type secretes hormone or

signaling molecule

Binds with receptor

on target cell

Signal transduced to a form that may

influence a metabolic pathway,

gene expression, or membrane properties

hormone action in germination
Hormone Action in Germination
  • Imbibed water stimulates cells of embryo to release gibberellin
    • Water moves giberellin to cells of aleurone (protein storing layer)
    • Water also activates protein digesting enzymes
  • In aleurone layer, hormone triggers transcription and translation of amylase genes to hydrolyze starch molecules
    • Digests starch into transportable sugar
  • Amylase moves into endosperm’s starch rich cells
    • Sugar monomers released from starch fuel aerobic respiration
  • ATP from aerobic respiration provides the energy for growth of the primary root and shoot
polar transport of auxin
Polar Transport of Auxin
  • Auxin concentration gradients start forming during early cell divisions of embryo sporophyte
  • Cells exposed to higher concentrations transcribe different genes than those exposed to lower concentrations
  • Help form plant parts (leaves) in expected patterns
  • Helps young cells elongate
polar transport of auxin26
Polar Transport of Auxin
  • Auxin concentration highest at source: apical meristem in a shoot (or coleoptile)
  • Auxin transported down, toward shoot’s base
    • Polar transport takes place in parenchyma cells
polar transport of auxin27
Polar Transport of Auxin
  • Auxin gives up hydrogen in each cell, which alters cytoplasmic pH
  • Membrane pumps activly transport H+ outside, which lowers pH of moist cell wall
  • Enzymes in cell wall become active at lower pH
polar transport of auxin28
Polar Transport of Auxin
  • Enzymes cleave crosslink's between microfibrils, which support the wall
  • Water is diffusing into the cell, turgor pressure builds against wall
  • Microfibrils now free to move apart, wall is free to expand
    • Ta-dah….cell lengthens!
  • pH change also activates transcription factors, after auxin exposure, proteins that help cell assume its new shape are synthesized
chapter 32 4

Chapter 32.4

Adjusting the Direction and Rates of Growth

response to gravity
Response to Gravity
  • Gravitroprism: growth response to gravity
    • Shoots grow up, roots grow down
  • Auxin, with growth-inhibiting hormone: may play a role in promoting or inhibiting growth in various regions of the plant
  • Statoliths: are unbound starch grains in plastids, respond to gravity and may trigger redistribution of auxin
response to light
Response to Light
  • Phototropism: growth response to light
  • Bending toward light is caused by elongation of cells (auxin stimulation) on the side of the plant NOT exposed to light
  • Phototropins: pigments that absorb blue wavelengths of light and signal the redistribution of auxin that initiates the elongation of cells
response to contact
Response to Contact
  • Thigmotropism: shift in growth triggered by physical contact with surrounding objects
  • This response to auxin and ethylene is prevalent in climbing vines and in the tendrils that support some plants
    • Tendrils: new, modified leaves or stems
  • When cells at shoot tip touch stable object, cells on contact side stop elongating and cells on other side keep growing
  • Unequal rates of growth make vine or tendril curl around object
response to mechanical stress
Response to Mechanical Stress
  • Responses to the mechanical stress of strong winds explain why plants grown at higher elevations are stubbier than those at lower elevations
  • Grazing animals, growing outside vs. greenhouse can also inhibit plant growth
  • Human intervention such as shaking can inhibit plant growth
chapter 32 5

Chapter 32.5

Seasonal Shifts in Growth

seasonal shifts
Seasonal Shifts
  • Circadian Cycle: completed in 24 hour period
  • Photoperiodism: refers to biological response to alternations in the length of darkness relative to daylight during a circadian cycle
    • Ex. The number of hours plant spends in darkness and daylight shifts with seasons
seasonal shifts38
Seasonal Shifts
  • Biological Clocks: internal mechanisms that preset the time for recurring shifts in daily tasks or seasonal patterns of growth, development, and reproduction
seasonal shifts39
Seasonal Shifts
  • Phytochrome: blue-green pigment functions as a receptor for red and far-red light
    • Red light at sunrise causes phytochrome to shift from its inactive form (Pr) to its active form (Pfr)
    • Far-red light at sunset shifts to inactive form (Pr)
    • Longer the nights, longer the interval when phytochrome is inactive
    • Pfr can induce gene transcription
      • Can bring about seed germination, shoot elongation, branching, leaf expansion, and flower, fruit and seed formation, then dormancy
chapter 32 6

Chapter 32.6

When to Flower?

response to hours of darkness
Response to Hours of Darkness
  • Flowering process is keyed to changes in day length throughout the year
  • Cue is length of darkness
response to hours of darkness43
Response to Hours of Darkness
  • Short-day plants: flower in early spring or fall
    • Nights are longer than some critical value
  • Long-day plants: flower in summer
    • Nights are shorter than some critical value
  • Day-neutral plants: flower whenever they are mature enough to do so
response to hours of darkness44
Response to Hours of Darkness
  • Phytochrome is trigger for flowering
  • Detection of photoperiod (alternations in length of darkness relative to daylight) occurs in leaves, where hormones inhibit a shift from leaf growth to flower formation
revisiting the master genes
Revisiting the Master Genes
  • 3 groups of master genes A, B, C control formation of floral structures from whorls of a floral shoot
  • In response to photoperiods of other environmental cues, leaf cells transcribe a flowering gene
  • mRNA transcript travels in phloem to as-yet undifferentiated floral buds, where they are translated into FT protein
  • This signaling molecule with a transcription factor turn on master genes that cause undetermined bud of meristematic tissue to develop into a flower
vernalization
Vernalization
  • Vernalization: low temperature stimulation of flowering
  • Unless certain biennials and perennials are exposed to low temperatures, flowers will not form on their stems in spring
chapter 32 7

Chapter 32.7

Entering and Breaking Dormancy

abscission and senescence
Abscission and Senescence
  • Abscission: the dropping of leaves, flowers, fruits, other parts
  • Senescence: sum total of the processes leading to the death of plant parts or the whole plant
abscission and senescence49
Abscission and Senescence
  • Recurring cue is decrease in day length that triggers a decrease in auxin production
  • Cells in abscission zones produce ethylene, which causes cells to deposit suberin in their walls
  • Simultaneously, enzymes digest cellulose and pectin in the middle lamella to weaken the abscission zone
    • Lamella: cementing layer between plant cell walls
bud dormancy
Bud Dormancy
  • Dormancy occurs in autumn when days shorten, and growth stops in many trees and non-woody perennials
  • It will not resume until spring
bud dormancy51
Bud Dormancy
  • Strong cues for dormancy include short days, cold nights, and dry, nitrogen deficient soil
  • Requirement for multiple cues for dormancy has great adaptive value in preventing plant growth on occasional warm autumn days only to be killed later by frost
  • Dormancy broken by milder temperatures, rains, and nutrients
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