Chapter 32

Plant Growth and Development

AP BiologySpring 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 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 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 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 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 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

Plant Hormones and Other Signaling Molecules

Major Types of Plant Hormones Plant hormones have central roles in the

coordination of plant growth and development

Giberellins Acidic compounds synthesized in seeds and

young shoot tissues Promote stem elongation, germination and

starch hydrolysis Help induce flowering in some plants

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 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) 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) 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 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 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 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

Mechanisms of Plant Hormone Action

Signal Transduction Plants have pathways of cell communication

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 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 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 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 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

Adjusting the Direction and Rates of Growth

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 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 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 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

Seasonal Shifts in Growth

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 Shifts Biological Clocks: internal mechanisms that

preset the time for recurring shifts in daily tasks or seasonal patterns of growth, development, and reproduction

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

When to Flower?

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 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 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 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: 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

Entering and Breaking Dormancy

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 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 Dormancy occurs in autumn when days

shorten, and growth stops in many trees and non-woody perennials

It will not resume until spring

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