Transcript Chapter 32
AP Biology Spring 2011
Chapter 32
Plant Growth and Development
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