Chapter 38 Plant Growth and Regulation

Biology 102

Tri-County Technical College

Pendleton, SC

Seed Dormancy

• Seed is dormant if all developmental activity within has been suspended– Cells inside do NOT divide, expand, or differentiate

• Insures survival through unfavorable conditions and results in germination when conditions are favorable

• Adaptations include: long cold periods, internal clock, need for fire/heat, light and/or dark, annual seeds can skip a year, moisture (cypress)

Dormancy, cont.

• For embryo to begin developing, dormancy must be broken by physical mechanisms or leeching of inhibitors by water– Exposure to light, mechanical abrasion, fire

• As seed germinates (begins to develop), it first imbibes (takes up) water

• Growing embryo must obtain monomers for its development by digesting polysaccharides, fats, and proteins stored in cotyledon(s) or endosperm

Dormancy, cont.

• Release of GIBBERELLINS signals seeds to break dormancy and germinate

• Imbibed water stimulates gibberellin release (as does some environmental cues)

• In cereal grains, gibberellins stimulate germination and support growth by stimulating synthesis of α-amylase– Will digest stored starch making it available to embryo

and seedling

Cereal Grass Visual

Mono/Dicot Germination

• First step is imbibition (absorption of water) for seed germination in many plants– Hydration causes seed to swell and ruptures seed coat

– Triggers metabolic changes in embryo that cause it to resume growth

– Storage materials of endosperm/cotyledon(s) digested by enzymes and nutrients transferred to growing regions of embryo

• Radicle (embryonic root) emerges from seed

Germination, cont.

• Shoot tip breaks through soil surface• In many dicots, hook forms in the hypocotyl• Growth pushes hypocotyl above ground• Light stimulates hypocotyl to straighten,

raising cotyledons and epicotyl• Epicotyl then spreads first leaves which

become green and begin photosynthesis

Germination, cont.

• Germination may follow different methods depending on plant species

• In peas, hook forms in epicotyl and shoot tip lifted by elongation of epicotyl and straightening of hook

• Cotyledons of peas stay in the ground• In monocots, coleoptile pushes through soil and

shoot tip grows up through tunnel of tubular coleoptile

I want to see…

More, I want to see more…

The Hormones are Flowing…• Hormone is regulatory compound that acts at very

low [ ] at sites distant from where it is produce• Each plant hormone typically plays multiple

regulatory roles• AUXIN (indoleacetic acid, IAA) promotes

elongation of young developing shoots or coleoptiles

• Affects secondary growth by inducing vascular cambium cell division & differentiation of secondary xylem

Hormones, cont.

• Auxin promotes formation of adventitious roots

• Promotes fruit growth in many plants

• Used in herbicides (2,4-D synthetic auxin for dicots)

• Apical meristem is major site of production

Hormones III

• CYTOKININS are modified forms of adenine that stimulate cytokinesis

• Affect cell division and differentiation

• Influence apical dominance

• Serve as anti-aging hormones

• Manufactured in the roots

Hormones, IV• GIBBERELLINS are produced primarily in roots

and young leaves• Stimulate growth in leaves/stems; have little effect

on roots• Stimulate cell division & elongation in stems

(perhaps in conjunction with auxin)• Causes BOLTING-rapid growth of floral stems

which elevates flowers• Fruit development controlled by

gibberellins/auxins (Thompson seedless grapes)• Release causes seeds to break dormancy and

germinate

Hormones, V

• ABSCISIC ACID (ABA) produced in terminal bud; prepares plant for winter by suspending both primary/secondary growth

• Directs leaf primordia to develop scales to protect dormant buds

• Inhibits cell division in vascular cambium• Helps in seed dormancy• Also stress hormone (closing stomata in times of

water stress

Hormones, VI

• ETHYLENE is gaseous hormone that diffuses through air spaces between plant cells

• High auxin [ ]s induce release of ethylene which acts as growth inhibitor

• Senescence (aging) at cellular, organ, & whole plant level affected by ethylene– Important in fruit ripening and leaf abscission

Hormones, VII

• BRASSINOSTEROIDS promote elongation of stems/pollen tubes

• Promote vascular tissue differentiation• JASMONATES trigger defenses against

pathogens and herbivores• OLIGOSACCHARINS trigger defenses

against pathogens– Limit effects of high auxin [ ]s– Regulate cell differentiation

Hormones, VIII

• SALICYLIC ACID triggers resistance to pathogens

• SYSTEMIN causes jasmonate production in response to tissue damage

• IO #4 “Explain the probable mechanism by which gibberellins trigger seed germination was covered in detail in latter part of IO #1. Enough said……

Tropism

• Tropism is growth response that results in curvatures of whole plant organs toward or away from the stimuli

• Mechanism is differential rate of cell elongation on opposite sides of the organ

• Two stimuli that most profoundly affect plant growth are LIGHT and GRAVITY– Phototropism and Gravitropism

• Should mention thigmotropism

Role of Auxin

• Phototropism: cells on darker side of grass coleoptile elongate faster than cells on bright side due to asymmetric distribution of auxins moving down from shoot tip– May be different in other organs

• Shoot tip is site of photoreceptioin that triggers growth response

• PR sensitive to blue light is present in shoot tip– Believed to be yellow pigment related to riboflavin– Same receptor may be involved in other plant responses

to light

Role of Auxin, cont.

• Gravitropism: even in dark, auxin moves to lower side of tipped-over shoot

• Auxin moves downward in response to gravitational stimuli

• Higher auxin [ ] causes more rapid growth on lower side

• Tip curves upward

Role of Auxin, cont.

• Apical Dominance is [ ] of growth at tip of plant shoot where terminal bud partially inhibits axillary bud growth

• Cytokinins and auxin contribute to apical dominance through antagonistic mechanism

• Auxin from terminal bud restrains axillary bud growth causing the shoot to lengthen

• Cytokinins (from roots) stimulate axillary bud growth

Apical Dominance, cont.

• Auxin CANNOT suppress axillary bud growth once it begins

• Lower buds grow before higher ones since they are closer to cytokinin source than auxin source

• Auxin stimulates lateral root growth while cytokinins restrain it

• This stimulation-inhibition action balances plant growth since > in root system would signal plant to produce more shoots

Apical Dominance, III

• Auxin and cytokinins indirectly change [ ] of ethylene

• Levels of different nutrients in bud may also affect response to auxin/cytokinins

• Gibberellins also contribute to apical bud dominance

• Brassinosteriods are required for normal growth and development

Leaf Abscission

• High [ ]s of auxin induce release of ethylene which acts as growth inhibitor

• Senescence (aging) in plants occurs at cellular, organ, or whole plant level– Ethylene plays important role at each level– Xylem vessel elements/cork cells that die before

becoming fully functional– Leaf fall in autumn– Withering of flowers– Death of annuals after flowering

Leaf Abscission, cont.• Mechanics controlled by change in balance of

ethylene and auxin• Auxin decrease makes cells in abscission layer

more sensitive to ethylene• Cells then produce more ethylene which inhibits

auxin production• Ethylene induces synthesis of enzymes that digest

polysaccharides in cell walls further weakening the abscission layer

• Two most important stimuli for leaf abscission are shortening days and cooler temperatures

Leaf Abscission Visual

Leaf Abscission Visual II

Fruit Ripening

• Ethylene triggers senescence during fruit ripening• Aging cells then release more ethylene• Breakdown of cell walls and loss of chlorophyll• Signal to ripen then spreads from fruit to fruit

since ethylene is a gas• Use of ethylene is single most important use of a

plant hormone in agriculture and commerce

Parthenocarpy

• Fruit development normally depends on prior fertilization of the egg (ovum)

• In many species, treatment of unfertilized ovary with auxin or gibberrellins causes parthenocarpy

• Fruit formation without fertilization

• Dandelions, seedless grapes, and cultivated bananas

More of Ethylene• Role of ethylene in leaf abscission covered (IO 6)• Ethylene can be produced in all parts of plant• Called the senescence hormone• As fruit ripens, loses chlorophyll & cell walls

break down• Ethylene promotes both processes and causes

more ethylene to be produced (apple & barrel)• Also associated with apical hook of eudicots,

inhibition of stem elongation in general, and causing stem to lose sensitivity to gravitropic stimulation

Abscisic Acid & Stress

• Called stress hormone because it accumulates when plants deprived of water & possible role in maintaining winter dormancy of buds

• Is most common inhibitor of seed germination• Causes stomata to close and prevents stomatal

opening normally caused by light• Both processes involve ion channels in plasma

membrane of guard cells

Abscisic Acid, cont.

• First response of guard cell to abscisic acid is opening of calcium channels and entry of calcium into cell

• This calcium causes cell’s vacuole to release calcium too

• Increased [ ] of calcium leads to opening of potassium channels

• Release of K+ ions and of water causes guard cells to sag together

• Results in closing of the stomata

Photoreceptors

• Light-its presence or absence, intensity, color, and duration-provides cues to various conditions

• Light regulates many aspects of plant development• Seed gemination to shoot elongation to flowering

to etiolation• Photoreceptors interpret light, its duration, and its

wavelength distribution• Five phytochromes mediate effects of red and dim

blue light

Photoreceptors, cont.

• They are bluish proteins (pigments) found in CYTOSOL of plant cells

• Phytochromes help plants measure the length of darkness in a photoperiod

• Phytochrome is protein containing a chromophore (light-absorbing component) responsible for a plant’s response to photoperiod

• Phytochromes alternate between 2 photoreversible forms– Pr (P red) and Pfr (P far red)

Photoreceptors, cont.

• Plants synthesize Pr and if kept in dark, it remains

• If illuminated, some Pr converted to Pfr

• Pfr triggers many plant responses to light (seed germination)

• Shift in equilibrium indicates relative amounts of red light and far-red light present in sunlight

• Shifts in ratio may causes changes such as increased growth

Moving right on along…

• Far red converted back to red after sunset• Three or more blue-light receptors mediate effects

of higher-intensity light• Cryptochromes are yellow photoreceptor

pigments that absorb in blue/ultraviolet• Affect some of same developmental processes as

phytochromes (seedling development/flowering)• Cryptochromes located in plant nucleus• Phototropin (yellow protein) appears to be

photoreceptor for phototropism