CONTROL OF GROWTH BY HORMONES
Growth and organogenesis are controlled...
...by genes (independent of environment): e.g., number of primaryvascular bundles, general shape of a leaf or flower
...by genes interacting with environment: e.g., direction of growth(up, down, sideways, toward or away from light), timing ofabscission of leaves (in winter, cold, drought) and formation ofstorage organs; formation of symbiotic organs (root nodules, galls)
Hormones (chemical signals) provide clues to internal and externalevents
In plants, there are several families of hormones: auxin(s),gibberellin(s), cytokinin(s), abscisic acid (ABA), ethylene; alsoothers
Auxin
Discovery: phototropism of grass seedling "coleoptile",
modified leaf protecting true leaves of seedling
• in lab, cut off coleoptile, remove leaves
• coleoptile grows (several cm) without cell division, only
cell elongation
• tropic growth: bends towards light, upwards
• tip controls growth (rate and direction)--C. Darwin: tip
perceives light, transmits signal to region of growth in
shaft
Experiment to show that the tip produces a chemical that
stimulates growth: substance can be transferred on an
agar block
Chemical identified as "auxin": indole acetic acid (IAA)
(produced from the amino acid tryptophan)
IAA is produced in shoot tips, moves toward roots
It moves to the lower side or the side away from light
Effects of auxin:
• promotes cell enlargement (in young stems,
coleoptiles, leaves and roots)
• induces differentiation of xylem: cut off apex, stop
differentiation of xylem; add auxin, restore
differentiation; injure xylem, form bypass tracheids if
auxin present
• apical dominance: remove apex, get growth of
branches; add auxin, inhibit growth of branches
• promotes root initiation at base of shoot cuttings (from
natural basipetal movement and accumulation of IAA,
or from artificial application of a synthetic auxin)
newroots
The stimulation of elongation by auxin involvesa loosening of the cellulose microfibrils in thecell wall
Auxin stimulates the formation of roots in cuttings: the middle and right-hand holly cuttings were treated with naphthalene acetic acid, an auxin
Gibberellins
Discovery: Japan--rice with bakanae (foolish seedling
disease); Gibberella fujikuroi caused excessive growth
by overproducing a chemical, gibberellin
Effects of gibberellins
• promote stem elongation: e.g., rice stem with
bakanae; bolting of biennial rosettes (carrot, spinach)
• promote long day growth stimulation
• promote cold treatment growth stimulation
• promote seed germination
• promote mobilization of starch in germinating seeds
• induce differentiation of phloem
Gibberellin inducesbolting in carrot(Daucus carota)
No treatment Gibberellin Cold treatment
Gibberellin promotes mobilization of starch in
germinating seeds by inducing synthesis of
amylase
1350 BCE One of the earliest written records of a urine-based pregnancy test can be found in an ancient Egyptian document. A papyrus described a test in which a woman who might be pregnant could urinate on wheat and barley seeds over the course of several days: “If the barley grows, it means a male child. If the wheat grows, it means a female child. If both do not grow, she will not bear at all.” Testing of this theory in 1963 found that 70 percent of the time, the urine of pregnant women did promote growth, while the urine of non-pregnant women and men did not. Scholars have identified this as perhaps the first test to detect a unique substance in the urine of pregnant women, and have speculated that elevated levels of estrogens in pregnant women’s urine may have been the key to its success. There is a distant relationship between the chemical structures of estrogens and gibberellins; gibberellins are known to stimulate germination of seeds.
http://www.history.nih.gov/exhibits/thinblueline/timeline.html
Cytokinins
Discovery: factor in boiled DNA (or coconut milk)
promoting cell division in plant tissue culture
Effects of cytokinins
• Promotes cell division (in culture and in axillary
meristems)
• Promotes shoot formation in callus
• Inhibits senescence (e.g., bean leaves: cut sections in
water turn yellow, in cytokinin solution stay green)
CytokininInducesshoots
AuxinInducesroots
Cytokinindelays senescence
Abscisic acid
Discovery: factor promoting abscision of petioles in cotton
• Promotes dormancy in perennial buds: short days
induce leaves to make ABA; ABA stimulates formation
of specialized dormancy bud with bud scales
(modified leaves)
• Promotes dormancy in seeds: ABA in seed coat
inhibits germination; cold, soaking in water lower
[ABA] and release inhibition
• Promotes synthesis of storage protein genes during
seed formation
• Stimulates the closing of stomata: ABA is produced
by mesophyll cells in response to drought
Ethylene
Discovery: identified as a gas, H2C=CH2 , produced by
old and wounded plant tissues
Effects of ethylene
Promotes senescence: enzymatic degradation of protein,
chlorophyll in e.g., leaves
Promotes ripening in many fruits (this is senescence, too)
autocatalysis: ripe (or wounded) fruit produces
ethylene, which promotes further ripening (the "one bad
apple" hypothesis)
Promotes thigmotropism: characteristic of agitated
tissues: forms shorter, thicker stems and roots
by dis(re)-organization of microtubules, microfilaments,
leading to loss of polarized expansion of cells
Ethylene promotes fruit ripening: the tomatoes on the right were treated with air containing 0.01% ethylene for 3 days
Ethylene promotes thigmotropism: shorter, thicker stems
and roots
Other hormones
Related to stress responses, defense against pathogens,
etc.: wounding produces compounds that stimulate
defense in other parts of the plant
--brassinolide (steroid)
--jasmonic acid
--systemin (a polypeptide—18 amino acids)
--salicylic acid
--H2O2
--oligosaccharins (carbohydrate elicitors)
--phytosulfokines (polypeptides with tyr-SO4)
Summary
Plants, like animals, control and coordinate growth by producingand recognizing chemical signals
Well-studied signal molecules include:AuxinsGibberellinsCytokininsAbscisic acidEthylene
New discoveries include new organic molecules andpeptide hormones
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