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