CONTROL OF GROWTH BY HORMONES
Growth and organogenesis are controlled......by genes (independent of environment): e.g., number of primary vascular 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 of abscission of leaves (in winter, cold, drought) and formation of storage organs; formation of symbiotic organs (root nodules, galls) Hormones (chemical signals) provide clues to internal and external events In plants, there are several families of hormones: auxin(s), gibberellin(s), cytokinin(s), abscisic acid (ABA), ethylene; also others
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) new roots
The stimulation of elongation by auxin involves a loosening of the cellulose microfibrils in the cell wall
Auxin stimulates the formation of roots in cuttings: the middle and righthand 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 induces bolting 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)
Cytokinin Induces shoots Cytokinin delays senescence Auxin Induces roots
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 S timulates 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 --H 2 O 2 --oligosaccharins (carbohydrate elicitors) --phytosulfokines (polypeptides with tyr-so 4 )
Summary Plants, like animals, control and coordinate growth by producing and recognizing chemical signals Well-studied signal molecules include: Auxins Gibberellins Cytokinins Abscisic acid Ethylene New discoveries include new organic molecules and peptide hormones