Chapter 15 continued

• found in fungi and higher plants
• exogenous application causes hyper-elongation of stem tissue
• the only plant growth regulator (hormone) defined by its chemical structure

Physiological Roles of Gibberellins

• seed germination
• stimulation of alpha-amylase production
• flower development
• fruit development

There are over 80 known in the family of ent-gibberellane structures

• 1/3 are C₂₀ gibberellins
• 2/3 are C₁₉ gibberellins

If "A" is attached (e.g. GA₃; GA₈, etc.), they are naturally occurring and their structure is known. (Numbers are assigned in order of discovery)

GA₃ — Gibberellic Acid, one of the first isolated from fungi; commercially used

GA₁ & GA₂₀ (C₁₉ gibberellins)

• most active
• found in higher plants

Note — many gibberellins have NO biological activity, they are intermediates or byproducts

Even though 80+ have been identified, very limited number per species or plant organ

What Makes a GA active?

• COOH at carbon-7 — REQUIRED
• 3-B-Hydroxylation — increases activity
• 3-B, 13-digydroxylation — increases activity
• 1,2-unsaturation — increases activity
• If a GA has BOTH of these = HIGHEST ACTIVITY
• 3-B-OH
• 1,2-Unsat

Inactivation of a GA?

• -OH at Carbon-2

• Control of shoot elongation (fungal GA₃ in rice, for example) GA₃ works on INTACT plants (whereas auxin can be applied to cut stems, etc. and produce elongation)

• Zea Mays dwarf mutants exhibit normal (taller, hence longer internodes) when treated with GA₃ (exogenously)
• No response to other hormones (e.g. auxin or cytokinin)
• Biochemical and radioisotope studies confirm these results for GA₃

• usually environmentally induced by photoperiod (e.g. in spinach) or
• photoperiod + low temp. (e.g. in cabbage)

Therefore, gibberellins are the limiting factor for internode growth in rosettes

And

Environmental conditions can induce formation of active forms of GA.

In spinach, as day length changes, levels of GA₁₉ : GA₂₀ change

• High GA19 : Low GA20 > short internodes
• Low GA19 : High GA20 > Long internodes (bolting)

Inhibitors of Stem Elongation — synthetic chemicals such as ancymidol, AMO-1618, Alar, mimic dwarf genes, inhibit biosynthesis of active GA’s (sprayed on may plants to suppress internode growth)

Seed Germination: Gibberellins in embryo promotes digestion of starch in endosperm by stimulating production of alpha-amylase

Exogenous vs. Endogenous

Hard to find out what controls these and IF GA’s play a role:

• Flowering
• Juvenile phases
• Sexual expression in flowers

Cytokinins

• N6 — substituted derivatives of adenine (purine of the nucleic acids)
• Stimulate Cell division in tissue cultures

Tissue Cultures =

stem, leaf, pith, or cortex tissue + media, form a callus

after a long period the tissues become habituated (produce their own cytokinins) (do not revert back)

the gene for synthesis of cytokinin is switched on

Influences morphogenesis (formation of certain structures)

This is how plantlets are induced to grow in tissue cultures (micro propagation)

Crown Gall = tumerous growths

• after infection of plant stem and root wounds w/ a bacterium (Agrobacterium tumefaciens)
• genes are "switched on" and the infected cells produce cytokinins, which cause the growth of tumors
• killing bacteria w/ high temp. does not then affect the Gall’s ability to continue producing cytokinins

Ti plasmid, a virus, is part of this bacteria that is tumor inducing

• it is NOT expressed in the bacteria, only in the PLANT
• coded to produce EXCESS AUXINS, CYTOKININ & OPINES (N-compounds)
• used in transformation experiments, because it moves so readily from the bacteria’s DNA to the plant cell’s DNA

Cytokinins — role in senescence

• exogenous application delays senescence
• sharp decrease in mobilization of cytokinin from root to petioles (endogenous) noted during senescence
• studies using recombinant DNA, transformation with the Ti plasmid, using tmr gene to promote heat shock proteins turns on cytokinin production, when plant is subjected to high heat briefly.

• prevents vivipary (germination of seeds w/ immature embryos, or of seeds still w/in the fruit) (level of ABA in seed must be higher than 4ug/g to prevent vivipary
• fluridone treatment induces vivipary
• carotenoids and ABA have the same biochemical pathway in early stages of synthesis, but —
• addition of exogenous ABA can inhibit the premature germination of the seeds, and —
• ABA inhibits opening of stomata (higher levels cause complete closure)
• Evidence — exogenous application of high ABA to plants w/ low levels of ABA causes all stomata close

Therefore, regulation of water balance is a function of ABA

• Plenty of water > low levels of ABA maintained in plants
• Water stress > dramatically increases conc. Of ABA (40x in 30 min as, e.g. in text)

Produced in tissues under stress, H2C = CH2, plays a role in senescence and abscission, fruit ripening, and flower development

• Promotes abscission of leaves, flowers, and fruits by formation of the enzyme that causes cell wall degradation
• Fruit Ripening:
• Degradation of chlorophyll
• Induction of other pigments
• Enzymatic digestion of pectin (softening occurs due to breakdown of middle lamella)
• Starches & acids > sugars

• Sex Expression

How is ethylene, a common gas (and an air pollutant) studied?

• Gas Chromatograph : Quan. Analysis
• Ethylene-releasing agents, such as ethephon > at physiological (cellular) pH > ethylene is released

Ethylene’s other effects (are these natural or artifacts?):

• Stimulates stem, root, petiole, peduncle elongation in some plants
• C₂H₄ stimulate GA production in rice; BUT inhibits root and soot elongation in peas.
• Abnormal growth effects:
• Stimulates stem tissues to swell
• Stimulates downward curvature of leaves (epinasty)

What does this indicate?

• Promotes seed germination
• Inhibits buds from opening
• Reduces apical dominance (axillary buds grow)
• Inhibits root development