EFB530 Plant Physiology

Gibberellins (GAs)

Discovery of GA

GA was discovered in the 1930's-50's as the active compound produced by the fungus Gibberella fujikuroi which causes a disease on rice that makes the plants grow very tall called "foolish seedling disease"

While we defined the auxins by their biological activity, for example in the Avena curvature assay, the gibberellins are defined by their chemical structure

• there are >110 compounds in plants with a structure of a gibberellin, but not all of those have biological activity
• these have been numbered as they have been purified and characterized, GA₁-> (GA stands for gibberellin A; gibberellic acid is GA₃)

Gibberellins are 19 or 20 carbon-containing compounds which are diterpenoids

• synthesized by the assembly of 4 isoprene (5 carbon) units
  

Active GAs can be generated from inactive precursors or active GAs can be converted to inactive compounds

• GAs can also be inactivated by conjugation to sugars (usually glucose)

GAs are synthesized in seeds and in growing vegetative tissues (young leaves, stems)

Primary effect

The primary effect of GA is to induce stem elongation

• most prominent effect is on long-day plants that grow as a rosette, long days stimulate conversion from vegetative growth to floral production=bolting
• GA application can induce stem elongation of the rosettes and premature flowering (for example in cabbage, lettuce, spinach)
• dwarf plants are especially sensitive to GA, such as dwarf corn; also pea, bean stems
• GA₁ is the active compound in inducing stem elongation
• stem elongation is the result of both increased cell division and cell elongation
• GA causes cell wall loosening (as with auxin), but not by wall acidification
• different plants show different sensitivities to applied GA

1) GA induces production of alpha-amylase in cereal seeds

• the storage reserves in cereals is in the endosperm, is mainly starch
• this must be broken down to glucose by alpha- and beta-amylase, which are produced by the aleurone cells (surrounding the endosperm) and are induced upon germination
• this induction is dependent on the presence of the embryo, but if the embryo is removed and GA is applied, amylase is induced
• at the start of germination, the embryo sends a mobile signal (GA) to the aleurone cells to induce amylase production and initiate release of storage reserves
• this germination process resulting in the break down of starch to glucose is called malting in the case of barley, first step in beer production

2) GA is involved in the switch from juvenile growth to mature growth

• this switch can include a change in leaf shape, growth habit, or flowering
• application of GA₃ can induce the reversion from mature to juvenile form in English ivy, application of GA₄ + GA₇ can cause premature flowering in conifers (juvenile->mature)

3) GA treatment can cause increased fruit growth and development

• used commercially to induce growth of apple fruit, but especially to induce growth of seedless table grapes

4) GA is involved in sex determination in some plants

• in maize, GA seems to repress maleness (stamen development) and promote femaleness (pistil development) [maize is a monoecious plant with male flowers and female flowers on the same plant]
• in cucumber, spinach - GA has the opposite effect - it promotes maleness

GA synthesis inhibitors (paclobutrazol, brand name Bonzi; also uniconazole (Sumagic)) are often used in horticulture or in urban forestry to limit stem growth

• used to keep chrysanthemums and poinsettias short, stocky
• used to keep tree branches from growing into power lines

The GID1 protein is a GA receptor

• When bound to GA, it interacts with GID2 to attach ubiquitin to DELLA-type proteins, tagging them to be degraded
• DELLA proteins act to repress the expression of genes induced by GA - DELLA proteins include SLR, GAI, RGA and others
• defective mutants of those DELLA proteins can be constitutive GA response mutants

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