Gibberellins

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Gibberellins

The Nature of Gibberellins

Unlike the classification of auxins which are classified on the basis of function, gibberellins are classified on the basis of structure as well as function. All gibberellins are derived from the ent-gibberellane skeleton. The structure of this skeleton derivative along with the structure of a few of the active gibberellins are shown above. The gibberellins are named GA1....GAn in order of discovery. Gibberellic acid, which was the first gibberellin to be structurally characterised , is GA3. There are currently 136 GAs identified from plants, fungi and bacteria.

GA's are widespread and so far ubiquitous in both flowering (angiosperms) and non-flowering (gymnosperms) plants as well as ferns.

Gibberellin History

Gibberellin Biosynthesis and Metabolism

Gibberellins are diterpenes synthesized from acetyl CoA via the mevalonic acid pathway. They all have either 19 or 20 carbon units grouped into either four or five ring systems. The fifth ring is a lactone ring as shown in the structures above attached to ring A. Gibberellins are believed to be synthesized in young tissues of the shoot and also the developing seed. It is uncertain whether young root tissues also produce gibberellins. There is also some evidence that leaves may be the source of some biosynthesis (Sponsel, 1995; Salisbury and Ross). The pathway by which gibberellins are formed is outlined below and illustrated in figure1.
3 acetyl CoA molecules are oxidized by 2 NADPH molecules to produce 3 CoA molecules as a side product and mevalonic acid.
Mevalonic acid is then Phosphorylated by ATP and decarboxylated to form isopentyl pyrophosphate.
4 of these molecules form geranylgeranyl pyrophosphate which serves as the donor for all GA carbon atoms.
This compound is then converted to copalylpyrophosphate which has 2 ring systems
Copalylpyrophosphate is then converted to kaurene which has 4 ring systems
Subsequent oxidations reveal kaurenol (alcohol form), kaurenal (aldehyde form), and kaurenoic acid respectively.
Kaurenoic acid is converted to the aldehyde form of GA12 by decarboxylation. GA12 is the 1st true gibberellane ring system with 20 carbons.
From the aldehyde form of GA12 arise both 20 and 19 carbon gibberellins but there are many mechanisms by which these other compounds arise.
Certain commercial chemicals which are used to stunt growth do so in part because they block the synthesis of gibberellins. Some of these chemicals are Phosphon D, Amo-1618, Cycocel (CCC), ancymidol, and paclobutrazol. During active growth, the plant will metabolize most gibberellins by hydroxylation to inactive conjugates quickly with the exception of GA3. GA3 is degraded much slower which helps to explain why the symptoms initially associated with the hormone in the disease bakanae are present. Inactive conjugates might be stored or translocated via the phloem and xylem before their release (activation) at the proper time and in the proper tissue (Arteca, 1996; Sponsel, 1995).


Functions of Gibberellins

Active gibberellins show many physiological effects, each depending on the type of gibberellin present as well as the species of plant. Some of the physiological processes stimulated by gibberellins are outlined below (Davies, 1995; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992).

Stimulate stem elongation by stimulating cell division and elongation.

Stimulates bolting/flowering in response to long days.

Breaks seed dormancy in some plants which require stratification or light to induce germination.

Stimulates enzyme production (a-amylase) in germinating cereal grains for mobilization of seed reserves.

Induces maleness in dioecious flowers (sex expression).

Can cause parthenocarpic (seedless) fruit development.

Can delay senescence in leaves and citrus fruits.

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