Gibberellins

NATURAL OCCURRENCE AND PHYSIOLOGICAL ACTIVITY 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. More than 100 members of this group of plant hormones are now known. They all share gibbane ring structures and are either dicarboxylic (CB20B) or monocarboxylic (CB19B), they have all been assigned ‘gibberellin numbers’ (GABxB) and are usually referred to by these rather than by conventional chemical nomenclature. No plant appears to possess all of the gibberellins, some have only been found in fungi and some only in higher plants; nor are the various gibberellins equally active, some are precursors and some catabolites of active gibberellins. GAB1B (1) is the most active gibberellin in the promotion of cell elongation. Very few gibberellins are available commercially and GAB3B (2) or a mixture of GAB4 B(3) and GAB7B (4) have been used most frequently in plant culture.

Gibberellins are involved in a wide range of developmental responses. These include promotion of elongation in stems and grass leaves, due in part to activation of the intercalary meristem. Another important role of gibberellins is the induction of hydrolytic enzymes such as α-amylase and protease in the seeds of grasses and cereals, hence facilitating endosperm mobilisation. Other roles in some plants include the promotion of seed germination, bolting of rosette plants, sex determination, fruit development and the control of juvenility.

BIOSYNTHESIS OF GIBBERELLINS

The biosynthetic pathway(s) for gibberellins are very complex. All start from isopentenyl diphosphate which in response to soluble cyclases produces ent-kaurene(in plastids). Membrane monoxygenases then convert this to the common precursor GAB12B aldehyde which - in a series of steps involving hydroxylases and oxidases - yields the active gibberellins (see Hedden, 1999). Very little is known about the early steps in gibberellin signal transduction. It is clear however that later steps involve selective gene transcription and de novo protein synthesis.

Inhibitors of biosynthesis and action

Because very little is known about the mode of action of gibberellins it is doubtful that the action of any of the substances known to affect developmental responses involving these growth regulators is due to effects early in signal transduction. On the other hand, much is known about a wide range of synthetic substances, often called 'antigibberellins', which act by blocking biosynthetic pathways.

hese were in general developed to achieve desirable agricultural outcomes - for example dwarfing of cereals to prevent lodging.These substances fall into four categories (see Rademacher, 2000). A number of quaternary ammonium, phosphonium and sulphonium salts act by inhibiting the cyclisation process. Examples of this type are chlormequat chloride (CCC) (5) and AMO 1618 (6). Certain heterocyclic nitrogen-containing compounds such as ancymidol (7), paclobutrazol (8), uniconazole-P (9) and tetcyclasis (10) appear to act by inhibiting ent-kaurene oxidase. A further group of inhibitors are the acylcyclohexanedione derivatives, for example prohexadione (11) and daminozide (12), which affect the later steps of gibberellin biosynthesis involving hydroxylases. While the inhibitors may be useful tools, it should be noted that none are absolutely specific and may affect other biosynthetic pathways such as those for sterols and abscisic acid. Lastly, the inhibitor, 16,17-dihydro GAB5B (13) and related structures appear to act by mimicking the natural substrates.