13.3.1—
Introduction
Gibberellins may be defined as compounds having a gibbane skeleton and biological activity in stimulating cell division or cell elongation or both (various structures are given in Fig. 13.3). Gibberelins also may possess other biological
Figure 13.3
Structure of gibbane and the conversion
of GA4 to common gibberellins.
activities such as induction of nucleic acid and enzyme synthesis. Presently there are more than forty known gibberellins which have been isolated from green plants or the fungus Gibberella fujikuroi (Cherry, 1973). Since the late 1960's three to four new gibberellins have been identified each year. Thus, it seems that great progress is being made in the identification of naturally-occurring gibberellins; on the other hand the mechanism of action of gibberellins is still unknown. Plant physiologists are therefore confronted with a very wide array of gibberellins, several of which often occur in the same tissue, and the question arises as to their function within the cells.
Work beginning in the 1960's on the possible regulation of gene transcription by gibberellins was initially successful and achieved wide-spread acceptance. Later work raised many doubts, however, and at the present time, many workers in the field reject the concept that gibberellins regulate physiological processes through the control of gene transcription. Alternative mechanisms involving an action of gibberellins on lipids and membranes are now being proposed.