GIBBERELLIN FORMATION IN MICROORGANISMS

Several microorganisms possess the capacity of synthesizing gibberellins (GAs) in axenic culture. GA concentrations in the range of approximately 20 to 200 milligrams per litre of culture filtrate are produced by wild-type strains of the following fungi: Gibberella fujikuori (GA(3), GA(4), GA(7), GA(1) and others), Sphaceloma manihoticola and other species of this genus (GA(4), GA(9) and others), Phaeosphaeria sp. (GA(1), GA(4), GA(9) and others). Neurospora crassa is capable of producing GA(3) in the range of micrograms per kilogram of mycelium. Nanogram amounts per litre of culture are present in fermentations of the bacteria Rhizobium phaseoli (GA(1), GA(4), GA(9), GA(20)) and in Azospirillum lipoferum and A. brasilense (GA(1), GA(3)). Of the high-producing organisms, G. fujikuroi and the Sphaceloma spp. appear to have an almost identical GA metabolism except that Sphaceloma is, in particular, unable to produce GA(7) and GA(1). Phaeosphaeria sp. converts GA(9) via GA(4) or GA(20) into GA(1), reactions not known from G. fujikuroi. Generally however, GA metabolism in these organisms appears to be very similar to the one known from higher plants. Most likely, the GAs formed play no hormonal or other immediate physiological role in the producing organism and can, thus, be regarded as secondary metabolites. On the other hand, evidence is available that GA-producing microorganisms often induce reactions in host plants which are beneficial to their growth.