AZOSPIRILLUM-BRASILENSE PRODUCES GIBBERELLIN IN PURE CULTURE ON CHEMICALLY-DEFINED MEDIUM AND IN COCULTURE ON STRAW

Rhizobacteria have been shown to produce gibberellin (GA)-like substances, but this evidence has been obtained primarily from analysis by bioassay, usually on chemically-defined medium, and always in systems containing a single microorganism. We examined, by capillary gas chromatography-mass spectrometry, the production of GAs by Azospirillum brasilense (1) in pure culture on chemically-defined medium and (2) in co-culture with Trichoderma harzianum, a fungus which degrades cellulose and controls some bacterial and fungal phytopathogens, in a barley straw decomposition culture. Pure cultures of Azospirillum brasilense contained Ga3 > iso-Ga3 much greater than GA1, but no precursors thereto were detected. In the Azospirillum + Trichoderma straw culture 0.5 ng Ga1 g-1 straw was measured. This is direct, analytical evidence that Azospirillum brasilense produces GA in the presence of complex biochemicals and another microorganism.