PRODUCTION OF GIBBERELLIN-LIKE SUBSTANCES BY BACTERIA AND ACTINOMYCETES

The production of gibberellin-like substances by bacteria and actinomycetes was investigated. Most of the bacterial cultures tested produced a gibberellic acid-like substance (A 3 ) in amounts varying from 1 to 14 μg/liter. The production of a gibberellin-A 9 -like compound was much more limited, 6 out of 15 cultures yielding small amounts. Six out of 11 actinomycetes showed evidence of A 3 synthesis; two produced A 9 . Antigibberellin substances co-chromatographing with A 3 were detected in culture filtrates of Bacillus polymyxa and two actionmycetes. Cultural conditions and concentration of the ingredients of the medium influenced gibberellin synthesis by Agrobacterium radiobacter. Production of these substances was not growth linked and none was detected in stationary cultures. Microbial synthesis of the gibberellins was demonstrated also by means of assays with the dwarf maize mutants d 1 , d 2 , and d 5 .As added to soil was rapidly inactivated and it was postulated that the plant could derive maximum benefit from microbial synthesis of gibberellins and related substances only at the root surface—the rhizoplane.