The plant‐growth‐promoting rhizobacteria Bacillus pumilus and Bacillus licheniformis produce high amounts of physiologically active gibberellins

The plant‐growth‐promoting rhizobacteria (PGPR), Bacillus pumilus and Bacillus licheniformis, isolated from the rhizosphere of alder ( Alnus glutinosa [L.] Gaertn.) have a strong growth‐promoting activity. Bioassay data showed that the dwarf phenotype induced in alder seedlings by paclobutrazol (an inhibitor of gibberellin [GA] biosynthesis) was effectively reversed by applications of extracts from media incubated with both bacteria and also by exogenous GA 3 . Full‐scan gas chromatography‐mass spectrometry analyses on extracts of these media showed the presence of GA 1 , GA 3 , GA 4 and GA 20 , in addition to the isomers 3‐ epi ‐GA 1 and iso ‐GA 3 . Isotope dilution analysis indicated that epi ‐GA 1 was an artefact. Likewise, iso ‐GA 3 is also probably an artifact spontaneously formed during extraction and/or analysis. In both culture media, GA 1 was present in higher concentrations (130–150 ng ml −1 ) than GA 3 (50–60 ng ml −1 ), GA 4 (8–12 ng ml −1 ) and GA 20 (2–3 ng ml −1 ). The data indicated that culture of both bacteria accumulate bioactive C19‐gibberellins in relative high amounts and that these GAs appear to be physiologically active in the host plant. The evidence suggests that the promotion of stem elongation induced by the PGPR could be mediated by bacterial GAs.