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

The plant‐growth‐promoting rhizobacteria (PGPR), Bacilluspumilus 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 GA₃. Full‐scan gas chromatography‐mass spectrometry analyses on extracts of these media showed the presence of GA₁, GA₃, GA₄and GA₂₀, in addition to the isomers 3‐epi‐GA₁ and iso‐GA₃. Isotope dilution analysis indicated that epi‐GA₁ was an artefact. Likewise, iso‐GA₃ is also probably an artifact spontaneously formed during extraction and/or analysis. In both culture media, GA₁ was present in higher concentrations (130–150 ng ml⁻¹) than GA₃ (50–60 ng ml⁻¹), GA₄ (8–12 ng ml⁻¹) and GA₂₀ (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.