Cytokinin production by Paenibacillus polymyxa

The production of hormones has been suggested to be one of the mechanisms by which plant growth-promoting rhizobacteria (PGPR) stimulate plant growth. To evaluate whether the free-living soil bacterium, Paenibacillus polymyxa, releases the hormone group cytokinins and, if so, their identity, the content of cytokinins in the growth media, before and after cultivation of this bacterium, was determined by immunoaffinity chromatography (IAC). This method allows the isolation of almost all known cytokinins and their metabolites. Separation and characterization were done by high performance liquid chromatography (HPLC) with on-line ultraviolet (UV) detection, and final identification was by gas chromatography-mass spectrometry. Iso-pentenyladenine (iP) was identified in the two defined media used for the cultivation of P. polymyxa, but not earlier than at its late stationary growth. A third medium, supplemented with yeast extract, contained iso-pentenyladenine riboside (iPR) and some additional cytokinin-like substances before inoculation. When the same medium was sampled after the cultivation of P. polymyxa up to its logarithmic growth phase, the cytokinin concentration had decreased. After prolonged cultivation of P. polymyxa, small amounts of iP appeared in all three media, and iPR had disappeared from the yeast-containing medium, which indicates that the bacterium can metabolize cytokinins.