Cytokinin production by Paenibacillus polymyxa

The production of hormones has been suggested to be one of the mechanisms b y 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 con tent of cytokinins in the growth media, before and after cultivation of thi s bacterium, was determined by immunoaffinity chromatography (IAC). This me thod allows the isolation of almost all known cytokinins and their metaboli tes. Separation and characterization were done by high performance liquid c hromatography (HPLC) with on-line ultraviolet (UV) detection, and final ide ntification was by gas chromatography-mass spectrometry. Isopentenyladenine (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 med ium, supplemented with yeast extract, contained iso-pentenyladenine ribosid e (iPR) and some additional cytokinin-like substances before inoculation. W hen the same medium was sampled after the cultivation of P. polymyxa up to its logarithmic growth phase: the cytokinin concentration had decreased. Af ter prolonged cultivation of P. polymyxa, small amounts of iP appeared in a ll three media, and iPR had disappeared from the yeast-containing medium, w hich indicates that the bacterium can metabolize cytokinins. (C) 1999 Elsev ier Science Ltd. All rights reserved.