AMINO-ACID SYNTHESIS IS NECESSARY FOR TOMATO ROOT COLONIZATION BY PSEUDOMONAS-FLUORESCENS STRAIN WCS365

In this work the bio-availability of amino acids For the root-colonizi ng Pseudomonas fluorescens strain WCS365 in the tomato rhizosphere was studied. The amino acid composition of axenically collected tomato ro ot exudate was determined, The results show that aspartic acid, glutam ic acid, isoleucine, leucine, and lysine are the major amino acid comp onents, The concentrations of individual amino acids in the rhizospher e of gnotobiotically grown tomato plants were estimated and considered to be too low to support growth of rhizosphere micro-organisms to num bers usually found ill the tomato rhizosphere. To test this experiment ally, mutants of P. fluorescens WCS365 auxotrophic for the! amino acid s leucine, arginine, histidine, isoleucine plus valine, and tryptophan were isolated after mutagenesis with Tn5lacZ. Root Lip colonization o f these mutants was measured after inoculation of germinated tomato se eds and subsequent growth in a gnotobiotic quartz sand system (M. Simo ns, A. J. van der Bij, I. Brand, L. A. de Weger, C. a. Wijffelman, and B. J. J. Lugtenberg. 1996. Gnotobiotic system for studying rhizospher e colonization by plant growth-promoting Pseudomonas bacteria, Mol. Pl ant-Microbe Interact, 9:600-607), In contrast to the wild-type strain, none of the five amino acid auxotrophs tested was able to colonize th e tomato root tip, neither alone nor after co-inoculation with the wil d-type strain. However, addition of the appropriate amino acid to the system restored colonization by the auxotrophic mutants, usually to wi ld-type levels. Analysis of the root base showed that cells of anxotro phic mutants were still present there. The results show that, although amino acids are present in root exudate, the bio-availability of the tested amino acids is too low to support root tip colonization by auxo trophic mutants of P. fluorescens strain WCS365. The genes that are re quired for amino acid synthesis are therefore necessary for root colon ization. Moreover, these compounds apparently play no major role as nu trients in the tomato rhizosphere.