M. Simons et al., AMINO-ACID SYNTHESIS IS NECESSARY FOR TOMATO ROOT COLONIZATION BY PSEUDOMONAS-FLUORESCENS STRAIN WCS365, Molecular plant-microbe interactions, 10(1), 1997, pp. 102-106
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.