Is L-arabinose important for the endophytic lifestyle of Pseudomonas spp.?

Twenty endophytic bacteria were isolated from surface-sterilized stems and roots of cucumber plants. After removal of potential siblings and human pathogens, the remaining seven strains were identified based on their 16S rDNA as Pseudomonas fluorescens (2 strains) and P. putida (5 strains). Three strains, namely P. fluorescens CS1, P. fluorescens CR2 and P. putida CR3, were able to suppress tomato foot and root rot (TFRR). Special attention was paid to the characterization of the BIOLOG carbon oxidation profiles of the isolated pseudomonads in order to identify nutrients which might be important for their endophytic lifestyle. Comparative analysis of the profiles of these seven strains with those of seven rhizospheric Pseudomonas spp. revealed that endophytes were able to oxidize L-arabinose and 2,3-butanediol significantly more often than the rhizospheric group. An independent growth experiment performed in tubes using L-arabinose and 2,3-butanediol as sole carbon sources showed the same results as seen using BIOLOG for L-arabinose, but not for 2,3-butanediol. Since L-arabinose is one of the most abundant sugars in xylem of cucumber plants and was not detected in their rhizosphere, our data suggest that utilization of L-arabinose might be a trait contributing to the endophytic lifestyle of the isolated Pseudomonas endophytes.