Systemic induction of phytoalexins in cucumber in response to treatments with fluorescent pseudomonads

The Pseudomonas putida isolate BTP1 and its sid(-) mutant M3 were recently reported to protect cucumber against Pythium aphanidermatum root rot. This protection was mainly associated with an accumulation of antifungal phenolics in the treated roots. In this study, analyses of root extracts from split-root experiments showed that these phytoalexins were produced systemically. Indeed, several antifungal molecules accumulated similarly in both treated and nontreated root parts of plants protected against P. aphanidermatum with BTP1 or M3. In addition, analyses of leaf samples also revealed increased amounts of fungitoxic molecules in PGPR-treated plants, although the nature of these molecules appeared to be different from those detected in roots. The antifungal compounds isolated both from roots and leaves were mainly detected in acid-hydrolyzed extracts containing aglycones. These results suggest that PGPR can elicit phytoalexins systemically in cucumber and that the overall defence response is not based on a single phytoalexin but is chemically complex and organ-specific.