Pseudomonas putida induces resistance to Fusarium oxysporum f. sp. radicis-lycopersici in tomato plants by activating expression of defense-related genes

Plant growth-promoting rhizobacteria (PGPR) may prevent attack from pathogenic microorganisms by eliciting induced systemic resistance (ISR). In the present work, Pseudomonas putida isolate TR21/1 showed significant biological control of tomato seedlings inoculated with Fusarium oxysporum f.sp. radicis-lycopersici (FORL). Here, the SA-responsive genes PR-1, PR-4, PR-6 and CH9 were downregulated upon induction of ISR by P. putida strain TR21/1 and induced when bacterized tomato roots were inoculated with FORL. This indicates that SAR involves the accumulation of SA-responsive genes but ISR does not. Similarly, expression of ET-regulated genes such as ACO1, ACO3, ACO4 were not induced in ISR-expressing tomato roots and P. putida treatment induced only ACO2 expression suggesting that ACO2 expression is involved in ISR-expressing tomato seedlings. In contrast, the infection of ISR expressing plants by FORL strongly induced ACO3, ACO2, and ACO1 indicating the transcriptional regulation of ACO genes in response to FORL attack which may be related to possible ethylene synthesis in response to pathogen. Here P. putida treatment increased ETR1 gene expression in roots and this induction was upregulated in presence of FORL indicating that ETR1 plays a role in the protection of plants against FORL by reducing ethylene sensitivity. Activation of SA-and ET- regulated genes in bacterized plants in the presence of FORL implies that not only SA but other signals as well, may play an important role in inducing resistance.