Induced resistance in tomato plant against Alternaria solani by lipopeptides and siderophores isolated from Bacillus subtilis strain 30B-B6

In the framework of a sustainable model of agriculture, several beneficial microorganisms have been reported these last decades for their abilities to protect crops against pathogens. They constitute an alternative to the use of synthetic pesticides. The bacterium Bacillus subtilis 30B-B6 is able to stimulate tomato plant defenses against early blight (EB), an important disease caused by the fungus Alternaria solani. Lipopeptides (LPs) and siderophores, two bioactive compounds synthesized by the strain 30B-B6, are highly suspected to elicit defense response in tomato plants. The present work aims to investigate the potential of B. subtilis-produced LPs and siderophores to elicit tomato systemic resistance. Results from a bioassay under controlled conditions indicated that root application of the beneficial strain 30B-B6 six hours prior foliar A. solani infection significantly protected plants by 70% against EB compared with controls. Similarly, a root application with concentrated LPs (100 mg/L) and siderophores produced by the strain 30B-B6 led to a significant protection index of 53% compared with controls. The three leaves above the ones inoculated by A. solani were collected 0, 6, 24 and 72 hours post pathogen inoculation (hpi). Total RNA was extracted, reversed transcribed and selectively amplified by qPCR. Expression of five genes linked to tomato natural defenses was analyzed : PR1 and PR2b for their involvement in systemic acquired resistance ; Lox and ERF1 as markers of the induced systemic resistance ; NPR1 for being associated to the cross-talk between these two pathways of resistance. Interestingly, infected plants treated with the strain 30B-B6 or with LPs/siderophores showed different NPR1 transcripts levels 6 and 24 hpi. Additionally, A. solani might have played a significant role in gene induction. LPs/siderophores can contribute to protect tomato from EB but their involvement in resistance induction by B. subtilis 30B-B6 needs to be further investigated.