Nonribosomal lipopeptides from Bacillus subtilis: potential resistance inducers and/or biopesticides for wheat crop?

Abstract: The PhytoBio INTERREG IV project involves several research teams (University ofLille, University of Champagne Ardenne, University of Gent, University of Liège, and University of Littoral Côte d’Opale) that aims at developing and studying biosurfactants of bacterial origin. Phytobio focuses on cyclic nonribosomal lipopeptides (LPs), which could be used as resistance inducers or biopesticides against plant pathogens and could be considered as an potential alternative to chemical pesticides. Our contribution to PhytoBio aims at characterizing mycosubtilin, surfactin and fengycin, three bacterial LPs produced by the rhizobacterium Bacillus subtilis, for their activity against two fungal pathogens of wheat responsible for two of the most important foliar diseases, Blumeria graminis f.sp. tritici (Bgt),an obligate parasitic fungus responsible for powdery mildew and Mycosphaerella graminicola (Mg), an hemibiotrophic fungus causing septoria leaf blotch. An in vitro study showed that surfactin had no direct effect on germination and on fungal growth of Mg. However fengycin slightly decreased the conidial germination rate and impaired Mg growth, whereas mycosubtilin completely inhibited both Mg conidial germination and fungal growth. On the other hand, no direct antifungal effect of LPs was observed in vitro on Bgt conidia germination. Moreover, a preventive treatment in planta with surfactin and mycosubtilin resulted at least in 41% and 44% protection levels against Bgt. RT-qPCR was then performed in order to study the ability of surfactin and mycosubtilin to induce the expression of defence-related genes in either non-infectious conditions or in the presence of Bgt. For instance, in non-infectious conditions, both surfactine and mycosubtilin induced the expression of a peroxydase encoding gene (POX381). However, in infectious conditions, only surfactin induced the expression of this gene in response to Bgt. Depending on the targeted pathogen and on the type of LP, these molecules could exhibit direct antifungal effect and/or defence induction, the later occurring through either elicitation or potentialisation.