Effect of surfactin and fengycin on the induction of immune defence responses in monocotyledons (Brachypodium distachyon and Triticum aestivum)

Abstract

Stimulation of plant immunity using biocontrol agents is a topic of growing interest to develop a more sustainable agriculture. In this context, surfactin (srf) and fengycin (fgc), two cyclic lipopeptides (CLPs) produced by bacteria of the Bacillus genus, showed interesting potential as biocontrol agents. Plant immune stimulation by CLPs is thought to be triggered by a disturbance of plant plasma membrane organization, which could represent a novel process in plant immunity. While CLPs-triggered early defence responses have been extensively described in dicots, research on monocots remains limited. In this work, we developed strategies to evaluate the biocontrol potential against Z. tritici of these CLPs in T. aestivum and B. distachyon, two monocots species. We first observed a direct inhibition effect of fgc and srf on the mycelial growth of Z. tritici. Then, we optimized a protocol for isolating root protoplasts from the two species for investigating CLPs-triggered immune responses in monocots. Using protoplasts to measure the impact of CLPs on membrane physical state, we observed a rigidifying effect of srf on wheat plasma membrane while fgc fluidified and formed pores in plasma membrane of both species. Even though these results confirmed the ability of CLPs to disturb the plasma membrane in monocotyledons, no influx of calcium ion, one hallmark of defence early events, was detected in protoplasts in response to srf and fgc treatments. Finally, we attempted to evaluate the reduction of disease incidence in plants in presence of these CLPs. Even though the procedure still requires optimisation to allow quantitative observation of disease reduction, we did not observe inhibition of Z. tritici spore germination on wheat leaves. While the results obtained need to be confirmed after some procedure adjustment, this work provides some tools and elements to decipher the biocontrol activities of Bacillus CLPs in monocotyledons.