Biocontrol agents (BCAs) based on plant growth promoting rhizobacteria have recently been developed as alternatives to chemical pesticides. Among those beneficial bacteria, Bacillus spp. are one of the most promising BCAs. A wide range of bioactive secondary metabolites (BSMs) are involved in biocontrol via antibiosis to phytopathogens and/or via elicitation of systemic resistance in their host plants. This chapter illustrates the diversity of pathosystems in which BCA based on Bacillus spp. have proved effective. It describes the mechanisms underpinning this biocontrol activity via production of a wide range of enzymes, proteins and small-size BSMs. As these BSMs are clearly involved in pathogen control, we emphasise the importance of understanding the ecological factors influencing their production. In the last part of the chapter, we highlight the potential interactions between Bacillus spp. and other soil microorganisms in developing consortia of biocontrol agents combining species with synergistic activities for plant health improvement.
Cyclic lipopeptides are key bioactive secondary metabolites produced by some plant beneficial rhizobacteria such as Pseudomonas and Bacillus. They exhibit antimicrobial properties, promote induced systemic resistance in plants and support key developmental traits including motility, biofilm formation and root colonization. However, our knowledge about the fate of lipopeptides once released in the environment and especially upon contact with neighboring rhizobacteria remains limited. Here, we investigated the enzymatic degradation of Bacillus and Pseudomonas cyclic lipopeptides by Streptomyces venezuelae. We observed that Streptomyces is able to degrade the three lipopeptides surfactin, iturin and fengycin upon confrontation with of B. velezensis in vitro and in planta according to specific mechanisms. S. venezuelae was also able to degrade the structurally diverse sessilin, tolaasin, orfamide, xantholisin and putisolvin-type lipopeptides produced by Pseudomonas, indicating that this trait is likely engage in the interaction with various competitors. Furthermore, the degradation of CLPs is associated with the release of free amino and fatty acids and was found to enhance Streptomyces growth, indicating a possible nutritional utilization. Thereby, this work stresses on how the enzymatic arsenal of S. venezuelae may contribute to its adaptation to BSMs-driven interactions with microbial competitors. The ability of Streptomyces to degrade exogenous lipopeptides and feed on them adds a new facet to the implications of the degradation of those compounds by Streptomyces, where linearization of surfactin was previously reported as a detoxification mechanism. Additionally, we hypothesize that lipopeptide-producing rhizobacteria and their biocontrol potential are impacted by the degradation of their lipopeptides as observed with the polarized motility of B. velezensis, avoiding the confrontation zone with Streptomyces and the loss of antifungal properties of degraded iturin. This work illustrates how CLPs, once released in the environment, may rapidly be remodeled or degraded by members of the bacterial community, with potential impacts on CLP-producing rhizobacteria and the biocontrol products derived from them.
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