Molecular crosstalk between plant beneficial rhizobacteria: identification of Pseudomonas compounds influencing the growth and antimicrobial potential of Bacillus velezensis

Abstract

Bacilli are among the most prolific bacteria regarding the potential to form a wide array of Bioactive Secondary (specialized) Metabolites (BSMs) including metal chelators, hormones, and antimicrobials. This is particularly true for members of the rhizosphere-dwelling and plant-associated species B. velezensis. Some isolates belonging to this species are among the most promising bacteria to be used as biocontrol agents to protect plants against phytopathogens and some of the BSM are clearly involved in this biocontrol activity. According to some recent works including those performed in the MiPI lab, it appears that BSM production by Bacillus may undergo unanticipated changes upon interspecies(kingdom) interactions. However few publications reported outcomes from Bacillus interactions with other highly competitive soil bacterial species and therefore, it is poorly known whether this bacterium is able to establish molecular cross-talks with other species which could markedly impact expression of their respective secondary metabolome.

Our global objective is to better appreciate how far the expression of this BSM arsenal can be modulated upon interaction with other rhizosphere bacteria and what are the molecular signals involved. More specifically, we selected Pseudomonas sp. strain CMR12a as interacting partner because this bacterium is also a good competitor producing a range of BSM and is well characterized regarding its genomic content. B. velezensis GA1 and Pseudomonas sp. CMR12a actually interact in a multifaceted way as on one hand, the perception of some Pseudomonas metabolites triggers the production of antimicrobial PKS by Bacillus and on the other hand, Pseudomonas is able to inhibit the growth of Bacillus. In this work, we demonstrate that the cyclic lipopeptide sessilin is the molecule secreted by P. sp. CMR12a mainly responsible for the antimicrobial activity toward B.velezensis GA1. We also illustrate that sessilin toxicity is neutralized in the presence of surfactin, a lipopeptide synthesized by Bacillus, probably via chemical binding leading to co-precipitation of the two molecules. This represents a new ecological role for surfactin acting as a kind of chemical barrier, which add to the multiple avoidance strategies of Bacillus. Moreover, our data strongly suggest that iron deficiency induced by P. sp. CMR12a siderophores acts as trigger perceived by B. velezensis GA1 which in response, stimulates the production of antibacterial PKS. These results not only illustrate a new facet of siderophore-mediated competitive interactions that may occur between two rhizosphere bacteria but also show that B. velezensis can mount an aggressive response upon sensing competitors. In its humble contribution, this work thus highlights the tremendous complexity of interspecies interactions that may occur in the rhizosphere. Considering B. velezensis specifically, these unsuspected outcomes probably impact not only its ecological fitness but also its biocontrol potential.