Guillaume Henry

Insights into the Defense-Related Events Occurring in Plant Cells Following Perception of Surfactin-Type Lipopeptide from Bacillus subtilis

Multiple strains of Bacillus subtilis were demonstrated to stimulate plant defense responses, and cyclic lipopeptides may be involved in the elicitation of this induced systemic resistance phenomenon. Here, we further investigated molecular events underlying the interaction between such lipopeptides and plant cells. Addition of surfactin but not fengycin or iturin in the micromolar range to tobacco cell suspensions induced defense-related early events such as extracellular medium alkalinization coupled with ion fluxes and reactive oxygen species production. Surfactin also stimulated the defense enzymes phenylalanine ammonia lyase and lipoxygenase and modified the pattern of phenolics produced by the elicited cells. The occurrence of these surfactin-elicited early events is closely related to Ca(2+) influx and dynamic changes in protein phosphorylation but is not associated with any marked phytotoxicity or adverse effect on the integrity and growth potential of the treated tobacco cells. Reduced activity of some homologues also indicates that surfactin perception is dictated by structural clues in both the acyl moiety and cyclic peptide part. Our results suggest that these molecules could interact without irreversible pore formation but in a way sufficient to induce disturbance or transient channeling in the plasma membrane that can, in turn, activate a biochemical cascade of molecular events leading to defensive responses. The present study sheds new light not only on defense-related events induced following recognition of amphiphilic lipopeptides from Bacillus spp. but also more globally on the way elicitors from beneficial bacteria can be perceived by host plant cells.

Plant Defense Stimulation by Natural Isolates of Bacillus Depends on Efficient Surfactin Production

Some plant-associated Bacillus strains produce induced systemic resistance (ISR) in the host, which contributes to their protective effect against phytopathogens. Little is known about the variety of elicitors responsible for ISR that are produced by Bacillus strains. Working with a particular strain, we have previously identified the surfactin lipopeptide as a main compound stimulating plant immune-related responses. However, with the perspective of developing Bacillus strains as biocontrol agents, it is important to establish whether a central role of surfactin is generally true for isolates belonging to the B. subtilis/amyloliquefaciens complex. To that end, we set up a comparative study involving a range of natural strains. Their secretomes were first tested for triggering early defense events in cultured tobacco cells. Six isolates with contrasting activities were further evaluated for ISR in plants, based both on macroscopic disease reduction and on stimulation of the oxylipin pathway as defense mechanism. A strong correlation was found between defense-inducing activity and the amount of surfactin produced by the isolates. These results support the idea of a widespread role for surfactin as a nonvolatile elicitor formed by B. subtilis/amyloliquefaciens, and screening for strong surfactin producers among strains naturally secreting multiple antibiotics could be an efficient approach to select good candidates as biopesticides.

The bacterial lipopeptide surfactin targets the lipid fraction of the plant plasma membrane to trigger immune-related defence responses

The lipopeptide surfactin secreted by plant‐beneficial bacilli has crucial biological functions among which the ability to stimulate immune‐related responses in host tissues. This phenomenon is important for biological control of plant diseases but its molecular basis is still poorly understood. In this work, we used various approaches to study the mechanism governing the perception of this biosurfactant at the plant cell surface. Combining data on oxidative burst induction in tobacco cells, structure/activity relationship, competitive inhibition, insertion kinetics within plant membranes and thermodynamic determination of binding parameters on model membranes globally indicates that surfactin perception relies on a lipid‐driven process at the plasma membrane level. Such a sensor role of the lipid bilayer is quite uncommon considering that plant basal immunity is usually triggered upon recognition of microbial molecular patterns by high‐affinity proteic receptors.

The Roles of Cyclic Lipopeptides in the Biocontrol Activity of Bacillus subtilis

Bacillus subtilis species are well-known and extensively-used beneficial rhizobacteria for biocontrol of plant diseases. Their interest arises from their ability to generate a broad array of bioactive metabolites among which three families of cyclic lipopeptides (CLPs). These CLPs display a huge diversity of structures and physico-chemical and biological properties which probably account for an important part of the biocontrol potential of the producing strains. Beside their antimicrobial properties, they are also involved in colonization and motility as well as in the systemic stimulation of immune system of the host plant. We summarize here the current knowledge of CLPs activities and focus on the recent findings in the context of biocontrol.