The role of plasma membrane mechanics and organization in the stimulation of plant defense by Bacillus lipopeptides

Cyclic lipopeptides produced by beneficial bacilli are promising candidates to develop biocontrol strategies in agriculture and reduce the use of conventional chemical pesticides. These biological molecules have shown ability to reduce the disease infection in several pathosystems through two main mechanisms: a direct antagonism against plant pathogens and/or the stimulation of plant immunity. Their activities rely on their amphiphilic properties, enabling them to readily insert into plasma membrane lipids. While the molecular mechanism for antagonistic activities are quite-well understood, involving the alteration of cell plasma membrane integrity and pore formation, little is known about the molecular basis underlying the stimulation of plant immunity through the interaction of lipopeptides with plasma membrane lipids. The present thesis work was therefore dedicated to unraveling how plasma membrane lipids contribute to the detection of lipopeptides by plant cells and subsequently activate plant immunity.
The first part of the research used surfactin as a model to study plant immune stimulation by Bacillus lipopeptides, given its well-documented ability to induce systemic resistance in plants. A combination of functional assays in planta and biophysical experiments led to a molecular description of a unique lipid-mediated sensing mechanism of surfactin by plant cells. Compared to the conventional description of plant immunity relying on protein receptors, the surfactin sensing process is based on its interaction with plasma membrane sphingolipids. This interaction induces a disturbance of plasma membrane lipid environment detected via mechanosensitive channels that subsequently trigger a specific immune signature with a low transcriptional change, ultimately leading to the establishment of a systemic resistance in plants.
In the second part, fengycin, another lipopeptides produced by beneficial bacilli with potential as biocontrol agent, was compared to surfactin in terms of antifungal and plant eliciting activities, and effects on membrane lipid properties. The two lipopeptides exhibited differences in their biological activities, which were correlated with their distinct effects on membrane lipids. While surfactin primarily influences the packing of membrane lipids, fengycin interacts with membrane lipid through a solubilizing/permeabilizing mechanism, with little effect on the packing of membrane lipids. This led to the actcivation of a plant immune response that is important for surfactin and less pronounced for fengycin. In contrast, fengycin displays direct antifungal properties not observed with surfactin. This section therefore illustrated the pivotal role of the mechanism of interaction with membrane lipids in determining the biological activities of both lipopeptides and, more broadly, amphipathic molecules.
The final part of the research focused on the role of the plant cell wall in the mechanosensing of surfactin. This structure is an inherent component of plant cell mechanics and possibly affects plant mechanosensing. By comparing responses to surfactin in root protoplasts fully devoid of cell wall with those of protoplasts with a partially recovered cell wall, the experiments revealed a mitigating impact of the cell wall on cell responses to surfactin. To further investigate the impact of the cell wall on the mechanical disturbance capacity of surfactin, efforts were made to develop a microfluidic device for studying the plant cell deformability. Although the optimal device was not achieved during the thesis, the foundations have beens laid for future research in this area.
Altogether, the results of this thesis shed light on a yet underestimated role of plasma membrane lipids in plant immunity. It also opens many research outlooks, aiming to better understand the contribution of the different plant lipid species in the mechanosensing mechanism and to identify markers indicating a state of induced resistance in plants. Ultimately, the knowledge gained from this thesis regarding the lipid-mediated biological activities of amphipathic molecules will contribute to their potential use as novel biocontrol agents for more sustainable strategies in agriculture