Plant membrane lipids as key actors of the sensing mechanism of the elicitor surfactin in plant cells

Cyclic lipopeptides (CLPs) represent a vast and structurally heterogeneous group of secondary metabolites retaining key ecological functions for commensal and beneficial plant-associated bacteria such as Bacillus and Pseudomonas. CLPs also contribute to the biocontrol activity of selected isolates notably by eliciting the so-called induced resistance in the host plant leading to self-protection against phytopathogens. Surfactin (SRF) produced by beneficial bacilli is the bacterial CLP best described as plant immunity elicitor but the molecular mechanism behind its perception by plant cells remains largely unknown.
Previous data strongly suggested that SRF sensing at the plant plasma membrane (PPM) level is mainly mediated by some interaction with the lipid fraction of the membrane rather than detection via membrane protein receptors as described for pattern-triggered immunity upon detection of pathogens. To investigate the role of PPM lipids in SRF sensing mechanism, we combined functional assays on Arabidopsis thaliana root protoplasts with experimental and in silico biophysics on PPM biomimetic membrane models.

Isothermal titration calorimetry and molecular modelling first revealed a higher binding of SRF for liposomes containing sphingolipids. This interaction SRF-lipids leads to a thinning of the membrane, observed with neutron reflectivity and atomic force microscopy, and a disturbance of membrane mechanics revealed by a decrease in membrane fluidity in both liposomes and root protoplasts of A.thaliana wild type (Col-0). Moreover, the loh1 mutant (LONGEVITY ASSURANCE 1 HOMOLOG1), impacted in its PM sphingolipid content, showed a strong reduction in SRF-induced ROS burst, one hallmark of plant early immune event, supporting the importance of sphingolipids in SRF sensing.

The involvement of PPM lipids in SRF sensing and the disturbance of PPM mechanics by SRF leads us to investigate the importance in SRF sensing of the PPM-located mechanosensitive proteins, the Mid1-Complementing Activity (mca) channels and MscS-like (msl) channels. Interestingly, a lower calcium response, another hallmark of early immune event, is observed in root protoplasts from mca and msl channels-depleted mutants upon SRF treatment compared to Col-0 protoplasts.

Altogether, our results are in accordance with an SRF eliciting mechanism triggered by an activation of MS channel through a disturbance of PM lipids organization. They suggest that such lipid-mediated mechanosensing represents a new aspect of plant immune stimulation by beneficial bacterial molecules.