Mechanistic aspect of the perception of the elicitor surfactin at the plant plasma membrane

Mechanosensitive (MS) ionic channels are proteins able to transduce change in membrane tension due to mechanical stimuli into ion fluxes. In plant plasma membrane (PM), two families of MS channels are currently reported, the Mid1-Complementing Activity (MCA) channels and MscS-like (MSL) channels, and recent reports have shown their involvement in cell wall damage perception, callose response against bacteria invasion and programmed cell death following cell swelling.
Surfactin (SRF) is a lipopeptide produced by plant beneficial bacilli able to trigger immunity in plants. Results have shown a sensing mechanism independent of protein pattern-recognition receptors commonly known for plant elicitors. As SRF is able to insert into the lipid fraction of plant PM, its perception by plant was assumed to be a lipid-driven process but the detailed sensing mechanism is not yet understood. We hypothesise that SRF disturbs PM organisation and mechanical properties and consequently activate MS proteins located in plant PM.
To test our hypothesis, analyses of cytosolic calcium variation were first performed in root protoplasts of Arabidopsis thaliana from wild type plants and MCA and MSL channels depleted mutant. The results have shown a lower cytosolic calcium increase under SRF treatment in protoplasts devoid of MCA or MSL channels. Then, the disturbance of membrane lipid organisation by SRF was evaluated through membrane fluidity measurements on root protoplasts in presence or absence of SRF. A change in membrane fluidity was observed in presence of SRF.
Altogether, our results are in accordance with an SRF eliciting mechanism triggered by an activation of MS channel through a PM mechanical disturbance. The detailed molecular mechanism at the PM level will be investigated by using specific asymmetric membrane models in combination with fluorescence imaging.