Surfactin activates defense in Arabidopsis via a non-canonical pathway

Surfactin, a cyclic lipopeptide produced by beneficial bacteria of the genus Bacillus, activates defense responses in Arabidopsis thaliana through remodeling of the plasma membrane of root cells. The mechanism involves interaction with sphingolipids, increased lateral membrane tension, and activation of mechanosensitive ion channels. The process resulted in induced systemic resistance against Botrytis cinerea, a necrotrophic fungus that causes gray mold.

Scientists investigated how bacterial lipopeptides act on the plant cell surface. They used surfactin as a model. The molecule is already among the most studied bacterial compounds in inducing resistance against phytopathogens.

In the trials, root treatment with 10 micrometers of surfactin reduced foliar infection by B. cinerea. Treated plants accumulated more camalexin in infected leaves. This compound acts as a phytoalexin in Arabidopsis. The role of camalexin in disease control appeared in the pad3 mutant, which is deficient in its synthesis. In this material, surfactin-induced resistance did not occur.

Aerial part

Surfactin also prepared the aerial parts for stronger immune responses. Leaves of plants treated at the roots showed a greater apoplastic burst of reactive oxygen species after the detection of chitooligosaccharide CO8, used as a fungal standard. A similar response occurred with flg22, a bacterial standard.

The study showed differences between the response to surfactin and immunity triggered by classical pattern recognition receptors. Surfactin did not generate the typical apoplastic burst of reactive oxygen species in roots. Instead, it promoted an increase in reactive oxygen species inside cells. It also induced a calcium response, alkalinization of the medium, depolarization of the plasma membrane, and transient ionic currents.

RNAseq analysis indicated a limited transcriptional response. Researchers recorded 246 differentially expressed genes after surfactin elicitation. This number was lower than the values ​​observed in treatments with flg22 and chitin, which are associated with thousands of genes. According to the study, genes typical of initial immune signaling or defense mechanisms did not undergo extensive alteration after treatment.

Tests with mutants

Scientists tested mutants of Arabidopsis with alterations in known components of immunity by patterns. Surfactin maintained activity in materials without receptors, co-receptors, or cytoplasmic kinases related to the classical perception of MAMPs. This result indicates distinct perception of the canonical PTI pathway.

The central step involved the interaction of surfactin with plasma membrane lipids. Assays with biomimetic liposomes indicated a greater affinity of the molecule for membranes containing glucosylceramides, a type of sphingolipid. Molecular dynamics simulations indicated a preferential approach of surfactin to these molecules, especially the glucose moiety.

The role of glucosylceramides also appeared in mutant plants. In the loh1 mutant, deficient in complex sphingolipids, the intracellular response to reactive oxygen species decreased. The same material showed less systemic immune activation and less resistance to B. cinerea. The moca1 mutant, with altered GIPCs but without reduced glucosylceramides, maintained a response to surfactin.

Measurements with probes

The insertion of surfactin into the membrane resulted in bilayer thinning, lateral redistribution of lipid components, and increased rigidity in the polar head region. Measurements with probes indicated increased lateral membrane tension in root protoplasts and root epidermal cells. This effect decreased in the loh1 mutant, reinforcing the involvement of glucosylceramides.

The study linked this remodeling to the activation of mechanosensitive ion channels. The blocker GsMTX-4 reduced calcium and reactive oxygen species responses induced by surfactin. msl4/5/6/9/10 and mca1/2 mutants, linked to mechanosensitive channels in the plasma membrane, also showed a reduced response to the lipopeptide. Membrane depolarization almost disappeared in these genotypes.

These mutants also lost some of their ability to mount systemic resistance and systemic immune activation after root canal treatment with surfactin. The data support the interpretation that mechanosensitive channels residing in the plasma membrane participate in the transduction of the signal generated by the insertion of the lipopeptide.

More information at doi.org/10.1038/s41477-026-02270-3