Perception of free cutin monomers by plant cells

Enzymatic degradation of plant cuticles by fungal pathogens results in the release of free cutin monomers. The hypothesis that free cutin monomers are recognized by plant cells as endogenous stress‐related signals was tested in a model system consisting of cultured potato cells. Addition of cutin monomers in the micromolar range induced a transient alkalinization of the culture medium, similar to that observed with chitin or chitotetraose that served as positive control. The cutin monomers tested varied considerably in their potential to induce alkalinization, the most and least active compounds being cis ‐9,10‐epoxy‐18‐hydroxystearic acid and palmitic acid, respectively. n ,16‐dihydroxypalmitic acid ( n = 8, 9 or 10) was found to be the major component of potato leaf cuticle and was among the most active cutin monomers. 9,10‐Dihydroxystearic acid, an analogue of the cutin monomer threo ‐9,10,18‐trihydroxystearic acid, exhibited biological activity in a stereoselective manner, only the naturally occurring threo ‐stereoisomer inducing a rapid and strong alkalinization response. Alkalinization of the culture medium was inhibited by addition of the protein‐kinase inhibitor K‐252a, and the onset of alkalinization was paralleled by changes in phosphorylation of specific proteins. The active cutin monomers also stimulated the production of the plant stress hormone ethylene and activated defence‐related genes at the mRNA level. The data provide evidence for a role of enzymatic breakdown products of plant cuticles as a new class of endogenous signal molecules.