The grapevine LysM receptor kinase VvLYK4-2 is a key player in chitosan-triggered immune responses

Chitooligosaccharides, such as chitin, are essential components of fungal cell walls and have thus naturally been selected as microbe-associated molecular patterns detected by plants to initiate defense mechanisms. These molecules are typically recognized by the lysin motif receptor-like kinases (LysM RLKs) at the plasma membrane. While chitin perception is well elucidated in Arabidopsis thaliana and other plant species, the recognition mechanisms of its deacetylated form, chitosan, remain poorly investigated despite its use as a biocontrol strategy to protect crops against pathogens. Here, we investigated the role of the two grapevine orthologs of AtLYK4, which participate in the tripartite complex for chitin perception in A. thaliana. Using a dual approach consisting of the functional complementation of the atlyk4/5 double mutant and CRISPR-Cas9 genome editing in Vitis vinifera, we showed that VvLYK4-2 is involved in both chitosan- and chitin-induced immune responses, encompassing MAPK phosphorylation and defense gene expression. Furthermore, grapevine in vitro plantlets lacking VvLYK4-2 exhibited a significantly reduced response to chitosan while retaining a low-intensity response to chitin, potentially due to the presence of VvLYK5-1. Finally, VvLYK4-2 produced in a heterologous system showed binding to chitosan oligomers and, to a lesser extent, to chitin oligomers. These findings indicate that this pattern recognition receptor plays a crucial role in the perception of chitosan oligomers and has thus potential for selective breeding purposes. This discovery may also help to better understand the partial lack of efficacy of chitosan-based plant defense stimulants used in viticulture.