Comprehensive analysis of the LysM protein family and functional characterization of the key LysM effector StLysM1, which modulates plant immunity in Setosphaeria turcica

LysM proteins contain the lysin domain (LysM), bind chitin and are found in various organisms including fungi. In phytopathogenic fungi, certain LysM proteins act as effectors to inhibit host immunity, thus increasing fungal virulence. However, our understanding of the LysM protein family in Setosphaeria turcica is limited. In this study, eight StLysM genes are identified and designated as StLysM1 to StLysM8. The analysis of sequence features indicates that five proteins (StLysM1, StLysM2, StLysM5, StLysM6, and StLysM7) are potential effectors. Phylogenetic analysis suggests that the StLysMs are divided into fungal/bacterial and fungus-specific subclasses. Domain architecture analysis reveals that the five StLysM effectors exclusively harbor the LysM domain, whereas the other three StLysM proteins contain additional functional domains. Sequence conservation analysis shows that the fungal-specific LysM domain sequences share the ⁸GDxTC¹² and ²⁹WNP³¹ motifs as well as three highly conserved cysteine residues. Conversely, the LysM domain sequences from the bacterial/fungal branch have few conserved sites. Moreover, expression profiling analysis shows that the StLysM1 gene is significantly upregulated during the infection of maize. Yeast secretion assays and transient expression experiments demonstrate that StLysM1 is a secreted protein that can suppress BAX/INF1-induced programmed cell death in Nicotiana benthamiana. Further functional analysis suggests that StLysM1 cannot interact with itself but it can bind chitin. The transient expression of StLysM1 inhibits the chitin-triggered plant immune response, increasing susceptibility to the phytopathogenic fungus Botrytis cinerea in N. benthamiana. This study reveals that the S. turcica LySM protein family consists of eight members, highlighting the significance of StLysM1 as a vital effector in regulating plant immunity. The results provide insight into StLysMs and establish a foundation for understanding the roles of StLysM proteins in the pathogenic process of S. turcica.