Variation in microbial feature perception in the Rutaceae family with immune receptor conservation in citrus

Although much is known about the responses of model plants to microbial features, we still lack an understanding of the extent of variation in immune perception across members of a plant family. In this work, we analyzed immune responses in Citrus and wild relatives, surveying 86 Rutaceae genotypes with differing leaf morphologies and disease resistances. We found that responses to microbial features vary both within and between members. Species in 2 subtribes, the Balsamocitrinae and Clauseninae, can recognize flagellin (flg22), cold shock protein (csp22), and chitin, including 1 feature from Candidatus Liberibacter species (csp22CLas), the bacterium associated with Huanglongbing. We investigated differences at the receptor level for the flagellin receptor FLAGELLIN SENSING 2 (FLS2) and the chitin receptor LYSIN MOTIF RECEPTOR KINASE 5 (LYK5) in citrus genotypes. We characterized 2 genetically linked FLS2 homologs from "Frost Lisbon" lemon (Citrus ×limon, responsive) and "Washington navel" orange (Citrus ×aurantium, nonresponsive). Surprisingly, FLS2 homologs from responsive and nonresponsive genotypes were expressed in Citrus and functional when transferred to a heterologous system. "Washington navel" orange weakly responded to chitin, whereas "Tango" mandarin (C. ×aurantium) exhibited a robust response. LYK5 alleles were identical or nearly identical between the 2 genotypes and complemented the Arabidopsis (Arabidopsis thaliana) lyk4/lyk5-2 mutant with respect to chitin perception. Collectively, our data indicate that differences in chitin and flg22 perception in these citrus genotypes are not the results of sequence polymorphisms at the receptor level. These findings shed light on the diversity of perception of microbial features and highlight genotypes capable of recognizing polymorphic pathogen features.