A Sorghum BAK1/SERK4 Homolog Functions in PAMP-Triggered Immunity and Cell Death in Response to Colletotrichum sublineola Infection

Sorghum bicolor (L.) Moench is the fifth most important cereal crop and expected to gain prominence due to its versatility, low input requirements, and tolerance to hot and dry conditions. In warm and humid environments the productivity of sorghum is severely limited by the hemibiotrophic fungal pathogen Colletotrichum sublineola, the causal agent of anthracnose. Cultivating anthracnose-resistant accessions is the most effective ane environmentally benign way to safeguard yield. A previous genome-wide association study for anthracnose resistance in the sorghum association panel (SAP) uncovered single nucleotide polymorphisms on chromosome 5 associated with resistance to anthracnose, including one located within the coding region of gene Sobic.005G182400. In this study, we investigated the molecular function of Sobic.005G182400 in response to C. sublineola infection. Conserved domain, phylogenetic, and structural analyses revealed that the protein encoded by Sobic.005G182400 shares significant structural similarity with the Arabidopsis BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) / SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE 4 (SERK4). Even though sequence analysis of four sorghum accessions showed no substantial variation in the coding region, accession SC1330, which carries the resistance allele, exhibited significantly higher expression of Sobic.005G182400 during early infection (≤24 hours). Co-expression network analysis identified the module associated with Sobic.005G182400 was enriched in genes involved in endocytosis, autophagy, and vesicle transport. Gene regulatory network analysis further suggested that Sobic.005G182400 regulates genes required for BAK1/SERK4-mediated cell death via protein glycosylation. Together, these findings indicate that Sobic.005G182400 encodes a protein with similarity to Arabidopsis BAK1/SERK4 that enables PAMP-triggered immunity and regulates cell death.