Genome-wide identification and expression analysis of the Citrus malectin domain-containing receptor-like kinases in response to arbuscular mycorrhizal fungi colonization and drought

Malectin-like domain-containing receptor-like kinases (MRLKs) compose a big and diverse protein family in plants that plays significant roles in biotic and abiotic stress response. However, the MRLK family members in Citrus, as well as their roles during drought stress (abiotic stress) and arbuscular mycorrhizal fungi (AMF) colonization, remain relatively unknown. In the present study, we characterized the MRLK family genes in Citrus. By analyzing the presence of malectin-like domains and searching through the Citrus genome, we identified 47 CsMRLKs located within nine chromosomes, with most CsMRLKs mainly located on chromosome 1. These 47 CsMRLK family members were classified into 6 subfamilies with distinct malectin domains. Gene expression analysis showed that all 47 CsMRLKs were expressed in the roots and exhibited differential expression patterns. During the drought stress response, CsMRLK6, 7, 8, 9, 10, 11, 19, 25, 26, and 40 were downregulated. AMF colonization downregulated the expression of CsMRLK6, 10, 11, 12, 14, 17, 20, 30, 35, 43, and 47 and upregulated the expression of CsMRLK2, 5, 13, 21, 22, 28, 29, 32, 38, 41, 45, and 46. In addition, AMF colonization and drought stress showed a synergistic effect, in which the downregulated genes became even more weakened and the upregulated genes became even more enhanced. The present study offers useful perception into the Citrus MRLK gene family and its responses in AMF colonization and drought stress.