CitMYB84: A promising transcription factor for the citrus fight against Candidatus Liberibacter asiaticus

Huanglongbing (HLB) is a serious and devastating disease of the citrus industry, primarily caused by Candidatus Liberibacter asiaticus (CLas). Citrus breeding for HLB resistance urgently requires the identification of genes that confer resistance to its causal agent. Here, the function of CitMYB84, an R2R3-MYB transcription factor of citrus, was investigated in response to CLas. The results showed that CitMYB84 was located on chromosome 3 of the Citrus sinensis genome, and contained only one exon. CitMYB84 protein was an unstable hydrophilic protein consisting of 254 amino acids and containing two R-structures at the N-terminus. Its molecular weight and theoretical isoelectric point were 28.1 kDa and 8.8, respectively. Its secondary structure was composed of α-helix (29.5%) and irregular curl (68.5%). CitMYB84 protein shared high degree of homology with Arabidopsis thaliana AtMYB70, AtMYB77, AtMYB73, and AtMYB44, with identities ranging from 64.7% to 77.9%. CitMYB84 promoter contained both pathogen-responsive elements (dof box, W box, and GT1 box) and phytohormone-responsive elements (TCA-element, ABRE, and CGTCA-motif). Gene expression analysis showed that CitMYB84 was expressed preferentially in roots, and was increased to 7.9 times in CLas-infected citrus leaves compared to healthy ones. It was up-regulated by salicylic acid (SA) and ethephon, down-regulated by abscisic acid, and changed by methyl jasmonate. The overexpression vector pLGN-CitMYB84 was successfully constructed and transformed into the hairy roots of CLas-free and CLas-infected Ziyang Xiangcheng citrus (C. junos) through Rhizobium rhizogenes. CLas content was significantly decreased by 97.4% in the CitMYB84-overexpressed hairy roots compared with the control. Meanwhile, the overexpressed healthy hairy roots had increased SA, total phenolics, flavonoids, and callose. These results indicated that CitMYB84 could enhance citrus resistance to CLas and provide a foundation for further research.