Genome-Wide Identification of the DnaJ Gene Family in Citrus and Functional Characterization of ClDJC24 in Response to Citrus Huanglongbing

Citrus Huanglongbing (HLB) is the most destructive citrus disease worldwide. The etiological agent responsible for this disease is "<i>Candidatus</i> Liberibacter asiaticus" (<i>C</i>Las), a phloem-restricted bacterium transmitted by psyllid vectors. To date, effective practical strategies for curing Citrus HLB remain elusive. Additionally, no susceptibility genes associated with HLB have been identified in Citrus species, thereby complicating the application of gene-editing techniques such as CRISPR-Cas9 to enhance resistance to HLB. The co-chaperone <i>DnaJ</i> plays a crucial role in protein folding and the regulation of various physiological activities, and it is also associated with multiple pathological processes. <i>DnaJ</i> has been extensively studied in many species, including Arabidopsis, rice, and wheat. However, there is limited information available regarding the DnaJ gene family in citrus. In this study, we conducted a comprehensive genome-wide analysis of the DnaJ family genes in various Citrus species. The Citrus genome was identified to contain 86 <i>DnaJ</i> genes, which were unevenly distributed across nine chromosomes. Phylogenetic analysis indicated that these genes could be classified into six distinct groups. Furthermore, transcriptomic analysis revealed that nine <i>DnaJ</i> genes exhibited significantly higher induction in HLB-infected samples relative to non-HLB-infected Citrus. <i>Cis</i>-acting elements within the promoters of <i>DnaJ</i> genes were also examined, revealing the presence of hormone and defense/stress responsiveness elements (TC-rich) distributed on the <i>ClDJC24</i> gene. The results were validated using quantitative real-time PCR (qRT-PCR). Additionally, the silencing of <i>ClDJC24</i> suggested that this gene negatively regulates disease resistance in Citrus. Our study provided useful clues for further functional characterization and constructed a theoretical foundation for disease-resistant breeding in Citrus.