Genome-wide identification of papain-like cysteine proteases in <i>Citrus sinensis</i> and expression analysis in response to <i>Candidatus</i> Liberibacter asiaticus

Papain-like cysteine proteases (PLCPs) play important roles in plant-pathogen interactions. In citrus, PLCPs are targeted by effectors from <i>Candidatus</i> Liberibacter asiaticus (CLas), the causal agent of Huanglongbing (HLB). To study interactions between <i>Citrus sinensis</i> and CLas, we identified 33 candidate <i>CsPLCP</i> genes by HMMsearch with the Peptidase_C1 domain (PF00112), and further confirmed them using local BLAST with PLCP sequences from <i>Arabidopsis thaliana</i>. The 33 <i>CsPLCPs</i> were classified into 9 subfamilies. Analysis of the cis-acting elements in their promoters indicated that <i>CsPLCPs</i> are responsive to various phytohormones and stressors. Further qPCR analysis indicated that several <i>CsPLCPs</i> were up-regulated during CLas infection. Subcellular localization analysis indicated that five CsPLCPs were localized to the cytoplasm and nucleus, while CsXBCP2 was localized only to the cytoplasm. In summary, this study provides a sound basis for understanding the roles of <i>CsPLCPs</i> in the Citrus-CLas interaction.