Backgroud Huanglongbing (HLB) is currently one of the most destructive citrus disease worldwide. It is caused by Candidatus Liberibacter asiaticus (CLas), a nonculturable alphaproteobacterium, which it resides exclusively in the phloem tissues. Therefore, understanding the early CLas-responsive proteins in citrus petiole where pathogenic bacteria colonized will help to investigate plant resistance to the pathogen.Results In this study, a comparative proteomic approach was applied to identify

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... petiole proteins associated with the response to CLas infection. A total of 777 proteins were differentially expressed in response to CLas. Among them, 499 proteins were up-regulated and 278 were down-regulated. Among the most highly up-regulated differentially expressed proteins (DEPs) were salicylate carboxymethyltransferase, ubiquitin carboxyl-terminal hydrolase 13, trans-resveratrol di-O-methyltransferase, linoleate 13S-lipoxygenase 2-1, granule-bound starch synthase 1 and thaumatin-like proteins. While the most highly downregulated DEPs were oxygen-evolving enhancer proteins, ribulose bisphosphate carboxylase/oxygenase activase, peroxidases and photosystem reaction center subunits. The results of qPCR analysis of a number of indicated DEPs and western blotting further validated four representative DEPs, including salicylate carboxymethyltransferase, linoleate 13S-lipoxygenase 2-1, granule-bound starch synthase 1 and photosystem I reaction center subunit showed that most of detected DEPs were positively correlated with their mRNA and protein levels.Conclusions Our comparative proteomic analysis first profiling reveals early and primary proteome alterations in CLas-infected citrus petiole, where pathogens reside in. The DEPs results demonstrate that CLas infection could promote the carbohydrate metabolism, depress the photosystem and activate/inhibit defense responses. 3