Transcriptome Profile Analysis of Citrus Canker-Resistance Tangerine Ponkan (Citrus reticulata) Compared to Citrus Canker-Susceptible Sweet Orange Bahia (Citrus sinensis)

Citrus canker, one of the most devastating diseases of citrus plants, is caused by the bacterium Xanthomonas citri subsp. citri (Xac). There is no resistant citrus variety to this disease, and susceptibility levels vary according to specific plant genotypes. While “Ponkan” tangerine (Citrus reticulata) is more resistant, some varieties are more susceptible to Xac, such as sweet orange “Bahia” (Citrus sinensis). To study the transcriptional profile of canker disease in these two citrus varieties, ‘Ponkan’ and 'Bahia' plants were infected by Xac with the aim to identify genes involved in the disease resistance. Leaves of ‘Ponkan’ and ‘Bahia’ were inoculated with a suspension of Xac (treatment) or sterile water (control) and samples were collected at 24, 48 and 72 hours after inoculation. Total RNA was isolated and cDNA was synthetized for high throughput sequencing of RNA-seq in a HiScanSQ System (Illumina). Differential gene expression analysis followed by gene ontology (GO) was performed for annotation of unigenes, including molecular functions, biological processes and cellular components. 'Ponkan' and 'Bahia' expressed a group of 53 and 9 exclusive genes that are differentially induced, respectively. Several genes were found to be involved in known biological processes including defense against pathogens, protein degradation, cell signaling and detoxification of xenobiotics. These biological processes can be involved in defense mechanisms conferring resistance to ‘Ponkan’ against Xac. This connection between variation in gene expression and plant susceptibility to Xac infection seems to be related to a rapid gene regulation in response to pathogen infection.