CsBZIP40, a BZIP transcription factor in sweet orange, plays a positive regulatory role in citrus bacterial canker response and tolerance

Citrus bacterial canker (CBC) caused by Xanthomonas citri subsp. citri (Xcc) is a systemic bacterial disease that affects citrus plantations globally. Biotic stress in plants has been linked to a group of important transcription factors known as Basic Leucine Zippers (BZIPs). In this study, CsBZIP40 was functionally characterized by expression analysis, including induction by Xcc and hormones, subcellular localization, over-expression and RNAi silencing. CsBZIP40 belongs to group D of the CsBZIP family of transcription factors and localizes in the nucleus, potentially serving as a transcriptional regulator. In wild type (WT) plants CsBZIP40 can be induced by plant hormones in addition to infection by Xcc which has given insight into its involvement in CBC. In the present study, over-expression of CsBZIP40 conferred resistance to Xcc while its silencing led to Xcc susceptibility. Both over-expression and RNAi affected salicylic acid (SA) production and expression of the genes involved in the SA synthesis and signaling pathway, in addition to interaction of CsBZIP40 with CsNPR1, as detected by a GST pull-down assay. Taken together, the results of this study confirmed the important role of CsBZIP40 in improving resistance to citrus canker through the SA signaling pathway by the presence of NPR1 to activate PR genes. Our findings are of potential value in the breeding of tolerance to CBC in citrus fruits.