Temporal Transcription Profiling of Sweet Orange in Response to PthA4-Mediated Xanthomonas citri subsp. citri Infection

Citrus canker, caused by Xanthomonas citri subsp. citri, is a devastating disease of most commercial citrus varieties. In our previous study, we analyzed the transcriptional response of 'Valencia' sweet orange to X. citri subsp. citri wild-type and pthA4 mutant infection at 48 h postinoculation (hpi). Using microarray analysis, two PthA4 targets, CsLOB1 and CsSWEET1, were identified. We have shown that PthA4 binds to the effector binding element (EBE) of CsLOB1 and activates gene expression of this susceptibility gene. However, how PthA4 modulates host genes at different stages of infection remains to be determined. In this study, we compared the transcriptional profiles between citrus leaf tissue inoculated with Xcc306 and those inoculated with a pthA4-deletion mutant strain (Xcc306∆pthA4) at 6, 48, and 120 hpi. At both 48 and 120 hpi, the PthA4-mediated infection significantly upregulated expression of a variety of genes involved in cell-wall degradation and modification, DNA packaging, G-protein, protein synthesis, sucrose metabolism, and cell division functions, while the downregulated genes were mainly enriched in photosynthesis, transport, secondary metabolism, cytochrome P450, and various plant defense-associated mechanisms. To validate microarray results, gene expression of 26 genes representing genes associated with cell-wall-associated, immunity system, and carbohydrate metabolism was confirmed using quantitative reverse-transcription polymerase chain reaction. Expression patterns of these genes at 48 and 120 hpi were consistent with the microarray results. We also identified putative EBE for PthA4 (EBEPthA4) in the promoter regions of multiple genes upregulated by PthA4, to which PthA4 might bind directly to control their gene expression. Our study provided a dynamic picture of citrus genes regulated by PthA4 during the X. citri subsp. citri infection of citrus leaves at different stages. This study will be useful in further understanding the virulence mechanism of X. citri subsp. citri and identifying potential targets of PthA4.