Transcriptional Characteristics of Xa21-mediated Defense Responses in Rice

Bacterial blight,caused by Xanthomonas oryzae pv.oryzae（Xoo）,is the most destructive bacterial disease of rice.The cloned rice gene Xa21 confers resistance to a broad spectrum of Xoo races.To identify genes involved in Xa21-mediated immunity,a whole-genome oligonucleotide microarray of rice was used to profile the expression of rice genes between incompatible interactions and mock treatments at 0,4,8,24,72 and 120 h post inoculation（hpi） or between incompatible and compatible interactions at 4 hpi,respectively.A total of 441 differentially expressed genes,designated as XDGs（Xa21 mediated differentially expressed genes）,were identified.Based on their functional annotations,the XDGs were assigned to 14 categories,including defense-related,signaling,transcriptional regulators.Most of the defense-related genes belonged to the pathogenesis-related gene family,which was induced dramatically at 72 and 120 hpi.Interestingly,most signaling and transcriptional regulator genes were downregulated at 4 and 8 hpi,suggesting that negative regulation of cellular signaling may play a role in the Xa21-mediated defense response.Comparison of expression profiles between Xa21- and other R gene-mediated defense systems revealed interesting common responses.Representative XDGs with supporting evidences were also discussed.