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The genus <i>Xanthomonas</i> is an important group of Gram-negative plant pathogenic bacteria, which infects approximately 124 monocotyledonous and 268 dicotyledonous plants. The genus, <i>Xanthomonas</i>, has become an important model organism for studying plant-microbe interactions and for understanding bacterial pathogenicity and virulence mechanisms. Among the diseases caused by members of the genus <i>Xanthomonas</i>, citrus canker is one of the most serious diseases of most commercial citrus cultivars resulting in significant losses worldwide. This devastating disease is caused by <i>Xanthomonas citri</i> subsp. <i>citri</i>. <i>X. citri</i> subsp. <i>citri</i> is spread primarily by rain splash; it enters its hosts through wounds and natural openings such as stomata and forms distinctive raised, necrotic lesions surrounded by oily, watersoaked margins and yellow chlorotic rings on leaves, stems, and fruit. Eventually, the epidermis ruptures, and massive numbers of bacterial cells emerge to the plant surface, where the bacteria are readily available for rain splash and repeated infection cycles. To understand the virulence mechanisms of <i>X. citri</i> subsp. <i>citri</i> underneath the infection cycle, we conducted genomewide microarray and RNA-Seq analyses to characterize the critical regulators including HrpX, HrpG, RpfG, and RpfC, which are critical for the pathogenicity of <i>X. citri</i> subsp. <i>citri</i>. How <i>X. citri</i> subsp. <i>citri</i> coordinates its diverse virulence factors will be presented. Genome wide mutagenesis was used to further characterize the genetic determinants of the virulence mechanism of <i>X. citri</i> subsp. <i>citri</i>. Several novel virulence factors of <i>X. citri</i> subsp. <i>citri</i> will be presented.<p><p>Keywords: