## 7 Influence of Iron and Other Metals on HrpG/HrpX Regulation  
Iron sensing, uptake, and the maintenance of intracellular iron homeostasis play a crucial part in xanthomonads virulence [85, 86, 87, 88, 89, 90]. Multiple evidence supports that iron starvation plays a significant role in the regulation of the HrpG/HrpX regulon inside the host. Microarray-based transcriptome analysis showed induced expression of multiple _hrp_ genes, including _hrpG_ and _hrpX_, of _Xcc_ under low iron conditions [91]. Further promoter probe analyses using GUS reporter fusions demonstrated upregulation of _hrp_ gene cluster and _hrpG_ in _Xcc_ under iron depletion [90]. Supplementation of exogenous iron to the _hrp_-inducing media XCM2 drastically suppresses the expression of _hrpG_ and the _hrp_ gene cluster in _Xcc_[71; 90]. Accordingly, external supplementation of iron suppresses the HR induction on tomato leaves by _Xcc_ and _Xcc_[71; 90]. However, high iron availability does not affect HR induction by _Xoo_[90]. In addition, _Xcc_ also displayed a drastic reduction in virulence on cabbage leaves under iron-replete condition, indicating that the pathogenicity of _Xcc_ is activated by iron depletion [90]. Regulation of iron and metal homeostasis is multilayered and harbors several key regulators. In addition to responding to excess/depletion of iron and other metals, these regulators were shown to control many cellular functions and traits associated with pathogenicity. XibR, an orphan response regulator, binds with ferric iron and regulates several virulence-associated functions, including iron uptake and metabolism, T3 SS and T3 Es expression, chemotaxis, and motility in response to iron availability in _Xcc_[91]. Deletion mutant of _xibR_ in _Xcc_ displays a substantial reduction in both _in planta_ colonization and virulence on cabbage [91] (Figure 2). In contrast, Ferric Uptake Regulator (Fur) binds with ferrous iron and regulates multiple cellular functions including iron homeostasis [92]. _fur_ mutant of _Xoo_ showed hypersensitivity to oxidative stress and reduced virulence on rice plants [85]. The Zinc Uptake Regulator (Zur) transcriptional regulator is involved in the suppression of zinc uptake to maintain cellular zinc homeostasis. This regulator was also found to indirectly upregulate _hrpX_ expression in _Xcc_ under low iron conditions [93] (Figure 2). The ColS/CoIR (VgrS/VgrR) two-component system was reported to play a critical role in iron and metal homeostasis and virulence regulation of multiple _Xanthomonas_ spp. [58; 59; 60]. The ColS/CoIR system is essential for pathogenicity and disruption of this system eliminates the ability to colonize the host and cause disease [58; 59; 60]. The ColS/CoIR system was identified to respond to the availability of iron, zinc, manganese, and cadmium and was found to be critical for survival in excess/depletion of iron and other metals [58; 59; 94]. ColS is a membrane-bound receptor histidine kinase that senses extracytoplasmic iron limitation in the periplasm, while its cognate response regulator ColR detects intracellular iron excess [94]. The ColS/CoIR system controls many cellular processes and pathogenicity-associated traits, such as biofilm formation, lipopolysaccharide-charide production, catalase activity, iron uptake, tolerance to environmental stresses, and _hrp_ gene expression [59; 60; 94] (Figure 2). Therefore, its effect on bacterial pathogenesis is multilayered and does not seem to be a result of an alteration of one particular trait. Apart from indirect transcriptional control of _hrp_ and T3E genes, the interaction of the ColS/CoIR system with the _HrpG/HrpX_ region is not fully understood. The ColS sensor does not interact or directly induce the phosphorylation of HrpG and ColR-mediated interaction was not identified with the promoter sequences of _hrpG_ or _hrpX_. The transcriptional regulator HpaR, which is positively regulated by HrpG/HrpX, was identified to directly control the _colS/coIR_ operon [57] (Figure 2).