## 6 Metabolic Regulation of the HrpG/HrpX Regulon  
The expression and protein accumulation of HrpG and HrpX are highly dependent on the environment and the metabolic state of the bacterium. The two regulators are strongly expressed upon reaching the plant apoplast [61; 62; 63; 64; 65]. By utilizing GFP promoter fusions, [PERSON] et al. [61] demonstrated that _hrpG_, _hrpX_, and T3 SS translocator _hrpF_ of _Xeu_ are specifically induced upon entering tomato substomatal cavities, whereas their expression was repressed on the leaf surface. The transcriptional expression of _hrpG_, _hrpX_, and their target genes _in planta_ was examined in numerous studies which usually focused on a time frame of 24-48 h post-inoculation [61; 62; 63; 64; 65]. However, several direct and indirect studies indicates that _hrp_ expression starts much earlier, if not instantly upon the introduction of the bacteria into the plant environment. [PERSON] et al. have shown that _hrpE_ is highly induced in _Xcc_ 4 h after inoculation of cabbage [64]. In addition, effector delivery monitored through CyaA reporter fusions or transcriptional expression of TALE-induced plant susceptibility gene suggests that effectors can be actively delivered by the T3 SS into the host cells between 12-24 h post-inoculation [66; 67]. Induction of the _hrpG/hrpX_ regulon is mediated through sensing the plant environment. Supplementing minimal media with rice extract or cabbage xylem sap almost immediately promoted the upregulation of _hrpG_ and _hrpX_ in _Xoo_ and _Xcc_[64; 68]. Undefined rich media such as NB and LB suppress _hrp_ gene expression, whereas several _hrp_-inducing plant-mimicking artificial media were found to induce _hrp_ gene expression. These defined media are of slightly acidic pH supplemented with trace metals, amino acids, and low molecular weight carbon sources such as mono- and di-saccharides or carbonic acids. Species specific responses were identified in different types of media: XVM1 and XVM2 media, which are based on fructose and sucrose as a carbon source and methionine (for XVM1) or casamino acids (for XVM2) as a nitrogen source, induces _hrp_ gene expression in pepper and citrus pathogens _Xeu_ and _Xcci_[37; 69]. XOM2, which uses xylose as a carbon source and methionine as the sole supplemented amino acid, promotes _hrp_ expression in the rice pathogen _Xoo_[70]. MME and XCM2 media, which use either glutamate or succinate as the main carbon source and are supplemented with a low abundance of casamino acids and magnesium (in XCM2), were found to be effective for _hrp_ expression in _Xcc_[71]. The differential responses of different _Xanthomonas_ bacteria to different media suggest that, while the key regulatory elements of the T3 SS are conserved, the metabolic regulation of the _hrp_ is different in _Xanthomonas_ spp. This differential regulation is possibly linked with the unique niche occupied by each species. It should be noted that discrepancies between the transcriptional responses of _Xanthomonas_ to artificial plant-mimicking media and transcriptional responses within the plant environment were observed. For instance, GntR-family transcriptional regulator HpaR1/YtrA of _Xcc_ and _Xcci_ negatively regulates the T3 SS associated genes in plant-mimicking media while positively regulates their expression _in planta_[63; 72] (Figure 2). The metabolic regulation of _hrp_ gene expression is multilayered and integrates multiple environmental factors. The repression of _hrp_ gene expression by rich undefined media suggests that the response is repressed by certain metabolites or activated by starvation. Stringent response in bacteria is induced by stress or metabolite starvation [73] and is mediated by internal levels of guanosine pentaphosphate [(p)ppGpp], which is synthesized and degraded by RelA and SpoT [73]. It was recently reported that deletion of _relA_ and _spoT_ homologs in _Xcci_ completely abolished pathogenicity in citrus and caused a significant reduction in the expression of _hrpG_, _hrpX_, and their downstream targets [74].