The HrpG/HrpX Regulon of Xanthomonads—An Insight to the Complexity of Regulation of Virulence Traits in Phytopathogenic Bacteria

Abstract

Bacteria of the genus Xanthomonas cause a wide variety of economically important diseases in most crops. The virulence of the majority of Xanthomonas spp. is dependent on secretion and translocation of effectors by the type 3 secretion system (T3SS) that is controlled by two master transcriptional regulators HrpG and HrpX. Since their discovery in the 1990s, the two regulators were the focal point of many studies aiming to decipher the regulatory network that controls pathogenicity in Xanthomonas bacteria. HrpG controls the expression of HrpX, which subsequently controls the expression of T3SS apparatus genes and effectors. The HrpG/HrpX regulon is activated in planta and subjected to tight metabolic and genetic regulation. In this review, we cover the advances made in understanding the regulatory networks that control and are controlled by the HrpG/HrpX regulon and their conservation between different Xanthomonas spp.

The Methyltransferase HemK Regulates the Virulence and Nutrient Utilization of the Phytopathogenic Bacterium Xanthomonas citri Subsp. citri

“…The transcriptome profile data presented above revealed the significantly decreased expression of 43 genes associated with T3SS in the ∆ hemK mutant. These genes included 24 genes of the whole hrp / hrc gene cluster coding for the core structural component of T3SS apparatus ( hrpF , XAC0395 , hpaB , hrpE , hrpD6 , hrpD5 , hpaA , hrcS , hrcR , hrcQ , hpaP , hrcV , hrcU , hrpB1 , hrpB2 , hrcJ , hrpB4 , hrpB5 , hrcN , hrpB7 , hrcT , hrcC , hpa1 and hpa2 ), 18 genes encoding for putative T3SS-associated effectors ( hpaF , XAC0419 , avrBs2 , xopR , avrXacE1 , xopX , xopI , xopN / HopAU1 , hrpW , xopK , xopL , XAC3230 , XAC3666 , xopQ / HopQ1 , xopAU , xopAV , xopAP , and xopS ), and hrpX , encoding a transcriptional regulator of virulence ( Figure 6 A, Supplementary Table S1 ) [ 15 , 33 ].…”

“…In this study, the transcriptomic analysis showed that HemK positively regulated the expression of T3SS genes, including 24 hrp genes. HrpX is the master regulator of T3SS and its effector genes in Xanthomonas [ 15 , 33 , 42 ]. Our qRT-PCR analysis further confirmed that a number of T3SS effector genes, including hrpB2 , hpaA , hrcC , hrpE , and virulence genes ( avrXacE1 and avrBs2 ), are down-regulated in ∆ hemK .…”

Plant immunity: Rice XA21-mediated resistance to bacterial infection

“…In most cases, HrpX-dependent transcription requires a PIP (plant-inducible promoter) box sequence, which forms the binding site for HrpX ( 63 – 70 ). HrpX synthesis is governed by a complex regulatory network that converges on the response regulator HrpG, which activates hrpX transcription ( 71 – 73 ). Thus, the HrpX protein directly activates hrp and xop gene transcription initiation.…”

“…Thus, the HrpX protein directly activates hrp and xop gene transcription initiation. As a global regulator of pathogenicity, HrpX also regulates a diverse array of other microbial “virulence factors” that facilitate infection ( 71 , 73 , 74 ).…”

Xanthomonas oryzae Pv. oryzicola Response Regulator VemR Is Co-opted by the Sensor Kinase CheA for Phosphorylation of Multiple Pathogenicity-Related Targets

“…The effectors also mediate host specificity and pathogenicity by triggering disease in susceptible or hypersensitive response (HR) in the resistant hosts ( Jacques et al, 2016 ). The Xoc T3SS strain is encoded by the hrp ( h ypersensitive r esponse and p athogenicity) gene cluster and controlled by the HrpG/HrpX regulon ( Teper et al, 2021 ). The expression of T3SS apparatus genes and effectors is regulated by HrpX expression which is under the control of HrpG.…”