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.
Highlights
Xanthomonas is a large genus of gamma-proteobacteria
We review the environmental factors and regulatory networks associated with virulence regulation in xanthomonads controlled by the HrpG/HrpX regulon
HrpG and hrpX are found in all reported pathogenic Xanthomonas spp. with the exception of X. albilineans and X. sacchari that lack the hrp2 type 3 secretion system (T3SS) gene cluster [1], and they are regarded as the master regulators of the T3SS system in xanthomonads [3]
Summary
Xanthomonas is a large genus of gamma-proteobacteria. Xanthomonas isolates are mostly plant-associated, and many are important plant pathogens that cause devastating effects on yield [1]. 4. Genomic Organization of hrpG and hrpX hrpG and hrpX are found in all reported pathogenic Xanthomonas spp. with the exception of X. albilineans and X. sacchari that lack the hrp T3SS gene cluster [1], and they are regarded as the master regulators of the T3SS system in xanthomonads [3]. Genomic Organization of hrpG and hrpX hrpG and hrpX are found in all reported pathogenic Xanthomonas spp. with the exception of X. albilineans and X. sacchari that lack the hrp T3SS gene cluster [1], and they are regarded as the master regulators of the T3SS system in xanthomonads [3] Homologs of these two regulators were found to function in a similar manner in phytopathogenic bacteria such as Ralstonia solanacearum, Burkholderia Pseudomallei, and Acidovorax citruli [19,43,44,45,46]. All the information available in this review will only cover the HrpG/HrpX regulatory network of group 2 xanthomonads
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