Abstract:In bacteria, 2-component regulatory systems (TCSs) are the critical information-processing pathways that link stimuli to specific adaptive responses. Signals perceived by membrane sensors, which are generally histidine kinases, are transmitted by response regulators (RRs) to allow cells to cope rapidly and effectively with environmental challenges. Over the past few decades, genes encoding components of TCSs and their responsive proteins have been identified, crystal structures have been described, and signaling mechanisms have been elucidated. Here, we review recent findings and interesting breakthroughs in bacterial TCS research. Furthermore, we discuss structural features, mechanisms of activation and regulation, and cross-regulation of RRs, with a focus on the largest RR family, OmpR/PhoB, to provide a comprehensive overview of these critically important signaling molecules.
Like other bacteria, Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial leaf blight disease in rice, possesses intracellular signalling systems, known as two-component regulatory systems (TCSs), which regulate pathogenesis and biological processes. Completion of the genome sequences of three Xoo strains has facilitated the functional study of genes, including those of TCSs, but the biological functions of most Xoo TCSs have not yet been uncovered. To identify TCSs involved in Xoo pathogenesis, we generated knockout strains lacking response regulators (RRs, a cytoplasmic signalling component of the TCS) and examined the virulence of the RR knockout strains. This study presents a knockout strain (detR(-) ) lacking the PXO_04659 gene which shows dramatically reduced virulence relative to the wild-type. Our studies to elucidate detR function in Xoo pathogenesis revealed a reduction in extracellular polysaccharide (EPS), intolerance to reactive oxygen species (ROS) and deregulation of iron homeostasis in the detR(-) strain. Moreover, gene expression of regulatory factors, including other RRs and transcription factors (TFs), was altered in the absence of DetR protein, as determined by reverse transcription-polymerase chain reaction (RT-PCR) and/or real-time quantitative RT-PCR analyses. All evidence leads to the conclusion that DetR is essential for Xoo virulence through the regulation of the Xoo defence system including EPS synthesis, ROS detoxification and iron homeostasis, solely or cooperatively with other regulatory factors.
Bacterial blight (BB) disease caused by Xanthomonas oryzae pv. oryzae (Xoo) drives severe yield and quality losses in rice (Oryza sativa Xa1, Xa3/Xa26, xa5, xa13, Xa21, and Xa27). Here we employ a transcriptomics approach to elucidate the Xa21-, NH1-(NPR1 homolog 1) (NH1)-, and NRR-(negative regulator of disease resistance) mediated defense response to Xoo. Among the candidate genes, we focused on 288 genes showing significant change in at least two of the above comparisons to support the association with an enhanced defense response. Gene Ontology enrichment analysis for this gene list revealed that response to biotic stimulus was 25.0-fold more enriched compared to the control, well qualifying the candidate genes for enhanced resistant response. The biotic stress overview installed in the MapMan toolkit was used to identify diverse components consisting of defense signaling pathways such as genes involved in disease resistance, redox, signaling, regulation of transcription, pathogenesis-related functions, secondary metabolism, and protein degradation. Of these, we validated the expression patterns of genes related to regulation of transcription and pathogenesis-related functions and suggest a functional network model for WRKY transcription factors mediating defense signaling pathways against Xoo. We expect that our analysis will contribute to increasing the depth of knowledge on the molecular mechanism for enhanced disease resistance against bacterial blight disease in rice.
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