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Expression of hrpG and activation of response regulator HrpG are controlled by distinct signal cascades in Ralstonia solanacearum

  • Bacterial and Phytoplasma Diseases
  • Published:
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Abstract

The transcriptional regulator HrpB activates the entire hrp regulon in the plant pathogen Ralstonia solanacearum. Through a complex multigene regulatory cascade PrhA–PrhR/PrhI–PrhJ–HrpG, expression of hrpB is induced in a hrp-inducing, nutrient-poor medium and in response to contact between the bacterium and plant cell. In this study, we analyzed the expression levels of these regulatory genes and hrpB using lacZ reporter strains grown in three different conditions: in a nutrient-rich or nutrient-poor medium and co-cultivated with Arabidopsis thaliana seedlings. We found that prhA and prhIR were expressed constitutively. Expression of prhJ and hrpG was PrhA-dependent in all three conditions. Despite the high level of hrpG expression in all cases, hrpB was induced only when the bacteria were co-cultivated with A. thaliana seedlings or grown in nutrient-poor medium. A mutation in the predicted phosphorylation site of hrpG greatly reduced the function of HrpG. From these results, we conclude that the prhA-dependent regulatory cascade controls the expression of hrpG, and a new cascade, which is induced by a signal from plant cells, activates HrpG by phosphorylation. Only when both signal cascades are effective is full expression of hrpB induced. We speculate that the metabolic status of the bacteria in the nutrient-poor medium also contributes to the second cascade.

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Acknowledgments

This work was supported in part by KAKENHI (Grant-in-Aid for Scientific Research) from Japan Society for the Promotion of Science (16658020 to Y.H. and 17380031 to K.O.).

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Authors and Affiliations

  1. Laboratory of Plant Pathology and Biotechnology, Kochi University, Nankoku, Japan

    Takeshi Yoshimochi, Akinori Kiba & Yasufumi Hikichi

  2. Research Institute of Molecular Genetics, Kochi University, Nankoku, Kochi, 783-8502, Japan

    Yong Zhang & Kouhei Ohnishi

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  1. Takeshi Yoshimochi
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  2. Yong Zhang
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  3. Akinori Kiba
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  4. Yasufumi Hikichi
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  5. Kouhei Ohnishi
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Correspondence to Kouhei Ohnishi.

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Yoshimochi, T., Zhang, Y., Kiba, A. et al. Expression of hrpG and activation of response regulator HrpG are controlled by distinct signal cascades in Ralstonia solanacearum . J Gen Plant Pathol 75, 196–204 (2009). https://doi.org/10.1007/s10327-009-0157-1

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  • DOI: https://doi.org/10.1007/s10327-009-0157-1

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