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HpaP sequesters HrpJ, an essential component of Ralstonia solanacearum virulence that triggers necrosis in Arabidopsis.

The Gram-negative bacterium Ralstonia solanacearum, the causal agent of bacterial wilt, is a worldwide major crop pathogen whose virulence strongly relies on a type III secretion system (T3SS). This extracellular apparatus allows the translocation of proteins, called type III effectors (T3Es), directly into the host cells. To date, very few data are available in plant pathogenic bacteria concerning the role played by type III-secretion regulators at the post-translational level. We have demonstrated that HpaP, a putative type III secretion substrate specificity switch (T3S4) protein of R. solanacearum controls T3E secretion. To better understand the role of HpaP on type III secretion control, we analyzed the secretomes of the GMI1000 wild-type strain as well as the hpaP mutant using mass spectrometry experiment (LC-MS/MS). The secretomes of both strains appeared to be very similar and highlighted the modulation of the secretion of few type III substrates. Interestingly, only one type III associated protein, HrpJ, was identified as specifically secreted by the hpaP mutant. HrpJ appeared to be an essential component of the T3SS, essential for type III secretion and pathogenicity. We further showed that HrpJ is specifically translocated in planta by the hpaP mutant and that HrpJ can physically interact with HpaP. Moreover, confocal microscopy experiments demonstrated a cytoplasmic localization for HrpJ once in planta. When injected in Arabidopsis thaliana leaves, HrpJ is able to trigger a necrosis on 16 natural accessions. A Genome-Wide Association (GWA) mapping revealed a major association peak with 12 highly significant SNPs located on a plant acyl-transferase.