A LysR-Type Transcriptional Regulator Links Phosphonate Metabolism to Cool Virulence of Ralstonia solanacearum in Tomato.

Ralstonia solanacearum, the causal agent of bacterial wilt, is a major threat to solanaceous crops in tropical and subtropical regions worldwide. While most strains are pathogenic at high temperatures (>28°C), a subset can cause disease at low temperatures (16-18°C), posing a risk to temperate solanaceous crops. Previous comparative genomics of cool virulent with regular tropical strains identified candidate transcriptional regulators associated with cool virulence. In this work, we tested function in virulence to tomato at high and low temperatures of four of the candidate transcription regulators identified previously and demonstrated that a LysR-type transcriptional regulator (Rsp673_21385) in the cool-virulent strain P673 significantly contributes to virulence at 18°C but not at 30°C. This regulator is located upstream of a conserved gene cluster resembling a 2-aminoethylphosphonate (2-AEP) degradation operon. This cluster codes for an alternative pathway recently characterized. P673 was unable to utilize phosphonate as a sole carbon source, however it used 2-AEP as a source of phosphorus in a temperature dependent manner. Relative gene expression analysis of representative genes in the cluster showed significant upregulation at 18°C in the wild-type strain but not in the regulator mutant, indicating that the LysR regulator activates expression of the putative 2-AEP degradation cluster under cool temperatures. These findings reveal a novel temperature-dependent virulence factor and highlight the need to explore the regulon controlled by Rsp673_21385, providing new insights into the adaptation of R. solanacearum to temperate environments.