Ralstonia solanacearum nlpD (RSc1206) contributes to host adaptation

Ralstonia solanacearum is a soil-borne xylem-inhibited pathogen and causes a deadly wilting disease on a wide range of crops. This bacterium can also colonize many weeds and native plants without necessarily causing symptoms, leading to devastating epidemiological consequences; however, little is known about the biology. A R. solanacearum mutant harbouring a transposon insertion in the annotated gene RSc1206 was previously shown to retain virulence in tomato but displayed reduced virulence in model weed host Arabidopsis. In this study, we investigated the function of RSc1206 in bacterial pathogenesis in weed hosts. RSc1206 encodes a putative protein that is homologous to E. coli NlpD, and the organization of RSc1206-related gene cluster is mostly conserved among representative gram-negative bacteria analyzed. We therefore designate it R. solanacearum gene nlpD. Microscopic data revealed that the nlpD mutant had defects in cell separation and envelope integrity. An increased sensitivity to hydrophobic toxic chemicals confirmed that the cell integrity of the nlpD mutant was damaged. In addition, the activity of the nlpD mutant in biofilm formation and motility was reduced. However, enzymatic activity and tobacco pathogenesis assays revealed that this mutant functioned normally in the Type II and III secretion systems. Trans-mutation and trans-complementation analyses further verified that disruption of nlpD function was responsible for the observed defects. Our study provides evidence that NlpD contributes to R. solanacearum cell integrity and pathogenesis in weed host Arabidopsis.