Loss of glutamate dehydrogenase in Ralstonia solanacearum alters dehydrogenase activity, extracellular polysaccharide production and bacterial virulence

Abstract

Metabolism in Ralstonia solanacearum, which causes lethal wilt on Solanaceous plants, is poorly understood. In this study, we selected a Tn5-inserted mutant of R. solanacearum SL341 showing various phenotypic changes and altered virulence. When the gdhA gene encoding NAD(P)+-dependent glutamate dehydrogenase was disrupted, the gdhA mutant of SL341 (SL341P2) was defective in red colony development on tetrazolium chloride-amended medium and showed less extracellular polysaccharide (EPS) production. The growth rate of the gdhA mutant on rich medium did not differ from that of the wild-type strain; however, its growth on minimal medium with glutamate as the sole carbon source was completely inhibited. SL341P2 was also defective in the oxidation of several carbon sources compared to the wild type. All the observed defects of SL341P2 gdhA mutant were fully or partially restored by providing the gdhA gene in trans. The gdhA mutant showed reduced virulence after soil-soaking inoculation of tomato plants, both on susceptible tomato cultivar Moneymaker and on the well-known bacterial-wilt-resistant cultivar Hawaii 7996. The delayed disease development by the gdhA mutant was due to slower multiplication of the mutant bacteria than wild type in tomato plants. Taken together, these results indicate that GdhA is required for diverse metabolic functions in R. solanacearum, including normal production of the virulence factor EPS, as well as normal bacterial growth in planta and full virulence on tomato plants.