Ralstonia solanacearum Uses Inorganic Nitrogen Metabolism for Virulence, ATP Production, and Detoxification in the Oxygen-Limited Host Xylem Environment
Genomic data predict that, in addition to oxygen, the bacterial plant pathogen Ralstonia solanacearum can use nitrate (NO3 (-)), nitrite (NO2 (-)), nitric oxide (NO), and nitrous oxide (N2O) as terminal electron acceptors (TEAs). Genes encoding inorganic nitrogen reduction were highly expressed during tomato bacterial wilt disease, when the pathogen grows in xylem vessels. Direct measurements found that tomato xylem fluid was low in oxygen, especially in plants infected by R. solanacearum. Xylem fluid contained ~25 mM NO3 (-), corresponding to R. solanacearum's optimal NO3 (-) concentration for anaerobic growth in vitro. We tested the hypothesis that R. solanacearum uses inorganic nitrogen species to respire and grow during pathogenesis by making deletion mutants that each lacked a step in nitrate respiration (ΔnarG), denitrification (ΔaniA, ΔnorB, and ΔnosZ), or NO detoxification (ΔhmpX). The ΔnarG, ΔaniA, and ΔnorB mutants grew poorly on NO3 (-) compared to the wild type, and they had reduced adenylate energy charge levels under anaerobiosis. While NarG-dependent NO3 (-) respiration directly enhanced growth, AniA-dependent NO2 (-) reduction did not. NO2 (-) and NO inhibited growth in culture, and their removal depended on denitrification and NO detoxification. Thus, NO3 (-) acts as a TEA, but the resulting NO2 (-) and NO likely do not. None of the mutants grew as well as the wild type in planta, and strains lacking AniA (NO2 (-) reductase) or HmpX (NO detoxification) had reduced virulence on tomato. Thus, R. solanacearum exploits host NO3 (-) to respire, grow, and cause disease. Degradation of NO2 (-) and NO is also important for successful infection and depends on denitrification and NO detoxification systems.
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