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Mixed messages: The role of nitric oxide in Ralstonia solanacearum Type III Secretion and virulence

Connor Hendrich: University of Wisconsin

<div><em>Ralstonia solanacearum </em>(Rs) is a xylem-dwelling bacterial plant pathogen that can infect a wide range of hosts around the world. The xylem fluid of tomato, a model Rs host, is low in oxygen but high in nitrate, and nitrogen metabolism is an important factor in <em>Rs</em> biology. Nitric oxide (NO), a potent signal and metabolic byproduct that is produced by both <em>Rs</em> and its plant hosts is positively correlated with the <em>Rs</em> Type III Secretion System (T3SS), a key <em>Rs </em>virulence factor. Both exogenously applied and endogenously produced NO induced T3SS expression in a range of conditions. NO induces T3SS gene expression in a dose-dependent manner. Induction of T3SS is specific to NO, and is not shared by other toxins or stressors. We used an NO-reactive fluorescent dye to measure NO levels in tomato xylem and found that the levels of NO in the xylem are around 20 uM. <em>In vitro</em>, T3SS can be induced by NO concentrations above 20 uM, implying that the physiological levels of NO present in tomato xylem may be sufficient to induce <em>Rs </em>T3SS gene expression <em>in planta</em>. These findings suggest that <em>Rs</em>, and potentially other xylem dwelling microbes, experience NO stress in the xylem, and may use the presence of NO as a signal to recognize and adapt to their in-host niche.</div>