Extracellular ATP Induces the Accumulation of Superoxide via NADPH Oxidases in Arabidopsis

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Abstract
Extracellular ATP can serve as a signaling agent in animal cells, and, as suggested by recent reports, may also do so in plant cells. In animal cells it induces the production of reactive oxygen species through the mediation of NADPH oxidase. Similarly, here we report that in leaves of Arabidopsis (Arabidopsis thaliana), applied ATP, but not AMP or phosphate, induces the accumulation of superoxide (O2−) in a biphasic, dose-dependent manner, with a threshold at 500 nm ATP. This effect did not require ATP hydrolysis for it was mimicked by ATPγS. ATP also induced increased levels of Arabidopsis respiratory burst oxidase homolog D (AtrbohD) mRNA, but ATP-treated plants that had disrupted AtrbohD and AtrbohF genes did not accumulate O2−, indicating that NADPH oxidases are responsible for the induced O2− accumulation. Inhibitors of mammalian P2-type ATP receptors abolished ATP-induced O2− production, suggesting that the ATP effects may be mediated through P2-like receptors in plants. Cytosolic Ca2+ and calmodulin are likely to help transduce the ATP responses, as they do in animal cells, because a Ca2+ channel blocker, a Ca2+ chelator, and calmodulin antagonist all reduced ATP-induced O2− accumulation. Furthermore, ATP treatment enhanced the expression of genes that are induced by wounds and other stresses. The ATP measured at wound sites averaged 40 μm, well above the level needed to induce O2− accumulation and gene expression changes. Transgenic plants overexpressing an apyrase gene had reduced O2− production in response to applied ATP and wounding. Together, these data suggest a possible role for extracellular ATP as a signal potentially in wound and stress responses.