Calcium-Dependent Protein Kinases Regulate the Production of Reactive Oxygen Species by Potato NADPH Oxidase

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Abstract
Reactive oxygen species (ROS) are implicated in plant innate immunity. NADPH oxidase (RBOH; for Respiratory Burst Oxidase Homolog) plays a central role in the oxidative burst, and EF-hand motifs in the N terminus of this protein suggest possible regulation by Ca2+. However, regulatory mechanisms are largely unknown. We identified Ser-82 and Ser-97 in the N terminus of potato (Solanum tuberosum) St RBOHB as potential phosphorylation sites. An anti-phosphopeptide antibody (pSer82) indicated that Ser-82 was phosphorylated by pathogen signals in planta. We cloned two potato calcium-dependent protein kinases, St CDPK4 and St CDPK5, and mass spectrometry analyses showed that these CDPKs phosphorylated only Ser-82 and Ser-97 in the N terminus of St RBOHB in a calcium-dependent manner. Ectopic expression of the constitutively active mutant of St CDPK5, St CDPK5VK, provoked ROS production in Nicotiana benthamiana leaves. The CDPK-mediated ROS production was disrupted by knockdown of Nb RBOHB in N. benthamiana. The loss of function was complemented by heterologous expression of wild-type potato St RBOHB but not by a mutant (S82A/S97A). Furthermore, the heterologous expression of St CDPK5VK phosphorylated Ser-82 of St RBOHB in N. benthamiana. These results suggest that St CDPK5 induces the phosphorylation of St RBOHB and regulates the oxidative burst.