Protein phosphorylation is a prerequisite for the Ca2+-dependent activation of Arabidopsis NADPH oxidases and may function as a trigger for the positive feedback regulation of Ca2+ and reactive oxygen species.

Reactive oxygen species (ROS) produced by NADPH oxidases play critical roles in signalling and development. Given the high toxicity of ROS, their production is tightly regulated. In Arabidopsis, respiratory burst oxidase homologue F (AtrbohF) encodes NADPH oxidase.

Synergistic activation of the Arabidopsis NADPH oxidase AtrbohD by Ca2+ and phosphorylation.

Plant respiratory burst oxidase homolog (rboh) proteins, which are homologous to the mammalian 91-kDa glycoprotein subunit of the phagocyte oxidase (gp91(phox)) or NADPH oxidase 2 (NOX2), have been implicated in the production of reactive oxygen species (ROS) both in stress responses and during development. Unlike mammalian gp91(phox)/NOX2 protein, plant rboh proteins have hydrophilic N-terminal regions containing two EF-hand motifs, suggesting that their activation is dependent on Ca(2+). However, the significance of Ca(2+) binding to the EF-hand motifs on ROS production has been unclear.