Protein phosphatase PP2C19 controls hypocotyl phototropism through the phosphorylation modification of NONPHOTOTROPIC HYPOCOTYL3 in <i>Arabidopsis</i>

Plants exhibit shoot growth in the direction of the light source to facilitate photosynthesis, known as positive phototropism. In Arabidopsis hypocotyl phototropism, it is thought that a gradient of the signal intensity of the blue light (BL) photoreceptor phototropin1 (phot1) between the light-irradiated and shaded sides leads to the differential growth of hypocotyls. The intensity of phot1 signal is regulated not only by the protein kinase activity of phot1 but also by the phosphorylation status of the NONPHOTOTROPIC HYPOCOTYL3 (NPH3) protein, which has a dark form and a BL form of the phosphorylation modification. Previous studies have shown that phot1 drives the forward reaction from the dark form to the BL form of NPH3. However, the molecular mechanism underlying the reverse reaction remains unknown. Here, we show that protein phosphatase PP2C19 controls the reverse reaction that converts the BL form of NPH3 to the dark form of NPH3. The PP2C19 protein possesses the protein phosphatase type 2C (PP2C) domain, two cyclic nucleoside monophosphate (cNMP)-binding domains, and the protein kinase domain. Similar to phot1 and NPH3, PP2C19 localizes to the plasma membrane, and its PP2C domain is necessary and sufficient for PP2C19 function in hypocotyl phototropism. The pp2c19 mutants show abnormalities in second positive hypocotyl phototropism with a delay in the reverse reaction of NPH3 phosphorylation modification. The present study suggests that continuous BL irradiation induces an equilibrium state of the reversible reaction of NPH3 phosphorylation, which acts as a phot1 signaling gradient with phot1 kinase activity to induce the second positive phototropism.