Involvement of PP6-type protein phosphatase in hypocotyl phototropism in Arabidopsis seedlings.

Recently, we reported that the D6 protein kinase subfamily, which belongs to the AGCVIII kinase family, is a critical component of hypocotyl phototropism in Arabidopsis seedlings. Furthermore, we demonstrated that AGC1-12, which is also a member of the AGCVIII kinase family, is involved in both the pulse-induced first positive phototropism and gravitropism in Arabidopsis hypocotyls. Those results indicated that phosphorylation control is an important mechanism in phototropic signaling. As phosphorylation regulation is controlled by both kinases and phosphatases, we investigated the roles of phosphatases in hypocotyl phototropism. Our physiological analysis, which was performed using Arabidopsis mutants, indicated that the flower-specific, phytochrome-associated protein phosphatase family, which functions as a catalytic subunit of protein phosphatase 6 (PP6), is involved in both the pulse-induced first positive phototropism and the time-dependent second positive phototropism, although it is not necessary for the continuous-light-induced second positive phototropism. These results suggest that not only kinases, but also phosphatases play critical roles in hypocotyl phototropism to control phosphorylation status and that PP6-type protein phosphatases may act antagonistically with AGCVIII protein kinases on the same targets, such as PIN-formed proteins.