Phosphorylation of plasma membrane aquaporin regulates temperature-dependent opening of tulip petals

The opening and closing of tulip petals was reproduced in the dark by changing the temperature from 5 degreesC to 20 degreesC for opening and 20 degreesC to 5 degreesC for closing. The opening process was accompanied by (H2O)-H-3 transport through the stem from the incubation medium to the petals. A Ca(2+)channel blocker and a Ca2+-chelator inhibited petal opening and (H2O)-H-3 transport. Several proteins in the isolated plasma membrane fraction were phosphorylated in the presence of 25 muM Ca2+ at 20 degreesC. The 31-kDa protein that was phosphorylated, was suggested immunologically as the putative plasma membrane aquaporin (PM-AQP). This phosphorylated PM-AQP clearly reacted with the anti-phospho-Ser. In-gel assay revealed the presence of a 45-kDa Ca2+-dependent protein kinase in the isolated plasma membrane. Phosphorylation of the putative PM-AQP was thought to activate the water channel composed of PM-AQP Dephosphorylation of the phosphorylated PM-AQP was also observed during petal closing at 5 degreesC, suggesting the inactivation of the water channel.