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

Abstract

The opening and closing of tulip petals was reproduced in the dark by changing the temperature from 5°C to 20°C for opening and 20°C to 5°C for closing. The opening process was accompanied by 3H 2O transport through the stem from the incubation medium to the petals. A Ca2+-channel blocker and a Ca2+-chelator inhibited petal opening and 3H2O transport. Several proteins in the isolated plasma membrane fraction were phosphorylated in the presence of 25 μM Ca2+ at 20°C. 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 Ca 2+-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 5C, suggesting the inactivation of the water channel.