Phytochrome and calcium regulated protein phosphorylation in Sorghum bicolor

Irradiation with red light of Sorghum bicolor seedlings stimulated in vitro phosphorylation of 55 kD and several other soluble polypeptides in a devel opment-dependent manner. The red light stimulated phosphorylation of 55 kD polypeptide was more in 6-day-old etiolated plants as compared to 5-day-old plants, The in vitro phosphorylation of 55 kD polypeptide was enhanced fur ther when calcium was added to the extracts obtained from red light irradia ted tissues of 6-day-old seedlings. This effect was inhibited in the presen ce of calmodulin inhibitors. There was no significant stimulation in the ph osphorylation of this polypeptide by calcium in 5-day-old and 7-day-old eti olated plants. Besides 55 kD, the phosphorylation of several other polypept ides was either stimulated or inhibited by light, calcium and calmodulin in hibitors suggesting involvement of both kinases and phosphatases in light-m ediated phosphorylation.