Involvement of phytochrome(s), Ca2+ and phosphorylation in light-dependentcontrol of transcript levels for plastid genes (psbA, psaA and rbcL) in rice (Oryza sativa)

In the present investigation, an attempt has been made to understand the me chanism of light signal transduction in regulation of steady-state transcri pt levels of the plastid genes psbA, psaA and rbcL in rice (Oryza sativa L. ssp, indica cv, Pusa 169) seedlings. Red light irradiation of 5-day-old et iolated seedlings for 5 min upregulates the transcript levels of the above- mentioned plastid genes, while far-red light exposure subsequent to red lig ht reverses this effect, indicating the involvement of phytochrome(s). Ca2 and the Ca2+ ionophore, A23187 increase the level of transcripts in the da rk-grown seedlings. Ca2+ chelators (EGTA and BAPTA) as well. as Ca2+ channe l blockers (nifedipine and verapamil) suppress the accumulation of all tran scripts in light. This suggests the involvement of Ca2+ in the photoregulat ion of plastid genes. Okadaic acid, a specific inhibitor of protein phospha tase 1 and 2A (PP1 and PP2A), and sodium fluoride (NaF), a general inhibito r of protein phosphatases, significantly prevent the light-induced increase in transcript levels of the plastid genes. Staurosporine, a protein kinase inhibitor, causes an increase in the transcript levels in darkness and a f urther enhancement in conjunction with light. The results suggest that ligh t signal transduction downstream to phytochrome(s) involves Ca2+ and phosph orylation for controlling the pattern of plastid gene expression. Phosphory lation inhibits light-activated gene expression in chloroplasts.