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

Abstract

In the present investigation, an attempt has been made to understand the mechanism of light signal transduction in regulation of steady-state transcript 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 etiolated seedlings for 5 min upregulates the transcript levels of the above-mentioned plastid genes, while far-red light exposure subsequent to red light reverses this effect, indicating the involvement of phytochrome(s). Ca²⁺ and the Ca²⁺ ionophore, A23187 increase the level of transcripts in the dark-grown seedlings. Ca²⁺ chelators (EGTA and BAPTA) as well as Ca²⁺ channel blockers (nifedipine and verapamil) suppress the accumulation of all transcripts in light. This suggests the involvement of Ca²⁺ in the photoregulation of plastid genes. Okadaic acid, a specific inhibitor of protein phosphatase 1 and 2A (PP1 and PP2A), and sodium fluoride (NaF), a general inhibitor 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 further enhancement in conjunction with light. The results suggest that light signal transduction downstream to phytochrome(s) involves Ca²⁺ and phosphorylation for controlling the pattern of plastid gene expression. Phosphorylation inhibits light-activated gene expression in chloroplasts.