Regulation of plastid gene expression in rice involves calcium and protein phosphatases/kinases for signal transduction

Abstract

Spatial control of expression of plastid-encoded photosynthesis-related genes, psbA, psaA and rbcL, has been studied in rice (Oryza sativa L.) seedlings. Whereas roots accumulate negligible amount of mRNAs of these genes, shoots show high transcript levels. A gradient in expression of these genes is seen even in lower, middle and upper segments of leaf sheath, with lower segment showing the least amount of mRNA-being nearer to the meristematic zone. The steady-state transcript levels in lower segment are significantly reduced on treatment with EGTA. The inhibitory effect of EGTA could be reversed by addition of calcium during treatment. Nifedipine and verapamil-the calcium channel blockers-also decrease the levels of transcripts. Okadaic acid, the specific inhibitor of protein phosphatases 1 and 2A (PPI and PP2A), and a general inhibitor of protein phosphatases, NaF, inhibit the steady-state transcript levels of psaA, psbA and rbcL significantly. Staurosporine, the protein kinase inhibitor, increases the steady-state transcript levels of these genes. These results suggest the importance of calcium and protein phosphorylation in expression of genes during chloroplast biogenesis. However, the inter-relationship of calcium effects and protein phosphorylation remains to be worked out.