Photorespiration and light act in concert to regulate the expression of the nuclear gene for chloroplast glutamine synthetase.

In Pisum sativum, distinct chloroplast and cytosolic forms of glutamine synthetase (GS) are encoded by homologous nuclear genes that are differentially expressed in vivo (Tingey, S. V., Tsai, F.-Y., Edwards, J. W., Walker, E. L., and Coruzzi, G. M. 1988. J. Biol. Chem. 263, 9651-9657). In leaves, light selectively affects the expression of the nuclear gene for chloroplast GS2. Differences in the maximal levels of GS2 mRNA in etiolated plants treated with red or white light indicate that only part of the white-light-induced accumulation of GS2 mRNA is due to a phytochrome-mediated response. The kinetics of GS2 mRNA accumulation in response to white-light illumination of etiolated or dark-adapted green plants indicates that GS2 mRNA accumulates more rapidly in plants containing mature, photosynthetically competent chloroplasts. Other evidence that GS2 mRNA levels are affected by the metabolic status of chloroplasts concerns the selective induction of GS2 mRNA in plants grown under conditions that result in the production of photorespiratory ammonia. These results indicate that the light-induced accumulation of GS2 mRNA in leaves results from the action of phytochrome as well as light-induced changes in chloroplast metabolism.