Differential Effects of Nitrate and Light on the Expression of Glutamine Synthetases and Ferredoxin-Dependent Glutamate Synthase in Maize

摘要

The effects of nitrate and light on the expression of genes for glutamine synthetase (GS) isoproteins and ferredoxin-dependent glutamate synthase (Fd-GOGAT) were studied in different organs of maize seedlings by analyzing the levels of the respective polypeptides and mRNAs. In roots, the levels of plastidic GS and of a novel, root-specific GS molecule localized in the extraplastidic compartment were increased markedly by nitrate, whereas Fd-GOGAT and cytosolic GS remained at their initial levels. Ammonia was not effective in inducing the plastidic GS and Fd-GOGAT but it did induce the novel GS isoprotein. In leaves, cytosolic and plastidic GSs and Fd-GOGAT were present in both mesophyll cells (MC) and bundle sheath cells (BSC). Upon addition of nitrate, the level of plastidic GS increased preferentially in MC, and upon exposure of etiolated seedlings to light, the levels of plastidic GS and Fd-GOGAT increased in BSC in a coordinated manner. The relationship between the expression of genes for GSs and Fd-GOGAT and the physiological role of the GS/GOGAT cycle is discussed in terms of the characteristics of nitrogen metabolism in roots, MC, and BSC.