Dissecting Light Repression of the Asparagine Synthetase gene (AS1) in Arabidopsis

Abstract

In plants, asparagine is a key amino acid used to mobilize assimilated nitrogen from sources to sinks. Inorganic nitrogen assimilated initially into glutamine is converted into asparagine under certain metabolic conditions (see Figure 1). Biochemical, physiological and molecular studies suggest that asparagine biosynthesis in plants is a dynamic process that is tightly regulated throughout development and by environmental factors such as light (Sieciechowicz et al., 1988; Lam et al., 1994; Tsai and Coruzzi, 1990,1991). Physiological studies on pea and Arabidopsis have shown that when plants are dark-adapted, asparagine levels are elevated and asparagine becomes the predominant amino acid transported in the phloem (Urquhart and Joy, 1982; Schultz, 1994; Lam et al., 1995). Asparagine has a higher nitrogen to carbon ratio than glutamine, hence it is a more economical nitrogen transport compound in plants grown in the dark, when carbon skeletons are limiting. As such asparagine is the preferred nitrogen transport compound in dark- adapted plants (see Figure 1). By contrast, asparagine synthesis is repressed by light as evidenced by changes in amino acid levels and gene expression studies.