Correlation of ASN2 Gene Expression with Ammonium Metabolism in Arabidopsis

In Arabidopsis, asparagine (Asn) synthetase is encoded by a small gene family (ASN1, ASN2, and ASN3). It has been shown that ASN1 and ASN2 exhibit reciprocal gene expression patterns toward light and metabolites. Moreover, changes in total free Asn levels parallel the expression of ASN1, but not ASN2. In this study, we show that ASN2 expression correlates with ammonium metabolism. We demonstrate that the light induction of ASN2 is ammonium dependent. The addition and removal of ammonium exerted fast and reciprocal effects on the levels of ASN2 mRNA, specifically under light-grown conditions. NaCl and cold stress increased cellular free ammonium and ASN2 mRNA levels in a coordinated manner, suggesting that the effects of stress on ASN2 expression may be mediated via accumulation of ammonium. The correlation between ASN2 and cellular ammonium metabolism was further demonstrated by analysis of ASN2 transgenic plants. When plants were grown on Murashige and Skoog medium containing 50 mM ammonium, ASN2 overexpressors accumulated less endogenous ammonium compared with the wild-type Colombia-0 and ASN2 underexpressors. When plants were subjected to high-light irradiance, ammonium levels built up. Under such conditions, ASN2 underexpressors accumulated more endogenous ammonium than the wild-type Colombia-0 and ASN2 overexpressors. These results support the notion that ASN2 is closely correlated to ammonium metabolism in higher plants.