ASN2 is a key enzyme in asparagine biosynthesis under ammonium sufficient conditions

The modification and enhancement of plant amino acid accumulation is potentially beneficial in terms of the production of suitable crop plants for food or feeds, and also with respect to possible environmental improvement via the effective use of nitrogen nutrition. It is also expected that modifies plants will produce beneficial nitrogenous compounds, such as proteins, secondary metabolites, or nucleic acids. The size of amino acid pools in plants is strictly controlled by certain environmental factors such as light and nutrition. In order to gain an understanding of the mechanisms that control amino acid biosynthesis and metabolism, we investigated the diurnal changed in amino acid contents and also how the amino acid contents change with the progression of growth. We observed that almost all the amino acids in plant leaves undergo diurnal changes and that the pattern of these changes was different in young and old stages. We focused on correlation of the asparagine content and the expression of ASN2 asparagine synthetase, since under our experimental conditions their fluctuating patterns were found to be similar. The asparagine contents of ASN2-overexpressing and -underexpressing plants were increased and decreased, respectively, when they were grown under normal light and nutrient conditions. These changes in asparagine content were marked when the plants were grown under conditions where ammonium was the sole nitrogen source. It has been previously reported that ASN2 expression and ammonium metabolism are correlated. Our findings and these previous observations suggest that ASN2 functions as a regulator of asparagine biosyntheses and metabolism and that it mediates the effective use of nitrogen under ammonium-sufficient conditions.