Arabidopsis chloroplasts dissimilate L-arginine and L-citrulline for use as N source.

When aseptically grown on defined medium with either L-arginine, L-citrulline, or nitrate as the sole N source, Arabidopsis plants grew and developed normally. Three catabolic activities, L-arginine iminohydrolase, L-ornithine carbamoyltransferase, and carbamate kinase, were found in stromal fractions of purified Arabidopsis chloroplasts. These activities dissimilate L-arginine and/or L-citrulline into L-ornithine, ammonium, bicarbonate, and ATP. In physiological tests with purified, intact Arabidopsis chloroplasts, L-[guanido-14C]arginine was rapidly taken up and about 10% was decomposed, releasing 14CO2. Therefore, chloroplasts can take up and dissimilate L-arginine. In principle, chloroplast arginine dissimilation allows Arabidopsis to use L-arginine and/or L-citrulline as general N sources for growth. However, plants rarely encounter exogenous L-arginine and/or L-citrulline in amounts exceeding their biosynthetic needs. Therefore, L-arginine and L-citrulline might serve as endogenous N sources.