Abstract
Citrulline is an efficient hydroxyl radical-scavenging compatible solute which accumulates in the leaves of wild watermelon during drought/strong light stresses. In plants, citrulline metabolic pathway is composed of eleven enzymes. However, how these enzymes are regulated for massive accumulation of citrulline in wild watermelon remains unclear. In this study, we determined the changes in activities of all the eleven enzymes in wild watermelon leaves during drought. The activities of the first and second enzymes in the citrulline pathway, N-acetylglutamate synthase and N-acetylglutamate kinase, and activity of the enzyme forming carbamoyl carrier, carbamoyl phosphate synthetase, increased approximately 7-, 7-, and 3-fold, respectively, during drought. In contrast, the activity of citrulline-catabolism enzyme, argininosuccinate synthase, decreased one-fifth under drought. These results suggest that citrulline accumulation is triggered by the increase in the influx of substrates into the synthetic pathway, and decrease in the efflux of citrulline into the catabolic pathway.
