Abstract
Citrulline is an efficient hydroxyl radical scavenger which accumulates in the leaves of wild watermelon under drought. Previously, we found that the activities of four key enzymes in citrulline metabolic pathway are modulated under drought. In this study, we focused on the regulatory mechanisms for acetylglutamate kinase (AGK) activity, which is up-regulated under drought. Enzymatic assays using leaf crude extracts showed that AGK from unstressed leaves was strongly feedback-regulated by arginine. In contrast, AGK from stressed leaves was insensitive to arginine inhibition, implying that PII protein, a regulator of carbon and nitrogen status, is involved in this regulation. Moreover, addition of PII protein to crude extracts from unstressed leaves relieved the inhibitory effect of arginine on AGK activity. Furthermore, immunoblot analysis revealed that abundance of PII protein increased in wild watermelon leaves under drought. These results suggest that PII protein plays pivotal roles in citrulline accumulation in wild watermelon.
