Citrulline function as an efficient hydroxyl radical scavenger: Implication for the drought-tolerance of wild watermelon.

Abstract

Wild watermelon plants from the Botswana desert have an ability to survive under severe drought conditions. Our previous report has revealed that drought stress induces accumulation of citrulline in the leaves up to 24 µmol/ gFW, corresponds to concentration of 600 mM if present in the cytosol. Activities of cytosolic metabolic enzymes were not inhibited by 600 mM citrulline, indicating that physiological concentration of citrulline does not interfere with cellular metabolisms. In order to address possible protective roles of accumulated citrulline, free radical scavenging activity of citrulline was examined in vitro. Salicylate competitive trapping assay demonstrated that the second-order rate constant for the reaction of citrulline with hydroxyl radical was estimated to be approximately 3×109 M-1 s-1, suggesting that citrulline is one of the most effective scavengers among known compatible solutes. Moreover, the physiologically relevant concentration of citrulline exhibited significant protective effects on an enzyme and DNA from free radical attack. These data suggest that citrulline, as the novel compatible solute, contributes to the tolerance against oxidative stress under drought condition in wild watermelon plants.