Effects of two arginine enantiomers (L and D) pre-treatment on some physiological parameters and the expression of arginase 1 gene in tomato plants under drought Stress

Arginine (Arg) is reported to contribute in reduction of destructive effects of biotic and abiotic stresses through three pathways and their end products (proline, polyamine and nitric oxide). Two isomers of arginine (L and D) have been applied as pre-treatment and then tomato plants were subjected to drought stress. The effects of these isomers on some physiological parameters and expression of arginase 1 (ARG I) (one of the enzymes in proline biosynthesis) pathway have been investigated. The results showed that leaf relative water content (RWC) of tomato plants under drought stress decreased and pre-treatment with L-Arg improved RWC, but D-Arg did not. Drought stress caused a significant decrease in photosynthetic pigments but arginine had no effect on these parameters. Sugar content and lipid peroxidation level increased under stress. However, arginine treatment caused decrease in lipid peroxidation while it did not change sugar content significantly. Proline also significantly increased under drought stress and L-Arg many time increased it content. Increment in expression of arginase1 may be the cause for this increment. On the other hand, the expression of arginase1 and proline content with D-Arg was less obvious. The results of the present study indicate that in tomato plant in addition of L-Arg, arginase can use D-Arg substrate to produce proline. However D-Arg has been reported as an analog of L-Arg which could block ADC pathway. Our molecular and biochemical results showed that this enantiomer could not inhibit arginase pathway.