Abstract
The beneficial roles of melatonin (Mel) and gibberellic acid (GA
3
) in the biosynthesis of photosynthetic pigments, osmoregulation, and methylglyoxal (MG) detoxification and antioxidant system were studied in tomato (
Solanum lycopersicum
L. cv. Five Star) seedlings under NaCl stress. The exogenous application of Mel (100 µM) and GA
3
(1.4 µM) together more efficiently affected growth performance of seedlings under salt stress. The decreased chlorophyll (Chl) degradation and Chl-degrading enzyme (chlorophyllase) activity in seedlings receiving Mel plus GA
3
resulted in increased Chl content by upregulating Chl synthesizing enzyme (
δ
-aminolevulinic acid dehydratase) under salinity. Exogenous Mel plus GA
3
suppressed the overproduction of reactive oxygen species (ROS; superoxide and hydrogen peroxide) and activity of glycolate oxidase. Application of Mel with GA
3
reduced MG content by enhancing the activity of enzymes (glyoxalase I and glyoxalase II) involved in the MG detoxification system. Both Mel and GA
3
together protected seedlings from ROS induced damage by regulating Δ
1
-pyrroline-5-carboxylate synthetase activity, and content of proline (Pro) and glycine betaine (GB). Seedlings receiving Mel + GA
3
exhibited a substantial upregulated activity of catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase, monodehydroascorbate reductase, glutathione peroxidase, polyphenol oxidase and lipoxygenase, and redox homeostasis that reduced oxidative damage induced by salinity. These outcomes advocate that Mel and GA
3
played beneficial roles in Chl, Pro and GB biosynthesis, and improved redox homeostasis, and MG detoxification and antioxidant system under salt stress.