Exogenous Melatonin Alleviates Heat-Induced Oxidative Damage in Strawberry (Fragaria x ananassa Duch. cv. Ventana) Plant

Melatonin is a signaling molecule that is involved in inducing plants & apos; abiotic stress tolerance. To determine the possible effects of the melatonin pre-treatment on thermotolerance in strawberry (Fragaria x ananassa Duch.), the heat-sensitive cultivar Ventana was subjected to high temperatures (35 degrees C and 40 degrees C) for 10 h after pre-treatment with 0, 50, and 100 mu M melatonin. High temperature increased malondialdehyde and H2O2 contents and reduced relative water content, carotenoid content, and catalase and ascorbate peroxidase activities, which led to a marked reduction in chlorophyll fluorescence. However, pre-treatment with melatonin at 100 mu M decreased heat injury symptoms and induced antioxidant mechanisms in heat-sensitive cultivar Ventana, increasing heat tolerance. The results showed that when the melatonin pre-treated strawberry plants were exposed to high temperature (40 degrees C) for 2 h, the levels of FaTHsfA2a and HSP90 mRNA significantly increased, but after heat treating at 40 degrees C for 5 and 10 h, their mRNA levels were as similar as the control. The results support the hypothesis that melatonin acts as an important signaling molecule during heat stress to induce protective mechanisms via up-regulating the expression of defense HSF (FaTHsfA2a, FaTHSFB1a) and HSP (HSP90) genes.