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As an indoleamine molecule, melatonin mediates many physiological processes in plants. We investigated its role in regulating growth, ion homeostasis, and the response to oxidative stress in Malus hupehensis Rehd. under high-salinity conditions. Stressed plants had reduced growth and a marked decline in their net photosynthetic rates and chlorophyll contents. However, pretreatment with 0.1μm melatonin significantly alleviated this growth inhibition and enabled plants to maintain an improved photosynthetic capacity. The addition of melatonin also lessened the amount of oxidative damage brought on by salinity, perhaps by directly scavenging H(2) O(2) or enhancing the activities of antioxidative enzymes such as ascorbate peroxidase, catalase, and peroxidase. We also investigated whether melatonin might control the expression of ion-channel genes under salinity. Here, MdNHX1 and MdAKT1 were greatly up-regulated in the leaves, which possibly contributed to the maintenance of ion homeostasis and, thus, improved salinity resistance in plants exposed to exogenous melatonin.

The objectives of this study were to test the effects of exogenous melatonin on apple (Malus domestica Borkh. cv. Golden Delicious) leaves and investigate its possible physiological role in delaying leaf senescence. Detached leaves treated with 10 mm melatonin solutions clearly showed a slowing in their process of dark-induced senescence, as evidenced by both biochemical and molecular parameters. Melatonin delayed the normal reduction in chlorophyll content and maximum potential photosystem II efficiency (F(v) /F(m) ). It also suppressed the transcript levels of a key chlorophyll degradation gene, pheide a oxygenase (PAO), and the senescence-associated gene 12 (SAG12). This outcome was thought to be because of the enhanced antioxidant capabilities of melatonin. Indeed, H(2) O(2) accumulation was inhibited by exogenous melatonin, which might have resulted from direct reactive oxygen species scavenging by melatonin and a great enhancement of ascorbate peroxidase (APX; EC 1.11.1.11), which acted on both mRNA and protein activity levels. Melatonin treatment led to the maintenance of higher contents of ascorbic acid (AsA) and glutathione (GSH) but less dehydroascorbate (DHA) and oxidized glutathione (GSSG) compared with the control, possibly through its regulation of the AsA-GSH cycle.

We examined whether exogenously applied melatonin could improve resistance to Marssonina apple blotch (Diplocarpon mali) by apple [Malus prunifolia (Willd.) Borkh. cv. Donghongguo]. This serious disease leads to premature defoliation in the main regions of apple production. When plants were pretreated with melatonin, resistance was increased in the leaves. We investigated the potential roles for melatonin in modulating levels of hydrogen peroxide (H2O2), as well the activities of antioxidant enzymes and pathogenesis-related proteins during these plant-pathogen interactions. Pretreatment enabled plants to maintain intracellular H2O2 concentrations at steady-state levels and enhance the activities of plant defence-related enzymes, possibly improving disease resistance. Because melatonin is safe and beneficial to animals and humans, exogenous pretreatment might represent a promising cultivation strategy to protect plants against this pathogen infection.

Abscisic acid (ABA)-, stress-, and ripening-induced (ASR) proteins are reported to be involved in abiotic stresses. However, it is not known whether ASR genes confer drought stress tolerance by utilizing the antioxidant system. In this study, a wheat ASR gene, TaASR1, was cloned and characterized. TaASR1 transcripts increased after treatments with PEG6000, ABA and H2O2. Overexpression of TaASR1 in tobacco resulted in increased drought/osmotic tolerance, which was demonstrated that transgenic lines had lesser malondialdehyde (MDA), ion leakage (IL) and reactive oxygen species (ROS), but higher relative water content (RWC) and superoxide dismutase (SOD) and catalase (CAT) activities than wild type (WT) under drought stress. Overexpression of TaASR1 in tobacco also enhanced the expression of ROS-related and stress-responsive genes under osmotic stress. In addition, transgenic lines exhibited improved tolerance to oxidative stress by retaining more effective antioxidant system. Finally, TaASR1 was localized in the cell nucleus and functioned as a transcriptional activator. Taken together, our results showed that TaASR1 functions as a positive factor under drought/osmotic stress, involved in the regulation of ROS homeostasis by activating antioxidant system and transcription of stress-associated genes.