Abstract
Oxidative stress is a major source of damage of plants exposed to adverse environments. We examined the effect of exogenous melatonin (MT) in limiting of oxidative stress caused by methyl viologen (MV; paraquatin) in apple leaves (Malus domestica Borkh.). When detached leaves were pre-treated with melatonin, their level of stress tolerance increased. Under MV treatment, melatonin effectively alleviated the decrease in chlorophyll concentrations and maximum potential Photosystem II efficiency while also mitigating membrane damage and lipid peroxidation when compared with control leaves that were sprayed only with water prior to the stress experiment. The melatonin-treated leaves also showed higher activities and transcripts of antioxidant enzymes superoxide dismutase, peroxidase, and catalase. In addition, the expression of genes for those enzymes was upregulated. Melatonin-synthesis genes MdTDC1, MdT5H4, MdAANAT2, and MdASMT1 were also upregulated under oxidative stress in leaves but that expression was suppressed in response to 1 mM melatonin pretreatment during the MV treatments. Therefore, we conclude that exogenous melatonin mitigates the detrimental effects of oxidative stress, perhaps by slowing the decline in chlorophyll concentrations, moderating membrane damage and lipid peroxidation, increasing the activities of antioxidant enzymes, and changing the expression of genes for melatonin synthesis.
Highlights
Environmental stress is becoming one of the most severe agricultural problems affecting plant growth and crop yield [1]
We explored the effect of different melatonin concentrations and monitored changes in the activities of several antioxidant enzymes, levels of H2O2, and the expression of genes involved in melatonin synthesis
We found that exogenous melatonin can preserve the content of chlorophyll by downregulating the expression of PAO, a chlorophyll degradation gene located in the envelope membrane of gerontoplasts [27]
Summary
Environmental stress is becoming one of the most severe agricultural problems affecting plant growth and crop yield [1]. Methyl viologen is a strong auto-oxidable electron acceptor in Photosystem I (PSI); its presence in the chloroplasts of light-exposed plants has several important consequences [4]. It catalyzes the generation of superoxide radicals by accepting electrons from PSI, inhibiting the reduction of ferredoxin, and transfers the electrons to oxygen-forming superoxide radicals that, via a disproportionation reaction, are converted to H2O2, leading to oxidative stress [4,5]. Methyl viologen can cause lipid peroxidation, protein denaturation, inhibition of photosynthesis, and electrolyte leakage due to the loss of membrane integrity [6,7]. In the presence of MV, antioxidant enzymes are usually induced in leaves due to the generation of superoxide [9]
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