Melatonin Positively Influences the Photosynthetic Machinery and Antioxidant System of Avena sativa during Salinity Stress

Abstract

Recent studies have demonstrated melatonin protects various crops against abiotic stresses. However, the effects of melatonin on the photosynthetic apparatus of stressed plants is poorly characterized. We investigated the effects of melatonin pretreatment on photosynthesis and tolerance to salinity stress in Avena sativa (oat) plants. Oat plants were exposed to four treatments (three replicate pots per treatment): well-watered (WW; control); watered with 300 mM salt solution for 10 days (NaCl); pretreated with 100 µM melatonin solution for 7 days then watered normally for 10 days (Mel+W); or pretreated with 100 µM melatonin for 7 days then 300 mM salt for 10 days (Mel+NaCl). Considerable differences in growth parameters, chlorophyll content, stomatal conductance, proline accumulation, lipid peroxidation, electrolyte leakage, and growth parameters were observed between groups. Genes encoding three major antioxidant enzymes were upregulated in the Mel+NaCl group compared to the other groups. Chlorophyll-a fluorescence kinetic analyses revealed that almost all photosynthetic parameters were improved in Mel+NaCl plants compared to the other treatments. Analysis of genes encoding the major extrinsic proteins of photosystem II (PSII) revealed that PsbA, PsbB, PsbC, and PsbD (but not PsbO) were highly upregulated in Mel+NaCl plants compared to the other groups, indicating melatonin positively influenced photosynthesis under control conditions and salt stress. In addition, melatonin upregulated stress-responsive NAC transcription factor genes in plants exposed to salt stress. These findings suggest melatonin pretreatment improves photosynthesis and enhances salt tolerance in oat plants.

Chlorophyll-a Fluorescence Analysis Reveals Differential Response of Photosynthetic Machinery in Melatonin-Treated Oat Plants Exposed to Osmotic Stress

“…The MPEG plant group exhibited a notably increased proline build-up in the leaf tissue compared to that of the PEG plant group, indicating that proline is necessary for stress resistance ( Figure 2B). Such an increase in the proline content in the melatonin-treated plants subjected to stress conditions is in agreement with earlier studies [7,10,23]. Besides estimating the proline content in the leaf tissues of the plant groups, qRT-PCR was also performed to analyze the expression of the genes responsible for proline biosynthesis to further investigate the effect of melatonin treatment on proline biosynthesis during PEG-induced osmotic stress, at the molecular level.…”

“…The effects of PEG-induced osmotic stress on the morphology of the oat plants with or without melatonin treatment (Figure 2A) appeared to be similar to the plants that were subjected to salinity stress, as described in our recent work [10]. The significant effects of PEG were observed in oat plants regarding their morphological parameters, such as root and shoot lengths and FW.…”

“…Recently, Sun et al [32] demonstrated that melatonin together with cold priming raises the rate of photosynthetic and stomatal conductance in wheat plants under conditions of cold stress. Recently, a similar regulation of stomatal conductance has also been reported in oat plants under salinity stress [10]. Comparable to stomatal conductance, another important marker used to assess the abiotic stress tolerance in plants is electrolyte leakage.…”

“…Recent studies have demonstrated the protective roles played by melatonin in different plants under different conditions of stress [9,11,[21][22][23][24]. Nevertheless, very few studies have discussed the impact of melatonin on some of the basic photosynthetic parameters under stress conditions [9,10,25]. In this study, we examined the effects of melatonin pretreatment on photosynthetic components by performing comprehensive analyses of the critical photosynthetic parameters derived through the Chl-a fluorescence transient kinetics.…”

“…Several studies have demonstrated the involvement of melatonin in various growth and developmental processes, from seed germination to photosynthesis [5]. The influence exerted by melatonin has also been studied extensively in various crops under almost all types of abiotic stress [6][7][8][9][10].…”