| Melatonin (N-acetyl-5-methoxytryptamine) is an indole amine which was long thought to be a classic hormone in animals. It is an endogenously generated molecule in pineal gland of vertebrates that can synchronize circadian and circannual rhythm. Melatonin is a natural antioxidant that can detoxify, neutralize, and metabolize reactive species. What’s more, melatonin can regulate antioxidant system enzymes to further eliminate ROS. Melatonin has now been found to exist in plants as well. Plants can take up melatonin from the culture medium. Plants have the mechanisms to synthesize melatonin from tryptophan. Melatonin biosynthesis in plants occurs via four consecutive enzymatic steps. Melatonin is a plant growth regulator which can make the in vitro leaves expand and promote root regeneration.Cucumber is a vegetable crop which needs a lot of water along growth. Water deficiency can inhibit the plant growth, decrease the yield and quality. Melatonin alleviated the seed germination rate which was decreased by osmotic stress. Osmotic stress seriously limits the cucumber root development. Under osmotic stress, root viability, the volume and the diameter of root were decreased. Melatonin treatment increased lateral root formation. The root:shoot ratio represents the nutrient allocation of the plants. PEG treatment restricted the ability of seedlings to take up water and slightly increased the root:shoot ratio. Melatonin treatments increased the volume of root and whole seedlings and inverted the proportion of root to shoot volume.Water deficiency induced a reduction in photosynthesis. No significant drought-induced change was detected at the level of the intercellular concentration of carbon dioxide. The improvement of photosynthetic rates was likely due to melatonin-induced metabolic changes. Polyethylene glycol severely inhibited the ability of cucumber plants to take up water, causing significant degradation of chlorophyll. Melatonin treatment significantly inhibited chlorophyll degradation in a concentration dependent manner. The photochemical efficiency of open PSⅡ reaction centers (Fv’/Fm’ratio), the quantum yield of PSII electron transport (ΦPSⅡ), and the electron transport rate (ETR) decreased significantly under water stress. In contrast, the nonphotochemical quenching (NPQ) was enriched; indicating enhanced thermal energy dissipation at PSII. A transmission electron microscope revealed that chloroplasts were normal and exhibited long ellipsoidal shapes in the control groups. Meanwhile, water stress resulted in swollen chloroplasts. The number of abnormal chloroplasts was lower in melatonin treated samples.Oxidative stress is considered a major damaging factor in plants exposed to drought. A significant increase in H2O2and·OH content was induced by PEG treatment; however, melatonin-treated plants showed relatively low levels of H2O2and OH. Excess ROS can cause foliar lipid peroxidation which can damage the membrane system. Water stress induced high levels of EL and MDA which were reduced by melatonin treatment. Melatonin and several of its metabolites are known endogenous free radical scavengers. SOD, POD, and CAT showed significant rises in activity with the melatonin treatment. Melatonin in plants functions as the first line of defense against internal and environmental oxidative stressors.Melatonin modulates root system architecture especially stimulates lateral root formation. In this study, RNA sequencing was employed to explore the mechanism of melatonin-induced lateral root formation in cucumber. Based on their expression pattern, peroxidase-related genes were selected as the candidates to be involved in the melatonin response. Several transcription factor families might play important roles in lateral root formation processes. Melatonin affected the root pattern in an auxin-independent manner. Melatonin modulates root development in an indirect way, mostly by regulating the reactive oxygen species generation and transcription factors. |
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