Abstract
Alfalfa (Medicago sativa L.) is an important forage crop, and salt stress is a major limiting factor in its yield. Melatonin (MT) is a multi-regulatory molecule in plants. We showed that basal MT content was positively correlated with the salt tolerance degree of different alfalfa varieties. MT and its precursor 5-HT fully recovered seed germination while partially ameliorated seedling growth of salt-stressed alfalfa. The 5-HT showed some divergent effects from MT with regards to growth amelioration under salinity. Salt stress caused stunted plant growth in soil culture, while MT ameliorated it by elevating plant height, fresh weight, branching number, and chlorophyll content. Silencing of a putative MT receptor, MsPMTR1, which was shown to be membrane-localized, abolished the ameliorative effects of MT on salt-stressed alfalfa seedling growth, while overexpression of MsPMTR1 improved plant growth under salt stress. The RNA sequencing analysis showed that nine pathway genes were specifically induced by MT treatment compared with salt stress. These MT-responsive differentially expressed genes include basal metabolic pathway genes, such as “ribosome, elongation factor,” “sugar and lipid metabolism,” and “photosynthesis” and stress-related genes encoding “membrane integrity” related proteins, heat shock protein, peroxidase/oxidoreductase, and protease. Several abiotic stress response-related genes, such as DRE, ARF, HD-ZF, MYB, and REM were repressed by NaCl treatment while induced by MT treatment. In summary, we demonstrated the importance of MsPMTR1 in MT-mediated salt tolerance in alfalfa, and we also analyzed the regulatory mechanism of MT during alfalfa seed germination under salt stress.
Highlights
Soil salinization is one of the most serious environmental stresses to crop production
Melatonin content was reported to be as high as 16 ng/g in alfalfa seeds, and the content dramatically decreased to 133 pg/g after germination, showing its role during germination (Aguilera et al, 2015)
We identified high-tolerant (Weiner), medium-tolerant (Algonquin and Golden Empress), and sensitive (Bingchi) varieties to pursue our study
Summary
Soil salinization is one of the most serious environmental stresses to crop production. Salt stress influences around 20% of the cultivated lands and nearly 50% of all irrigated lands worldwide (Zhu, 2001). Salinity affects plant growth and development through osmotic, ionic, and oxidative stresses. Seed germination is the critical stage in the whole life cycle of plants, and it is an extremely vulnerable stage to salt damage (Ibrahim, 2016). Salt stress causes morphological impairments and retards vegetative growth. While low or moderate salt concentration affects plant growth via osmotic stress, the high salt concentration would render the plant to ionic stress. The excessive Na+ accumulation leads to membrane injury by oxidative stress (Isayenkov and Maathuis, 2019)
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