Seed priming with melatonin improves salt tolerance in cotton through regulating photosynthesis, scavenging reactive oxygen species and coordinating with phytohormone signal pathways

Abstract Seed priming is a useful seed pretreatment technology, which is widely applied in crop cultivation. Salt stress is one of the important environmental stresses that always inhibited cotton (Gossypium hirsutum L.) growth. However, melatonin could alleviate abiotic and biotic stress in plants. In this study melatonin solution was used for seed priming and the role of melatonin priming in cotton seedlings tolerance to salt stress was investigated. As a widely extended upland cotton cultivar in China, CCRI12 was used as a material to study the effects of seed priming with melatonin on the seedling tolerance to the salt stress. Through morphological characterization, 25 μM melatonin primed seedlings exhibited the best phenotype after 14 days salt stress treatment. To reveal the mechanism of melatonin priming alleviating the salt stress, the photosynthesis indexes and antioxidant enzymes activities were detected. It was displayed that seedlings from 25 μM melatonin primed seeds showed better photosynthetic efficiency and stronger ability to scavenge reactive oxygen species. Besides, transcription sequencing analysis, it was found that almost all the photosynthesis-related genes in the melatonin primed seedlings were significantly down-regulated after 48 h salt stress treatment. It might be a way to decrease the photosynthetic rate to adapt salt stress and mitigate the damage. Moreover, amounts of genes encoding antioxidant enzymes were up-regulated by melatonin priming under salt stress, which was in accordance with the result of enzymes activities. Furthermore, a large number of genes participating in eight phytohormones signal pathways were significantly and differentially expressed by melatonin priming under salt stress, which suggested that melatonin could cooperated with phytohormone signal pathways to respond against salt stress. Therefore, it was indicated that melatonin priming might improve the tolerance of cotton seedling by regulating photosynthetic efficiency, enhancing the active oxygen scavenging system and coordinating to phytohormone signal pathways. Therefore, the application of melatonin, enhancing seedling tolerance to salt stress displayed its great potential for cotton production in the salinized land.