Nutrient stress harms plant growth and yield. Melatonin is a biologically active, multifunctional hormone that relieves abiotic stress in plants. Although previous studies have shown that melatonin plays an important role in improving nutrient-use efficiency, the mechanism of its regulation of nutrient stress remains unclear. In this study, melatonin was applied to apple plants under nutrient stress, and morphological indices, physiological and biochemical indices, and stomatal morphology were evaluated. The response of apple plants to nutrient deficiency and the melatonin mechanism to alleviate nutrient stress were analyzed by combining ionome, transcriptome, and metabolome. The results showed that exogenous melatonin significantly alleviated the inhibitory effect of nutritional stress on the growth of apple plants by regulating stomatal morphology, improving antioxidant enzyme activity, promoting ion absorption, and utilizing and changing the absorption and distribution of minerals throughout the plant. The transcriptome results showed that melatonin alleviated nutrient stress and promoted nutrient absorption and utilization by regulating glutathione metabolism and upregulating some metal ion transport genes. The metabolome results indicated that levels of oxalic acid, L-ascorbic acid, anthocyanins (cyanidin-3-O-galactoside), lignans (lirioresinol A and syringaresinol), and melatonin significantly increased after exogenous melatonin was applied to plants under nutrient stress. These differentially expressed genes and the increase in beneficial metabolites may explain how melatonin alleviates nutrient stress in plants.
Dopamine has demonstrated promise as a stress-relief substance. However, the function of dopamine in Cd tolerance and its mechanism remains largely unknown. The current study was performed to investigate the mechanism of dopamine on alleviating apple Cd stress through regular application of CdCl2 and dopamine solution to potting soil. The results indicated that dopamine significantly reduced reactive oxygen species (ROS) and Cd accumulation and alleviated the inhibitory effect of Cd stress on the growth of apple plants through activation of the antioxidant system, enhancement of photosynthetic capacity, and regulation of gene expression related to Cd absorption and detoxification. The richness of the rhizosphere microbial community increased, and community composition and assembly were affected by dopamine treatment. Network analysis of microbial communities showed that the numbers of nodes and total links increased significantly after dopamine treatment, while the keystone species shifted. Linear discriminant analysis effect size indicated that some biomarkers were significantly enriched after dopamine treatment, suggesting that dopamine induced plants to recruit potentially beneficial microorganisms (Pseudoxanthomonas, Aeromicrobium, Bradyrhizobium, Frankia, Saccharimonadales, Novosphingobium, and Streptomyces) to resist Cd stress. The co-occurrence network showed several metabolites that were positively correlated with relative growth rate and negatively correlated with Cd accumulation, suggesting that potentially beneficial microorganisms may be attracted by several metabolites (L-threonic acid, profenamine, juniperic acid and (3β,5ξ,9ξ)-3,6,19-trihydroxyurs-12-en-28-oic acid). Our results demonstrate that dopamine alleviates Cd stress in apple trees by recruiting beneficial microorganisms to enhance the physiological resilience revealed. This study provides an effective means to reduce the harm to agricultural production caused by heavy metals.
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