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Malus hupehensis, as an apple rootstock, is an economically important tree species popular due to its excellent fruit yield and stress resistance. Nitrogen is one of the critical limiting factors of plant growth and fruit yield, so it is crucial to explore new methods to improve nitrogen use efficiency. Melatonin and dopamine, as multifunctional metabolites, play numerous physiological roles in plants. We analyzed the effects of exogenous melatonin and dopamine treatments on the growth, root system architecture, nitrogen absorption, and metabolism of M. hupehensis when seedlings were exposed to nitrate-deficient conditions. Under low nitrate stress, plant growth slowed, and chlorophyll contents and 15NO3– accumulation decreased significantly. However, the application of 0.1 μmol/L melatonin or 100 μmol/L exogenous dopamine significantly reduced the inhibition attributable to low nitrate levels during the ensuing period of stress treatment, and the effect of dopamine was more obvious. In addition to modifying the root system architecture of nitrate-deficient plants, exogenous melatonin and dopamine also changed the uptake, transport, and distribution of 15NO3–. Furthermore, both exogenous melatonin and dopamine enhanced tolerance to low nitrate stress by maintaining the activity of enzymes (NR, NiR, GS, Fd-GOGAT, and NADH-GOGAT) and the transcription levels of related genes involved in leaf and root nitrogen metabolism. We also found that exogenous melatonin and dopamine promoted the expression of nitrate transporter genes (NRT1.1, NRT2.4, NRT2.5, and NRT2.7) in nitrate-deficient plant leaves and roots. Our results suggest that both exogenous melatonin and dopamine can mitigate low nitrate stress by changing the root system architecture, promoting the absorption of nitrate, and regulating the expression of genes related to nitrogen transport and metabolism. However, according to a comprehensive analysis of the results, exogenous dopamine plays a more significant role than melatonin in improving plant nitrogen use efficiency.

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.

This study explored the contributions of melatonin and dopamine to the uptake and utilization of nitrogen and the formation of rhizosphere microbial communities in ‘Tianhong 2’/M. hupehensis, with the goal improving plant resistance to drought stress. Drought stress was formed by artificially controlling soil moisture content. And melatonin or dopamine solutions were applied to the soil at regular intervals for experimental treatment. After 60 days of treatment, plant indices were determined and the structure of the rhizosphere microbial community was evaluated using high-throughput sequencing technology. The findings revealed two ways through which melatonin and dopamine alleviate the inhibition of growth and development caused by drought stress by promoting nitrogen uptake and utilization in plants. First, melatonin and dopamine promote the absorption and utilization of nitrogen under drought stress by directly activating nitrogen transporters and nitrogen metabolism-related enzymes in the plant. Second, they promote the absorption of nitrogen by regulating the abundances of specific microbial populations, thereby accelerating the transformation of the soil nitrogen pool to available nitrogen that can be absorbed directly by plant roots and utilized by plants. These findings provide a new framework for understanding how melatonin and dopamine regulate the uptake and utilization of nitrogen in plants and improve their ability to cope with environmental disturbances.

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.