| Salt and drought are two main serious problems for sustainable agriculture, which influence plant growth and crop yield in agricultural production in many regions of the world. It is estimated that 20% of all cultivated land and nearly half of the irrigated land is susceptible to the influence of high salt concentrations, greatly reducing plant yields to well below their genetic potential. Northwest China has the largest region for apple production. However, annual precipitation in this region is less and extreme imbalance. Arid become the main factor of apple development in the region. In addition, most of this region is soil saline. Secondary metabolism is an important aspect of plant metabolism. In recent years, a lot of secondary metabolism substance was found to improve the resistance of plants, such as betaine, salicylic acid and melatonin. However, rare research is about the relationship of melatonin and dopamine on Malus stress- resistance.In this study, apple rootstocks and varieties were used to study the regulation function and mechanism of melatonin and dopamine on drought and salt resistance of apple. The results provided a theoretical basis for the biological function and stress resistance improvement of apple. The main results are documented as follows.1. The mitigation effects of exogenous melatonin on salinity-induced stress in Malus hupehensis Rehd. Melatonin as an indoleamine molecule mediates many physiological processes in plants. In this study, we investigated the role of melatonin in regulating growth, oxidative stress and ion homeostasis in M. hupehensis Rehd under salinity conditions. Salinity stress inhibited plant growth and made the net photosynthetic rates and chlorophyll contents decline remarkably, while 0.1 μM melatonin pretreatment significantly alleviated this growth inhibition and maintained certain photosynthetic capacity. Addition of melatonin also relieved the oxidative damages brought by salinity, maybe through direct scavenging H2O2 or enhancing the activities of antioxidative enzymes such as ascorbate peroxidase(APX), catalase(CAT) and peroxidase(POD). We also investigated the possible regulation of melatonin on gene expressions of ion channels under salinity. Its great up-regulation of Md NHX1 and Md AKT1 expressions in leaves may contribute to maintain ion homeostasis and thus strengthen salinity resistance by melatonin.2. Melatonin pretreatment significantly increases the tolerance of both drought-tolerant Malus prunifolia and drought-sensitive M. hupehensis plants. Its beneficial effects include better water conservation in leaves, less electrolyte leakage, steady chlorophyll contents, and greater photosynthetic performance under stress conditions. Melatonin selectively down-regulates Md NCED3, an abscisic acid(ABA) synthesis gene, and up-regulates its catabolic genes, Md CYP707A1 and Md CYP707A2, thereby reducing ABA contents in drought-stressed plants. Melatonin also directly scavenges H2O2 and enhances the activities of antioxidant enzymes to detoxify H2O2 indirectly. These two mechanisms work synergistically to improve the functions of stomata, i.e., causing them to re-open. Plants can effectively regulate their water balance under drought conditions by up-regulating the expression of melatonin synthesis genes Md TDC1, Md AANAT2, Md T5H4, and Md ASMT1. Therefore, inducing melatonin production is an important mechanism by which plants can counteract the influence of this abiotic stressor.3. Dopamine alleviates salt-induced stress in M. hupehensis. We investigated its role in regulating growth, ion homeostasis, and the response to salinity in M. hupehensis Rehd. Both hydroponics and field-pot experiments were conducted under saline conditions. Salt-stressed plants had reduced growth and a marked decline in their net photosynthetic rates, values for FV / FM, and chlorophyll contents. However, pretreatment with 100 μM or 200 μM dopamine significantly alleviated this inhibition and enabled plants to maintain their photosynthetic capacity. In addition to changing stomatal behavior, supplementation with dopamine positively influenced the uptake of K, N, P, S, Cu, and Mn ions, but had an inhibitory effect on Na and Cl uptake, the balance of which is responsible for managing the response to salinity by Malus plants. Dopamine pretreatment also controlled the burst of hydrogen peroxide, possibly through direct scavenging and by enhancing the activities of antioxidative enzymes and the capacity of the ascorbate-glutathione cycle. We also investigated whether dopamine might regulate SOS pathway genes under salinity. Here, Md HKT1, Md NHX1, and Md SOS1 were greatly up-regulated in roots and leaves, which possibly contributed to the maintenance of ion homeostasis and, thus, improved salinity resistance in plants exposed earlier to exogenous dopamine. These results support our conclusion that dopamine alleviates salt-induced stress not only at the level of antioxidant defense but also by regulating other mechanisms of ion homeostasis.4. Dopamine alleviates drought-induced stress in apple. Two-year-old trees of‘Naganofuji No. 2’apple grafted onto M. hupehensis were studied. We compared differences when ‘Nagano Fuji No. 2’ apple was grown under normal or dopamine pre-treated conditions. Under a short term drought period, our data for leaf relative water content and relative electrolyte leakage demonstrated that pre-treated with dopamine improved the tolerance of plants to a water deficit under field conditions. Moreover, net photosynthesis, stomatal conductance, intercellular CO2 concentration, and chlorophyll contents were markedly reduced in the DR group, whereas the declines in those values were only minor in dopamine pre-treated plants. Dopamine also directly scavenges H2O2 and enhances the activities of antioxidant enzymes and upregulated the antioxidant enzymes genes to detoxify H2O2 indirectly. Plants can effectively regulate their water balance under drought conditions by up-regulating the expression of dopamine synthesis genes Md Ty Dc1. Therefore, inducing dopamine production is an important mechanism by which plants can counteract the influence of this abiotic stressor.5. Global transcriptome profiles of apple leaf responses to exogenous dopamine under drought stress were generated using Illumina RNA-sequencing. Two-year-old trees of‘Naganofuji No. 2’apple grafted onto M. hupehensis were studied. We studied the differential gene expression analysis using Illumina RNA-Sequencing technology after a short term drought and dopamine treatment. In total 48845 were functionally annotated in the Malus genome reference, accounting for 85.04% of the Malus reference genome. The new gene number was 3481. 1052 significant differently expressed genes were detected between DR group and DADR group. Through DGE analysis, we found that 643 genes were up-regulated and 409 genes down-regulated in response to exogenous dopamine treatment. The most three signicifant Kyoto Encyclopedia of Genes and Genomes(KEGG) pathway were nitrogen metabolism, plant-pathogen interaction and photosynthesis. Most significance DEGS in biological process of Gene Ontology were in regulation of transcription factors, osmotic stress and oxidative stress, carbohydrate metabolism, hydrogen peroxide metabolic process and programmed cell death. |
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