Response of exogenous melatonin on transcription and metabolism of soybean under drought stress.

Amino acid metabolism is an important factor in regulating nitrogen source assimilation and source/sink transport in soybean. Melatonin can improve plant stress resistance, but whether it affects amino acid metabolism is not known. Therefore, this study investigated whether exogenous melatonin had an effect on amino acid metabolism of soybean under drought conditions and explored its relationship with yield. The treatments were normal water supply treatment (WW), drought stress treatment (D), drought stress and melatonin treatment group (D + M), sprayed with 100 µmol/L melatonin. The effects of melatonin on amino acid metabolism and grain filling were studied by physiological and omics experiments using Kangxian 9 (drought-sensitive variety) and Suinong 26 (drought-resistant variety) soybean cultivars. The results showed that drought stress decreased the activity of carbon and nitrogen metabolizing enzymes, which inhibited the accumulation of dry matter and protein, and decreased the yield. In the drought-sensitive soybean variety, glycoenzymes and amino acid synthetases synthetic genes were upregulated in melatonin-treated soybeans, hence carbon and nitrogen metabolism enzyme activity increased, increasing the carbohydrate and amino acid contents simultaneously. This resulted in higher dry matter and yield than drought-stressed soybean not treated with melatonin. In the drought-resistant variety, the grain weight per plant increased by 7.98% and 6.57% in 2020 and 2021, respectively, while it increased by 23.20% and 14.07% in the drought-sensitive variety during the respective years. In conclusion, melatonin treatment can enhance the activity of nitrogen and carbon metabolism and amino acid content by upregulating the expression of soybean metabolic pathway and related genes, thus increasing the yield of soybean under drought stress.

Effect of nitrogen application levels on photosynthetic nitrogen distribution and use efficiency in soybean seedling leaves.

BACKGROUND: Nitrogen nutrition is strongly associated with crop growth and development. Nitrogen application level affects leaf size as well as nitrogen content and distribution, and thus affects photosynthetic nitrogen-use efficiency (PNUE) and yield. In this study, soybean varieties "Jinyuan 55" and "Keshan 1" were treated with nitrogen as urea at: N0, 0 kg hm-2; N0.5, 60 kg hm-2; N1, 120 kg hm-2; and N1.5, 180 kg hm-2. We compared the effect of nitrogen level on plant morphology, biomass, photosynthetic physiology, nitrogen distribution, PNUE, and other soybean seedling leaf characteristics. RESULTS: Maximum carboxylation and electron transfer, net photosynthetic rates, and PNUE of both soybean varieties showed initial significant increases with increasing nitrogen application rate and subsequent stabilization. PNUE, carboxylation system components, electron transport components, and non-photosynthetic system distribution ratios in the photosynthetic system increased and subsequently decreased with increased nitrogen application rate. The nitrogen ratio between carboxylation and electron transport systems was positively correlated with PNUE in both soybean varieties. The nitrogen ratio in light-harvesting and non-photosynthetic systems showed a linear negative correlation with PNUE. CONCLUSIONS: Overall, an appropriate nitrogen level maintained a high photosynthetic nitrogen ratio, whereas low- or high-nitrogen conditions increased or decreased the nitrogen ratio in non-photosynthetic and photosynthetic systems, respectively, thus decreasing the PNUE and photosynthetic capacity. Moreover, increased nitrogen application rate led to a decreased nitrogen ratio in the light-harvesting system and an increased nitrogen ratio of electron transport and carboxylation systems. Our results provide a theoretical basis for optimizing leaf nitrogen distribution, determining optimum nitrogen levels, and promoting soybean seedling growth.

Postharvest treatments with MnCl2 and ZnCl2 reduce enzymatic browning and enhance antioxidant accumulation in soya bean sprout.

Soya bean sprout is a nutrient-abundant vegetable. However, enzymatic browning of soya bean sprouts during storage remains a challenge. In this study, the effects of treatment with MnCl2 or ZnCl2 on the browning index, antioxidant nutrient accumulation, total antioxidant capacity and enzyme activities of phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT) were investigated in soya bean sprouts after storage at 4 °C and 90% relative humidity for 0, 7, 14 and 21 days. The results showed that postharvest treatment with 1, 2 and 10 mM MnCl2 or ZnCl2 profoundly retarded enzymatic browning in soya bean sprouts to different extents. Compared with the control, the 10 mM MnCl2 and ZnCl2 treatments drastically enhanced ascorbic acid, total thiol and phenolic content, and enhanced FRAP (ferric-reducing ability of plasma) antioxidant capacity in stored soya bean sprouts. Moreover, the MnCl2 and ZnCl2 treatments enhanced SOD, CAT and PAL but decreased PPO and POD activities compared with the control. In addition, the Mn and Zn content in soya bean sprouts significantly increased, by approximately two- to threefold, compared with the control. This study provides a new method for improving the nutrient quality of soya bean sprouts based on postharvest Mn or Zn supplementation.

Exogenous Melatonin Modulates Physiological Response to Nitrogen and Improves Yield in Nitrogen-Deficient Soybean (Glycine max L. Merr.).

Melatonin (MT) is a key plant growth regulator. To investigate its effect at different growth stages on the yield of soybean under nitrogen deficiency, 100 µM MT was applied to soybean supplemented with zero nitrogen (0N), low nitrogen (LN), and control nitrogen (CK) levels, during the plant vegetative growth (V3) and filling (R5) stages. This study revealed that the application of MT mainly enhanced the nitrogen fixation of plants by increasing the root nodule number and provided more substrates for glutamine synthetase (GS) under 0N supply. However, under the LN supply, more ammonium was assimilated through the direct promotion of nitrate reductase (NR) activity by MT. MT enhanced the activity of ammonium-assimilation-related enzymes, such as GOGAT and GDH, and the expression of their coding genes, promoted the synthesis of chlorophyll and amino acids, and increased the photosynthetic capacity under nitrogen deficiency. Exogenous MT directly upregulated the expression of genes involved in the photosynthetic system and stimulated dry-matter accumulation. Thus, MT alleviated the inhibitory effect of nitrogen deficiency on soybean yield. This mitigation effect was better when MT was applied at the V3 stage, and the seed weight per plant increased by 16.69 and 12.20% at 0N and LN levels, respectively. The results of this study provide a new theoretical basis to apply MT in agriculture to improve the resilience of soybean plants to low nitrogen availability.

The mechanisms underlying melatonin improved soybean seedling growth at different nitrogen levels.

To investigate the function of melatonin (MT) on nitrogen uptake and metabolism in soybean, six groups of treatments, with and without 100µM melatonin were conducted at low, normal, and high nitrogen levels (1.5, 7.5, and 15mM, respectively). The related indexes of nitrogen metabolism and the antioxidant system of seedlings were measured and analysed. Results indicated that MT could enhance the level of nitrogen metabolism by upregulating the coding genes of enzymes related to nitrogen metabolism and increasing total nitrogen content, especially under low nitrogen levels. Under high nitrogen conditions, the addition of MT not only accelerated ammonium assimilation and utilisation by enhancing the activity of glutamine synthetase involved in ammonium assimilation, but also reduced the extent of membrane lipid peroxidation to alleviate the degree of damage by improving the activity of antioxidant enzymes. In addition, MT enhanced soybean growth with positive effects in morphological changes at different nitrogen levels, including significantly increased stem diameter, total leaf area, and root nodule number, and biomass accumulation. Finally, biomass accumulation increased under low, normal, and high nitrogen levels by 9.80%, 14.06%, and 11.44%, respectively. The results suggested that MT could enhance the soybean tolerance to low and excessive N treatments.

Exogenous melatonin reduces the inhibitory effect of osmotic stress on antioxidant properties and cell ultrastructure at germination stage of soybean.

Understanding the relationship between exogenous melatonin and water deficit stress is crucial for alleviating the effects of water deficit stress at germination stage of soybean (Glycine max (L.) Merrill) in agriculture. This study investigated the effects of exogenous melatonin on soybean antioxidant properties and cell ultrastructure under water deficit stress induced by polyethylene glycol (PEG) 6000. The drought-sensitive soybean variety Suinong 26 was used as the material to study the effects of different concentrations of melatonin (0, 300, 500 µmol·L-1) soaking soybean seeds under drought stress (PEG-6000: 3% and 6%). The results showed that the germination rate (GR), germination potential (GP), germination index (GI) and radicle shape of soybean were affected negatively to different degrees under PEG stress. Moreover, stress induced by different PEG concentrations overproduced the content of reactive oxygen species (H2O2, O2·-) in cells, leading to increased lipid membrane peroxidation as electrolyte leakage (EL) and malondialdehyde (MDA) content, which resulted in impaired cell integrity. However, after seeds soaking with melatonin, the lipid peroxidation of the cell membrane was reduced, and the activities of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) further increased to minimize the excessive generation of ROS. Similar results were obtained for soluble protein and proline, that may help in regulating the osmotic pressure and maintain cellular integrity. With the interaction of these enzymes, compared with 300 µmol·L-1 melatonin, 500 µmol·L-1 melatonin could more effective to remove the ROS and reduce cell peroxidation. Overall, 500 µmol·L-1 melatonin performed better than 300 µmol·L-1. In conclusion, the seed soaking with melatonin promoted the germination of soybean seeds under water stress.

Transcriptomic and metabolomic profiling of melatonin treated soybean (Glycine max L.) under drought stress during grain filling period through regulation of secondary metabolite biosynthesis pathways.

There is a growing need to enhance the productivity of soybean (Glycine max L.) under severe drought conditions in order to improve global food security status. Melatonin, a ubiquitous hormone, could alleviate drought stress in various plants. Earlier, we demonstrated that exogenous melatonin treatment could enhance the tolerance of drought-treated soybean. However, the underlying mechanisms by which this hormone exerts drought resistance is still unclear. The present study used transcriptomic and metabolomic techniques to determine some critical genes and pathways regulating melatonin response to drought conditions. Results showed that exogenous melatonin treatment could increase relative water content and decrease electrolyte leakage in the leaves and increase seed yield under drought stress. Transcriptomic analysis showed that there were 852 core differentially expressed genes (DEGs) that were regulated by drought stress and melatonin in soybean leaves. The most enriched drought-responsive genes are mainly involved in the 'biosynthesis of secondary metabolites'. Metabolomic profiling under drought stress showed higher accumulation levels of secondary metabolites related to drought tolerance after exogenous melatonin treatment. Also, we highlighted the vital role of the pathways including phenylpropanoid, flavonoid, isoflavonoid, and steroid biosynthesis pathways for improvement of drought tolerance in soybean by exogenous melatonin treatment. In all, findings from this study give detailed molecular basis for the application of melatonin as a drought-resistant agent in soybean cultivation.

Exogenous melatonin reduces the inhibitory effect of osmotic stress on photosynthesis in soybean.

Understanding the relationship between exogenous melatonin and water deficit stress is crucial for achieving high yields and alleviating the effects of water deficit stress on soybean (Glycine max (L.) Merrill) plants in agriculture. This study investigated the effects of exogenous melatonin on soybean photosynthetic capacity under water deficit stress induced by polyethylene glycol (PEG) 6000. We conducted a potting experiment in 2018 using the soybean (Glycine max L. Merrill) cultivar Suinong 26. We identified the impacts of a concentration of PEG 6000 simulating drought (15%, w/v) and an appropriate melatonin concentration (100 µmol/L) on the growth of soybean seedlings and flowering stages in a preliminary test. We applied exogenous melatonin by foliar spraying and root application to determine the effects on leaf photosynthesis during water deficit stress. Our results indicated that 15% PEG 6000 had an obvious inhibitory effect on the growth of soybean seedlings and flowering stages, causing oxidative stress and damage due to reactive oxygen species (ROS) (H2O2 and O2·-) accumulation and potentially reducing air exchange parameters and photosystem II (PSII) efficiency. The application of exogenous melatonin significantly relieved the inhibitory effects of PEG 6000 stress on seedlings and flowering growth, and gas exchange parameters, potentially improved PSII efficiency, improved the leaf area index (LAI) and the accumulation of dry matter, slowed down oxidative stress and damage to leaves by increasing the activity of antioxidant enzymes (SOD, POD, and CAT), reduced the content of malondialdehyde (MDA), and ultimately improved soybean yield. Overall, the results of this study demonstrated that application of exogenous melatonin at the seedlings and flowering stages of soybean is effective in alleviating plant damage caused by water deficit stress and improving the drought resistance of soybean plants. In addition, the results showed that application of exogenous melatonin by root is superior to foliar spraying.