Evidence that melatonin promotes soybean seedlings growth from low-temperature stress by mediating plant mineral elements and genes involved in the antioxidant pathway

Melatonin alleviated cadmium accumulation and toxicity by modulating phytohormonal balance and antioxidant metabolism in rice

Cadmium (Cd) contamination is an eminent dilemma that jeopardizes global food safety and security, especially through its phytotoxicity in rice; one of the most edible crops. Melatonin (MET) has emerged as a protective phytohormone in stress conditions, but the defensive role and underlying mechanisms of MET against Cd toxicity in rice still remain unclear. To fulfill this knowledge gap, the present study is to uncover the key mechanisms for MET-mediated Cd-stress tolerance in rice. Cd toxicity significantly reduced growth by hindering the process of photosynthesis, cellular redox homeostasis, phytohormonal imbalance, and ultrastructural damages. Contrarily, MET supplementation considerably improved growth attributes, photosynthetic efficiency, and cellular ultrastructure as measured by gas exchange elements, chlorophyll content, reduced Cd accumulation, and ultrastructural analysis via transmission electron microscopy (TEM). MET treatment significantly reduced Cd accumulation (39.25%/31.58%), MDA (25.87%/19.45%), H2O2 (17.93%/9.56%), and O2 (29.11%/27.14%) levels in shoot/root tissues, respectively, when compared with Cd treatment. More importantly, MET manifested association with stress responsive phytohormones (ABA and IAA) and boosted the defense mechanisms of plant by enhancing the activities of ROS-scavenging antioxidant enzymes (SOD; superoxide dismutase, POD; peroxidase, CAT; catalase, APX; ascorbate peroxidase) and as well as regulating the key stress-responsive genes (OsSOD1, OsPOD1, OsCAT2, OsAPX1), thereby reinstate cellular membrane integrity and confer tolerance to ultrastructural damages under Cd-induced phytotoxicity. Overall, our findings emphasized the potential of MET as a long-term and cost-effective approach to Cd remediation in paddy soils, which can pave the way for a healthier and more environmentally conscious agricultural sector.

Melatonin-mediated development and abiotic stress tolerance in plants

Melatonin is a multifunctional molecule that has been widely discovered in most plants. An increasing number of studies have shown that melatonin plays essential roles in plant growth and stress tolerance. It has been extensively applied to alleviate the harmful effects of abiotic stresses. In view of its role in regulating aspects of plant growth and development, we ponder and summarize the scientific discoveries about seed germination, root development, flowering, fruit maturation, and senescence. Under abiotic and biotic stresses, melatonin brings together many pathways to increase access to treatments for the symptoms of plants and to counteract the negative effects. It has the capacity to tackle regulation of the redox, plant hormone networks, and endogenous melatonin. Furthermore, the expression levels of several genes and the contents of diverse secondary metabolites, such as polyphenols, terpenoids, and alkaloids, were significantly altered. In this review, we intend to examine the actions of melatonin in plants from a broader perspective, explore the range of its physiological functions, and analyze the relationship between melatonin and other metabolites and metabolic pathways.

Melatonin-mediated temperature stress tolerance in plants

Global climate changes cause extreme temperatures and a significant reduction in crop production, leading to food insecurity worldwide. Temperature extremes (including both heat and cold stresses) is one of the most limiting factors in plant growth and development and severely affect plant physiology, biochemical, and molecular processes. Biostimulants like melatonin (MET) have a multifunctional role that acts as a "defense molecule" to safeguard plants against the noxious effects of temperature stress. MET treatment improves plant growth and temperature tolerance by improving several defense mechanisms. Current research also suggests that MET interacts with other molecules, like phytohormones and gaseous molecules, which greatly supports plant adaptation to temperature stress. Genetic engineering via overexpression or CRISPR/Cas system of MET biosynthetic genes uplifts the MET levels in transgenic plants and enhances temperature stress tolerance. This review highlights the critical role of MET in plant production and tolerance against temperature stress. We have documented how MET interacts with other molecules to alleviate temperature stress. MET-mediated molecular breeding would be great potential in helping the adverse effects of temperature stress by creating transgenic plants.

Exogenous 5-aminolevulinic acid alleviates low-temperature damage by modulating the xanthophyll cycle and nutrient uptake in tomato seedlings

5-Aminolevulinic acid (ALA), an antioxidant existing in plants, has been widely reported to participate in the process of coping with cold stress of plants. In this study, exogenous ALA promoted the growth of tomato plants and alleviated the appearance of purple tomato leaves under low-temperature stress. At the same time, exogenous ALA improved antioxidant enzyme activities, SlSOD gene expression, Fv/Fm, and proline contents and reduced H₂O₂ contents, SlRBOH gene expression, relative electrical conductivity, and malondialdehyde contents to alleviate the damage caused by low temperature to tomato seedlings. Compared with low-temperature stress, spraying exogenous ALA before low-temperature stress could restore the indicators of photochemical quenching, actual photochemical efficiency, electron transport rate, and nonphotochemical quenching to normal. Exogenous ALA could increase the total contents of the xanthophyll cycle pool, the positive de-epoxidation rate of the xanthophyll cycle and improved the expression levels of key genes in the xanthophyll cycle under low-temperature stress. In addition, we found that exogenous ALA significantly enhanced the absorption of mineral nutrients, promoted the transfer and distribution of mineral nutrients to the leaves, and improved the expression levels of mineral nutrient absorption-related genes, which were all conducive to the improved adaptation of tomato seedlings under low-temperature stress. In summary, the application of exogenous ALA can increase tomato seedlings' tolerance to low-temperature stress by improving the xanthophyll cycle and the ability of the absorption of mineral nutrients in tomato seedlings.

Phytomelatonin and plant mineral nutrition

Plant mineral nutrition is critical for agricultural productivity and for human nutrition; however, the availability of mineral elements is spatially and temporally heterogeneous in many ecosystems and agricultural landscapes. Nutrient imbalances trigger intricate signalling networks that modulate plant acclimation responses. One signalling agent of particular importance in such networks is phytomelatonin, a pleiotropic molecule with multiple functions. Evidence indicates that deficiencies or excesses of nutrients generally increase phytomelatonin levels in certain tissues, and it is increasingly thought to participate in the regulation of plant mineral nutrition. Alterations in endogenous phytomelatonin levels can protect plants from oxidative stress, influence root architecture, and influence nutrient uptake and efficiency of use through transcriptional and post-transcriptional regulation; such changes optimize mineral nutrient acquisition and ion homeostasis inside plant cells and thereby help to promote growth. This review summarizes current knowledge on the regulation of plant mineral nutrition by melatonin and highlights how endogenous phytomelatonin alters plant responses to specific mineral elements. In addition, we comprehensively discuss how melatonin influences uptake and transport under conditions of nutrient shortage.

Comparative Physiological and Transcriptomic Analyses of Improved Heat Stress Tolerance in Celery (Apium Graveolens L.) Caused by Exogenous Melatonin

Melatonin (MT) is crucial in plant growth, development, and response to stress. Celery is a vegetable that grows in a cool climate, and a hot climate can deteriorate its growth, yield, and quality. This study investigates the effect of exogenous melatonin on celery physiology. Transcriptional levels were analyzed by spraying celery with exogenous MT before exposing it to high temperatures. The regulatory mechanism of exogenous MT-mediated heat tolerance was examined. The results show that the exogenous MT reduced the thermal damage state of celery seedlings, as well as the malondialdehyde (MDA) content and relative conductivity (REC), increasing the oxidase activity, the osmotic regulatory substances, and chlorophyll, enhancing the leaf transpiration and the light energy utilization efficiency. We examined the mechanism of exogenous MT in mitigating high-temperature damage using the transcriptome sequencing method. A total of 134 genes were expressed differently at high temperature in the celery treated with MT compared with the untreated celery. Functional annotation analysis revealed that the differentially expressed genes were abundant in the “pyruvate metabolism” pathway and the “peroxidase activity” pathway. According to the pathway-based gene expression analysis, exogenous MT can inhibit the upregulation of pyruvate synthesis genes and the downregulation of pyruvate consumption genes, preventing the accumulated pyruvate from rapidly upregulating the expression of peroxidase genes, and thereby enhancing peroxidase activity. RT-qPCR verification showed a rising encoding peroxidase gene expression under MT treatment. The gene expression pattern involved in pyruvate anabolism and metabolism agreed with the abundant transcriptome expression, validating the physiological index results. These results indicate that the application of exogenous MT to celery significantly enhances the ability of plant to remove reactive oxygen species (ROS) in response to heat stress, thereby improving the ability of plant to resist heat stress. The results of this study provide a theoretical basis for the use of MT to alleviate the damage caused by heat stress in plant growth and development.

Melatonin Induced Cold Tolerance in Plants: Physiological and Molecular Responses

Cold stress is one of the most limiting factors for plant growth and development. Cold stress adversely affects plant physiology, molecular and biochemical processes by determining oxidative stress, poor nutrient and water uptake, disorganization of cellular membranes and reduced photosynthetic efficiency. Therefore, to recover impaired plant functions under cold stress, the application of bio-stimulants can be considered a suitable approach. Melatonin (MT) is a critical bio-stimulant that has often shown to enhance plant performance under cold stress. Melatonin application improved plant growth and tolerance to cold stress by maintaining membrane integrity, plant water content, stomatal opening, photosynthetic efficiency, nutrient and water uptake, redox homeostasis, accumulation of osmolytes, hormones and secondary metabolites, and the scavenging of reactive oxygen species (ROS) through improved antioxidant activities and increase in expression of stress-responsive genes. Thus, it is essential to understand the mechanisms of MT induced cold tolerance and identify the diverse research gaps necessitating to be addressed in future research programs. This review discusses MT involvement in the control of various physiological and molecular responses for inducing cold tolerance. We also shed light on engineering MT biosynthesis for improving the cold tolerance in plants. Moreover, we highlighted areas where future research is needed to make MT a vital antioxidant conferring cold tolerance to plants.

The molecular events underpinning cultivar differences in melatonin counteracting salt damage in Phaseolus vulgaris

Melatonin (N -acetyl-5-methoxytryptamine) plays important roles in multiple stress responses, especially under salt stress. However, cultivar differences in melatonin mediated salt stress tolerance are unclear. Phaseolus vulgaris L. (common bean) cultivars Jiyin 1 (JY, salt-tolerant) and Xuliyabai (XL, salt-sensitive) were used in this study. Exogenous melatonin significantly improved root growth under salt stress in JY, but had little effect on XL. Physiology analysis showed significant differences in activities of antioxidant enzymes (superoxide, SOD; and catalase, CAT) and malondialdehyde content between JY and XL. Meanwhile, the change of ABA content in JY and XL root was opposite in salt plus melatonin treatment. Comparative root transcriptomes of JY and XL revealed 3505 and 668 differentially expressed genes (DEGs) regulated by salt stress and melatonin. The most enriched melatonin-responsive genes under salt stress are mainly involved in regulation of transcription, oxidation-reduction process, transcription factor activity, oxidoreductase activity. In addition, melatonin induced more obvious changes of DEGs in JY than that in XL under salt condition. Melatonin also significantly induced 41 DEGs only in JY, including signal transduction genes, transcription factors, ubiquitin protein ligases, ion homeostasis and osmotic adjustment genes etc. This study uncovered the molecular mechanism of cultivar difference of melatonin response under salt stress in common bean.

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.

Melatonin in Plant Defense against Abiotic Stress

Abiotic stress adversely affects plant growth and metabolism and as such reduces plant productivity. Recognized as a major contributor in the production of reactive oxygen species (ROS), it hinders the growth of plants through induction of oxidative stress. Biostimulants such as melatonin have a multifunctional role, acting as a defense strategy in minimizing the effects of oxidative stress. Melatonin plays important role in plant processes ranging from seed germination to senescence, besides performing the function of a biostimulant in improving the plant’s productivity. In addition to its important role in the signaling cascade, melatonin acts as an antioxidant that helps in scavenging ROS, generated as part of different stresses among plants. The current study was undertaken to elaborate the synthesis and regulation of melatonin in plants, besides emphasizing its function under various abiotic stress namely, salt, temperature, herbicides, heavy metals, and drought. Additionally, a special consideration was put on the crosstalk of melatonin with phytohormones to overcome plant abiotic stress.

Gibberellins and auxin regulate soybean hypocotyl elongation under low light and high-temperature interaction

Soybean is an important oilseed crop grown globally. However, two examples of environmental stresses that drastically regulate soybean growth are low light and high-temperature. Emerging evidence suggests a possible interconnection between these two environmental stimuli. Low light and high-temperature as individual factors have been reported to regulate plant hypocotyl elongation. However, their interactive signal effect on soybean growth and development remains largely unclear. Here, we report that gibberellins (GAs) and auxin are required for soybean hypocotyl elongation under low light and high-temperature interaction. Our analysis indicated that low light and high-temperature interaction enhanced the regulation of soybean hypocotyl elongation and that the endogenous GA3 , GA7 , indole-3-acetic acid (IAA), and indole-3-pyruvate (IPA) contents significantly increased. Again, analysis of the effect of exogenous phytohormones and biosynthesis inhibitors treatments showed that exogenous GA, IAA, and paclobutrazol (PAC), 2, 3, 5,-triiodobenzoic acid (TIBA) treatments significantly regulated soybean seedlings growth under low light and high-temperature interaction. Further qRT-PCR analysis showed that the expression level of GA biosynthesis pathway genes (GmGA3ox1, GmGA3ox2 and GmGA3) and auxin biosynthesis pathway genes (GmYUCCA3, GmYUCCA5 and GmYUCCA7) significantly increased under (i) low light and high-temperature interaction and (ii) exogenous GA and IAA treatments. Altogether, these observations support the hypothesis that gibberellins and auxin regulate soybean hypocotyl elongation under low light and high-temperature stress interaction.