Overexpression of MdASMT9, an N-acetylserotonin methyltransferase gene, increases melatonin biosynthesis and improves water-use efficiency in transgenic apple

Overexpression of FERONIA receptor kinase MdMRLK2 regulates lignin accumulation and enhances water use efficiency in apple under long-term water deficit condition

Water use efficiency (WUE) is crucial for apple tree fitness and survival, especially in response to climatic changes. The receptor-like kinase FERONIA is reportedly an essential regulator of plant stress responses, but its role in regulating WUE under water deficit conditions is unclear. Here, we found that overexpressing the apple FERONIA receptor kinase gene, MdMRLK2, enhanced apple WUE under long-term water deficit conditions. Under drought treatment, 35S::MdMRLK2 apple plants exhibited higher photosynthetic capacity and antioxidant enzyme activities than wild-type (WT) plants. 35S::MdMRLK2 apple plants also showed increased biomass accumulation, root activity, and water potential compared to WT plants. Moreover, MdMRLK2 physically interacts with and phosphorylates cinnamoyl-CoA reductase 1, MdCCR1, an enzyme essential for lignin synthesis, at position Ser260. This interaction likely contributed to increased vessel density, vascular cylinder area, and lignin content in 35S::MdMRLK2 apple plants under drought conditions. Therefore, our findings reveal a novel function of MdMRLK2 in regulating apple WUE under water deficit conditions.

MdASMT9-mediated melatonin biosynthesis enhances basal thermotolerance in apple plants

High temperatures negatively impact the yield and quality of fruit crops. Exogenous melatonin (MT) application has been shown to enhance heat tolerance, but the response of endogenous MT to heat stress, particularly in perennial fruit trees, remains unclear. The present study investigated the effects of high temperatures on transgenic apple plants overexpressing the MT biosynthesis gene N-acetylserotonin methyltransferase 9 (MdASMT9). Endogenous MT protected transgenic plants from heat stress by increasing antioxidant enzyme activity and scavenging reactive oxygen species (ROS), and protecting the chloroplasts from damage. Application of MT and overexpression of MdASMT9 also reduced abscisic acid accumulation through promoting MdWRKY33-mediated transcriptional inhibition of MdNCED1 and MdNCED3, thus inducing stomatal opening for better heat dissipation. Furthermore, MT-enhanced autophagic activity through promoting MdWRKY33-mediated transcriptional enhancement of MdATG18a under heat stress. These findings provide new insights into the regulation of endogenous MT and its role in improving basal thermotolerance in perennial fruit trees.

Melatonin confers tolerance to nitrogen deficiency through regulating MdHY5 in apple plants

Nitrogen (N) is an essential nutrient for crop growth and development, significantly influencing both yield and quality. Melatonin (MT), a known enhancer of abiotic stress tolerance, has been extensively studied. However, its relationship with nutrient stress, particularly N deficiency, and the underlying regulatory mechanisms of MT on N absorption remain unclear. In this study, exogenous MT treatment was found to improve the tolerance of apple plants to N deficiency. Apple plants overexpressing the MT biosynthetic gene N-acetylserotonin methyltransferase 9 (MdASMT9) were used to further investigate the effects of endogenous MT on low-N stress. Overexpression of MdASMT9 improved the light harvesting and heat transfer capability of apple plants, thereby mitigating the detrimental effects of N deficiency on the photosynthetic system. Proteomic and physiological data analyses indicated that MdASMT9 overexpression enhanced the trichloroacetic acid cycle and positively modulated amino acid metabolism to counteract N-deficiency stress. Additionally, both exogenous and endogenous MT promoted the transcription of MdHY5, which in turn bound to the MdNRT2.1 and MdNRT2.4 promoters and activated their expression. Notably, MT-mediated promotion of MdNRT2.1 and MdNRT2.4 expression through regulating MdHY5, ultimately enhancing N absorption. Taken together, these findings shed light on the association between MdASMT9-mediated MT biosynthesis and N absorption in apple plants under N-deficiency conditions.

Experimental approaches in studying active biomolecules modulating fruit ripening: Melatonin as a case study

Phytohormones are naturally occurring small organic molecules found at low concentrations in plants. They perform essential functions in growth and developmental processes, from organ initiation to senescence, including fruit ripening. These regulatory molecules are studied using different experimental approaches, such as performing exogenous applications, evaluating endogenous levels, and/or obtaining genetically modified lines. Here, we discuss the advantages and limitations of current experimental approaches used to study active biomolecules modulating fruit ripening, focusing on melatonin. Although melatonin has been implicated in fruit ripening in several model fruit crops, current knowledge is affected by the different experimental approaches used, which have given different and sometimes even contradictory results. The methods of application and the doses used have produced different results in studies based on exogenous applications, while different measurement methods and ways of expressing results explain most of the variability in studies using correlative analyses. Furthermore, studies on genetically modified crops have focused on tomato (Solanum lycopersicum L.) plants only. However, TILLING and CRISPR methodologies are becoming essential tools to complement the results from the experimental approaches described above. This will not only help the scientific community better understand the role of melatonin in modulating fruit ripening, but it will also help develop technological advances to improve fruit yield and quality in major crops. The combination of various experimental approaches will undoubtedly lead to a complete understanding of the function of melatonin in fruit ripening in the near future, so that this knowledge can be effectively transferred to the field.

Fruit crops combating drought: Physiological responses and regulatory pathways

Drought is a common stress in agricultural production. Thus, it is imperative to understand how fruit crops respond to drought and to develop drought-tolerant varieties. This paper provides an overview of the effects of drought on the vegetative and reproductive growth of fruits. We summarize the empirical studies that have assessed the physiological and molecular mechanisms of the drought response in fruit crops. This review focuses on the roles of calcium (Ca2+) signaling, abscisic acid (ABA), reactive oxygen species signaling, and protein phosphorylation underlying the early drought response in plants. We review the resulting downstream ABA-dependent and ABA-independent transcriptional regulation in fruit crops under drought stress. Moreover, we highlight the positive and negative regulatory mechanisms of microRNAs in the drought response of fruit crops. Lastly, strategies (including breeding and agricultural practices) to improve the drought resistance of fruit crops are outlined.

Exogenous Melatonin Enhances the Yield and Secondary Metabolite Contents of Prunella vulgaris by Modulating Antioxidant System, Root Architecture and Photosynthetic Capacity

Melatonin (MT) plays a number of key roles in regulating plant growth and secondary metabolite accumulation. Prunella vulgaris is an important traditional Chinese herbal medicinal plant which is used for the treatment of lymph, goiter, and mastitis. However, the effect of MT on the yield and medicinal component content of P. vulgaris remains still unclear. In this research, we have examined the influence of different concentrations of MT (0, 50, 100, 200, 400 μM) on the physiological characteristics, secondary metabolite contents, and yield of P. vulgaris biomass. The results showed that 50-200 μM MT treatment had a positive effect on P. vulgaris. MT treatment at 100 μM greatly increased the activities of superoxide dismutase and peroxidase, the contents of soluble sugar and proline, and obviously decreased the relative electrical conductivity, the contents of malondialdehyde and hydrogen peroxide of leaves. Furthermore, it markedly promoted the growth and development of the root system, increased the content of photosynthetic pigments, improved the performance of photosystems I and II and the coordination of both photosystems, and enhanced the photosynthetic capacity of P. vulgaris. In addition, it significantly increased the dry mass of whole plant and spica and promoted the accumulation of total flavonoids, total phenolics, caffeic acid, ferulic acid, rosmarinic acid, and hyperoside in the spica of P. vulgaris. These findings demonstrated that the application of MT could effectively activate the antioxidant defense system of P. vulgaris, protect the photosynthetic apparatus from photooxidation damage, and improve the photosynthetic capacity and the root absorption capacity, thereby promoting the yield and accumulation of secondary metabolites in P. vulgaris.

Genetic and evolutionary dissection of melatonin response signaling facilitates the regulation of plant growth and stress responses

The expansion of gene families during evolution could generate functional diversity among their members to regulate plant growth and development. Melatonin, a phylogenetically ancient molecule, is vital for many aspects of a plant's life. Understanding the functional diversity of the molecular players involved in melatonin biosynthesis, signaling, and metabolism will facilitate the regulation of plant phenotypes. However, the molecular mechanism of melatonin response signaling elements in regulating this network still has many challenges. Here, we provide an in-depth analysis of the functional diversity and evolution of molecular components in melatonin signaling pathway. Genetic analysis of multiple mutants in plant species will shed light on the role of gene families in melatonin regulatory pathways. Phylogenetic analysis of these genes was performed, which will facilitate the identification of melatonin-related genes for future study. Based on the abovementioned signal networks, the mechanism of these genes was summarized to provide reference for studying the regulatory mechanism of melatonin in plant phenotypes. We hope that this work will facilitate melatonin research in higher plants and finely tuned spatio-temporal regulation of melatonin signaling.

Ectopic expression of HIOMT improves tolerance and nitrogen utilization efficiency in transgenic apple under drought stress

Melatonin enhances plant tolerance to various environmental stressors. Although exogenous application of melatonin has been investigated, the role of endogenous melatonin metabolism in the response of apples to drought stress and nutrient utilization remains unclear. Here, we investigated the effects of ectopically expressing the human melatonin synthase gene HIOMT on transgenic apple plants under drought stress conditions. The tolerance of transgenic apple lines that ectopically expressed HIOMT improved significantly under drought conditions. After 10 days of natural drought stress treatment, the transgenic apple plants showed higher relative water content, chlorophyll levels and Fv/Fm, and lower relative electrolyte leakage and hydrogen peroxide accumulation, than wild-type plants. The activities of peroxidase, superoxide dismutase and catalase, as well as genes in the ascorbate-glutathione cycle, increased more in transgenic apple plants than in the wild-type. The ectopic expression of HIOMT also markedly alleviated the inhibitory effects of long-term drought stress on plant growth, photosynthetic rate and chlorophyll concentrations in apple plants. The uptake and utilization of 15N increased markedly in the transgenic lines under long-term moderate drought stress. Drought stress sharply reduced the activity of enzymes involved in nitrogen metabolism, but ectopic expression of HIOMT largely reversed that response. The expression levels of genes of nitrogen metabolism and uptake were more upregulated in transgenic apple plants than the wild-type. Overall, our study demonstrates that ectopic expression of HIOMT enhanced the tolerance of apple plants to drought stress, and transgenic apple plants showed improved growth due to higher nutrient utilization efficiency under drought conditions.

Exogenous treatment with melatonin enhances waterlogging tolerance of kiwifruit plants

Waterlogging stress has an enormous negative impact on the kiwifruit yield and quality. The protective role of exogenous melatonin on water stress has been widely studied, especially in drought stress. However, the research on melatonin-induced waterlogging tolerance is scarce. Here, we found that treatment with exogenous melatonin could effectively alleviate the damage on kiwifruit plants in response to waterlogging treatment. This was accompanied by higher antioxidant activity and lower ROS accumulation in kiwifruit roots during stress period. The detection of changes in amino acid levels of kiwifruit roots during waterlogging stress showed a possible interaction between melatonin and amino acid metabolism, which promoted the tolerance of kiwifruit plants to waterlogging. The higher levels of GABA and Pro in the roots of melatonin-treated kiwifruit plants partly contributed to their improved waterlogging tolerance. In addition, some plant hormones were also involved in the melatonin-mediated waterlogging tolerance, such as the enhancement of ACC accumulation. This study discussed the melatonin-mediated water stress tolerance of plants from the perspective of amino acid metabolism for the first time.

Introducing melatonin to the horticultural industry: physiological roles, potential applications, and challenges

Melatonin (N-acetyl-5-methoxytryptamine) is an emerging biomolecule that influences horticultural crop growth, flowering, fruit ripening, postharvest preservation, and stress protection. It functions as a plant growth regulator, preservative and antimicrobial agent to promote seed germination, regulate root system architecture, influence flowering and pollen germination, promote fruit production, ensure postharvest preservation, and increase resistance to abiotic and biotic stresses. Here, we highlight the potential applications of melatonin in multiple aspects of horticulture, including molecular breeding, vegetative reproduction, production of virus-free plants, food safety, and horticultural crop processing. We also discuss its effects on parthenocarpy, autophagy, and arbuscular mycorrhizal symbiosis. Together, these many features contribute to the promise of melatonin for improving horticultural crop production and food safety. Effective translation of melatonin to the horticultural industry requires an understanding of the challenges associated with its uses, including the development of economically viable sources.

Exogenous Melatonin Protects Lime Plants from Drought Stress-Induced Damage by Maintaining Cell Membrane Structure, Detoxifying ROS and Regulating Antioxidant Systems

Lime is an important commercial product in tropical and subtropical regions, where drought stress is becoming one of the most severe environmental challenges in the agricultural sector. Melatonin is an antioxidant molecule that helps plants regulate their development and respond to a variety of stresses. In this research, the effects of exogenous melatonin treatments were evaluated at different concentrations (0, 50, 100, and 150 μM) on biochemical aspects and gene expression in two species of lime plants (“Mexican lime” and “Persian lime”) under normal (100% field capacity (FC)) and drought stress conditions (75% and 40% FC). The experiments were factorial and based on a completely randomized design (CRD) with four replicates. Drought stress caused electrolyte leakage (EL) as well as accumulations of hydrogen peroxide (H2O2) and malondialdehyde (MDA), indicating the occurrence of damage to cellular membranes. In contrast, the melatonin pretreatment at various concentrations reduced the levels of EL, H2O2 and MDA while mitigating the negative effects of drought stress on the two lime species. The application of melatonin (100-μM) significantly increased the level of proline content and activity of antioxidant enzymes in plants under drought stress compared to control plants. According to real-time PCR analysis, drought stress and melatonin treatment enhanced the expression of genes involved in ROS scavenging, proline biosynthesis, and cell redox regulation in both species, as compared to their respective controls. According to these findings, melatonin is able to detoxify ROS and regulate antioxidant systems, thereby protecting lime plants from drought stress-induced damages.