RNA-seq Analysis of Overexpressing Ovine AANAT Gene of Melatonin Biosynthesis in Switchgrass

Functions and prospects of melatonin in plant growth, yield, and quality

Melatonin (N-acetyl-5-methoxytryptamine) is an indole molecule widely found in animals and plants. It is well known that melatonin improves plant resistance to various biotic and abiotic stresses due to its potent free radical scavenging ability while being able to modulate plant signaling and response pathways through mostly unknown mechanisms. In recent years, an increasing number of studies have shown that melatonin plays a crucial role in improving crop quality and yield by participating in the regulation of various aspects of plant growth and development. Here, we review the effects of melatonin on plant vegetative growth and reproductive development, and systematically summarize its molecular regulatory network. Moreover, the effective concentrations of exogenously applied melatonin in different crops or at different growth stages of the same crop are analysed. In addition, we compare endogenous phytomelatonin concentrations in various crops and different organs, and evaluate a potential function of phytomelatonin in plant circadian rhythms. The prospects of different approaches in regulating crop yield and quality through exogenous application of appropriate concentrations of melatonin, endogenous modification of phytomelatonin metabolism-related genes, and the use of nanomaterials and other technologies to improve melatonin utilization efficiency are also discussed.

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.

Integrated morphological, physiological and omics analyses reveal the arylalkylamine N-acetyltransferase (AANAT) gene contributing to growth, flowering and defence in switchgrass (Panicum virgatum L.)

Arylalkylamine N-acetyltransferase (AANAT) catalyses the acetylation of serotonin, a rate-limiting process in melatonin biosynthesis. To obtain better insight into the underlying mechanism of AANAT's actions in switchgrass growth, flowering and defence, we performed integrated morphological, physiological and omics analyses between overexpressed oAANAT transgenic lines in wild-type and transgenic control (expressing only the empty vector) plants. We showed that oAANAT played pivotal roles in modulating plant growth through its regulation of cell elongation, and regulating flowering through photoperiod and GA pathways. In relation to photosynthesis, oAANAT promoted photosynthetic efficiency primarily through regulating leaf anatomical structures, stomatal development and chlorophyll metabolism. Moreover, oAANAT overexpression can trigger a number of defence responses or strategies, including antioxidant enzymatic properties, non-enzymatic capacity, significantly activated phenylpropanoid biosynthesis, and adaptive morphological characteristics. This study unveils the possible molecular mechanisms underlying oAANAT dependent melatonin functions in switchgrass, providing an important starting point for further analyses.

Melatonin biosynthesis pathways in nature and its production in engineered microorganisms

Melatonin is a biogenic amine that can be found in plants, animals and microorganism. The metabolic pathway of melatonin is different in various organisms, and biosynthetic endogenous melatonin acts as a molecular signal and antioxidant protection against external stress. Microbial synthesis pathways of melatonin are similar to those of animals but different from those of plants. At present, the method of using microorganism fermentation to produce melatonin is gradually prevailing, and exploring the biosynthetic pathway of melatonin to modify microorganism is becoming the mainstream, which has more advantages than traditional chemical synthesis. Here, we review recent advances in the synthesis, optimization of melatonin pathway. l-tryptophan is one of the two crucial precursors for the synthesis of melatonin, which can be produced through a four-step reaction. Enzymes involved in melatonin synthesis have low specificity and catalytic efficiency. Site-directed mutation, directed evolution or promotion of cofactor synthesis can enhance enzyme activity and increase the metabolic flow to promote microbial melatonin production. On the whole, the status and bottleneck of melatonin biosynthesis can be improved to a higher level, providing an effective reference for future microbial modification.

Phytomelatonin: An Emerging Regulator of Plant Biotic Stress Resistance

Melatonin has diverse functions in plant development and stress tolerance, with recent evidence showing a beneficial role in plant biotic stress tolerance. It has been hypothesized that pathogenic invasion causes the immediate generation of melatonin, reactive oxygen species (ROS), and reactive nitrogen species (RNS), with these being mutually dependent, forming the integrative melatonin-ROS-RNS feedforward loop. Here we discuss how the loop, possibly located in the mitochondria and chloroplasts, maximizes disease resistance in the early pathogen ingress stage, providing on-site protection. We also review how melatonin interacts with phytohormone signaling pathways to mediate defense responses and discuss the evolutionary context from the beginnings of the melatonin receptor-mitogen-activated protein kinase (MAPK) cascade in unicellular green algae, followed by the occurrence of phytohormone pathways in land plants.

Effects of AANAT overexpression on the inflammatory responses and autophagy activity in the cellular and transgenic animal levels

To explore the anti-inflammatory activity of endogenous produced melatonin, a melatonin-enriched animal model (goat) with AANAT transfer was successfully generated with somatic cell nuclear transfer (SCNT) technology. Basically, a pIRES2-EGFP-AANAT expression vector was constructed and was transferred into the female fetal fibroblast cells (FFCs) via electrotransfection and then the nuclear of the transgenic FFC was transferred to the eggs of the donor goats. The peripheral blood mononuclear cells (PBMCs) of the transgenic offspring expressed significantly higher levels of AANAT and melatonin synthetic function than those PBMCs from the wild-type (WT) animals. After challenge with lipopolysaccharide (LPS), the transgenic PBMCs had increased autophagosomes and LC3B expression while they exhibited suppressed production of the proinflammatory cytokines, IL1B and IL12 (IL12A-IL12B/p70), compared to their WT. The mechanistic analysis indicated that the anti-inflammatory activity of endogenous melatonin was mediated by MTNR1B (melatonin receptor 1B). MTNR1B stimulation activated the MAPK14 signaling pathway to promote cellular macroautophagy/autophagy, thus, suppressing the excessive inflammatory response of cellular. However, when the intact animals challenged with LPS, the serum proinflammatory cytokines were significantly higher in the transgenic goats than that in the WT. The results indicated that endogenous melatonin inhibited the MAPK1/3 signaling pathway and ROS production, subsequently downregulated gene expression of BECN1, ATG5 in PMBCs and then suppressed the autophagy activity of PBMCs and finally elevated levels of serum proinflammatory cytokines in transgenic animals, Herein we provided a novel melatonin-enriched animal model to study the potential effects of endogenously produced melatonin on inflammatory responses and autophagy activity.

Phytomelatonin: a universal abiotic stress regulator

Melatonin, a derivative of tryptophan, was first detected in plant species in 1995 and it has been shown to be a diverse regulator during plant growth and development, and in stress responses. Recently, great progress has been made towards determining the detailed functions of melatonin in plant responses to abiotic stress. Melatonin priming improves plant tolerance to cold, heat, salt, and drought stresses through regulation of genes involved in the DREB/CBF, HSF, SOS, and ABA pathways, respectively. As a scavenger of free radicals, melatonin also directly detoxifies reactive oxygen species, thus alleviating membrane oxidation. Abiotic stress-inhibited photosynthesis is partially recovered and metabolites accumulate in the presence of melatonin, leading to improved plant growth, delayed leaf senescence, and increased stress tolerance. In this review, we summarize the interactions of melatonin with phytohormones to regulate downstream gene expression, protein stabilization, and epigenetic modification in plants. Finally, we consider the need for, and approaches to, the identification of melatonin receptors and components during signaling transduction pathways.

Ameliorative effects of melatonin on dark-induced leaf senescence in gardenia (Gardenia jasminoides Ellis): leaf morphology, anatomy, physiology and transcriptome

Cut gardenia (Gardenia jasminoides Ellis) foliage is widely used as a vase material or flower bouquet indoors; however, insufficient indoor light accelerates its senescence, which shortens its viewing time. In this study, applying melatonin to delay gardenia leaf senescence when exposed to extremely low light condition (darkness), and the results showed that 1.0 mM was the effective concentration. At this concentration, chlorophyll contents and chlorophyll fluorescence parameters (Fv/Fm, Fv/F0 and Y(II)) increased, while the carotenoid and flavonoid contents decreased. Meanwhile, stress physiological indices decreased in response to exogenous melatonin application, whereas an increase in glutamine synthetase activity, water and soluble protein contents was observed. Moreover, exogenous melatonin application also reduced leaf programmed cell death under darkness, increased the endogenous melatonin level, expression levels of tryptophan decarboxylase gene, superoxide dismutase and catalase activities and the ascorbate-glutathione cycle, and maintained more intact anatomical structures. Furthermore, transcriptome sequencing revealed that various biological processes responded to exogenous melatonin application, including carbohydrate metabolism, amino acid metabolism, lipid metabolism, plant hormone signal transduction and pigment biosynthesis. Consequently, dark-induced leaf senescence in gardenia was significantly delayed. These results provided a better understanding for improving the ornamental value of cut gardenia foliage using melatonin.

Overexpression of rice serotonin N-acetyltransferase 1 in transgenic rice plants confers resistance to cadmium and senescence and increases grain yield

While ectopic overexpression of serotonin N-acetyltransferase (SNAT) in plants has been accomplished using animal SNAT genes, ectopic overexpression of plant SNAT genes in plants has not been investigated. Because the plant SNAT protein differs from that of animals in its subcellular localization and enzyme kinetics, its ectopic overexpression in plants would be expected to give outcomes distinct from those observed from overexpression of animal SNAT genes in transgenic plants. Consistent with our expectations, we found that transgenic rice plants overexpressing rice (Oryza sativa) SNAT1 (OsSNAT1) did not show enhanced seedling growth like that observed in ovine SNAT-overexpressing transgenic rice plants, although both types of plants exhibited increased melatonin levels. OsSNAT1-overexpressing rice plants did show significant resistance to cadmium and senescence stresses relative to wild-type controls. In contrast to tomato, melatonin synthesis in rice seedlings was not induced by selenium and OsSNAT1 transgenic rice plants did not show tolerance to selenium. T₂ homozygous OsSNAT1 transgenic rice plants exhibited increased grain yield due to increased panicle number per plant under paddy field conditions. These benefits conferred by ectopic overexpression of OsSNAT1 had not been observed in transgenic rice plants overexpressing ovine SNAT, suggesting that plant SNAT functions differently from animal SNAT in plants.

Comparative Transcriptomic Analyses of Differentially Expressed Genes in Transgenic Melatonin Biosynthesis Ovine HIOMT Gene in Switchgrass

Melatonin serves pleiotropic functions in prompting plant growth and resistance to various stresses. The accurate biosynthetic pathway of melatonin remains elusive in plant species, while the N-acetyltransferase and O-methyltransferase were considered to be the last two key enzymes during its biosynthesis. To investigate the biosynthesis and metabolic pathway of melatonin in plants, the RNA-seq profile of overexpression of the ovine HIOMT was analyzed and compared with the previous transcriptome of transgenic oAANAT gene in switchgrass, a model plant for cellulosic ethanol production. A total of 946, 405, and 807 differentially expressed unigenes were observed in AANAT vs. control, HIOMT vs. control, and AANAT vs. HIOMT, respectively. Two hundred and seventy-five upregulated and 130 downregulated unigenes were detected in transgenic oHIOMT line comparing with control, including the significantly upregulated (F-box/kelch-repeat protein, zinc finger BED domain-containing protein-3) genes, which were potentially correlated with enhanced phenotypes of shoot, stem and root growth in transgenic oHIOMT switchgrass. Several stress resistant related genes (SPX domain-containing membrane protein, copper transporter 1, late blight resistance protein homolog R1A-6 OS etc.) were specifically and significantly upregulated in transgenic oHIOMT only, while metabolism-related genes (phenylalanine-4-hydroxylase, tyrosine decarboxylase 1, protein disulfide-isomerase and galactinol synthase 2 etc.) were significantly upregulated in transgenic oAANAT only. These results provide new sights into the biosynthetic and physiological functional networks of melatonin in plants.