Melatonin Rescues Photosynthesis and Triggers Antioxidant Defense Response in Cucumis sativus Plants Challenged by Low Temperature and High Humidity

Mitigating effects of Methyl Jasmonate on photosynthetic inhibition and oxidative stress of pepper (Capsicum annuum L) seedlings under low temperature combined with low light

Low temperature combined with low light (LL) is a critical abiotic stress that restricting plant growth and yield of pepper (Capsicum annuum L.). Methyl jasmonate (MeJA) is considered with potential benefits for improving plant stress resistance; however, the physiological mechanisms underlying the adaptation of pepper to LL stress have not been explored. This study aimed to investigate the potential mitigating effects of foliar MeJA (200 μmol L-1) application on pepper seedlings subjected to LL stress (10/5 °C, 100 μmol m-2 s-1) for 168 h. Our results indicated that the application of exogenous MeJA reduced the negative effect on growth inhibition of pepper seedlings caused by LL stress, significantly increased chlorophyll contents and photosynthetic capacity as a result of improved photosynthesis rate. In addition, MeJA reduced the accumulation of reactive oxygen species and malondialdehyde contents induced by LL stress, while enhancing the activities of superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, dehydroascorbate reductase, and monodehydroascorbate reductase as a result of upregulated expression levels of antioxidant enzyme genes (CaSOD, CaPOD, CaCAT, CaAPX, CaGR, CaDHAR, and CaMDHAR). Additionally, it increased the ascorbic acid and reduced glutathione content, while reducing oxidized glutathione content, thereby preventing membrane lipid peroxidation and protecting plants from oxidative damage under LL stress. Furthermore, seedlings treated with MeJA exhibited significantly enhanced soluble sugar and soluble protein contents in leaves. Taken together, present findings indicate that MeJA application may serve as an effective strategy for mitigating LL-induced oxidative stress by maintaining plant growth, enhancing chlorophyll fluorescence, upregulating the antioxidant defence system, optimizing ascorbate-glutathione cycle, and osmotic adjustment.

Exogenous 24-epibrassinolide mitigates damage in grape seedlings under low-temperature stress

Grapes are cultivated worldwide and have a high economic value as fruit trees. However, winter frost damage and spring cold damage have limited the sustainability of the table grape industry. A novel plant growth regulator, 24-epibrassinolide (EBR), exhibits an essential regulatory function in plant life cycles, especially in its unique mechanism against various environmental stresses. We treated 'Lihongbao' grapes with exogenous EBR (0.2 μM), brassinazole (BRZ, 10 μM), EBR + BRZ (0.2 μM +10 μM), and deionized water (CK). We investigated the effect of exogenous EBR on 'Lihongbao' grape seedlings under low-temperature stress (4°C) at different periods (0 h, 12 h, 24 h, 48 h, and 96 h). We explored physiological mitigation mechanisms of exogenous EBR in grape seedlings with low-temperature injury by observing the impacts of EBR treatment on the physical and biochemical indices such as phenotypes and anatomical structures, photosynthetic characteristics, chlorophyll fluorescence parameters, antioxidant systems, and osmoregulatory substances. Exogenous EBR had an inhibitory effect on cold stress in grape seedlings at different treatment periods compared with the CK group. Based on plant phenotype and anatomical structure, the leaves of the grape seedlings treated with exogenous EBR showed no signs of water loss or wilting. At 96 h under low-temperature stress, the lower epidermal thickness (LET), palisade tissue thickness (PT), palisade-to-sea ratio (P/S), and blade structural compactness (CTR) of the exogenous EBR-treated grape leaves were significantly reduced by 6.71%, 19.59%, 14.52%, and 11.65% compared with the CK group, respectively. Chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Chl total), carotenoids (carotenoid), transpiration rate (Tr), and stomatal conductance (Gs) contents of exogenous EBR-treated grape leaves were significantly upregulated by 30.24%, 48.52%, 39.75%, 34.67%, 704.66%, and 277.27%, respectively. The intercellular CO2 concentration (Ci) and non-photosynthetic burst coefficient (NPQ) of exogenous EBR-treated grape leaves were significantly downregulated by 16.29% and 25.83%, respectively. Glutathione (GSH) contents of exogenous EBR-treated grape leaves were significantly upregulated by 33.63%, superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) activities of exogenous EBR-treated grape leaves were significantly increased by 42.70%, 27.60%, and 28.64%, respectively. However, hydrogen peroxide (H2O2), superoxide anion (O2·-), and malondialdehyde (MDA) contents of exogenous EBR-treated grape leaves were reduced by 29.88%, 23.66%, and 47.96%, respectively, and significantly. Catalase (CAT) activity of exogenous EBR-treated grape leaves significantly increased by 15.03%. Soluble sugar and free proline contents increased by 5.29% and 19.44%, respectively, and significantly. Exogenous EBR could effectively alleviate growth inhibition caused by regulating the antioxidant system indices in grape seedlings under cold temperature. The results offer a theoretical basis for enhancing grape cold tolerance.

Integrated roles of nitric oxide and melatonin in enhancing chromium resilience in cotton plants: modulation of thiol metabolism and antioxidant responses

Chromium (Cr) is a hazardous metal found in various oxidation states, posing significant environmental and health risks. The study focuses on understanding how melatonin (MT), known for its diverse functions, including stress alleviation and antioxidant properties, interacts with nitric oxide (NO) to regulate sulfur metabolism and enhance cotton resilience under Cr toxicity. Cr toxicity negatively affected plant growth and photosynthesis and induced oxidative stress. MT treatment ameliorated these effects by enhancing photosynthetic pigments and gas exchange traits and upregulating the activities of antioxidant enzymes and the expression of FeSOD, CuZnSOD, and APX1. Moreover, MT reduced Cr accumulation in leaves, protecting photosynthetic organs from direct toxicity. Additionally, MT promotes the level of sulfur-based defense substances like glutathione (GSH) and phytochelatins (PC), which are crucial for detoxifying heavy metals. This is achieved by upregulating genes involved in cysteine metabolism (CYC1, CYC2, CAS1, CAS2, DES1, DES2, and SSCS), increasing cysteine availability for GSH and PC synthesis, and enhancing Cr sequestration in vacuoles. However, when the inhibitor of MT biosynthesis (p-CPA) and NO scavenger (cPTIO) were used along with MT in Cr-stressed plants, they hindered the stimulatory effects of MT. The study highlights the importance of the NO-MT interaction in mediating these protective effects, indicating a potential signaling role for NO in plant defense mechanisms against Cr toxicity. Overall, the findings reveal that MT fertilizing may serve as an effective strategy to enhance Cr resistance in cotton plants, with NO potentially playing a signaling role in this response pathway.

Low temperature and high humidity affect dynamics of chlorophyll biosynthesis and secondary metabolites in Cucumber

BACKGROUND

During the cold season, low temperature (LT) and high relative humidity (HRH) are significant environmental factors in greenhouses and plastic tunnels, often hindering plant growth and development. The chlorophyll (Chl) biosynthesis inhibitory mechanisms under LT and HRH stress are still widely unclear. To understand how cucumbers seedlings respond to LT and HRH stress, we investigated the impact of these stressors on Chl biosynthesis.

RESULTS

Our results revealed that individual LT, HRH and combined LT + HRH stress conditions affected chlorophyll a, b, total chlorophyll and carotenoid content, reducing the levels of these pigments. The levels of Chlorophyll precursors were also markedly reduced under LT and HRH stresses, with the greatest reduction observed in cucumber seedlings exposed to LT + HRH conditions (9/5℃, 95%HRH). The activities of glutamate-1-semialdehyde transaminase (GSA-AT), ALA dehydratase (ALAD), Mg-chelatase, and protochlorophyllide oxidoreductase (POR) were increased under individual LT, HRH, conditions but decreased by combination of LT + HRH stress condition. In addition, Chl biosynthesis related genes (except PBG) were upregulated by the HRH stress but were significantly downregulated under the LT + HRH stress condition in cucumber seedlings. Furthermore, the content of phenols, flavonoids and phenolic acids (cinnamic acid and caffeic acid) were significantly surged under LT + HRH treatment over the control. Histochemical observation showed higher O2- and H2O2 content in cucumber leaves during the LT and HRH stress.

CONCLUSION

The results indicate that LT + HRH stress significantly impairs chlorophyll biosynthesis in cucumber seedlings by drastically reducing pigment accumulation, altering enzyme activity and gene expression. Additionally, LT + HRH stress induces oxidative damage, which further exacerbates the decline in chlorophyll content and affects overall cucumber metabolism.

Interaction of melatonin and H2S mitigates NaCl toxicity summer savory (Satureja hortensis L.) through Modulation of biosynthesis of secondary metabolites and physio-biochemical attributes

As versatile signaling molecules, melatonin (ML) and hydrogen sulfide (H2S) are well-known for their roles in response to abiotic stresses. However, their cross-talk to the regulation of biochemical defence responses and secondary metabolite synthesis during salinity has received less attention. Here, the role of ML-H2S interplay in inducing defensive responses and the biosynthesis of essential oil compounds in summer savoury plants under NaCl treatment was investigated. NaCl treatment, by increasing Na accumulation, disrupting nitrogen metabolism, and inducing oxidative stress, lowered photosynthetic pigments and savoury growth. NaCl treatment also resulted in a decrease in γ-terpinene (10.3%), α-terpinene (21.9%), and p-cymene (15.3%), while an increase in carvacrol (9.1%) was observed over the control. ML and ML + H2S increased the activity of antioxidant enzymes and the level of total phenols and flavonoids, resulting in decreased levels of hydrogen peroxide and superoxide anion and alleviation of oxidative damage under salinity. ML and ML + H2S increased K uptake and restored K/Na homeostasis, thus protecting the photosynthetic apparatus against NaCl-induced toxicity. ML and ML + H2S treatments also improved nitrate/ammonium homeostasis and stimulated nitrogen metabolism, leading to improved summer savoury adaptation to NaCl stress. ML and ML + H2S changed the composition of essential oils, leading to an increase in the monoterpene hydrocarbons and oxygenated monoterpenes in plants stressed with NaCl. However, the addition of an H2S scavenger, hypotaurine, inhibited the protective effects of the ML and ML + H2S treatments under NaCl stress, which could confirm the function of H2S as a signaling molecule in the downstream defence pathway induced by ML.

The Roles of Mepiquate Chloride and Melatonin in the Morpho-Physiological Activity of Cotton under Abiotic Stress

Cotton growth and yield are severely affected by abiotic stress worldwide. Mepiquate chloride (MC) and melatonin (MT) enhance crop growth and yield by reducing the negative effects of abiotic stress on various crops. Numerous studies have shown the pivotal role of MC and MT in regulating agricultural growth and yield. Nevertheless, an in-depth review of the prominent performance of these two hormones in controlling plant morpho-physiological activity and yield in cotton under abiotic stress still needs to be documented. This review highlights the effects of MC and MT on cotton morpho-physiological and biochemical activities; their biosynthetic, signaling, and transduction pathways; and yield under abiotic stress. Furthermore, we also describe some genes whose expressions are affected by these hormones when cotton plants are exposed to abiotic stress. The present review demonstrates that MC and MT alleviate the negative effects of abiotic stress in cotton and increase yield by improving its morpho-physiological and biochemical activities, such as cell enlargement; net photosynthesis activity; cytokinin contents; and the expression of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase. MT delays the expression of NCED1 and NCED2 genes involved in leaf senescence by decreasing the expression of ABA-biosynthesis genes and increasing the expression of the GhYUC5, GhGA3ox2, and GhIPT2 genes involved in indole-3-acetic acid, gibberellin, and cytokinin biosynthesis. Likewise, MC promotes lateral root formation by activating GA20x genes involved in gibberellin catabolism. Overall, MC and MT improve cotton's physiological activity and antioxidant capacity and, as a result, improve the ability of the plant to resist abiotic stress. The main purpose of this review is to present an in-depth analysis of the performance of MC and MT under abiotic stress, which might help to better understand how these two hormones regulate cotton growth and productivity.

SlTDC1 Overexpression Promoted Photosynthesis in Tomato under Chilling Stress by Improving CO2 Assimilation and Alleviating Photoinhibition

Chilling causes a significant decline in photosynthesis in tomato plants. Tomato tryptophan decarboxylase gene 1 (SlTDC1) is the first rate-limiting gene for melatonin (MT) biosynthesis and is involved in the regulation of photosynthesis under various abiotic stresses. However, it is not clear whether SlTDC1 participates in the photosynthesis of tomato under chilling stress. Here, we obtained SlTDC1 overexpression transgenic tomato seedlings, which showed higher SlTDC1 mRNA abundance and MT content compared with the wild type (WT). The results showed that the overexpression of SlTDC1 obviously alleviated the chilling damage to seedlings in terms of the lower electrolyte leakage rate and hydrogen peroxide content, compared with the WT after 2 d of chilling stress. Moreover, the overexpression of SlTDC1 notably increased photosynthesis under chilling stress, which was related to the higher chlorophyll content, normal chloroplast structure, and higher mRNA abundance and protein level of Rubisco and RCA, as well as the higher carbon metabolic capacity, compared to the WT. In addition, we found that SlTDC1-overexpressing seedlings showed higher W_k (damage degree of OEC on the PSII donor side), φ_Eo (quantum yield for electron transport in the PSII reaction center), and PI_ABS (photosynthetic performance index) than WT seedlings after low-temperature stress, implying that the overexpression of SlTDC1 decreased the damage to the reaction center and donor-side and receptor-side electron transport of PSII and promoted PSI activity, as well as energy absorption and distribution, to relieve the photoinhibition induced by chilling stress. Our results support the notion that SlTDC1 plays a vital role in the regulation of photosynthesis under chilling stress.

Melatonin Role in Plant Growth and Physiology under Abiotic Stress

Phyto-melatonin improves crop yield by mitigating the negative effects of abiotic stresses on plant growth. Numerous studies are currently being conducted to investigate the significant performance of melatonin in crops in regulating agricultural growth and productivity. However, a comprehensive review of the pivotal performance of phyto-melatonin in regulating plant morpho-physiological and biochemical activities under abiotic stresses needs to be clarified. This review focused on the research on morpho-physiological activities, plant growth regulation, redox status, and signal transduction in plants under abiotic stresses. Furthermore, it also highlighted the role of phyto-melatonin in plant defense systems and as biostimulants under abiotic stress conditions. The study revealed that phyto-melatonin enhances some leaf senescence proteins, and that protein further interacts with the plant's photosynthesis activity, macromolecules, and changes in redox and response to abiotic stress. Our goal is to thoroughly evaluate phyto-melatonin performance under abiotic stress, which will help us better understand the mechanism by which phyto-melatonin regulates crop growth and yield.

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 enhanced low-temperature combined with low-light tolerance of pepper (Capsicum annuum L.) seedlings by regulating root growth, antioxidant defense system, and osmotic adjustment

Melatonin (MT) is an important biologically active hormone that plays a vital role in plant growth and development. In particular, it has been investigated for its roles in abiotic stress management. In this study, pepper seedlings were subjected to low-temperature combined with low-light stress (LL) (15/5°C, 100 μmol m^-2s^-1) prior to a foliar spray of 200mM MT for 168h to investigate the protective role of MT in pepper seedlings. Our results demonstrated that LL stress negatively affected root growth, and accelerated the accumulation of reactive oxygen species (ROS), including H₂O₂ and O 2 - , changed the osmolytes contents, and antioxidative system. However, these were reversed by exogenous MT application. MT effectively promoted the root growth as indicated by significant increase in root length, surface area, root volume, tips, forks, and crossings. In addition, MT reduced the burst of ROS and MDA contents induced by LL, enhanced the activities of SOD, CAT, POD, APX, DHAR, and MDHAR resulted by upregulated expressions of CaSOD, CaPOD, CaCAT, CaAPX, CaDHAR, and CaMDHAR, and elevated the contents of AsA and GSH, declined DHA and GSSH contents, which prevented membrane lipid peroxidation and protected plants from oxidative damages under LL stress. Furthermore, seedlings treated with MT released high contents of soluble sugar and soluble protein in leave, which might enhance LL tolerance by maintaining substance biosynthesis and maintaining cellular homeostasis resulted by high levels of osmolytes and carbohydrate in the cytosol. Our current findings confirmed the mitigating potential of MT application for LL stress by promoting pepper root growth, improving antioxidative defense system, ascorbate-glutathione cycle, and osmotic adjustment.