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In recent decades, global crop production is being threatened by contamination of arable lands with vanadium (V). Among many stress-relief substances, melatonin (ME) is a widely studied biomolecule acting as an antioxidant under stress conditions. The current study was aimed to investigate the response of tomato seedlings towards vanadium stress, along with the circumventing role of ME by promoting V stress tolerance in tomato seedlings. Our results revealed that accentuated inhibition of growth and biomass were caused by V (40 mg/L) stress, mainly due to impairments of photosynthetic systems, root traits, and mineral homeostasis. Conversely, notable reinforcement of plant growth parameters was seen with ME (100 µM) application, with improved chlorophyll content, root morphology, mineral nutrient homeostasis, and gas exchange parameters, along with reduced V accumulation. Further, ME efficiently triggered the antioxidant enzymes activities, by restoring cellular integrity [reduced electrolyte leakage (EL) and malondialdehyde] and restricted production of superoxide (O2•−) and hydrogen peroxide (H2O2) radicals, mainly through regulation of antioxidant enzymes. The present study highlighted the potential role of ME in tomato, for circumventing V-induced phytotoxicity, mainly by boosting photosynthesis, biomass production, redox balance, nutrient uptake, and root traits. In conclusion, ME application restricted the V availability in plant, improved plant growth and, thus, provided an improved V stress tolerance. [ABSTRACT FROM AUTHOR]

Waterlogging (WL) is a major limiting factor in global crop production and seriously limits growth and yield improvement in low-lying rainfed regions. Melatonin (MT) is a vital phytohormone that functions as a "master regulator" in multiple facets related to plant growth and development, in addition to maintaining a potential role in response to stresses. However, the pharmacological role of MT in attenuating waterlogging stress in tomato largely remains elucidated. The objective of the current investigation is to justify the physiological regulatory mechanism of tomato seedlings exposed to WL and the putative functions of MT to mitigate the adverse effects of WL. Tomato seedlings were grown on substrate (peat: vermiculite, 2:1, v/v) and at the 4th leaf stage subjected to waterlogging stress for 10 days and seedlings were foliar sprayed with melatonin during waterlogging stress. The results revealed that WL significantly arrested tomato seedlings growth, and reduced pigment content coincided with enhanced leaf senescence. MT supplementation attenuated WL-induced oxidative damage through increasing osmoprotectants activity, elevating antioxidant enzyme functioning synchronized with inhibiting excess reactive oxygen species (ROS) production. The concentrations of MT (28 %), abscisic acid (ABA, 170 %), and 1-aminocyclopropane-1-carboxylic acid (ACC, 129 %) were increased, while indole acetic acid (IAA, 15 %), jasmonic acid (JA, 55 %) and gibberellic acid (GA 3 , 26 %) content were decreased in only WL seedlings roots relative to control seedlings. The core anaerobic respiration enzyme alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC) activity were elevated by 127 % and 163 %, respectively at day 10 in WL+MT received seedlings than control. WL treatment varyingly contributed on nutrient content, as evidenced that N+, K+, and Ca2+ content decreased, whereas Mn2+, Fe2+, and Mg2+ content increased and MT addition reversed their concentrations under similar stress conditions. Exogenous MT promoted WL-tolerance of tomato by positively suppressing respiratory burst oxidase homologs (RBOH)-regulating gene expression while up-regulating ethylene biosynthesis gene transcription. Most importantly, programmed cell death (PCD) regulated enzyme caspase-3 activity concurred with PCD-induced gene (caspase-3 , pirin , TBN1) expression significantly inhibited by MT application. In general, these findings reveal that external supplementation with MT can improve plant tolerance to WL through complex processes and multifaceted mechanisms. [Display omitted] • Melatonin and/or waterlogging triggers anaerobic respiratory enzyme activity. • Melatonin regulates hormonal balance during waterlogging. • Melatonin inhibited PCD activity under waterlogging. • Ethylene biosynthesis gene transcription increased under waterlogging condition. [ABSTRACT FROM AUTHOR]

Polyamine oxidases (PAOs) are key enzymes in polyamine metabolism and are related to the tolerance of plants to abiotic stresses. In this study, overexpression of cucumber (Cucumis sativus L.) PAO2 (CsPAO2) in Arabidopsis resulted in increased activity of the antioxidant enzyme and accelerated conversion from Put to Spd and Spm, while malondialdehyde content (MDA) and electrolyte leakage (EL) was decreased when compared with wild type, leading to enhanced plant growth under salt stress. Photosystem Ⅰ assembly 3 in cucumber (CsPSA3) was revealed as an interacting protein of CsPAO2 by screening yeast two-hybrid library combined with in vitro and in vivo methods. Then, CsPAO2 and CsPSA3 were silenced in cucumber via virus-mediated gene silencing (VIGS) with pV190 as the empty vector. Under salt stress, net photosynthetic rate (Pn) and transpiration rate (Tr) of CsPAO2-silencing plants were lower than pV190-silencing plants, and EL in root was higher than pV190-silencing plants, indicating that CsPAO2-silencing plants suffered more serious salt stress damage. However, photosynthetic parameters of CsPSA3-silencing plants were all higher than those of CsPAO2 and pV190-silencing plants, thereby enhancing the photosynthesis process. Moreover, CsPSA3 silencing reduced the EL in both leaves and roots when compared with CsPAO2-silencing plants, but the EL only in leaves was significantly lower than the other two gene-silencing plants, and conversion from Put to Spd and Spm in leaf was also promoted, suggesting that CsPSA3 interacts with CsPAO2 in leaves to participate in the regulation of salt tolerance through photosynthesis and polyamine conversion. [ABSTRACT FROM AUTHOR]

Soil salinization is the leading environmental factor that restricts crop growth. This study studied the effects of Bacillus cereus (B. cereus) on growth, photosynthesis, and antioxidant metabolism in salt stressed-cucumber seedlings. The results showed that B. cereus could maintain high activity in the high salt environment (4% NaCl). B. cereus significantly increased plant height, stem diameter, fresh weight, and dry weight of cucumber seedlings under salt stress, and increased root vitality, net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr) of cucumber seedlings under salt stress. B. cereus significantly increased the maximum photochemical quantum yield of photosystem II (Fv/Fm), the actual photochemical quantum yield (ΦPSII), and the quantum yield of regulatory energy dissipation Y (NPQ) under salt stress, which were 9.31%, 20.44%, and 5.22% higher than those under salt stress, respectively. The quantum yield of non-regulatory energy dissipation Y (NO) was reduced by 19.81%. Superoxidase (SOD), peroxidase (POD), and catalase (CAT) activities in leaves and roots of cucumber seedlings were significantly increased by B. cereus under salt stress. Compared with salt stress, SOD activities in leaves were significantly increased by 1.70% and 6.32% on the first and third days after treatment. At 1 d, 3 d, and 5 d after treatment, SOD activity in roots increased by 3.06%, 11.24%, and 3.00%, POD activity in leaves increased by 113.38%, 38.81%, and 52.89%, respectively. The POD activity in roots increased by 56.79% and 10.92% on the third and fifth days after treatment, the CAT activity in leaves increased by 8.50% and 25.55%, and the CAT activity in roots increased by 30.59% and 84.45%. Under salt stress, the H2O2 and MDA contents of seedlings treated with B. cereus decreased significantly. Compared with salt stress, the proline content in leaves decreased by 12.69%, 3.90%, and 13.12% at 1 d, 3 d, and 5 d, respectively, while the proline content in roots decreased by 44.94% and 60.08% at 3 d and 5 d, respectively. These results indicated that B. cereus could alleviate salt-induced inhibition of growth and photosynthesis by regulating antioxidant metabolism of cucumber seedlings and thus enhancing salt tolerance of cucumber seedlings. [ABSTRACT FROM AUTHOR]

Crops around the world are facing a diversity of environmental problems, of which high temperatures are proving to be the most serious threat to crops. Polyamine putrescine (Put) acts as a master growth regulator that contributes to optimal plant growth and development and increased stress tolerance. Here, the current study aimed to elucidate how Put functions in regulating chlorophyll (Chl) metabolism, oxidative stress, and antioxidant defense, as well as to characterize the expression of genes related to heat stress in tomato seedlings under such stress. The results revealed that Put treatment significantly attenuates heat-induced damage by promoting biomass production, increasing photosynthetic efficiency, and inhibiting excessive production of oxidative stress markers. Heat stress markedly decreased the Chl content in the tomato leaf and accelerated the leaf yellowing process. However, Put-treated tomato seedlings showed a higher Chl content, which could be associated with the functions of Put in elevating PBGD activity (Chl biosynthesis enzyme) and suppressing the activity of the Chl catabolic enzyme (Chlase and MDCase). Under high-temperature stress, the expression levels of the gene encoding factors involved in Chl biosynthesis and Chl catabolism were significantly down- and upregulated, respectively, and this trend was reversed in Put-treated heat-stressed seedlings. In addition, exogenous application of Put boosted the activity of antioxidant enzymes, along with the levels of expression of their encoding genes, only in plants that were heat stressed. Furthermore, the expression levels of heat-shock-related genes (HSP90, HSP70, and HsfA1) were elevated in Put-treated, high-temperature-stressed tomato seedlings. Taken together, our results indicate that Put treatment significantly increases the heat tolerance of tomato seedlings, by elevating Chl concentrations and suppressing Chl catabolic enzyme activity, modulating endogenous free PA content, increasing antioxidant defense efficiency, and upregulating the expression of heat-shock-related genes. [ABSTRACT FROM AUTHOR]

Photosynthesis is a fundamental biosynthetic process in plants that can enhance carbon absorption and increase crop productivity. Heat stress severely inhibits photosynthetic efficiency. Melatonin is a bio-stimulator capable of regulating diverse abiotic stress tolerances. However, the underlying mechanisms of melatonin-mediated photosynthesis in plants exposed to heat stress largely remain elucidated. Our results revealed that melatonin treatment (100 μM) in tomato seedlings increased the endogenous melatonin levels and photosynthetic pigment content along with upregulated of their biosynthesis gene expression under high-temperature stress (42 °C for 24 h), whereas heat stress significantly decreased the values of gas exchange parameters. Under heat stress, melatonin boosted CO 2 assimilation, i.e., V c,max (maximum rate of ribulose-1,5-bisphosphate carboxylase, Rubisco), and J max (electron transport of Rubisco generation) and also enhanced the Rubisco and FBPase activities, which resulted in upregulated photosynthetic related gene expression. In addition, heat stress greatly reduced the photochemical chemistry of photosystem II (PSII) and photosystem I (PSI), particularly the maximum quantum efficiency of PSII (Fv/Fm) and PSI (Pm). Conversely, melatonin supplementation increased the chlorophyll a fluorescence parameters led to amplifying the electron transport efficiency. Moreover, heat stress decreased the actual PSII efficiency (ΦPSII), electron transport rate (ETR) and photochemical quenching coefficient (qP), while increasing nonphotochemical quenching (NPQ); however, melatonin reversed these values, which helps to fostering the dissipation of excess excitation energy. Taken together, our results provide a concrete insight into the efficacy of melatonin-mediated photosynthesis performance in a high-temperature regime. • Melatonin increased photosynthetic pigment contents and CO 2 assimilation of tomato seedlings exposed to heat stress. • Melatonin protected the photosystem II and photosystem I reaction center and reduced photoinhibition under thermal stress. • Melatonin enhanced photosynthesis efficiency by elevating the Rubisco and FBPase enzyme activities. • Melatonin treatment efficiently enhanced the heat-induced reduction of chlorophyll a fluorescence photochemistry. [ABSTRACT FROM AUTHOR]

Tomato is an important vegetable that is highly sensitive to drought (DR) stress which impairs the development of tomato seedlings. Recently, melatonin (ME) has emerged as a nontoxic, regulatory biomolecule that regulates plant growth and enhances the DR tolerance mechanism in plants. The present study was conducted to examine the defensive role of ME in photosynthesis, root architecture, and the antioxidant enzymes' activities of tomato seedlings subjected to DR stress. Our results indicated that DR stress strongly suppressed growth and biomass production, inhibited photosynthesis, negatively affected root morphology, and reduced photosynthetic pigments in tomato seedlings. Per contra, soluble sugars, proline, and ROS (reactive oxygen species) were suggested to be improved in seedlings under DR stress. Conversely, ME (100 µM) pretreatment improved the detrimental-effect of DR by restoring chlorophyll content, root architecture, gas exchange parameters and plant growth attributes compared with DR-group only. Moreover, ME supplementation also mitigated the antioxidant enzymes [APX (ascorbate peroxidase), CAT (catalase), DHAR (dehydroascorbate reductase), GST (glutathione S-transferase), GR (glutathione reductase), MDHAR (monodehydroascorbate reductase), POD (peroxidase), and SOD (superoxide dismutase)], non-enzymatic antioxidant [AsA (ascorbate), DHA (dehydroascorbic acid), GSH (glutathione), and GSSG, (oxidized glutathione)] activities, reduced oxidative damage [EL (electrolyte leakage), H2O2 (hydrogen peroxide), MDA (malondialdehyde), and O2•− (superoxide ion)] and osmoregulation (soluble sugars and proline) of tomato seedlings, by regulating gene expression for SOD, CAT, APX, GR, POD, GST, DHAR, and MDHAR. These findings determine that ME pretreatment could efficiently improve the seedlings growth, root characteristics, leaf photosynthesis and antioxidant machinery under DR stress and thereby increasing the seedlings' adaptability to DR stress. [ABSTRACT FROM AUTHOR]

Arable land contamination with nickel (Ni) has become a major threat to worldwide crop production. Recently, melatonin has appeared as a promising stress-relief substance that can alleviate heavy metal-induced phytotoxicity in plants. However, the plausible underlying mechanism of melatonin function under Ni stress has not been fully substantiated in plants. Herein, we conducted an experiment that unveiled critical mechanisms in favor of melatonin-mediated Ni-stress tolerance in tomato. Ni stress markedly inhibited growth and biomass by impairing the photosynthesis, photosystem function, mineral homeostasis, root activity, and osmotic balance. In contrast, melatonin application notably reinforced the plant growth traits, increased photosynthesis efficiency in terms of chlorophyll content, upregulation of chlorophyll synthesis genes, i.e. POR , CAO , CHL G , gas exchange parameters, and PSII maximum efficiency (Fv/Fm), decreased Ni accumulation and increased mineral nutrient homeostasis. Moreover, melatonin efficiently restricted the hydrogen peroxide (H 2 O 2) and superoxide radical production and increased RBOH expression and restored cellular integrity (less malondialdehyde and electrolyte leakage) through triggering the antioxidant enzyme activities and modulating AsA-GSH pools. Notably, oxidative stress was effectively mitigated by upregulation of several defense genes (SOD , CAT , APX , GR , GST , MDHAR , DHAR) and melatonin biosynthesis-related genes (TDC , T5S , SNAT , ASMT). Besides, melatonin treatment enhanced secondary metabolites (phenols, flavonoids, and anthocyanin) contents along with their encoding genes (PAL , CHS) expression, and these metabolites potentially restricted excess H 2 O 2 accumulation. In conclusion, our findings deciphered the potential functions of melatonin in alleviating Ni-induced phytotoxicity in tomato through boosting the biomass production, photosynthesis, nutrient uptake, redox balance, and secondary metabolism. • Melatonin treatment inhibits Ni uptake both in roots and shoots and enhances plant growth and biomass production. • Under Ni-stress, melatonin increases photosynthetic efficiency, nutrients uptake and root activity and reduces osmotic damage. • Melatonin supplementation modulates antioxidant defense mechanism and redox homeostasis under Ni stress. • Melatonin triggers higher accumulation of secondary metabolites and upregualtion of their biosynthesis genes (PAL , CHS) expression and restrict ROS production. [ABSTRACT FROM AUTHOR]

High temperature is considered a critical abiotic stressor that is increasing continuously, which is severely affecting plant growth and development. The use of heat-resistant rootstock grafting is a viable technique that is practiced globally to improve plant resistance towards abiotic stresses. In this experiment, we explored the efficacy of bitter melon rootstock and how it regulates photosynthesis and the antioxidant defense system to alleviate heat stress (42 °C/32 °C) in cucumber. Our results revealed that bitter-melon-grafted seedlings significantly relieved heat-induced growth inhibition and photoinhibition, maintained better photosynthesis activity, and accumulated a greater biomass than self-grafted seedlings. We measured the endogenous polyamine and hydrogen peroxide (H2O2) contents to determine the inherent mechanism responsible for these effects, and the results showed that heat stress induced a transient increase in polyamines and H2O2 in the inner courtyard of grafted seedlings. This increment was greater and more robust in bitter-melon-grafted seedlings. In addition, the use of polyamine synthesis inhibitors MGBG (methylglyoxal bis-guanylhydrazone) and D-Arg (D-arginine), further confirmed that the production of H2O2 under heat stress is mediated by the accumulation of endogenous polyamines. Moreover, compared with other treatments, the bitter-melon-grafted seedlings maintained high levels of antioxidant enzyme activity under high temperature conditions. However, these activities were significantly inhibited by polyamine synthesis inhibitors and H2O2 scavengers (dimethylthiourea, DMTU), indicating that bitter melon rootstock not only maintained better photosynthetic activity under conditions of high temperature stress but also mediated the production of H2O2 through the regulation of the high level of endogenous polyamines, thereby boosting the antioxidant defense system and comprehensively improving the heat tolerance of cucumber seedlings. Taken together, these results indicate that grafting with a resistant cultivar is a promising alternative tool for reducing stress-induced damage. [ABSTRACT FROM AUTHOR]

Contamination by heavy metals has severely hampered agricultural productivity across the world. Cadmium (Cd) is one of the most dangerous heavy metals to plants. Melatonin (ME), a stress-relief hormone, has been extensively utilized to reduce phytotoxicity caused by heavy metals. In this work, we examined how ME reduced Cd damage in strawberry seedlings. The results revealed that Cd stress dramatically reduced growth characteristics (fresh and dry weight, root and shoot length), leaf gas exchange elements, and pigments content (chlorophyll a and chlorophyll b) in strawberry seedlings. In contrast, ME application efficiently enhanced growth and biomass production, increased net photosynthetic rate, and improved pigments molecules in strawberry seedling under Cd toxicity. Cadmium treatment effectively improved the level of malondialdehyde (MDA) and hydrogen peroxide (H 2 O 2), whereas ME strictly counterbalanced these oxidative stress markers. Furthermore, the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), were dominantly increased by ME. Additionally, ME supplementation significantly improved secondary metabolites (phenols, anthocyanin, and flavonoids) content, and these metabolites potentially restricted excess H 2 O 2 accumulation. In conclusion, our results revealed that ME might help to reduce Cd-induced phytotoxicity in strawberry seedlings by increasing growth, photosynthesis, redox homeostasis, and secondary metabolites. [Display omitted] • Melatonin enhanced cadmium stress tolerance in strawberry plants. • Melatonin stimulates antioxidant defense system in strawberry seedling under cadmium toxicity. • Melatonin increased photosynthesis apparatus in cadmium-stressed strawberry plants. • Melatonin as a supplement regulates defense mechanism in strawberry under cadmium stress. [ABSTRACT FROM AUTHOR]

• Melatonin improves salt stress tolerance in tomato seedling. • Melatonin activates antioxidant ability in tomato seedling under salt stress. • Melatonin promotes root architecture system in tomato seedling under salt stress. • Melatonin improves photosynthesis related parameters in salt-stressed tomato seedling. • Melatonin as a supplement modulates defense mechanism in tomato under salt stress. Tomato is considered an important vegetable crop for studying the response to abiotic stresses, which deteriorate the growth and development of plants, particularly salt stress. Melatonin is a crucial pleiotropic nontoxic signaling molecule that has a various role in modulating of plant responses to environmental stresses. The purpose of the study was to reckon the alleviating effects of melatonin on tomato plant growth and development in salinity condition. The results exhibited that the pretreatment of tomato seedlings with 100 μM melatonin for 3 days effectively improved the root architecture, photosynthetic pigments, photosynthetic assimilation and growth status of plants under subsequent salt stress (150 mM). The pretreatment slashed sodium ions concentration in leaf and stem by checking sodium ions transport from roots to shoot. Furthermore, melatonin notably surged potassium contents. Melatonin pretreatment (3 days) followed by salinity exposure (7 days) efficiently lowered the oxidative stress by checking the over accumulation of superoxide (O 2 •−) and hydrogen peroxide (H 2 O 2), reducing the malondialdehyde (MDA) content and electrolyte leakage (EL). This was associated with increased activities of enzymatic antioxidants [superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and ascorbate peroxidase (APX)] and non-enzymatic antioxidants [ascorbic acid (AsA) and glutathione (GSH)]. In conclusion, melatonin pretreatment significantly escalates the salinity tolerance of tomato seedlings by scavenging the excessive ROS and improving cellular membrane stability of, thus mitigating salinity-induced oxidative damage. [Display omitted] [ABSTRACT FROM AUTHOR]