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Heavy metal pollution often occurs together with organic contaminants. Brassinosteroids (BRs) induce plant tolerance to several abiotic stresses, including phenanthrene (PHE) and cadmium (Cd) stress. However, the role of BRs in PHE+Cd co-contamination-induced stress amelioration is unknown. Here, the interactive effects of PHE, Cd, and 24-epibrassinolide (EBR; a biologically active BR) were investigated in tomato plants. The application of Cd (100 µM) alone was more phytotoxic than PHE applied alone (100 µM); however, their combined application resulted in slightly improved photosynthetic activity and pigment content compared with Cd alone after a 40 d exposure. Accumulation of reactive oxygen species and membrane lipid peroxidation were induced by PHE and/or Cd; however, the differences in effect were insignificant between Cd and PHE+Cd. The foliar application of EBR (0.1 µM) to PHE- and/or Cd-stressed plants alleviated photosynthetic inhibition and oxidative stress by causing enhancement of the activity of the enzymes and related transcript levels of the antioxidant system, secondary metabolism, and the xenobiotic detoxification system. Additionally, PHE and/or Cd residues were significantly decreased in both the leaves and roots after application of EBR, more specifically in PHE+Cd-stressed plants when treated with EBR, indicating a possible improvement in detoxification of these pollutants. The findings thus suggest a potential interaction of EBR and PHE for Cd stress alleviation. These results advocate a positive role for EBR in reducing pollutant residues for food safety and also strengthening phytoremediation.

Heavy metal pollution not only decreases crop yield and quality, but also affects human health via the food chain. Ubiquitination‐dependent protein degradation is involved in plant growth, development, and environmental interaction, but the functions of ubiquitin‐ligase (E3) genes are largely unknown in tomato (Solanum lycopersicum L.). Here, we functionally characterized a RING E3 ligase gene, SlRING1, which positively regulates cadmium (Cd) tolerance in tomato plants. An in vitro ubiquitination experiment shows that SlRING1 has E3 ubiquitin ligase activity. The determination of the subcellular localization reveals that SlRING1 is localized at both the plasma membrane and the nucleus. Overexpression of SlRING1 in tomato increased the chlorophyll content, the net photosynthetic rate, and the maximal photochemical efficiency of photosystem II (Fv/Fm), but reduced the levels of reactive oxygen species and relative electrolyte leakage under Cd stress. Moreover, SlRING1 overexpression increased the transcript levels of CATALASE (CAT), DEHYDROASCORBATE REDUCTASE (DHAR), MONODEHYDROASCORBATE REDUCTASE (MDHAR), GLUTATHIONE (GSH1), and PHYTOCHELATIN SYNTHASE (PCS), which contribute to the antioxidant and detoxification system. Crucially, SlRING1 overexpression also reduced the concentrations of Cd in both shoots and roots. Thus, SlRING1‐overexpression‐induced enhanced tolerance to Cd is ascribed to reduced Cd accumulation and alleviated oxidative stress. Our findings suggest that SlRING1 is a positive regulator of Cd tolerance, which can be a potential breeding target for improving heavy metal tolerance in horticultural crops. [ABSTRACT FROM AUTHOR]

Salinity is a global environmental problem, restricting crop production in a vast area of agricultural land. Although epigallocatechin-3-gallate (EGCG), the most abundant polyphenol in tea plants, has a strong antioxidative action in vitro, the role of EGCG in the plant response to salt stress remains unknown. In the present study, using a series of EGCG concentrations (10, 100, and 1000 µM), we showed that EGCG could alleviate salt stress-induced inhibition in seed germination and root growth in tomato. Exogenous EGCG increased not only the seed germination rate, but also the germination energy and germination index under salt stress. The 150 mM NaCl treatment significantly increased lipid peroxidation in roots by excessive accumulation of reactive oxygen species (ROS). In contrast, EGCG treatment, particularly at 100 µM concentration, mitigated NaCl-induced oxidative stress as evidenced by the decreased H2O2 and malondialdehyde content in roots. Analysis of the antioxidant enzyme system reveals that EGCG increased the activity of superoxide dismutase, peroxidase, ascorbate peroxidase, and catalase under salt stress. Considering the beneficial effect of EGCG on seed germination, root growth, ROS scavenging, and antioxidant enzyme activity, the 100 µM EGCG treatment appears to be the most effective concentration of those tested under salt stress in tomato. Our results suggest that the EGCG-promoted tomato tolerance to salt stress is associated with the mitigation of oxidative stress through an efficient ROS scavenging mechanism by the action of enhanced antioxidant enzyme activity. Thus, the enhancement of plant tolerance by exogenous EGCG can potentially expand crop cultivation in saline soils. [ABSTRACT FROM AUTHOR]

Chromium (Cr) is one of the toxic elements that harms all forms of life, including plants. Industrial discharges and mining largely contribute to Cr release into the soil environment. Excessive Cr pollution in arable land significantly reduces the yield and quality of important agricultural crops. Therefore, remediation of polluted soil is imperative not only for agricultural sustainability but also for food safety. Arbuscular mycorrhizal fungi (AMF) are widespread soil-borne endophytic fungi that form mutualistic relationships with the vast majority of land plants. In mycorrhizal symbiosis, AMF are largely dependent on the host plant-supplied carbohydrates and lipids, in return, AMF aid the host plants in acquiring water and mineral nutrients, especially phosphorus, nitrogen and sulfur from distant soils, and this distinguishing feature of the two-way exchange of resources is a functional requirement for such mutualism and ecosystem services. In addition to supplying nutrients and water to plants, the AMF symbiosis enhances plant resilience to biotic and abiotic stresses including Cr stress. Studies have revealed vital physiological and molecular mechanisms by which AMF alleviate Cr phytotoxicity and aid plants in nutrient acquisition under Cr stress. Notably, plant Cr tolerance is enhanced by both the direct effects of AMF on Cr stabilization and transformation, and the indirect effects of AMF symbiosis on plant nutrient uptake and physiological regulation. In this article, we summarized the research progress on AMF and associated mechanisms of Cr tolerance in plants. In addition, we reviewed the present understanding of AMF-assisted Cr remediation. Since AMF symbiosis can enhance plant resilience to Cr pollution, AMF may have promising prospects in agricultural production, bioremediation, and ecological restoration in Cr-polluted soils. [ABSTRACT FROM AUTHOR]

Bisphenol A (BPA) is an emerging organic pollutant, widely distributed in environment. Plants can uptake and metabolize BPA, but BPA accumulation induces phytotoxicity. In this study, we administered dopamine, a kind of catecholamines with strong antioxidative potential, to unveil its role in cucumber tolerance to BPA stress. The results showed that exposure to BPA (20 mg L−1) for 21 days significantly reduced growth and biomass accumulation in cucumber seedlings as revealed by decreased lengths and dry weights of shoots and roots. While BPA exposure decreased the chlorophyll content, cell viability and root activity, it remarkably increased reactive oxygen species (ROS) accumulation, electrolyte leakage and malondialdehyde (MDA) content, suggesting that BPA induced oxidative stress in cucumber. However, exogenous dopamine application significantly improved the photosynthetic pigment content, root cell viability, growth and biomass accumulation, and decreased the ROS and MDA levels by increasing the activity of antioxidant enzymes under BPA stress. Further analysis revealed that dopamine application significantly increased the glutathione content and the transcripts and activity of glutathione S -transferase under co-administration of dopamine and BPA compared with only BPA treatment. Moreover, dopamine decreased the BPA content in both leaves and roots, suggesting that dopamine promoted BPA metabolism by enhancing the glutathione-dependent detoxification. Our results show that dopamine has a positive role against BPA phytotoxicity and it may reduce the risks-associated with the dietary intake of BPA through consumption of vegetables. Image 1 • Bisphenol A (BPA) causes phytotoxicity in cucumber by inducing oxidative stress. • Dopamine alleviates oxidative stress by improving antioxidant enzyme activity. • Dopamine enhances glutathione content and glutathione S -transferase activity. • Dopamine administration reduces BPA content in leaves and roots of cucumber. • Dopamine promotes metabolism of BPA and may improve food safety. [ABSTRACT FROM AUTHOR]

Trichoderma can enhance the metabolism of organophosphate pesticides in plants, but the mechanism is unclear. Here, we performed high-throughput transcriptome sequencing of roots upon Trichoderma asperellum (TM) inoculation and phoxim (P) application in tomato (Solanum lycopersicum L.). A total of 4059 differentially expressed genes (DEGs) were obtained, including 2110 up-regulated and 1949 down-regulated DEGs in P vs TM+P. COG and KOG analysis indicated that DEGs were mainly enriched in signal transduction mechanisms. We then focused on the pesticide detoxification pathway and screened out cytochrome P450 CYP736A12 as a putative gene for functional analysis. We suppressed the expression of CYP736A12 in tomato plants by virus-induced gene silencing and analyzed tissue-specific phoxim residues, oxidative stress markers, glutathione pool, GST activity and related gene expression. Silencing CYP736A12 significantly increased phoxim residue and induced oxidative stress in tomato plants, by attenuating the TM-induced increased activity of antioxidant and detoxification enzymes, redox homeostasis and transcripts of detoxification genes including CYP724B2 , GSH1 , GSH2 , GR , GPX , GST1 , GST2 , GST3 , and ABC. The study revealed a critical mechanism by which TM promotes the metabolism of phoxim in tomato roots, which can be useful for further understanding the Trichoderma -induced xenobiotic detoxification and improving food safety. [Display omitted] • Trichoderma asperellum (TM) reduced phoxim (P) residue in tomato plants. • CYP736A12 detoxifying P in tomato was screened out as a putative gene by RNA-seq. • Silencing CYP736A12 increased phoxim residue and induced oxidative stress. • TM enhanced antioxidants, redox homeostasis and detoxification genes via CYP736A12. • CYP736A12 is essential for TM-induced phoxim metabolism in tomato roots. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) is a toxic heavy metal, increasingly accumulating in the environment and its presence in various environmental compartments represents a significant risk to human health via the food chain. Epigallocatechin-3-Gallate (EGCG) is a prominent secondary metabolite, which can safeguard plants from biotic and abiotic stress. However, the role of EGCG in flavonoid synthesis, nutrient acquisition and reactive oxygen species (ROS) metabolism under Cd stress remains unclear. Here, we examined the effects of EGCG and Cd treatment on leaf photochemical efficiency, cell ultrastructure, essential element acquisition, antioxidant system, and secondary metabolism in tomato (Solanum lycopersicum L.). The results showed that O 2 •−, H 2 O 2 , and malondialdehyde levels increased after Cd treatment, but Fv/Fm decreased significantly, suggesting that Cd induced oxidative stress and photoinhibition. However, EGCG mitigated the adverse effects of Cd-induced phytotoxicity in both the roots and leaves. A decrease in ROS accumulation under EGCG + Cd treatment was mainly attributed to the significant enhancement in antioxidant enzyme activity, flavonoid content, and PHENYLALANINE AMMONIA-LYASE expression in roots. Moreover, EGCG reduced Cd content but increased some essential nutrient contents in tomato plants. Transmission electron microscopy-based observations revealed that EGCG treatment safeguards leaf and root cell ultrastructure under Cd stress. This implies that tomato plants subjected to Cd stress experienced advantageous effects upon receiving EGCG treatment. The present work elucidated critical mechanisms by which EGCG induces tolerance to Cd, thereby providing a basis for future investigations into environmentally sustainable agricultural practices in areas contaminated with heavy metals, for utilizing naturally occurring substances found in plants. • Epigallocatechin-3-gallate (EGCG) is a secondary metabolite and a potent biostimulant. • EGCG increased photochemical efficiency and alleviated Cd-induced oxidative stress. • EGCG increased uptake and translocation of essential plant nutrients under Cd stress. • EGCG enhanced Cd tolerance in roots by enhancing antioxidant enzymes and flavonoids. • EGCG alleviated Cd phytotoxicity in leaves by GSH-dependent detoxification mechanism. [ABSTRACT FROM AUTHOR]

Reactive oxygen species (ROS) are crucial signaling molecules in plants that play multifarious roles in prompt response to environmental stimuli. Despite the classical thoughts that ROS are toxic when accumulate in excess, recent advances in plant ROS signaling biology reveal that ROS participate in biotic and abiotic stress perception, signal integration, and stress-response network activation, hence contributing to plant defense and stress tolerance. ROS production, scavenging and transport are fine-tuned by plant hormones and stress-response signaling pathways. Crucially, the emerging plant hormone melatonin attenuates excessive ROS accumulation under stress, whereas ROS signaling mediates melatonin-induced plant developmental response and stress tolerance. In particular, RESPIRATORY BURST OXIDASE HOMOLOG (RBOH) proteins responsible for apoplastic ROS generation act downstream of melatonin to mediate stress response. In this review, we discuss promising developments in plant ROS signaling and how ROS might mediate melatonin-induced plant resilience to environmental stress. • Reactive oxygen species (ROS) are crucial signaling molecules in plants. • Melatonin is an antioxidant with ROS scavenging potential and a novel plant hormone. • Environmental stress triggers ROS production and melatonin synthesis in plants. • Melatonin can safeguard plants from a wide range of biotic and abiotic stress. • RBOH-derived apoplastic ROS act downstream of melatonin to mediate stress tolerance. [ABSTRACT FROM AUTHOR]

Highlights • COMT1 silencing reduces endogenous melatonin level in tomato leaves. • Melatonin deficiency aggravates high temperature-induced oxidative stress. • COMT1 silencing also impairs redox homeostasis under heat stress. • Exogenous melatonin increases endogenous melatonin in COMT1 -silenced plants. • Melatonin strengthens antioxidant defense and improves thermotolerance. Abstract Melatonin is a natural biostimulating molecule that occurs in a number of plant species. Despite a short history of phytomelatonin research, the critical roles of melatonin in plant stress responses have extensively been documented. However, the majority of such studies constitute 'pharmacological approach' and studies exploring the relevance of endogenous melatonin are scanty. As an attempt to unmask the role of endogenous melatonin in plant thermotolerance, we generated melatonin deficient tomato plants by silencing a gene, CAFFEIC ACID O-METHYLTRANSFERASE 1 (COMT1), involved in melatonin biosynthesis. We first examined the effect of exogenous melatonin and then performed a detailed analysis of the effect of melatonin deficiency on oxidative stress markers, enzymatic and non-enzymatic antioxidants, and redox homeostasis in tomato plants. The results showed that suppression of endogenous melatonin had an opposite effect of exogenous melatonin on tomato tolerance to high temperature stress. Endogenous melatonin deficiency aggravated high temperature-induced oxidative stress as evidenced by increased electrolyte leakage percentage, malondialdehyde concentration, and oxidized and insoluble protein accumulation in tomato leaves. These changes were accompanied with the significant reductions in the activity of ascorbate peroxidase and catalase, two key antioxidant enzymes that play prominent role in plant thermotolerance. Furthermore, silencing of COMT1 altered redox balance as reflected by the significantly decreased ratios of GSH:GSSG and AsA:DHA under high temperature stress. In contrast, exogenous melatonin augmented the endogenous melatonin level in COMT1 -silenced plants and alleviated the heat-induced oxidative stress. Our results suggest that an optimal endogenous melatonin level is critical, not only for its self-antioxidant capacity, but also for maintaining an efficient enzymatic antioxidant system and redox homeostasis under perturbed environmental conditions. [ABSTRACT FROM AUTHOR]

Chromium (Cr) is a toxic heavy metal for both animals and plants. The multifunctional signaling molecule melatonin can confer plant tolerance to heavy metal stress, but the mechanisms remain largely unknown. Here, we unveiled the critical role of the secondary metabolite anthocyanin in melatonin-induced Cr stress tolerance. Excess Cr caused severe phytotoxicity, which was manifested by leaf yellowing, stunted growth, reduced Fv/Fm, and increased accumulation of reactive oxygen species and malondialdehyde in a dose-dependent manner. Interestingly, leaf anthocyanin content increased under Cr stress and was the highest under 100 µM Cr (7.67-fold), while exogenous melatonin further increased anthocyanin accumulation with the highest being with 100 µM melatonin (by 90.72 %). In addition, exogenous melatonin increased endogenous melatonin content and alleviated Cr stress; however, suppression of melatonin accumulation aggravated Cr phytotoxicity and inhibited anthocyanin accumulation by downregulating the transcript levels of key structural genes. Melatonin also reduced the Cr content in roots and leaves. Crucially, suppression of anthocyanin biosynthesis by silencing an anthocyanin biosynthetic gene ANTHOCYANIDIN SYNTHASE (ANS) significantly compromised melatonin-induced anthocyanin accumulation and alleviation of Cr phytotoxicity, suggesting that anthocyanin potentially acts downstream of melatonin and its accumulation is essential for melatonin-induced Cr stress tolerance in tomato plants. [Display omitted] • Cr (VI) stress caused severe phytotoxicity and oxidative stress in tomato plants. • Exogenous melatonin increased both melatonin and anthocyanin content under Cr stress. • Suppression of endogenous melatonin accumulation reduced anthocyanin accumulation. • Melatonin alleviated phytotoxicity and reduced Cr content in tomato roots and leaves. • Silencing of ANTHOCYANIDIN SYNTHASE compromised melatonin-induced Cr stress tolerance. [ABSTRACT FROM AUTHOR]

• Mycorrhiza for vegetable (VT) is the best AMF for watermelon among 8 consortiums. • 300 spores/plant caused the highest dry weight, photosynthetic rate, and dependence. • ABA, IAA and GA 3 concentrations were maximum with VT inoculation at 300 spore/plant. • VT reduced Rhizoctonia solani disease incidence with a 59.7% control effect. • VT alleviated R. solani –induced oxidative stress by improving antioxidant defense. The beneficial effects of arbuscular mycorrhizal fungi (AMF) vary depending on the species of fungi and host plants. In this study, we tested 8 mycorrhizal consortiums to explore the best AMF conformation for watermelon plants. The results showed that colonization with mycorrhiza for vegetable (VT) resulted in the highest biomass accumulation, photosynthetic rate, and dependence in watermelon seedlings. VT increased the photosynthetic rate and dry weight by 68.6% and 63.4%, respectively, compared with non-AMF plants. Dose trial on inoculum density for VT revealed that 300 spores per plant caused the highest dry weight, photosynthetic rate, and dependence. Notably, different inoculum density differentially affected endogenous hormone levels, whereas 300 spores/plant caused the highest accumulation of abscisic acid, indole acetic acid, and gibberellic acid. When VT-inoculated plants were infected with soil-borne fungal pathogen Rhizoctonia solani , they showed only 18.4% disease incidence with a 59.7% control effect compared with the 45.7% disease incidence in control plants without VT inoculation. R. solani decreased the photosynthetic rate, chlorophyll a , chlorophyll b , and carotenoid content by 38.8%, 20.5%, 63.4%, and 33.6%, respectively, while VT colonization significantly improved those indexes. Moreover, VT significantly reduced the relative electrolyte leakage, H 2 O 2 accumulation, and lipid peroxidation, and increased the root activity and antioxidant enzymes activities and their transcripts in roots, suggesting mitigation of oxidative stress caused by R. solani. Our results suggest that VT at optimal dose could enhance watermelon resistance to R. solani and thus can be considered as a biological control agent for R. solani management in cucurbits. [ABSTRACT FROM AUTHOR]

• Low-or high-Fe inhibited cucumber growth and induced chlorosis and oxidative stress • Melatonin alleviated oxidative stress and improved photosynthesis under iron stress • Melatonin promoted the activity and transcripts of antioxidant enzymes • Melatonin stimulated biosynthesis of secondary metabolites under iron stress • Melatonin altered Fe uptake and expression of FRO2 and IRT1 under iron stress Iron is an essential element for plants and animals; however, deficit or excess iron can result in stress on plants. In this study, we unveiled the mechanisms of shoot-based tolerance to iron toxicity by melatonin (N -acetyl-5-methoxytryptamine) in cucumber plants. Both low and high iron (Fe) supply in hydroponics decreased growth and biomass accumulation, induced chlorosis and oxidative stress, and reduced chlorophyll content, photosynthesis rate and transpiration rate in cucumber leaves. Notably, the negative effect of low-Fe was more profound than that of high-Fe treatment. However, exogenous melatonin application alleviated those inhibitions in growth, biochemical and physiological parameters, which entailed an elevation of endogenous melatonin content and a reduction of electrolyte leakage, reactive oxygen species accumulation and lipid peroxidation by improving the activity and transcripts of antioxidant enzymes and secondary metabolism-related enzymes, and concentrations of phenols and flavonoids under low and high iron conditions after melatonin treatment. Analysis of iron content in leaves and roots revealed that melatonin significantly increased the iron content under low-Fe conditions, but it decreased the same under high-Fe conditions. Moreover, melatonin increased the transcript levels of FRO2 and IRT1 under low-Fe, but it decreased those transcripts under high-Fe, suggesting that melatonin plays a dual role in iron uptake under low and high iron conditions. This study expands the stress ameliorative role of melatonin in plants and may have potential implications in agronomic management of crops in low and high iron prone soils. [ABSTRACT FROM AUTHOR]