Tetratricopeptide repeat protein SlREC2 positively regulates cold tolerance in tomato.

Cold stress is a key environmental constraint that dramatically affects the growth, productivity, and quality of tomato (Solanum lycopersicum); however, the underlying molecular mechanisms of cold tolerance remain poorly understood. In this study, we identified REDUCED CHLOROPLAST COVERAGE 2 (SlREC2) encoding a tetratricopeptide repeat protein that positively regulates tomato cold tolerance. Disruption of SlREC2 largely reduced abscisic acid (ABA) levels, photoprotection, and the expression of C-REPEAT BINDING FACTOR (CBF)-pathway genes in tomato plants under cold stress. ABA deficiency in the notabilis (not) mutant, which carries a mutation in 9-CIS-EPOXYCAROTENOID DIOXYGENASE 1 (SlNCED1), strongly inhibited the cold tolerance of SlREC2-silenced plants and empty vector control plants and resulted in a similar phenotype. In addition, foliar application of ABA rescued the cold tolerance of SlREC2-silenced plants, which confirms that SlNCED1-mediated ABA accumulation is required for SlREC2-regulated cold tolerance. Strikingly, SlREC2 physically interacted with ß-RING CAROTENE HYDROXYLASE 1b (SlBCH1b), a key regulatory enzyme in the xanthophyll cycle. Disruption of SlBCH1b severely impaired photoprotection, ABA accumulation, and CBF-pathway gene expression in tomato plants under cold stress. Taken together, this study reveals that SlREC2 interacts with SlBCH1b to enhance cold tolerance in tomato via integration of SlNCED1-mediated ABA accumulation, photoprotection, and the CBF-pathway, thus providing further genetic knowledge for breeding cold-resistant tomato varieties.

The miR164a-NAM3 module confers cold tolerance by inducing ethylene production in tomato.

Because of a high sensitivity to cold, both the yield and quality of tomato (Solanum lycopersicum L.) are severely restricted by cold stress. The NAC transcription factor (TF) family has been characterized as an important player in plant growth, development, and the stress response, but the role of NAC TFs in cold stress and their interaction with other post-transcriptional regulators such as microRNAs in cold tolerance remains elusive. Here, we demonstrated that SlNAM3, the predicted target of Sl-miR164a/b-5p, improved cold tolerance as indicated by a higher maximum quantum efficiency of photosystem II (Fv/Fm), lower relative electrolyte leakage, and less wilting in SlNAM3-overexpression plants compared to wild-type. Further genetic and molecular confirmation revealed that Sl-miR164a/b-5p functioned upstream of SlNAM3 by inhibiting the expression of the latter, thus playing a negative role in cold tolerance. Interestingly, this role is partially mediated by an ethylene-dependent pathway because either Sl-miR164a/b-5p silencing or SlNAM3 overexpression improved cold tolerance in the transgenic lines by promoting ethylene production. Moreover, silencing of the ethylene synthesis genes, SlACS1A, SlACS1B, SlACO1, and SlACO4, resulted in a significant decrease in cold tolerance. Further experiments demonstrated that NAM3 activates SlACS1A, SlACS1B, SlACO1, and SlACO4 transcription by directly binding to their promoters. Taken together, the present study identified the miR164a-NAM3 module conferring cold tolerance in tomato plants via the direct regulation of SlACS1A, SlACS1B, SlACO1, and SlACO4 expression to induce ethylene synthesis.

Melatonin delays ABA-induced leaf senescence via H2 O2 -dependent calcium signalling.

Precocious leaf senescence can reduce crop yield and quality by limiting the growth stage. Melatonin has been shown to delay leaf senescence; however, the underlying mechanism remains obscure. Here, we show that melatonin offsets abscisic acid (ABA) to protect photosystem II and delay the senescence of attached old leaves under the light. Melatonin induced H2 O2 accumulation accompanied by an upregulation of melon respiratory burst oxidase homolog D (CmRBOHD) under ABA-induced stress. Both melatonin and H2 O2 induced the accumulation of cytoplasmic-free Ca2+ ([Ca2+ ]cyt ) in response to ABA, while blocking of Ca2+ influx channels attenuated melatonin- and H2 O2 -induced ABA tolerance. CmRBOHD overexpression induced [Ca2+ ]cyt accumulation and delayed leaf senescence, whereas deletion of Arabidopsis AtRBOHD, a homologous gene of CmRBOHD, compromised the melatonin-induced [Ca2+ ]cyt accumulation and delay of leaf senescence in Arabidopsis under ABA stress. Furthermore, melatonin, H2 O2  and Ca2+ attenuated ABA-induced K+ efflux and subsequent cell death. CmRBOHD overexpression and AtRBOHD deletion alleviated and aggravated the ABA-induced K+ efflux, respectively. Taken together, our study unveils a new mechanism by which melatonin offsets ABA action to delay leaf senescence via RBOHD-dependent H2 O2 production that triggers [Ca2+ ]cyt accumulation and subsequently inhibits K+ efflux and delays cell death/leaf senescence in response to ABA.

Melatonin mediates elevated carbon dioxide-induced photosynthesis and thermotolerance in tomato.

Increasing carbon dioxide (CO2 ) promotes photosynthesis and mitigates heat stress-induced deleterious effects on plants, but the regulatory mechanisms remain largely unknown. Here, we found that tomato (Solanum lycopersicum L.) plants treated with high atmospheric CO2 concentrations (600, 800, and 1000 µmol mol-1 ) accumulated increased levels of melatonin (N-acetyl-5-methoxy tryptamine) in their leaves and this response is conserved across many plant species, including Arabidopsis, rice, wheat, mustard, cucumber, watermelon, melon, and hot pepper. Elevated CO2 (eCO2 ; 800 µmol mol-1 ) caused a 6.8-fold increase in leaf melatonin content, and eCO2 -induced melatonin biosynthesis preferentially occurred through chloroplast biosynthetic pathways in tomato plants. Crucially, manipulation of endogenous melatonin levels by genetic means affected the eCO2 -induced accumulation of sugar and starch in tomato leaves. Furthermore, net photosynthetic rate, maximum photochemical efficiency of photosystem II, and transcript levels of chloroplast- and nuclear-encoded photosynthetic genes, such as rbcL, rbcS, rbcA, psaD, petB, and atpA, significantly increased in COMT1 overexpressing (COMT1-OE) tomato plants, but not in melatonin-deficient comt1 mutants at eCO2 conditions. While eCO2 enhanced plant tolerance to heat stress (42°C) in wild-type and COMT1-OE, melatonin deficiency compromised eCO2 -induced thermotolerance in comt1 plants. The expression of heat shock proteins genes increased in COMT1-OE but not in comt1 plants in response to eCO2 under heat stress. Further analysis revealed that eCO2 -induced thermotolerance was closely linked to the melatonin-dependent regulation of reactive oxygen species, redox homeostasis, cellular protein protection, and phytohormone metabolism. This study unveiled a crucial mechanism of elevated CO2 -induced thermotolerance in which melatonin acts as an essential endogenous signaling molecule in tomato plants.

Glutathione-dependent redox homeostasis is critical for chlorothalonil detoxification in tomato leaves.

Glutathione plays a critical role in plant growth, development and response to stress. It is a major cellular antioxidant and is involved in the detoxification of xenobiotics in many organisms, including plants. However, the role of glutathione-dependent redox homeostasis and associated molecular mechanisms regulating the antioxidant system and pesticide metabolism remains unclear. In this study, endogenous glutathione levels were manipulated by pharmacological treatments with glutathione synthesis inhibitors and oxidized glutathione. The application of oxidized glutathione enriched the cellular oxidation state, reduced the activity and transcript levels of antioxidant enzymes, upregulated the expression level of nitric oxide and Ca2+ related genes and the content, and increased the residue of chlorothalonil in tomato leaves. Further experiments confirmed that glutathione-induced redox homeostasis is critical for the reduction of pesticide residues. RNA sequencing analysis revealed that miRNA156 and miRNA169 that target transcription factor SQUAMOSA-Promoter Binding Proteins (SBP) and NUCLEAR FACTOR Y (NFY) potentially participate in glutathione-mediated pesticide degradation in tomato plants. Our study provides important clues for further dissection of pesticide degradation mechanisms via miRNAs in plants.

Light-dependent activation of HY5 promotes mycorrhizal symbiosis in tomato by systemically regulating strigolactone biosynthesis.

Light quality affects mutualisms between plant roots and arbuscular mycorrhizal fungi (AMFs), which modify nutrient acquisition in plants. However, the mechanisms by which light systemically modulates root colonization by AMFs and phosphate uptake in roots remain unclear. We used a range of approaches, including grafting techniques, protein immunoblot analysis, electrophoretic mobility shift assay, chromatin immunoprecipitation, and dual-luciferase assays, to unveil the molecular basis of light signal transmission from shoot to root that mediates arbuscule development and phosphate uptake in tomato. The results show that shoot phytochrome B (phyB) triggers shoot-derived mobile ELONGATED HYPOCOTYL5 (HY5) protein accumulation in roots, and HY5 further positively regulates transcription of strigolactone (SL) synthetic genes, thus forming a shoot phyB-dependent systemic signaling pathway that regulates the synthesis and accumulation of SLs in roots. Further experiments with carotenoid cleavage dioxygenase 7 mutants and supplementary red light confirm that SLs are indispensable in the red-light-regulated mycorrhizal symbiosis in roots. Our results reveal a phyB-HY5-SLs systemic signaling cascade that facilitates mycorrhizal symbiosis and phosphate utilization in plants. The findings provide new prospects for the potential application of AMFs and light manipulation to effectively improve nutrient utilization and minimize the use of chemical fertilizers and associated pollution.

SlFHY3 and SlHY5 act compliantly to enhance cold tolerance through the integration of myo-inositol and light signaling in tomato.

Plants have evolved sophisticated regulatory networks to cope with dynamically changing light and temperature environments during day-night and seasonal cycles. However, the integration mechanisms of light and low temperature remain largely unclear. Here, we show that low red : far-red ratio (LR : FR) induces FAR-RED ELONGATED HYPOCOTYL3 (SlFHY3) transcription under cold stress in tomato (Solanum lycopersicum). Reverse genetic approaches revealed that knocking out SlFHY3 decreases myo-inositol accumulation and increases cold susceptibility, whereas overexpressing SlFHY3 induces myo-inositol accumulation and enhances cold tolerance in tomato plants. SlFHY3 physically interacts with ELONGATED HYPOCOTYL5 (SlHY5) to promote the transcriptional activity of SlHY5 on MYO-INOSITOL-1-PHOSPHATE SYNTHASE 3 (SlMIPS3) and induce myo-inositol accumulation in tomato plants under cold stress. Disruption of SlHY5 and SlMIPS3 largely suppresses the cold tolerance of SlFHY3-overexpressing plants and myo-inositol accumulation in tomato. Furthermore, silencing of SlMIPS3 drastically reduces myo-inositol accumulation and compromises LR : FR-induced cold tolerance in tomato. Together, our results reveal a crucial role of SlFHY3 in LR : FR-induced cold tolerance in tomato and unravel a novel regulatory mechanism whereby plants integrate dynamic environmental light signals and internal cues (inositol biosynthesis) to induce and control cold tolerance in tomato plants.

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.

Unveiling Molecular Mechanisms of Nitric Oxide-Induced Low-Temperature Tolerance in Cucumber by Transcriptome Profiling.

Cucumber (Cucumis sativus L.) is one of the most popular cultivated vegetable crops but it is intrinsically sensitive to cold stress due to its thermophilic nature. To explore the molecular mechanism of plant response to low temperature (LT) and the mitigation effect of exogenous nitric oxide (NO) on LT stress in cucumber, transcriptome changes in cucumber leaves were compared. The results showed that LT stress regulated the transcript level of genes related to the cell cycle, photosynthesis, flavonoid accumulation, lignin synthesis, active gibberellin (GA), phenylalanine metabolism, phytohormone ethylene and salicylic acid (SA) signaling in cucumber seedlings. Exogenous NO improved the LT tolerance of cucumber as reflected by increased maximum photochemical efficiency (Fv/Fm) and decreased chilling damage index (CI), electrolyte leakage and malondialdehyde (MDA) content, and altered transcript levels of genes related to phenylalanine metabolism, lignin synthesis, plant hormone (SA and ethylene) signal transduction, and cell cycle. In addition, we found four differentially expressed transcription factors (MYB63, WRKY21, HD-ZIP, and b-ZIP) and their target genes such as the light-harvesting complex I chlorophyll a/b binding protein 1 gene (LHCA1), light-harvesting complex II chlorophyll a/b binding protein 1, 3, and 5 genes (LHCB1, LHCB3, and LHCB5), chalcone synthase gene (CSH), ethylene-insensitive protein 3 gene (EIN3), peroxidase, phenylalanine ammonia-lyase gene (PAL), DNA replication licensing factor gene (MCM5 and MCM6), gibberellin 3 beta-dioxygenase gene (GA3ox), and regulatory protein gene (NPRI), which are potentially associated with plant responses to NO and LT stress. Notably, HD-ZIP and b-ZIP specifically responded to exogenous NO under LT stress. Taken together, these results demonstrate that cucumber seedlings respond to LT stress and exogenous NO by modulating the transcription of some key transcription factors and their downstream genes, thereby regulating photosynthesis, lignin synthesis, plant hormone signal transduction, phenylalanine metabolism, cell cycle, and GA synthesis. Our study unveiled potential molecular mechanisms of plant response to LT stress and indicated the possibility of NO application in cucumber production under LT stress, particularly in winter and early spring.

Auxin is involved in arbuscular mycorrhizal fungi-promoted tomato growth and NADP-malic enzymes expression in continuous cropping substrates.

BACKGROUND: Despite significant limitations of growth medium reuse, a large amount of organic substrate is reused in soilless cultivation of horticultural crops in China. Arbuscular mycorrhizal fungi (AMF) can promote nutrient absorption and improve plant tolerance to biotic and abiotic stresses. However, the mechanisms governing the effects of AMF on crop growth in organic continuous cropping substrates have not been elucidated. RESULTS: In this study, we showed that the inoculation of AMF in continuous cropping substrates promoted growth and root development, and increased the root and NADP-malic enzyme (NADP-ME) activity of tomato seedlings. Root transcriptome analysis demonstrated that the plant hormone signal transduction pathway was highly enriched, and 109 genes that positively correlated with the AMF-inoculated plant phenotype were obtained by gene set enrichment analysis (GSEA), which identified 9 genes related to indole acetic acid (IAA). Importantly, the levels of endogenous IAA in tomato seedlings significantly increased after AMF inoculation. Furthermore, the application of AMF significantly increased the expression levels of NADP-ME1 and NADP-ME2, as well as the activity of NADP-ME, and enhanced the root activity of tomato seedlings in comparison to that observed without inoculation of AMF. However, these effects were blocked in plants treated with 2,3,5-triiodobenzoic acid (TIBA), a polar transport inhibitor of IAA. CONCLUSIONS: These results suggest that IAA mediates the AMF-promoted tomato growth and expression of NADP-MEs in continuous cropping substrates. The study provides convincing evidence for the reuse of continuous cropping substrates by adding AMF as an amendment.

Nitrogen forms and metabolism affect plant defence to foliar and root pathogens in tomato.

Nitrogen (N) influences a myriad of physiological processes while its effects on plant defences and the underlying mechanisms are largely unknown. Here, the interaction between tomato and pathogens was examined under four N regimes (sole NO3- or mixed NO3- /NH4+ of total 1 and 7 mM N, denoting low and high N regimes, respectively) followed by inoculation with two bacterial pathogens, Pseudomonas syringae and Ralstonia solanacearum. Tomato immunity against both pathogens was generally higher under low N as well as NO3- as the sole N source. The disease susceptibility was reduced by silencing N metabolism genes such as NR, NiR and Fd-GOGAT, while increased in NiR1-overexpressed plants. Further studies demonstrated that the N-modulated defence was dependent on the salicylic acid (SA) defence pathway. Low N as well as the silencing of N metabolism genes increased the SA levels and transcripts of its maker genes, and low N-enhanced defence was blocked in NahG transgenic tomato plants that do not accumulate SA, while exogenous SA application attenuated the susceptibility of OE-NiR1. The study provides insights into the mechanisms of how nitrogen fertilization and metabolism affect plant immunity in tomato, which might be useful for designing effective agronomic strategies for the management of N supply.

Overexpression of tomato RING E3 ubiquitin ligase gene SlRING1 confers cadmium tolerance by attenuating cadmium accumulation and oxidative stress.

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.

The SlWRKY81 transcription factor inhibits stomatal closure by attenuating nitric oxide accumulation in the guard cells of tomato under drought.

The WRKY transcription factors (TFs) play multifaceted roles in plant growth, development, and stress response. Previously, we found that SlWRKY81 negatively regulates tomato tolerance to drought; however, the mechanisms of stomatal regulation in response to drought remain largely unclear. Here, we showed that drought-induced upregulation in the SlWRKY81 transcripts induced photoinhibition and reduced the net photosynthetic rate in tomato leaves. However, silencing SlWRKY81 alleviated those inhibitions and minimized the drought-induced damage. A time-course of water loss showed that SlWRKY81 silencing significantly and consistently reduced leaf water loss, suggesting a role for SlWRKY81 in stomatal movement. Further analysis using light microscopy revealed that SlWRKY81 silencing significantly decreased stomatal aperture and increased the ratio of length to width of stomata under drought. Both biochemical assay and confocal laser scanning microscopy demonstrated that drought-induced upregulation in SlWRKY81 expression inhibited the nitric oxide (NO) accumulation in the guard cells, which was attributed to the simultaneous declines in the activity of nitrate reductase (NR) and NR expression in tomato leaves. The inspection of 3-kb sequences upstream of the predicted transcriptional start site of the NR identified three copies of the core W-box (TTGACC/T) sequence in the promoter region, indicating possible targets of SlWRKY81. Taken together, these data suggest that SlWRKY81 potentially represses NR transcription and thus reduces NO accumulation to attenuate stomatal closure and subsequent drought tolerance. These findings provide an improved understanding of the mechanism of WRKY-induced regulation of stomatal closure, which can be exploited in the future to enhance drought tolerance in crops.

Dynamics of cell wall structure and related genomic resources for drought tolerance in rice.

KEY MESSAGE: Cell wall plasticity plays a very crucial role in vegetative and reproductive development of rice under drought and is a highly potential trait for improving rice yield under drought. Drought is a major constraint in rice (Oryza sativa L.) cultivation severely affecting all developmental stages, with the reproductive stage being the most sensitive. Rice plants employ multiple strategies to cope with drought, in which modification in cell wall dynamics plays a crucial role. Over the years, significant progress has been made in discovering the cell wall-specific genomic resources related to drought tolerance at vegetative and reproductive stages of rice. However, questions remain about how the drought-induced changes in cell wall made by these genomic resources potentially influence the vegetative and reproductive development of rice. The possibly major candidate genes underlying the function of quantitative trait loci directly or indirectly associated with the cell wall plasticization-mediated drought tolerance of rice might have a huge promise in dissecting the putative genomic regions associated with cell wall plasticity under drought. Furthermore, engineering the drought tolerance of rice using cell wall-related genes from resurrection plants may have huge prospects for rice yield improvement. Here, we review the comprehensive multidisciplinary analyses to unravel different components and mechanisms involved in drought-induced cell wall plasticity at vegetative and reproductive stages that could be targeted for improving rice yield under drought.

Mechanisms of elevated CO2-induced thermotolerance in plants: the role of phytohormones.

Rising atmospheric CO2 is a key driver of climate change, intensifying drastic changes in meteorological parameters. Plants can sense and respond to changes in environmental parameters including atmospheric CO2 and temperatures. High temperatures beyond the physiological threshold can significantly affect plant growth and development and thus attenuate crop productivity. However, elevated atmospheric CO2 can mitigate the deleterious effects of heat stress on plants. Despite a large body of literature supporting the positive impact of elevated CO2 on thermotolerance, the underlying biological mechanisms and precise molecular pathways that lead to enhanced tolerance to heat stress remain largely unclear. Under heat stress, elevated CO2-induced expression of respiratory burst oxidase homologs (RBOHs) and reactive oxygen species (ROS) signaling play a critical role in stomatal movement, which optimizes gas exchange to enhance photosynthesis and water use efficiency. Notably, elevated CO2 also fortifies antioxidant defense and redox homeostasis to alleviate heat-induced oxidative damage. Both hormone-dependent and independent pathways have been shown to mediate high CO2-induced thermotolerance. The activation of heat-shock factors and subsequent expression of heat-shock proteins are thought to be the essential mechanism downstream of hormone and ROS signaling. Here we review the role of phytohormones in plant response to high atmospheric CO2 and temperatures. We also discuss the potential mechanisms of elevated CO2-induced thermotolerance by focusing on several key phytohormones such as ethylene. Finally, we address some limitations of our current understanding and the need for further research to unveil the yet-unknown crosstalk between plant hormones in mediating high CO2-induced thermotolerance in plants.

Phytonanotechnology applications in modern agriculture.

With the rapidly changing global climate, the agricultural systems are confronted with more unpredictable and harsh environmental conditions than before which lead to compromised food production. Thus, to ensure safer and sustainable crop production, the use of advanced nanotechnological approaches in plants (phytonanotechnology) is of great significance. In this review, we summarize recent advances in phytonanotechnology in agricultural systems that can assist to meet ever-growing demands of food sustainability. The application of phytonanotechnology can change traditional agricultural systems, allowing the target-specific delivery of biomolecules (such as nucleotides and proteins) and cater the organized release of agrochemicals (such as pesticides and fertilizers). An amended comprehension of the communications between crops and nanoparticles (NPs) can improve the production of crops by enhancing tolerance towards environmental stresses and optimizing the utilization of nutrients. Besides, approaches like nanoliposomes, nanoemulsions, edible coatings, and other kinds of NPs offer numerous selections in the postharvest preservation of crops for minimizing food spoilage and thus establishing phtonanotechnology as a sustainable tool to architect modern agricultural practices.

Ozone Induced Stomatal Regulations, MAPK and Phytohormone Signaling in Plants.

Ozone (O3) is a gaseous environmental pollutant that can enter leaves through stomatal pores and cause damage to foliage. It can induce oxidative stress through the generation of reactive oxygen species (ROS) like hydrogen peroxide (H2O2) that can actively participate in stomatal closing or opening in plants. A number of phytohormones, including abscisic acid (ABA), ethylene (ET), salicylic acid (SA), and jasmonic acid (JA) are involved in stomatal regulation in plants. The effects of ozone on these phytohormones' ability to regulate the guard cells of stomata have been little studied, however, and the goal of this paper is to explore and understand the effects of ozone on stomatal regulation through guard cell signaling by phytohormones. In this review, we updated the existing knowledge by considering several physiological mechanisms related to stomatal regulation after response to ozone. The collected information should deepen our understanding of the molecular pathways associated with response to ozone stress, in particular, how it influences stomatal regulation, mitogen-activated protein kinase (MAPK) activity, and phytohormone signaling. After summarizing the findings and noting the gaps in the literature, we present some ideas for future research on ozone stress in plants.

The HY5 and MYB15 transcription factors positively regulate cold tolerance in tomato via the CBF pathway.

The induction of C-repeat binding factors (CBFs) is crucial for plant survival at low temperatures. Therefore, understanding the mechanisms that regulate CBF transcription is vital for the future development of crops with increased cold tolerance. Here, we provide evidence for the existence of a LONG HYPOCOTYL 5 (HY5)-MYB15-CBFs transcriptional cascade that plays a crucial role in the cold response in tomato. The exposure of tomato plants to cold (4°C) increased the levels of HY5, MYB15 and CBFs transcripts. Moreover, mutations in HY5 or MYB15 decreased the levels of CBF transcripts. In contrast, overexpression of HY5 or MYB15 increased CBF transcript abundance. Crucially, the HY5 transcription factor activated the expression of MYB15 by directly binding to the promoter region, while both HY5 and MYB15 activated the expression of CBF1, CBF2 and CBF3. Taken together, these data show that HY5 can directly regulate CBF transcript levels, and also influence CBF expression indirectly via MYB15. The coordinated action of HY5 and MYB15 allows precise regulation of CBF expression and subsequent cold tolerance. These findings provide an improved understanding of the molecular mechanisms affording transcriptional regulation of CBFs, which can be exploited in the future to enhance cold tolerance in crops.

Role of Melatonin in Arbuscular Mycorrhizal Fungi-Induced Resistance to Fusarium Wilt in Cucumber.

Melatonin is a multifunctional molecule that confers tolerance to a number of biotic and abiotic stresses in plants. However, the role of melatonin in plant response to Fusarium oxysporum and the interaction with arbuscular mycorrhizal fungi (AMF) remain unclear. Here we show that exogenous melatonin application promoted the AMF colonization rate in cucumber roots, which potentially suppressed Fusarium wilt as evidenced by a decreased disease index and an increased control effect. Leaf gas exchange analysis revealed that Fusarium inoculation significantly decreased the net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentrations (Ci), and transpiration rate (Tr). Intriguingly, either melatonin application or AMF inoculation significantly increased the Pn, Gs, Tr, and dry biomass, and their combined treatment showed a more profound effect under Fusarium stress. Further analysis showed that Fusarium induced oxidative stress as evidenced by increased lipid peroxidation and electrolyte leakage. Conversely, either melatonin or AMF drastically attenuated the levels of malondialdehyde, H2O2, and electrolyte leakage in Fusarium-inoculated plants, and their combined treatment caused a further decrease. Fusarium inoculation decreased the activity and transcripts of superoxide dismutase and ascorbate peroxidase, and the content of glutathione and proline. Besides, the activity and transcripts of peroxidase and catalase, the content of phenols and flavonoids increased after Fusarium infection. Importantly, melatonin and/or AMF significantly increased those parameters with the greatest effect with their combined treatment under Fusarium stress. Our results suggest that a positive collaboration between melatonin and AMF enhances resistance to Fusarium wilt in cucumber plants.

Melatonin alleviates nickel phytotoxicity by improving photosynthesis, secondary metabolism and oxidative stress tolerance in tomato seedlings.

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 (H2O2) 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 H2O2 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.