Inhibition of biphasic ethylene production enhances tolerance to abiotic stress by reducing the accumulation of reactive oxygen species in Nicotiana tabacum

Physiological and metabolomic responses of the ethylene insensitive squash mutant etr2b to drought

The squash gain-of-function mutant etr2b disrupts the ethylene-binding domain of ethylene receptor CpETR2B, conferring partial ethylene insensitivity, changes in flower and fruit development, and enhanced salt tolerance. In this paper, we found that etr2b also confers a growth advantage as well as a physiological and metabolomic response that make the mutant better adapted to drought. Mutant plants had a higher root and leaf biomass than WT under both well-watered and drought conditions, but the reduction in growth parameters in response to drought was similar in WT and etr2b. Water deficit reduced all gas-exchange parameters in both WT and etr2b, but under moderate drought the mutant increased photosynthesis rate in comparison with control conditions, and showed a higher leaf CO2 concentration, transpiration rate, and stomata conductance than WT. The response of etr2b to drought indicates that ethylene is a negative regulator of plant growth under both control and drought. Since etr2b increased ABA content in well-watered plant, but prevented the induction of ABA production in response to drought, it is likely that the etr2b response under drought is not mediated by ABA. A 1H NMR metabolomic analysis revealed that etr2b enhances the accumulation of osmolytes (soluble sugars and trigonelline), unsaturated and polyunsaturated fatty acids, and phenolic compounds under drought, concomitantly with a reduction of malic- and fumaric-acid. The role of CpETR2B and ethylene in the regulation of these drought-protective metabolites is discussed.

Loss of ACO4 in petunia improves abiotic stress tolerance by reducing the deleterious effects of stress-induced ethylene

To investigate the role of ethylene (ET) in abiotic stress tolerance in petunia cv. 'Mirage Rose', petunia plants in which the ET biosynthesis gene 1-aminocyclopropane-1-carboxylic acid oxidase 4 (ACO4) was knocked out (phaco4 mutants) and wild-type (WT) plants were exposed to heat and drought conditions. Loss of function of ACO4 significantly delayed leaf senescence and chlorosis under heat and drought stress by maintaining the SPAD values and the relative water content, indicating a greater stress tolerance of phaco4 mutants than that of WT plants. This tolerance was related to the lower ET and reactive oxygen species levels in the mutants than in WT plants. Furthermore, the stress-induced expression of genes related to ET signal transduction, antioxidant and proline activities, heat response, and biosynthesis of abscisic acid was higher in the mutants than in WT plants, indicating a greater stress tolerance in the former than in the latter. These results demonstrate the deleterious effects of stress-induced ET on plant growth and provide a better physiological and molecular understanding of the role of stress ET in the abiotic stress response of petunia. Because the loss of function of ACO4 in petunia improved stress tolerance, we suggest that ACO4 plays a vital role in stress-induced leaf senescence and acts as a negative regulator of abiotic stress tolerance.

The local and systemic accumulation of ethylene determines the rapid defence responses induced by flg22 in tomato (Solanum lycopersicum L.)

Plant defence responses induced by the bacterial elicitor flg22 are highly dependent on phytohormones, including gaseous ethylene (ET). While the regulatory role of ET in local defence responses to flg22 exposure has been demonstrated, its contribution to the induction of systemic responses is not clearly understood. For this consideration, we examined the effects of different ET modulators on the flg22-induced local and systemic defence progression. In our experiments, ET biosynthesis inhibitor aminoethoxyvinyl glycine (AVG) or ET receptor blocker silver thiosulphate (STS) were applied 1 h before flg22 treatments and 1 h later the rapid local and systemic responses were detected in the leaves of intact tomato plants (Solanum lycopersicum L.). Based on our results, AVG not only diminished the flg22-induced ET accumulation locally, but also in the younger leaves confirming the role of ET in the whole-plant expanding defence progression. This increase in ET emission was accompanied by increased local expression of SlACO1, which was reduced by AVG and STS. Local ET biosynthesis upon flg22 treatment was shown to positively regulate local and systemic superoxide (O2.-) and hydrogen peroxide (H2O2) production, which in turn could contribute to ET accumulation in younger leaves. Confirming the role of ET in flg22-induced rapid defence responses, application of AVG reduced local and systemic ET, O2.- and H2O2 production, whereas STS reduced it primarily in the younger leaves. Interestingly, in addition to flg22, AVG and STS induced stomatal closure alone at whole-plant level, however in the case of combined treatments together with flg22 both ET modulators reduced the rate of stomatal closure in the older- and younger leaves as well. These results demonstrate that both local and systemic ET production in sufficient amounts and active ET signalling are essential for the development of flg22-induced rapid local and systemic defence responses.

Abiotic Stress in Crop Production

The vast majority of agricultural land undergoes abiotic stress that can significantly reduce agricultural yields. Understanding the mechanisms of plant defenses against stresses and putting this knowledge into practice is, therefore, an integral part of sustainable agriculture. In this review, we focus on current findings in plant resistance to four cardinal abiotic stressors—drought, heat, salinity, and low temperatures. Apart from the description of the newly discovered mechanisms of signaling and resistance to abiotic stress, this review also focuses on the importance of primary and secondary metabolites, including carbohydrates, amino acids, phenolics, and phytohormones. A meta-analysis of transcriptomic studies concerning the model plant Arabidopsis demonstrates the long-observed phenomenon that abiotic stressors induce different signals and effects at the level of gene expression, but genes whose regulation is similar under most stressors can still be traced. The analysis further reveals the transcriptional modulation of Golgi-targeted proteins in response to heat stress. Our analysis also highlights several genes that are similarly regulated under all stress conditions. These genes support the central role of phytohormones in the abiotic stress response, and the importance of some of these in plant resistance has not yet been studied. Finally, this review provides information about the response to abiotic stress in major European crop plants—wheat, sugar beet, maize, potatoes, barley, sunflowers, grapes, rapeseed, tomatoes, and apples.

Redox Signaling in Plant Heat Stress Response

The increase in environmental temperature due to global warming is a critical threat to plant growth and productivity. Heat stress can cause impairment in several biochemical and physiological processes. Plants sense and respond to this adverse environmental condition by activating a plethora of defense systems. Among them, the heat stress response (HSR) involves an intricate network of heat shock factors (HSFs) and heat shock proteins (HSPs). However, a growing amount of evidence suggests that reactive oxygen species (ROS), besides potentially being responsible for cellular oxidative damage, can act as signal molecules in HSR, leading to adaptative responses. The role of ROS as toxic or signal molecules depends on the fine balance between their production and scavenging. Enzymatic and non-enzymatic antioxidants represent the first line of defense against oxidative damage and their activity is critical to maintaining an optimal redox environment. However, the HS-dependent ROS burst temporarily oxidizes the cellular environment, triggering redox-dependent signaling cascades. This review provides an overview of the redox-activated mechanisms that participate in the HSR.

ACC deaminase-producing endophytic fungal consortia promotes drought stress tolerance in M.oleifera by mitigating ethylene and H2O2

INTRODUCTION

Drought has become more prevalent due to dramatic climate change worldwide. Consequently, the most compatible fungal communities collaborate to boost plant development and ecophysiological responses under environmental constraints. However, little is known about the specific interactions between non-host plants and endophytic fungal symbionts that produce growth-promoting and stress-alleviating hormones during water deficits.

METHODS

The current research was rationalized and aimed at exploring the influence of the newly isolated, drought-resistant, ACC deaminase enzyme-producing endophytic fungi Trichoderma gamsii (TP), Fusarium proliferatum (TR), and its consortium (TP+TR) from a xerophytic plant Carthamus oxycantha L. on Moringa oleifera L. grown under water deficit induced by PEG-8000 (8% osmoticum solution).

RESULTS

The current findings revealed that the co-inoculation promoted a significant enhancement in growth traits such as dry weight (217%), fresh weight (123%), root length (65%), shoot length (53%), carotenoids (87%), and chlorophyll content (76%) in comparison to control plants under water deficit. Total soluble sugars (0.56%), proteins (132%), lipids (43%), flavonoids (52%), phenols (34%), proline (55%), GA₃ (86%), IAA (35%), AsA (170%), SA (87%), were also induced, while H₂O₂ (-45%), ABA (-60%) and ACC level (-77%) was decreased by co-inoculation of TP and TR in M. oleifera plants, compared with the non-inoculated plants under water deficit. The co-inoculum (TP+TR) also induced the antioxidant potential and enzyme activities POX (325%), CAT activity (166%), and AsA (21%), along with a lesser decrease (-2%) in water potential in M. oleifera plants with co-inoculation under water deficit compared with non-inoculated control. The molecular analysis for gene expression unraveled the reduced expression of ethylene biosynthesis and signaling-related genes up to an optimal level, with an induction of antioxidant enzymatic genes by endophytic co-inoculation in M. oleifera plants under water deficit, suggesting their role in drought stress tolerance as an essential regulatory function.

CONCLUSION

The finding may alert scientists to consider the impacts of optimal reduction of ethylene and induction of antioxidant potential on drought stress tolerance in M. oleifera. Hence, the present study supports the use of compatible endophytic fungi to build a bipartite mutualistic symbiosis in M. oleifera non-host plants to mitigate the negative impacts of water scarcity in arid regions throughout the world.

Hyperhydricity in Plant Tissue Culture

Hyperhydricity is the most common physiological disorder in in vitro plant cultivation. It is characterized by certain anatomical, morphological, physiological, and metabolic disturbances. Hyperhydricity significantly complicates the use of cell and tissue culture in research, reduces the efficiency of clonal micropropagation and the quality of seedlings, prevents the adaptation of plants in vivo, and can lead to significant losses of plant material. This review considers the main symptoms and causes of hyperhydricity, such as oxidative stress, impaired nitrogen metabolism, and the imbalance of endogenous hormones. The main factors influencing the level of hyperhydricity of plants in vitro are the mineral and hormonal composition of a medium and cultivation conditions, in particular the aeration of cultivation vessels. Based on these factors, various approaches are proposed to eliminate hyperhydricity, such as varying the mineral and hormonal composition of the medium, the use of exogenous additives, aeration systems, and specific lighting. However, not all methods used are universal in eliminating the symptoms of hyperhydricity. Therefore, the study of hyperhydricity requires a comprehensive approach, and measures aimed at its elimination should be complex and species-specific.

Ethylene involvement in the regulation of heat stress tolerance in plants

Because of the rise in global temperature, heat stress has become a major concern for crop production. Heat stress deteriorates plant productivity and alters phenological and physiological responses that aid in precise monitoring and sensing of mild-to-severe transient heat stress. Plants have evolved several sophisticated mechanisms including hormone-signaling pathways to sense heat stimuli and acquire heat stress tolerance. In response to heat stress, ethylene, a gaseous hormone, is produced which is indispensable for plant growth and development and tolerance to various abiotic stresses including heat stress. The manipulation of ethylene in developing heat stress tolerance targeting ethylene biosynthesis and signaling pathways has brought promising out comes. Conversely increased ethylene biosynthesis and signaling seem to exhibit inhibitory effects in plant growth responses from primitive to maturity stages. This review mainly focuses on the recent studies of ethylene involvement in plant responses to heat stress and its functional regulation, and molecular mechanism underlying the plant responses in the mitigation of heat-induced damages. Furthermore, this review also describes the crosstalk between ethylene and other signaling molecules under heat stress and approaches to improve heat stress tolerance in plants.

Role of Ethylene Biosynthesis Genes in the Regulation of Salt Stress and Drought Stress Tolerance in Petunia

Ethylene plays a critical signaling role in the abiotic stress tolerance mechanism. However, the role of ethylene in regulating abiotic stress tolerance in petunia has not been well-investigated, and the underlying molecular mechanism by which ethylene regulates abiotic stress tolerance is still unknown. Therefore, we examined the involvement of ethylene in salt and drought stress tolerance of petunia using the petunia wild type cv. "Merage Rose" and the ethylene biosynthesis genes (PhACO1 and PhACO3)-edited mutants (phaco1 and phaco3). Here, we discovered that editing PhACO1 and PhACO3 reduced ethylene production in the mutants, and mutants were more sensitive to salt and drought stress than the wild type (WT). This was proven by the better outcomes of plant growth and physiological parameters and ion homeostasis in WT over the mutants. Molecular analysis revealed that the expression levels of the genes associated with antioxidant, proline synthesis, ABA synthesis and signaling, and ethylene signaling differed significantly between the WT and mutants, indicating the role of ethylene in the transcriptional regulation of the genes associated with abiotic stress tolerance. This study highlights the involvement of ethylene in abiotic stress adaptation and provides a physiological and molecular understanding of the role of ethylene in abiotic stress response in petunia. Furthermore, the finding alerts researchers to consider the negative effects of ethylene reduction on abiotic stress tolerance when editing the ethylene biosynthesis genes to improve the postharvest quality of horticultural crops.

Phytosphingosine induces systemic acquired resistance through activation of sphingosine kinase

Phytosphingosine (PHS) is a naturally occurring bioactive sphingolipid molecule. Intermediates such as sphingolipid long-chain bases (LCBs) in sphingolipid biosynthesis have been shown to have important roles as signaling molecules. PHS treatment caused rapid cell damage and upregulated the generation of reactive oxygen species (ROS) and ethylene in tobacco plants. These events were followed by the induction of sphingosine kinase (SphK) in a biphasic manner, which metabolized PHS to phytosphingosine-1-phosphate (PHS-1-P). On the other hand, a PHS treatment with a virulent pathogen, Phytophthora parasitica var. nicotianae (Ppn), alleviated the pathogen-induced cell damage and reduced the growth of Ppn. A Ppn infection increased the PHS and PHS-1-P levels significantly in the upper part of the leaves at the infection site at the later stage. In addition, Ppn increased the transcription levels of serine palmitoyltransferase (LCB1 and LCB2) for sphingolipid biosynthesis at the later stage, which was enhanced further by PHS. Moreover, the PHS treatment increased the transcription and activity of SphK, which was accompanied by prominent increases in the transcription levels of ROS-detoxifying enzymes and PR proteins in the later phase of the pathogen infection. Overall, the PHS-induced resistant effects were prominent during the necrotic stage of this hemibiotrophic infection, indicating that it is more beneficial for inhibiting the pathogenicity on necrotic cell death. Phosphorylated LCBs reduced the pathogen-induced cell damage significantly in this stage. These results suggest that the selective channeling of sphingolipids into phosphorylated forms has a pro-survival effect on plant immunity.

Plant defence mechanisms against mycotoxin Fumonisin B1

Fumonisin B1 (FB1) is the most harmful mycotoxin which prevails in several crops and affects the growth and yield as well. Hence, keeping the alarming consequences of FB1 under consideration, there is still a need to seek other more reliable approaches and scientific knowledge for FB1-induced cell death and a comprehensive understanding of the mechanisms of plant defence strategies. FB1-induced disturbance in sphingolipid metabolism initiates programmed cell death (PCD) through various modes such as the elevated generation of reactive oxygen species, lipid peroxidation, cytochrome c release from the mitochondria, and activation of specific proteases and nucleases causing DNA fragmentation. There is a close interaction between sphingolipids and defence phytohormones in response to FB1 exposure regulating PCD and defence. In this review, the model plant Arabidopsis and various crops have been presented with different levels of susceptibility and resistivity exposed to various concentration of FB1. In addition to this, regulation of PCD and defence mechanisms have been also demonstrated at the physiological, biochemical and molecular levels to help the understanding of the role and function of FB1-inducible molecules and genes and their expressions in plants against pathogen attacks which could provide molecular and biochemical markers for the detection of toxin exposure.

Salicylic acid application alleviates cadmium accumulation in brown rice by modulating its shoot to grain translocation in rice

Cadmium (Cd) contamination, which poses a serious threat to human health, has been recognized as a major threat to the agricultural system and crop production. Salicylic acid (SA) is a signaling molecule that plays an important role in against Cd toxicity. Previously, we found that spraying rice with SA could reduce the Cd accumulation in rice grains grown in Cd-contaminated soil. In this study, we studied the specific mechanism of SA spray on reducing Cd accumulation in rice grain. The results showed that treatment with SA could alleviate Cd toxicity in rice by increasing the activities of antioxidant enzymes that reduce hydrogen peroxide (H₂O₂) accumulation, but not by changing the pH, or total or available Cd of the soil. The key factor by which SA treatment reduced Cd accumulation in rice grains was by decreasing the Cd content in rice leaves at the flowering stage. This indicated that SA could modulate the Cd accumulation in shoots, reducing the Cd translocation to rice grains. Furthermore, SA could increase the H₂O₂ content, activating the SA-signaling pathway and modulating the expression levels of Cd transporters (OsLCT1 and OsLCD) in rice leaves to increase Cd tolerance and reduce Cd accumulation in the rice grain. Thus, spraying rice with SA may be effective measure to cope with Cd contamination in paddy soils.

The fungal elicitor AsES requires a functional ethylene pathway to activate the innate immunity in strawberry

Acremonium strictum Elicitor Subtilisin (AsES) is a fungal elicitor that activates innate immunity, conferring disease resistance in strawberry (Fragaria × ananassa Duch.), Arabidopsis and other plant species. The aim of the present work was to evaluate the involvement of the ethylene (ET) signalling pathway in AsES-mediated immune response in strawberry. Ethylene production and expression of the genes responsible for ET synthesis, perception and response were measured after AsES treatment. ROS (H₂ O₂ ) accumulation and immunity induced by AsES were studied after ET perception was blocked by 1-methylcyclopropene (1-MCP). Biochemical and molecular results showed that AsES induced a marked increase in local and systemic biosynthesis of ET, both in a biphasic manner. Blocking of ET perception by 1-MCP prior to AsES induction reduced production of ROS (H₂ O₂ ) and prevented AsES from eliciting defence against fungal pathogens having different lifestyles, such as Botrytis cinerea (necrotrophic) and Colletotrichum acutatum (hemibiotrophic). These findings contribute to elucidate the mode of action of the novel elicitor subtilase, AsES, specifically regarding the role of ET signalling in the activation of plant innate immunity, in addition to the multitude of processes regulated by ET in plants.

The Anti-Senescence Activity of Cytokinin Arabinosides in Wheat and Arabidopsis Is Negatively Correlated with Ethylene Production

Leaf senescence, accompanied by chlorophyll breakdown, chloroplast degradation and inhibition of photosynthesis, can be suppressed by an exogenous application of cytokinins. Two aromatic cytokinin arabinosides (6-benzylamino-9-β-d-arabinofuranosylpurines; BAPAs), 3-hydroxy- (3OHBAPA) and 3-methoxy- (3MeOBAPA) derivatives, have recently been found to possess high anti-senescence activity. Interestingly, their effect on the maintenance of chlorophyll content and maximal quantum yield of photosystem II (PSII) in detached dark-adapted leaves differed quantitatively in wheat (Triticum aestivum L. cv. Aranka) and Arabidopsis (Arabidopsisthaliana L. (Col-0)). In this work, we have found that the anti-senescence effects of 3OHBAPA and 3MeOBAPA in wheat and Arabidopsis also differ in other parameters, including the maintenance of carotenoid content and chloroplasts, rate of reduction of primary electron acceptor of PSII (Q_A) as well as electron transport behind Q_A, and partitioning of absorbed light energy in light-adapted leaves. In wheat, 3OHBAPA had a higher protective effect than 3MeOBAPA, whereas in Arabidopsis, 3MeOBAPA was the more efficient derivative. We have found that the different anti-senescent activity of 3OHBAPA and 3MeOBAPA was coupled to different ethylene production in the treated leaves: the lower the ethylene production, the higher the anti-senescence activity. 3OHBAPA and 3MeOBAPA also efficiently protected the senescing leaves of wheat and Arabidopsis against oxidative damage induced by both H₂O₂ and high-light treatment, which could also be connected with the low level of ethylene production.

Maintenance of photosynthetic capacity in flooded tomato plants with reduced ethylene sensitivity

Ethylene is considered one of the most important plant hormones orchestrating plant responses to flooding stress. However, ethylene may induce deleterious effects on plants, especially when produced at high rates in response to stress. In this paper, we explored the effect of attenuated ethylene sensitivity in the Never ripe (Nr) mutant on leaf photosynthetic capacity of flooded tomato plants. We found out that reduced ethylene perception in Nr plants was associated with a more efficient photochemical and non-photochemical radiative energy dissipation capability in response to flooding. The data correlated with the retention of chlorophyll and carotenoids content in flooded Nr leaves. Moreover, leaf area and specific leaf area were higher in Nr, indicating that ethylene would exert a negative role in leaf growth and expansion under flooded conditions. Although stomatal conductance was hampered in flooded Nr plants, carboxylation activity was not affected by flooding in the mutant, suggesting that ethylene is responsible for inducing non-stomatal limitations to photosynthetic CO₂ uptake. Upregulation of several cysteine protease genes and high protease activity led to Rubisco protein loss in response to ethylene under flooding. Reduction of Rubisco content would, at least in part, account for the reduction of its carboxylation efficiency in response to ethylene in flooded plants. Therefore, besides its role as a trigger of many adaptive responses, perception of ethylene entails limitations in light and dark photosynthetic reactions by speeding up the senescence process that leads to a progressive disassembly of the photosynthetic machinery in leaves of flooded tomato plants.

A brief appraisal of ethylene signaling under abiotic stress in plants

For years, ethylene has been known to humankind as the plant hormone responsible for fruit ripening. However, the multitasking aspect of ethylene is still being investigated as ever. It is one of the most diversified signaling molecules which acclimatize plant under adverse conditions. It promotes adventitious root formation, stem and petiole elongation, opening and closing of stomatal aperture, reduces salinity and metal stress, etc. Presence of ethylene checks the production and scavenging of reactive oxygen species by strengthening the antioxidant machinery. Meanwhile, it interacts with other signaling molecules and initiates a cascade of adaptive responses. In the present mini review, the biosynthesis and sources of ethylene production, interaction with other signaling molecules, and its exogenous application under different abiotic stresses have been discussed.

Functional switching of NPR1 between chloroplast and nucleus for adaptive response to salt stress

Salt stress causes rapid accumulation of nonexpressor of pathogenesis-related genes 1 (NPR1) protein, known as the redox-sensitive transcription coactivator, which in turn elicits many adaptive responses. The NPR1 protein transiently accumulates in chloroplast stroma under salt stress, which attenuates stress-triggered down-regulation of photosynthetic capability. We observed that oligomeric NPR1 in chloroplasts and cytoplasm had chaperone activity, whereas monomeric NPR1 in the nucleus did not. Additionally, NPR1 overexpression resulted in reinforcement of morning-phased and evening-phased circadian clock. NPR1 overexpression also enhanced antioxidant activity and reduced stress-induced reactive oxygen species (ROS) generation at early stage, followed with transcription levels for ROS detoxification. These results suggest a functional switch from a molecular chaperone to a transcriptional coactivator, which is dependent on subcellular localization. Our findings imply that dual localization of NPR1 is related to proteostasis and redox homeostasis in chloroplasts for emergency restoration as well as transcriptional coactivator in the nucleus for adaptation to stress.

The expression of several pepper fatty acid desaturase genes is robustly activated in an incompatible pepper-tobamovirus interaction, but only weakly in a compatible interaction

The replication of positive strand RNA viruses in plant cells is markedly influenced by the desaturation status of fatty acid chains in lipids of intracellular plant membranes. At present, little is known about the role of lipid desaturation in the replication of tobamoviruses. Therefore, we investigated the expression of fatty acid desaturase (FAD) genes and the fatty acid composition of pepper leaves inoculated with two different tobamoviruses. Obuda pepper virus (ObPV) inoculation induced a hypersensitive reaction (incompatible interaction) while Pepper mild mottle virus (PMMoV) inoculation caused a systemic infection (compatible interaction). Changes in the expression of 16 FADs were monitored in pepper leaves following ObPV and PMMoV inoculations. ObPV inoculation rapidly and markedly upregulated seven Δ12-FADs that encode enzymes putatively located in the endoplasmic reticulum membrane. In contrast, PMMoV inoculation resulted in a weaker but rapid upregulation of two Δ12-FADs and a Δ15-FAD. The expression of genes encoding plastidial FADs was not influenced neither by ObPV nor by PMMoV. In accordance with gene expression results, a significant accumulation of linoleic acid was observed by gas chromatography-mass spectrometry in ObPV-, but not in PMMoV-inoculated leaves. ObPV inoculation led to a marked accumulation of H₂O₂ in the inoculated leaves. Therefore, the effect of H₂O₂ treatments on the expression of six tobamovirus-inducible FADs was also studied. The expression of these FADs was upregulated to different degrees by H₂O₂ that correlated with ObPV-inducibility of these FADs. These results underline the importance of further studies on the role of pepper FADs in pepper-tobamovirus interactions.

Ethylene Enhances Seed Germination and Seedling Growth Under Salinity by Reducing Oxidative Stress and Promoting Chlorophyll Content via ETR2 Pathway

Alfalfa (Medicago sativa L.) is an important forage, and salinity is a major stress factor on its yield. In this study, we show that osmotic stress retards alfalfa seedling growth, while ionic/oxidative stress reduces its seed germination. Ethylene treatment can recover the germination rate of alfalfa seeds under salt stress, while ethylene inhibitor silver thiosulfate exacerbates salt effects. ETH reduces the accumulation of MDA and H₂O₂ and increases POD activity. ETH and ACC improve the salt tolerance of alfalfa by increasing proline content under salt stress. In contrast, STS inhibits alfalfa seed germination by reducing POD activity. NaCl treatment reduces chlorophyll content in alfalfa leaves, while ETH and ACC can increase the chlorophyll content and promote seedling growth. ETH promotes the growth of alfalfa in saline condition by reducing the expression of MsACO and MsERF8 genes, while increases its germination rate by upregulating MsERF11 gene. Silencing of MsETR2, a putative ethylene receptor gene in alfalfa, abolishes ethylene triggered tolerance to salt stress. In summary, we show that ethylene improves salt tolerance in alfalfa via MsETR2 dependent manner, and we also analyze the regulatory mechanism of ethylene during germination of alfalfa seeds under salt stress.

Regulation of L-proline biosynthesis, signal transduction, transport, accumulation and its vital role in plants during variable environmental conditions

Background

In response to various environmental stresses, many plant species synthesize L-proline in the cytosol and accumulates in the chloroplasts. L-Proline accumulation in plants is a well-recognized physiological reaction to osmotic stress prompted by salinity, drought and other abiotic stresses. L-Proline plays several protective functions such as osmoprotectant, stabilizing cellular structures, enzymes, and scavenging reactive oxygen species (ROS), and keeps up redox balance in adverse situations. In addition, ample-studied osmoprotective capacity, L-proline has been also ensnared in the regulation of plant improvement, including flowering, pollen, embryo, and leaf enlargement.

Scope and conclusions

Albeit, ample is now well-known about L-proline metabolism, but certain characteristics of its biological roles are still indistinct. In the present review, we discuss the L-proline accumulation, metabolism, signaling, transport and regulation in the plants. We also discuss the effects of exogenous L-proline during different environmental conditions. L-Proline biosynthesis and catabolism are controlled by several cellular mechanisms, of which we identify only very fewer mechanisms. So, in the future, there is a requirement to identify such types of cellular mechanisms.

The growth impairment of salinized fenugreek (Trigonella foenum-graecum L.) plants is associated to changes in the hormonal balance

Fenugreek is a legume cultivated for its medicinal value, especially in arid and semi-arid regions, where soil salinity is an increasing problem. In fact, salinity is one of the major environmental factors limiting plant growth and productivity. Plant hormones are known to play vital roles in the ability of the plants to acclimatize to varying environments by mediating growth, development, and nutrient allocation. Thus, to gain insights about the role of plant hormones in the growth responses of salinized fenugreek plants (Trigonella foenum-graecum L.), a medium-term experiment was conducted under moderate (100 mM NaCl) and high (200 mM NaCl) salinity levels. Results showed that moderate, but especially high salinity stress, impaired shoot growth, total leaf area and leaf number. Salinity also provoked a reduction in relative water content, stomatal conductance and photosynthesis-related pigments, but, surprisingly, photosynthetic rate increased in the leaves of fenugreek plants. Na accumulated in the leaves, particularly at high salinity levels, while most mineral nutrients decreased. Furthermore, important changes in the main hormone classes were observed, associated to growth reduction under salinity. The active cytokinin form, trans-zeatin, and active cytokinin and gibberellin concentrations decreased with salinity in the leaves of fenugreek plants, whereas the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid, accumulated in the roots of fenugreek plants, especially at high salinity levels. Importantly, leaf abscisic acid concentrations increased under salinity, which could limit leaf transpiration to adapt growth to the stressful conditions. Therefore, plant hormones seem to play a critical role in the growth responses of fenugreek plants under salinity stress and they could have potential interest in salt tolerance programmes for this species.

Preharvest application of ethephon and postharvest UV-B radiation improve quality traits of beetroot (Beta vulgaris L. ssp. vulgaris) as source of colourant

BACKGROUND

Betanins have become excellent replacers for artificial red-purple food colourants. Red beet (Beta vulgaris L. spp. vulgaris) known as beetroot, is a rich source of betalains, which major forms are betanin (red to purple) and vulgaxanthin (yellow). Betalains and phenolic compounds are secondary metabolites, accumulation of which is often triggered by elicitors during plant stress responses. In the present study, pre-harvest applications of ethephon (an ethylene-releasing compound) and postharvest UV-B radiation were tested as elicitors of betalains and phenolic compounds in two beetroot cultivars. Their effects on quality parameters were investigated, and the expression of biosynthetic betalain genes in response to ethephon was determined.

RESULTS

Ethephon was applied as foliar spray during the growth of beetroot, resulting in increased betanin (22.5%) and decreased soluble solids contents (9.4%), without detrimental effects on beetroot yield. The most rapid accumulation rate for betanin and soluble solids was observed between 3 and 6 weeks after sowing in both untreated and ethephon-treated beetroots. Overall, the expression of the betalain biosynthetic genes (CYP76AD1, CYP76AD5, CYP76AD6 and DODA1), determining the formation of both betanin and vulgaxanthin, increased in response to ethephon treatment, as did the expression of the betalain pathway activator BvMYB1. In the postharvest environment, the use of short-term UV-B radiation (1.23 kJ m- 2) followed by storages for 3 and 7 days at 15 °C resulted in increased betanin to vulgaxanthin ratio (51%) and phenolic content (15%).

CONCLUSIONS

The results of this study provide novel strategies to improve key profitability traits in betalain production. High betanin concentration and high betanin to vulgaxanthin ratio increase the commercial value of the colourant product. In addition, lowering soluble solids levels facilitates higher concentration of beetroot colour during processing. Moreover, we show that enhanced betanin content in ethephon-treated beetroots is linked to increased expression of betalain biosynthetic genes.

Modulation of Enhanced Antioxidant Activity by Hydrogen Sulfide Antagonization of Ethylene in Tomato Fruit Ripening

Ethylene (C₂H₄) and hydrogen sulfide (H₂S) play important physiological roles in regulating fruit ripening and senescence. The mechanism of H₂S in ethylene-induced tomato fruit ripening and senescence is still unknown. Here, we show that exogenous H₂S reduced the production of superoxide anion (·O₂^-), malondialdehyde (MDA), and H₂O₂ in tomato fruit. Further, additional H₂S was found to induce the activities of guaiacol peroxidase, catalase, ascorbate peroxidase, and superoxide dismutase compared with C₂H₄ treatment alone, whereas the activities of lipoxygenase, polyphenol oxidase, and phenylalanine ammonia lyase were adversely affected. Moreover, the expression of the antioxidant-encoding genes SlAPX2, SlCAT1, SlPOD12, and SlCuZnSOD was generally up-regulated with C₂H₄-H₂S cotreatment, compared with their expression after ethylene treatment. Thus, the present results suggest that exogenous H₂S acts as a fruit-ripening regulator by antagonizing the effect of ethylene, thereby providing a potential application for H₂S in the postharvest storage of fruit.

New Urea Derivatives Are Effective Anti-senescence Compounds Acting Most Likely via a Cytokinin-Independent Mechanism

Stress-induced senescence is a global agro-economic problem. Cytokinins are considered to be key plant anti-senescence hormones, but despite this practical function their use in agriculture is limited because cytokinins also inhibit root growth and development. We explored new cytokinin analogs by synthesizing a series of 1,2,3-thiadiazol-5-yl urea derivatives. The most potent compound, 1-(2-methoxy-ethyl)-3-1,2,3-thiadiazol-5-yl urea (ASES - Anti-Senescence Substance), strongly inhibited dark-induced senescence in leaves of wheat (Triticum aestivum L.) and Arabidopsis thaliana. The inhibitory effect of ASES on wheat leaf senescence was, to the best of our knowledge, the strongest of any known natural or synthetic compound. In vivo, ASES also improved the salt tolerance of young wheat plants. Interestingly, ASES did not affect root development of wheat and Arabidopsis, and molecular and classical cytokinin bioassays demonstrated that ASES exhibits very low cytokinin activity. A proteomic analysis of the ASES-treated leaves further revealed that the senescence-induced degradation of photosystem II had been very effectively blocked. Taken together, our results including data from cytokinin content analysis demonstrate that ASES delays leaf senescence by mechanism(s) different from those of cytokinins and, more effectively. No such substance has yet been described in the literature, which makes ASES an interesting tool for research of photosynthesis regulation. Its simple synthesis and high efficiency predetermine ASES to become also a potent plant stress protectant in biotechnology and agricultural industries.

H2O2 homeostasis in wild-type and ethylene-insensitive Never ripe tomato in response to salicylic acid treatment in normal photoperiod and in prolonged darkness

Ethylene proved to be an important modulator of salicylic acid (SA) signalling pathway. Since SA may regulate both the production and scavenging of hydrogen peroxide (H₂O₂), which show light-dependency, the aim of this study was to compare H₂O₂ metabolism in the leaves of SA-treated wild-type (WT) tomato (Solanum lycopersicum L. cv. Ailsa Craig) and in ethylene receptor Never-ripe (Nr) mutants grown in normal photoperiod or in prolonged darkness. H₂O₂ accumulation was higher in the WT than in the mutants in normal photoperiod after 1 mM SA treatment, while Nr leaves contained more H₂O₂ after light deprivation. The expression of certain superoxide dismutase (SOD) genes and activity of the enzyme followed the same tendency as H₂O₂, which was scavenged by different enzymes in the two genotypes. Catalase (CAT, EC 1.11.1.6) activity was inhibited by SA in WT, while the mutants maintained enhanced enzyme activity in the dark. Thus, in WT, CAT inhibition was the major component of the H₂O₂ accumulation elicited by 1 mM SA in a normal photoperiod, since the expression and/or activity of ascorbate (APX, EC 1.11.1.11) and guaiacol peroxidases (POD, EC 1.11.1.7) were induced in the leaves. The absence of APX and POD activation in mutant plants suggests that the regulation of these enzymes by SA needs functional ethylene signalling. While the block of ethylene perception in Nr mutants was overwritten in the transcription and activity of certain SOD and CAT isoenzymes during prolonged darkness, the low APX and POD activities led to H₂O₂ accumulation in these tissues.

The rice zebra3 (z3) mutation disrupts citrate distribution and produces transverse dark-green/green variegation in mature leaves

BACKGROUND

Rice zebra mutants are leaf variegation mutants that exhibit transverse sectors of green/yellow or green/white in developing or mature leaves. In most cases, leaf variegation is caused by defects in chloroplast biogenesis pathways, leading to an accumulation of reactive oxygen species in a transverse pattern in the leaves. Here, we examine a new type of leaf variegation mutant in rice, zebra3 (z3), which exhibits transverse dark-green/green sectors in mature leaves and lacks the typical yellow or white sectors.

RESULTS

Map-based cloning revealed that the Z3 locus encodes a putative citrate transporter that belongs to the citrate-metal hydrogen symport (CitMHS) family. CitMHS family members have been extensively studied in bacteria and function as secondary transporters that can transport metal-citrate complexes, but whether CitMHS family transporters exist in eukaryotes remains unknown. To investigate whether Z3 acts as a citrate transporter in rice, we measured citrate levels in wild-type leaves and in the dark-green and green sectors of the leaves of z3 mutants. The results showed that citrates accumulated to high levels in the dark-green sectors of z3 mutant leaves, but not in the green sectors as compared with the wild-type leaves.

CONCLUSIONS

These results suggest that leaf variegation in the z3 mutant is caused by an unbalanced accumulation of citrate in a transverse pattern in the leaves. Taking these results together, we propose that Z3 plays an important role in citrate transport and distribution during leaf development and is a possible candidate for a CitMHS family member in plants.

Interactions between salicylic acid and antioxidant enzymes tilting the balance of H2O2 from photorespiration in non-target crops under halosulfuron-methyl stress

Halosulfuron-methyl (HSM) is a safe, selective and effective sulfonylurea herbicide (SU) for the control of sedge and broadleaf weeds in sugarcane, corn, tomato, and other crops. The primary site of action is acetolactate synthase (ALS), a key enzyme of branched chain amino acids (BCAAs) synthesis. In addition to ALS inhibition, BCAAs deficiencies and oxidative damage may be involved in toxic effects of SUs. However, secondary targets of HSM relevant to plant physiological responses are unclear. In the present study, comparative growth inhibition and peroxidization injury between sensitive and tolerance crops were observed at biochemical and physiological levels suggesting involvement of H2O2, ethylene, salicylic acid (SA) in the oxidative stress responses to HSM. HSM caused accumulation of H2O2, stimulated photorespiration and consequent accumulation of SA that worsened the peroxidization injury to the sensitive C3 plant soybean (Glycine max). The growth inhibition at low concentrations of HSM could be lessened by supplementary BCAAs, reactive oxygen species scavengers or ethylene inducers, whereas the oxidation damage at high concentrations of HSM could not be reversed and ultimately lead to plant death. H2O2 at a low level stimulated the antioxidase system including glutathione S-transferase activities in the HSM-tolerant C4 maize (Zea mays), which contributes to HSM tolerance. H2O2 plays an important role on HSM stress responses in both HSM-sensitive and HSM-tolerant soybean and maize.

Prolonged dark period modulates the oxidative burst and enzymatic antioxidant systems in the leaves of salicylic acid-treated tomato

Salicylic acid (SA) is an important plant growth regulator playing a role in the hypersensitive reaction (HR) and the induction of systemic acquired resistance. Since the SA-mediated signalling pathways and the formation of reactive oxygen species (ROS) are light-dependent, the time- and concentration-specific induction of oxidative stress was investigated in leaves of tomato plants kept under light and dark conditions after treatments with 0.1mM and 1mM SA. The application of exogenous SA induced early superoxide- and H₂O₂ production in the leaves, which was different in the absence or presence of light and showed time- and concentration-dependent changes. 1mM SA, which induced HR-like cell death resulted in two peaks in the H₂O₂ production in the light but the first, priming peak was not detected in the dark. Unlike 0.1mM SA, 1mM SA application induced NADPH oxidase activity leading to increased superoxide production in the first hours of SA treatments in the light. Moreover, SA treatments inhibited catalase (CAT) activity and caused a transient decline in ascorbate peroxidase (APX), the two main enzymes responsible for H₂O₂ degradation, which led to a fast H₂O₂ burst in the light. Their activity as well as the expression of some isoenzymes of SOD and APX increased only from the 12th h in the illuminated samples. The activity of NADPH oxidase and expression SlRBOH1 gene encoding a NADPH oxidase subunit was much lower in the dark. In spite of low CAT and APX activity after SA treatments in the dark, the activation of guaiacol-dependent peroxidase (POD) could partially substitute H₂O₂ scavenging activity of these enzymes in the dark, which reduced the ROS burst and development of lesion formation in the leaves.

Delayed response to cold stress is characterized by successive metabolic shifts culminating in apple fruit peel necrosis

BACKGROUND

Superficial scald is a physiological disorder of apple fruit characterized by sunken, necrotic lesions appearing after prolonged cold storage, although initial injury occurs much earlier in the storage period. To determine the degree to which the transition to cell death is an active process and specific metabolism involved, untargeted metabolic and transcriptomic profiling was used to follow metabolism of peel tissue over 180 d of cold storage.

RESULTS

The metabolome and transcriptome of peel destined to develop scald began to diverge from peel where scald was controlled using antioxidant (diphenylamine; DPA) or rendered insensitive to ethylene using 1-methylcyclopropene (1-MCP) beginning between 30 and 60 days of storage. Overall metabolic and transcriptomic shifts, representing multiple pathways and processes, occurred alongside α-farnesene oxidation and, later, methanol production alongside symptom development.

CONCLUSIONS

Results indicate this form of peel necrosis is a product of an active metabolic transition involving multiple pathways triggered by chilling temperatures at cold storage inception rather than physical injury. Among multiple other pathways, enhanced methanol and methyl ester levels alongside upregulated pectin methylesterases are unique to peel that is developing scald symptoms similar to injury resulting from mechanical stress and herbivory in other plants.

Reversion of hyperhydricity in pink (Dianthus chinensis L.) plantlets by AgNO3 and its associated mechanism during in vitro culture

Hyperhydricity occurs frequently in plant tissue culture and can severely affect commercial micropropagation and genetic improvement of the cultured plantlets. Hyperhydric shoots are charaterzized by high water content, but how this occurs is still a subject of investigation. Silver ion (Ag⁺) can reduce the extent of hyperhydricity in plants, but its effect on the reversion of hyperhydric plantlets and the underlying mechanism of reversion has not been clarified. In this study, about 67% of the hyperhydric Dianthus chinensis L. plantlets were found to revert to normal condition when the plantlets were cultured in medium supplemented with 29.4μmolL^-1AgNO₃. Water content and hydrogen peroxide (H₂O₂) content in the guard cells of these plantlets were reduced, while stomatal aperture and water loss rate were increased. AgNO₃ also reduced the content of endogenous ethylene and expression of ethylene synthesis and ethylene signal transduction-associated genes. Reduced accumulation of ethylene consequently led to an increase in stomatal aperture mediated by decreased H₂O₂ content in the guard cells. These results adequately verified the role of AgNO₃ in the reversion of hyperhydricity in D. chinensis L. and also provided clues for exploring the cause of excessive water accumulation in hyperhydric plants.

A magic red coat on the surface of young leaves: anthocyanins distributed in trichome layer protect Castanopsis fissa leaves from photoinhibition

The presence of anthocyanins in young leaves plays an important role in mitigation against photodamage and allows leaves to grow and develop normally. Many studies have reported that foliar anthocyanins are distributed within the vacuoles of mesophyll cells, so we explored the novel defence style of anthocyanin-coated young leaves of Castanopsis fissa, a dominant subtropical forest tree species, via removable trichomes. Anthocyanins were distributed in C. fissa leaf trichomes, which produced a red coating for the young leaves. As young leaves developed and then matured, the thickness and density of the anthocyanin trichomes progressively decreased, the coating finally disappearing, allowing greater utilization of light by mature leaves. In addition to anthocyanins, the trichomes contained a remarkably high amount of phenolics, which enable the red coating to be more efficient in screening ultraviolet light. Compared with mature leaves, the young leaves exhibited lower photosynthetic ability, which was attributable to the reduced chlorophyll and Rubisco contents. Removal of the red coating had little effect on the photosynthetic capacity of young leaves. However, the young leaves without the coating suffered greater light-induced photoinhibition due to greater excess light entering the chloroplast and the production of H₂O₂ Our results suggest that the anthocyanin coating is photoprotective and this anthocyanin defence style may be a metabolically cost-effective way of adjusting the anthocyanin content in response to demand.

Ethylene Response Factor TERF1, Regulated by ETHYLENE-INSENSITIVE3-like Factors, Functions in Reactive Oxygen Species (ROS) Scavenging in Tobacco (Nicotiana tabacum L.)

The phytohormone ethylene plays a crucial role in the production and accumulation of reactive oxygen species (ROS) in plants under stress conditions. Ethylene response factors (ERFs) are important ethylene-signaling regulators functioning in plant defense responses against biotic and abiotic stresses. However, the roles of ERFs during plant adapting to ROS stress have not yet been well documented. Our studies previously reported that a tomato ERF transcription factor TERF1 functions in the regulation of plant ethylene responses and stress tolerance. Here, we report our findings regarding the roles of TERF1 in ROS scavenging. In this study, we revealed that the transcription of TERF1 is regulated by upstream EIN3-like (EIN3, ethylene-insensitive 3) regulators LeEIL3 and LeEIL4 in tomato (Solanum lycopersicum), and is also inducible by exogenous applied ROS-generating reagents. Ectopic expression of TERF1 in tobacco promoted the expression of genes involved in oxidative stress responses, including carbonic anhydrase functioning in hypersensitive defense, catalase and glutathione peroxidase catalyzing oxidative reactions, and GDP-D-mannose pyrophosphorylase functioning in ascorbic acid biosynthesis, reduced the ROS content induced by ethylene treatment, and enhanced stress tolerance of tobacco seedlings to hydrogen peroxide (H2O2). Cumulatively, these findings suggest that TERF1 is an ethylene inducible factor regulating ROS scavenging during stress responses.

Salt stress-induced production of reactive oxygen- and nitrogen species and cell death in the ethylene receptor mutant Never ripe and wild type tomato roots

The salt stress triggered by sublethal, 100 mM and lethal, 250 mM NaCl induced ethylene production as well as rapid accumulation of superoxide radical and H2O2 in the root tips of tomato (Solanum lycopersicum cv. Ailsa Craig) wild type and ethylene receptor mutant, Never ripe (Nr/Nr) plants. In the wild type plants superoxide accumulation confined to lethal salt concentration while H2O2 accumulated more efficiently under sublethal salt stress. However, in Nr roots the superoxide production was higher and unexpectedly, H2O2 level was lower than in the wild type under sublethal salt stress. Nitric oxide production increased significantly under sublethal and lethal salt stress in both genotypes especially in mutant plants, while peroxynitrite accumulated significantly under lethal salt stress. Thus, the nitro-oxidative stress may be stronger in Nr roots, which leads to the programmed death of tissues, characterized by the DNA and protein degradation and loss of cell viability under moderate salt stress. In Nr mutants the cell death was induced in the absence of ethylene perception. Although wild type roots could maintain their potassium content under moderate salt stress, K(+) level significantly declined leading to small K(+)/Na(+) ratio in Nr roots. Thus Nr mutants were more sensitive to salt stress than the wild type and the viability of root cells decreased significantly under moderate salt stress. These changes can be attributed to a stronger ionic stress due to the K(+) loss from the root tissues.

Current Understanding of the Interplay between Phytohormones and Photosynthesis under Environmental Stress

Abiotic stress accounts for huge crop losses every year across the globe. In plants, the photosynthetic machinery gets severely damaged at various levels due to adverse environmental conditions. Moreover, the reactive oxygen species (ROS) generated as a result of stress further promote the photosynthetic damage by inhibiting the repair system of photosystem II. Earlier studies have suggested that phytohormones are not only required for plant growth and development, but they also play a pivotal role in regulating plants’ responses to different abiotic stress conditions. Although, phytohormones have been studied in great detail in the past, their influence on the photosynthetic machinery under abiotic stress has not been studied. One of the major factors that limits researchers fromelucidating the precise roles of phytohormones is the highly complex nature of hormonal crosstalk in plants. Another factor that needs to be elucidated is the method used for assessing photosynthetic damage in plants that are subjected to abiotic stress. Here, we review the current understanding on the role of phytohormones in the photosynthetic machinery under various abiotic stress conditions and discuss the potential areas for further research.

A novel role of ethephon in controlling the noxious weed Ipomoea cairica (Linn.) Sweet

Several auxin herbicides, such as 2, 4-D and dicamba, have been used to eradicate an exotic invasive weed Ipomoea cairica in subtropical China, but restraining the re-explosion of this weed is still a challenge. Since ethylene is one of the major intermediate functioning products during the eradication process, we explored the possibility, mechanism and efficiency of using ethephon which can release ethylene to control Ipomoea cairica. The results of the pot experiment showed that 7.2 g /L ethephon could totally kill Ipomoea cairica including the stems and roots. The water culture experiment indicated that ethephon released an abundance of ethylene directly in leaves and caused increases in electrolyte leakage, 1-aminocyclopropane-1-carboxylic acid (ACC), abscisic acid (ABA) and H2O2 and decreases in chlorophyll content and photosynthetic activity, finally leading to the death of Ipomoea cairica. The field experiment showed that the theoretical effective concentration of ethephon for controlling Ipomoea cairica (weed control efficacy, WCE = 98%) was 4.06 g/L and the half inhibitory concentration (I50) was 0.56 g/L. More than 50% of the accompanying species were insensitive to the phytotoxicity of ethephon. Therefore, ethephon is an excellent alternative herbicide for controlling Ipomoea cairica.

Effects of combined ozone and cadmium stresses on leaf traits in two poplar clones

Information on plant responses to combined stresses such as ozone (O3) and cadmium (Cd) is scarce in tree species. On the other hand, high O3 concentrations in the atmosphere and heavy metal contaminations in water and soil simultaneously affect forest ecosystems. Toxic metals may exacerbate the consequences of air pollutants. In this research, two poplar clones, differently sensitive to O3 ("I-214" O3-tolerant and "Eridano" O3-sensitive), were grown for 5 weeks in pots supplied with 0 and 150 mg Cd kg(-1) soil and then exposed to a 15-day O3 fumigation (60 nl l(-1), 5 h a day) or supplied with charcoal-filtered air under the same conditions (referred to as control samples). The effects of the two stressors, alone or in combination, on Cd accumulation, photosynthetic capacity, ethylene emission and oxidative state were investigated in fully expanded leaves. Cadmium accumulation in leaves caused a reduction, but not complete failure, of photosynthesis in Eridano and I-214 poplar clones. The reduction in assimilation rate was more important following O3 fumigation. Stomatal aperture after O3 treatment, instead, increased in I-214 and decreased in Eridano. Overall, Cd treatment was effective in decreasing ethylene emission, whereas O3 fumigation increased it in both clones, although interacting with the metal treatment. Again, O3 fumigation induced a significant increase in ascorbate (ASA) + dehydroascorbate (DHA) content, which was strongly oxidised by O3, thus decreasing the redox state. On the other hand, Cd treatment had a positive effect on ASA content and redox state in I-214, but not in Eridano. Although Cd and O3 are known to share some common toxicity pathways, the combined effects induced distinct clone-specific responses, underlying the complexity of plant reactions to multiple stresses.

SuMoToRI, an Ecophysiological Model to Predict Growth and Sulfur Allocation and Partitioning in Oilseed Rape (Brassica napus L.) Until the Onset of Pod Formation

Sulfur (S) nutrition in rapeseed (Brassica napus L.) is a major concern for this high S-demanding crop, especially in the context of soil S oligotrophy. Therefore, predicting plant growth, S plant allocation (between the plant's compartments) and S pool partitioning (repartition of the mobile-S vs. non-mobile-S fractions) until the onset of reproductive phase could help in the diagnosis of S deficiencies during the early stages. For this purpose, a process-based model, SuMoToRI (Sulfur Model Toward Rapeseed Improvement), was developed up to the onset of pod formation. The key features rely on (i) the determination of the S requirements used for growth (structural and metabolic functions) through critical S dilution curves and (ii) the estimation of a mobile pool of S that is regenerated by daily S uptake and remobilization from senescing leaves. This study describes the functioning of the model and presents the model's calibration and evaluation. SuMoToRI was calibrated and evaluated with independent datasets from greenhouse experiments under contrasting S supply conditions. It is run with a small number of parameters with generic values, except in the case of the radiation use efficiency, which was shown to be modulated by S supply. The model gave satisfying predictions of the dynamics of growth, S allocation between compartments and S partitioning, such as the mobile-S fraction in the leaves, which is an indicator of the remobilization potential toward growing sinks. The mechanistic features of SuMoToRI provide a process-based framework that has enabled the description of the S remobilizing process in a species characterized by senescence during the vegetative phase. We believe that this model structure could be useful for modeling S dynamics in other arable crops that have similar senescence-related characteristics.

Involvement of ethylene in reversal of salt-inhibited photosynthesis by sulfur in mustard

Sulfur (S) assimilation results in the synthesis of cysteine (Cys), a common metabolite for the formation of both reduced glutathione (GSH) and ethylene. Thus, ethylene may have regulatory interaction with GSH in the alleviation of salt stress. The involvement of ethylene in the alleviation of salt stress by S application was studied in mustard (Brassica juncea cv. Pusa Jai Kisan). First, the effects of 0, 0.5, 1.0 and 2.0 mM SO4 (2) (-) were studied on photosynthetic and growth parameters to ascertain the S requirement as sufficient-S and excess-S for the plant. In further experiments, the effects of sufficient-S (1 mM SO4 (2) (-) ) and excess-S (2 mM SO4 (2) (-) ) were studied on the alleviation of salt stress-induced by 100 mM NaCl, and ethylene involvement in the alleviation of salt stress by S. Under non-saline condition, excess-S increased ethylene with less content of Cys and GSH and adversely affected photosynthesis and growth. In contrast, excess-S maximally alleviated salt stress due to high demand for S and optimal ethylene formation, which maximally increased GSH and promoted photosynthesis and growth. The involvement of ethylene in S-mediated alleviation of salt stress was further substantiated by the reversal of the effects of excess-S on photosynthesis by aminoethoxyvinylglycine (AVG), ethylene biosynthesis inhibitor. The studies suggest that plants respond differentially to the S availability under non-saline and salt stress and excess-S was more potential in the alleviation of salt stress. Further, ethylene regulates plants' response and excess S-induced alleviation of salt stress and promotion of photosynthesis.

Transcriptomic profiling revealed an important role of cell wall remodeling and ethylene signaling pathway during salt acclimation in Arabidopsis

Plants can successfully improve their resistance to previously lethal salinity stress by a short exposure to low levels of salt stress, a process known as salt acclimation (SA). In spite of its fundamental significance in theoretical study and agricultural practice, the molecular mechanisms underlying plant SA remain elusive. In this study, we found that salt acclimated Arabidopsis young seedlings can survive subsequent 200 mM NaCl stress. RNA-seq was performed to analyze the genome-wide transcriptional response under SA conditions. Among 518 differentially expressed genes (DEGs) under SA, 366 up-regulated genes were enriched for cell wall biosynthesis, osmoregulation, oxidative stress, or transcription factors. Seven DEGs participate in the synthesis of lignin and 24 DEGs encode plant cell wall proteins, suggesting the importance of cell wall remodeling under SA. Furthermore, in comparison to non-acclimated salt stress, 228 of 245 DEGs were repressed by acclimated salt stress, including many genes related to ethylene biosynthesis and signaling pathway. In addition, MAPK6, a major component of the ethylene signaling pathway, was found to play a crucial role in SA. Our transcriptomic analysis has provided important insight on the roles of transcription factors, cell wall remodeling, and the ethylene biosynthesis and signaling pathways during SA in Arabidopsis.

Ethylene reverses photosynthetic inhibition by nickel and zinc in mustard through changes in PS II activity, photosynthetic nitrogen use efficiency, and antioxidant metabolism

We investigated the influence of exogenously sourced ethylene (200 μL L(-1) ethephon) in the protection of photosynthesis against 200 mg kg(-1) soil each of nickel (Ni)- and zinc (Zn)-accrued stress in mustard (Brassica juncea L.). Plants grown with Ni or Zn but without ethephon exhibited increased activity of 1-aminocyclopropane carboxylic acid synthase, and ethylene with increased oxidative stress measured as H2O2 content and lipid peroxidation compared with control plants. The oxidative stress in Ni-grown plants was higher than Zn-grown plants. Under metal stress, ethylene protected photosynthetic potential by efficient PS II activity and through increased activity of ribulose-1,5-bisphosphate carboxylase and photosynthetic nitrogen use efficiency (P-NUE). Application of 200 μL L(-1) ethephon to Ni- or Zn-grown plants significantly alleviated toxicity and reduced the oxidative stress to a greater extent together with the improved net photosynthesis due to induced activity of ascorbate peroxidase and glutathione (GSH) reductase, resulting in increased production of reduced GSH. Ethylene formation resulting from ethephon application alleviated Ni and Zn stress by reducing oxidative stress caused by stress ethylene production and maintained increased GSH pool. The involvement of ethylene in reversal of photosynthetic inhibition by Ni and Zn stress was related to the changes in PS II activity, P-NUE, and antioxidant capacity was confirmed using ethylene action inhibitor, norbornadiene.

Ectopic expression of the human MutT-type Nudix hydrolase, hMTH1, confers enhanced tolerance to oxidative stress in arabidopsis

Oxidized nucleotides produced by oxidative stress cause DNA mutations and the production of abnormal proteins. Thus, mammalian cells have developed multiple MutT-type Nudix hydrolases that exhibit pyrophosphohydrolase activity toward oxidized nucleotides in the cytosol, mitochondria and nucleus. On the other hand, AtNUDX1 is the only MutT-type Nudix hydrolase in the cytosol of Arabidopsis plants. To clarify the physiological significance of the defenses against oxidatively induced DNA damage in plant organelles, we analyzed the effects of the ectopic expression of the human MutT-type Nudix hydrolase, hMTH1, which was localized in the cytosol (cyt-hMTH1), chloroplasts (chl-hMTH1) and mitochondria (mit-hMTH1) of Arabidopsis cells, on tolerance to oxidative stress. Tolerance to oxidative stress caused by heating and paraquat (PQ) treatment was higher in the mit-hMTH1 and chl-hMTH1 plants than in the control and cyt-hMTH1 plants. The accumulation of H2O2 and the frequency of dead cells were lower in the mit-hMTH1 and chl-hMTH1 plants under stressful conditions. The poly(ADP-ribosyl)ation (PAR) reaction, which regulates repair systems for damaged DNA, was activated in the mit-hMTH1 and chl-hMTH1 plants under heat stress and PQ treatment. Furthermore, DNA fragmentation, which caused programmed cell death, was clearly suppressed in the mit-hMTH1 and chl-hMTH1 plants under heat stress. These results demonstrated that the ectopic expression of hMTH1 in the chloroplasts and mitochondria of Arabidopsis enhanced oxidative stress tolerance by activating the PAR reaction and suppressing programmed cell death.

Lysophosphatidylcholine enhances susceptibility in signaling pathway against pathogen infection through biphasic production of reactive oxygen species and ethylene in tobacco plants

It was previously reported that the amounts of lysophosphatidylcholines (lysoPCs), which are naturally occurring bioactive lipid molecules, significantly increase following pathogen inoculation, as determined using ultraperformance liquid chromatography-quadrupole-time of flight/mass spectrometry analyses. Here, real-time quantitative RT-PCR was performed for the phospholipase A2 (PLA2) genes, Nt1PLA2 and Nt2PLA2, which are responsible for LysoPCs generation. The transcription level of Nt2PLA2 in pathogen-infected tobacco plants transiently peaked at 1h and 36 h, whereas induction of Nt1PLA2 transcription peaked at 36 h. A prominent biphasic ROS accumulation in lysoPC (C18:1(9Z))-treated tobacco leaves was also observed. Transcription of NtRbohD, a gene member of NADPH oxidase, showed biphasic kinetics upon lysoPC 18:1 treatment, as evidenced by an early transient peak in phase I at 1h and a massive peak in phase II at 12h. Each increase in NtACS2 and NtACS4 transcription, gene members of the ACC synthase family, was followed by biphasic peaks of ethylene production after lysoPC 18:1 treatment. This suggested that lysoPC (C18:1)-induced ethylene production was regulated at the transcriptional level of time-dependent gene members. LysoPC 18:1 treatment also rapidly induced cell damage. LysoPC 18:1-induced cell death was almost completely abrogated in ROS generation-impaired transgenic plants (rbohD-as and rbohF-as), ethylene production-impaired transgenic plants (CAS-AS and CAO-AS), and ethylene signaling-impaired transgenic plants (Ein3-AS), respectively. Taken together, pathogen-induced lysoPCs enhance pathogen susceptibility accompanied by ROS and ethylene biosynthesis, resulting in chlorophyll degradation and cell death. Expression of PR genes (PR1-a, PR-3, and PR-4b) and LOX3 was strongly induced in lysoPC 18:1-treated leaves, indicating the involvement of lysoPC 18:1 in the defense response. However, lysoPC 18:1 treatment eventually resulted in cell death, as evidenced by metacaspase gene expression. Therefore, a hypothesis is proposed that the antipathogenic potential of lysoPC 18:1 is dependent on how quickly it is removed from cells for avoidance of lysoPC toxicity.

Constitutive S-adenosylmethionine decarboxylase gene expression increases drought tolerance through inhibition of reactive oxygen species accumulation in Arabidopsis

Using subtractive hybridization analysis, the S-adenosylmethionine decarboxylase (SAMDC) gene from Capsicum annuum was isolated and renamed CaSAMDC. We generated independent transgenic Arabidopsis (Arabidopsis thaliana) lines constitutively expressing a 35S::CaSAMDC construct. Drought tolerance was significantly enhanced in Arabidopsis T4 transgenic homozygous lines as compared to wild-type (WT) plants. The levels of main polyamines (PAs) were more significantly increased in CaSAMDC-overexpressing transgenic plants after 6 h of drought stress as compared to stressed WT plants. Basal transcription of polyamine oxidase (PAO) showed at a much higher level in unstressed-transgenic plants as compared to unstressed WT plants. However, the difference in PAO transcription level between WT and transgenic plants was reduced after drought stress. Cellular accumulation of reactive oxygen species (ROS) was significantly reduced following drought stress in transgenic Arabidopsis plants as compared to WT plants. These results were in agreement with additional observations that stress-induced ROS generation, as determined by qRT-PCR analysis of NADPH oxidase (RbohD and RbohF), was significantly suppressed while transcription of ROS-detoxifying enzymes was notably elevated in transgenic lines in response to drought stress. Further, ROS-induced transcription of the metacaspase II gene was remarkably inhibited in transgenic plants. Collectively, these results suggest that drought stress tolerance due to reduction of ROS production and enhancement of ROS detoxification can be attributed to elevation of PAs.

GhTZF1 regulates drought stress responses and delays leaf senescence by inhibiting reactive oxygen species accumulation in transgenic Arabidopsis

Redox homeostasis is important for plants to be able to maintain cellular metabolism, and disrupting cellular redox homeostasis will cause oxidative damage to cells and adversely affect plant growth. In this study, a cotton CCCH-type tandem zinc finger gene defined as GhTZF1, which was isolated from a cotton cell wall regeneration SSH library in our previous research, was characterized. GhTZF1 was predominantly expressed during early cell wall regeneration, and it was expressed in various vegetative and reproductive tissues. The expression of GhTZF1 was substantially up-regulated by a variety of abiotic stresses, such as PEG and salt. GhTZF1 also responds to methyl jasmonate (MeJA) and H2O2 treatment. Overexpression of GhTZF1 enhanced drought tolerance and delayed drought-induced leaf senescence in transgenic Arabidopsis. Subsequent experiments indicated that dark- and MeJA-induced leaf senescence was also attenuated in transgenic plants. The amount of H2O2 in transgenic plants was attenuated under both drought conditions and with MeJA-treatment. The activity of superoxide dismutase and peroxidase was higher in transgenic plants than in wild type plants under drought conditions. Quantitative real-time PCR analysis revealed that overexpression of GhTZF1 reduced the expression of oxidative-related senescence-associated genes (SAGs) under drought conditions. Overexpression of GhTZF1 also enhanced oxidative stress tolerance, which was determined by measuring the expression of a set of antioxidant genes and SAGs that were altered in transgenic plants during H2O2 treatment. Hence, we conclude that GhTZF1 may serve as a regulator in mediating drought stress tolerance and subsequent leaf senescence by modulating the reactive oxygen species homeostasis.

Molecular hydrogen is involved in phytohormone signaling and stress responses in plants

Molecular hydrogen (H₂) metabolism in bacteria and algae has been well studied from an industrial perspective because H₂ is viewed as a potential future energy source. A number of clinical trials have recently reported that H₂ is a therapeutic antioxidant and signaling molecule. Although H₂ metabolism in higher plants was reported in some early studies, its biological effects remain unclear. In this report, the biological effects of H₂ and its involvement in plant hormone signaling pathways and stress responses were determined. Antioxidant enzyme activity was found to be increased and the transcription of corresponding genes altered when the effects of H₂ on the germination of mung bean seeds treated with phytohormones was investigated. In addition, upregulation of several phytohormone receptor genes and genes that encode a few key factors involved in plant signaling pathways was detected in rice seedlings treated with HW. The transcription of putative rice hydrogenase genes, hydrogenase activity, and endogenous H₂ production were also determined. H₂ production was found to be induced by abscisic acid, ethylene, and jasmonate acid, salt, and drought stress and was consistent with hydrogenase activity and the expression of putative hydrogenase genes in rice seedlings. Together, these results suggest that H₂ may have an effect on rice stress tolerance by modulating the output of hormone signaling pathways.

Sulphur limitation provokes physiological and leaf proteome changes in oilseed rape that lead to perturbation of sulphur, carbon and oxidative metabolisms

BACKGROUND

The decline in industrial emissions of sulphur (S) has led to a sulphate depletion in soil resulting in an alteration of crop performance. In oilseed rape, an S deficiency dramatically reduced the seed yield and/or quality. Paradoxically, little is known about the impact of sulphate limitation on oilseed rape leaf metabolism, despite it being a key determinant of growth. In order to identify the metabolic processes involved in the oilseed rape response to S restriction, an analysis of the young leaf proteome combined with a physiological study was carried out at the vegetative stage.

RESULTS

S limitation does not significantly reduce the total shoot biomass but inhibits growth and photosynthesis of young leaves. This photosynthesis decline is not due to a decrease in chlorophyll content, which remains similar to Control. The increase in anthocyanins and H(2)O(2) content in young leaves of S-limited plants suggests that S restriction leads to an oxidative stress. Proteomic analysis at 35 d of S limitation also revealed the induction of 12-oxophitodienoate reductase and ACC synthase, respectively involved in jasmonate and ethylene biosynthesis, two phytohormones that could be implicated in oxidative stress. Proteins involved in photosynthesis and carbon metabolism were also modulated by S restriction. In particular, the decrease in plastocyanin and ferredoxin-NADP reductase suggests that H(2)O(2) accumulation is associated with perturbation of the photosynthetic electron transport chain. The accumulation of chloroplastic Cu-Zn SOD reinforces the idea that an oxidative stress probably occurs in the chloroplast. Proteomic results suggest that the maintenance of chlorophyll in S-limited conditions is related to an accumulation of Water Soluble Chlorophyll binding Proteins, involved in the protection of chlorophyll against ROS. The accumulation of the catalytic α-subunit of chloroplastic ATP synthase suggests that energy production is maintained.

CONCLUSION

S limitation leads to photosynthesis and carbon metabolism disturbances that could be responsible for the oxidative stress observed in the young leaves of oilseed rape. Despite this, induction of proteins involved in oxidative stress resistance and energy production shows that the leaf capacity to capture and use photosynthetic active radiations for ATP production remains efficient for as long as possible.

Ethylene signaling in salt stress- and salicylic acid-induced programmed cell death in tomato suspension cells

Salt stress- and salicylic acid (SA)-induced cell death can be activated by various signaling pathways including ethylene (ET) signaling in intact tomato plants. In tomato suspension cultures, a treatment with 250 mM NaCl increased the production of reactive oxygen species (ROS), nitric oxide (NO), and ET. The 10(-3) M SA-induced cell death was also accompanied by ROS and NO production, but ET emanation, the most characteristic difference between the two cell death programs, did not change. ET synthesis was enhanced by addition of ET precursor 1-aminocyclopropane-1-carboxylic acid, which, after 2 h, increased the ROS production in the case of both stressors and accelerated cell death under salt stress. However, it did not change the viability and NO levels in SA-treated samples. The effect of ET induced by salt stress could be blocked with silver thiosulfate (STS), an inhibitor of ET action. STS reduced the death of cells which is in accordance with the decrease in ROS production of cells exposed to high salinity. Unexpectedly, application of STS together with SA resulted in increasing ROS and reduced NO accumulation which led to a faster cell death. NaCl- and SA-induced cell death was blocked by Ca(2+) chelator EGTA and calmodulin inhibitor W-7, or with the inhibitors of ROS. The inhibitor of MAPKs, PD98059, and the cysteine protease inhibitor E-64 reduced cell death in both cases. These results show that NaCl induces cell death mainly by ET-induced ROS production, but ROS generated by SA was not controlled by ET in tomato cell suspension.

Cross-talk between sulfur assimilation and ethylene signaling in plants

Sulfur (S) deficiency is prevailing all over the world and becoming an important issue for crop improvement through maximising its utilization efficiency by plants for sustainable agriculture. Its interaction with other regulatory molecules in plants is necessary to improve our understanding on its role under changing environment. Our knowledge on the influence of S on ethylene signaling is meagre although it is a constituent of cysteine (Cys) required for the synthesis of reduced glutathione (GSH) and S-adenosyl methionine (SAM), a precursor of ethylene biosynthesis. Thus, there may be an interaction between S assimilation, ethylene signaling and plant responses under optimal and stressful environmental conditions. The present review emphasizes that responses of plants to S involve ethylene action. This evaluation will provide an insight into the details of interactive role of S and ethylene signaling in regulating plant processes and prove profitable for developing sustainability under changing environmental conditions.