Cross-talk between sulfur assimilation and ethylene signaling in plants

Sulfate Availability and Hormonal Signaling in the Coordination of Plant Growth and Development

Sulfur (S), one of the crucial macronutrients, plays a pivotal role in fundamental plant processes and the regulation of diverse metabolic pathways. Additionally, it has a major function in plant protection against adverse conditions by enhancing tolerance, often interacting with other molecules to counteract stresses. Despite its significance, a thorough comprehension of how plants regulate S nutrition and particularly the involvement of phytohormones in this process remains elusive. Phytohormone signaling pathways crosstalk to modulate growth and developmental programs in a multifactorial manner. Additionally, S availability regulates the growth and development of plants through molecular mechanisms intertwined with phytohormone signaling pathways. Conversely, many phytohormones influence or alter S metabolism within interconnected pathways. S metabolism is closely associated with phytohormones such as abscisic acid (ABA), auxin (AUX), brassinosteroids (BR), cytokinins (CK), ethylene (ET), gibberellic acid (GA), jasmonic acid (JA), salicylic acid (SA), and strigolactones (SL). This review provides a summary of the research concerning the impact of phytohormones on S metabolism and, conversely, how S availability affects hormonal signaling. Although numerous molecular details are yet to be fully understood, several core signaling components have been identified at the crossroads of S and major phytohormonal pathways.

Genome-Wide Identification and Expression Analysis of 1-Aminocyclopropane-1-Carboxylate Synthase (ACS) Gene Family in Chenopodium quinoa

Ethylene plays an important role in plant development and stress resistance. The rate-limiting enzyme in ethylene biosynthesis is 1-aminocyclopropane-1-carboxylic acid synthase (ACS). C. quinoa (Chenopodium quinoa) is an important food crop known for its strong tolerance to abiotic stresses. However, knowledge regarding the ACS gene family in C. quinoa remains restricted. In this study, we successfully identified 12 ACS genes (CqACSs) from the C. quinoa genome. Through thorough analysis of their sequences and phylogenetic relationships, it was verified that 8 out of these 12 CqACS isozymes exhibited substantial resemblance to ACS isozymes possessing ACS activity. Furthermore, these eight isozymes could be categorized into three distinct groups. The four remaining CqACS genes grouped under category IV displayed notable similarities with AtACS10 and AtACS12, known as amido transferases lacking ACS activity. The CqACS proteins bore resemblance to the AtACS proteins and had the characteristic structural features typically observed in plant ACS enzymes. Twelve CqACS genes were distributed across 8 out of the 18 chromosomes of C. quinoa. The CqACS genes were expanded from segment duplication. Many cis-regulatory elements related with various abiotic stresses, phytohormones, and light were found. The expression patterns of ACS genes varied across different tissues of C. quinoa. Furthermore, the analysis of gene expression patterns under abiotic stress showed that CqACS genes can be responsive to various stresses, implying their potential functions in adapting to various abiotic stresses. The findings from this research serve as a foundation for delving deeper into the functional roles of CqACS genes.

Impact of foliar spray with Se, nano-Se and sodium sulfate on growth, yield and metabolic activities of red kidney bean

Sulfur (S) is an essential microelement for plants. Based on the chemical similarity between Se and S, selenium may affects sulphur uptake by plants. This work aimed at investigating the effect of foliar spray with sodium selenate, gum arabic coated selenium nanoparticles (GA-SeNPs ≈ 48.22 nm) and sodium sulfate on red kidney bean (Phaseolus vulgaris L.) plants. Each treatment was used at 0.0, 1, 5, 10 and 50 µM, alone or combination of sodium sulfate with either Se or nano-Se, each at 0.5, 2.5 and 5 µM concentrations. The effect of foliar spray on vegetative growth, seed quality, and some metabolic constituents of red kidney bean (Phaseolus vulgaris L.) plants were investigated. Selenium nanoparticles have been synthesized through the green route using gum arabic (as a stabilizing and coating agent. Foliar application of different concentrations of Se, nano-Se, Na2SO4 up to 10 μM and their interaction were effective in increasing the growth criteria (i.e. shoot and root lengths, plant fresh and dry weights, number of leaves and photosynthetic area (cm2 plant-1).There was also a significant increase in photosynthetic pigment contents, yield (i.e., 100-seed weight), total carbohydrate, crude proteins and mineral contents in both leaf as compared to their untreated control plants. Furthermore, interaction between sodium sulfate with nano-Se or Se, each at 5 µM significantly increased the vegetative growth, 100-seed weight, and pigment contents in leaves and improved the nutritional value and quality of red kidney bean seeds.

Brassinosteroid modulates ethylene synthesis and antioxidant metabolism to protect rice (Oryza sativa) against heat stress-induced inhibition of source‒sink capacity and photosynthetic and growth attributes

This study presents an exploration of the efficacy of brassinosteroids (BRs) and ethylene in mediating heat stress tolerance in rice (Oryza sativa). Heat is one of the major abiotic factors that prominently deteriorates rice production by influencing photosynthetic efficiency, source‒sink capacity, and growth traits. The application of BR (0.5 mM) and ethylene (200 μl l-1) either individually and/or in combination was found to alleviate heat stress-induced toxicity by significantly improving photosynthesis, source‒sink capacity and defense systems; additionally, it reduced the levels of oxidative stress markers and ethylene formation. The study revealed the positive influence of BR in promoting plant growth responses under heat stress through its interplay with ethylene biosynthesis and enhanced plant defense systems. Interestingly, treatment with the ethylene biosynthesis inhibitor aminoethoxyvinylglycine (AVG) substantiated that BR application to heat-stressed rice plants enhanced ethylene-dependent pathways to counteract the underlying adversities. Thus, BR action was found to be mediated by ethylene to promote heat tolerance in rice. The present study sheds light on the potential tolerance mechanisms which can ensure rice sustainability under heat stress conditions.

Characterization of MicroRNAs and Gene Expression in ACC Oxidase RNA Interference-Based Transgenic Bananas

Banana (Musa acuminata, AAA group) is a typically respiratory climacteric fruit. Previously, genes encoding ACC oxidase, one of the key enzymes in ethylene biosynthesis, Mh-ACO1 and Mh-ACO2 in bananas were silenced individually using RNAi interference technology, and fruit ripening of transgenic bananas was postponed. Here, the differential expression of miRNAs and their targeted mRNAs were analyzed in the transcriptomes of fruits at the third ripening stage, peel color more green than yellow, from the untransformed and RNAi transgenic bananas. Five significantly differentially expressed miRNAs (mac-miR169a, mac-miR319c-3p, mac-miR171a, mac-miR156e-5p, and mac-miR164a-5p) were identified. The predicted miRNA target genes were mainly enriched in six KEGG pathways, including 'sulfur relay system', 'protein digestion and absorption', 'histidine metabolism', 'pathogenic E. coli infection', 'sulfur metabolism', and 'starch and sucrose metabolism'. After ethylene treatment, the expression of ACC oxidase silencing-associated miRNAs was down-regulated, and that of their target genes was up-regulated along with fruit ripening. The evolutionary clustering relationships of miRNA precursors among 12 gene families related to fruit ripening were analyzed. The corresponding expression patterns of mature bodies were mainly concentrated in flowers, fruits, and leaves. Our results indicated that ethylene biosynthesis is associated with miRNAs regulating the expression of sulfur metabolism-related genes in bananas.

Melatonin Reverses High-Temperature-Stress-Inhibited Photosynthesis in the Presence of Excess Sulfur by Modulating Ethylene Sensitivity in Mustard

Melatonin is a pleiotropic, nontoxic, regulatory biomolecule with various functions in abiotic stress tolerance. It reverses the adverse effect of heat stress on photosynthesis in plants and helps with sulfur (S) assimilation. Our research objective aimed to find the influence of melatonin, along with excess sulfur (2 mM SO42-), in reversing heat stress's impacts on the photosynthetic ability of the mustard (Brassica juncea L.) cultivar SS2, a cultivar with low ATP-sulfurylase activity and a low sulfate transport index (STI). Further, we aimed to substantiate that the effect was a result of ethylene modulation. Melatonin in the presence of excess-S (S) increased S-assimilation and the STI by increasing the ATP-sulfurylase (ATP-S) and serine acetyltransferase (SAT) activity of SS2, and it enhanced the content of cysteine (Cys) and methionine (Met). Under heat stress, melatonin increased S-assimilation and diverted Cys towards the synthesis of more reduced glutathione (GSH), utilizing excess-S at the expense of less methionine and ethylene and resulting in plants' reduced sensitivity to stress ethylene. The treatment with melatonin plus excess-S increased antioxidant enzyme activity, photosynthetic-S use efficiency (p-SUE), Rubisco activity, photosynthesis, and growth under heat stress. Further, plants receiving melatonin and excess-S in the presence of norbornadiene (NBD; an ethylene action inhibitor) under heat stress showed an inhibited STI and lower photosynthesis and growth. This suggested that ethylene was involved in the melatonin-mediated heat stress reversal effects on photosynthesis in plants. The interaction mechanism between melatonin and ethylene is still elusive. This study provides avenues to explore the melatonin-ethylene-S interaction for heat stress tolerance in plants.

Genome-wide transcriptome analysis uncovers gene networks regulating fruit quality and volatile compounds in mango cultivar 'Tainong' during postharvest

Mango is one of the most economically important fruit; however, the gene regulatory mechanism associated with ripening and quality changes during storage remains largely unclear. This study explored the relationship between transcriptome changes and postharvest mango quality. Fruit quality patterns and volatile components were obtained using headspace gas chromatography and ion-mobility spectrometry (HS-GC-IMS). The changes in mango peel and pulp transcriptome were analyzed during four stages (pre-harvesting, harvesting, maturity, and overripe stages). Based on the temporal analysis, multiple genes involved in the biosynthesis of secondary metabolites were upregulated in both the peel and pulp during the mango ripening process. Moreover, cysteine and methionine metabolism related to ethylene synthesis were upregulated in the pulp over time. Weighted gene co-expression network analysis (WGCNA) further showed that the pathways of pyruvate metabolism, citrate cycle, propionate metabolism, autophagy, and SNARE interactions in vesicular transport were positively correlated with the ripening process. Finally, a regulatory network of important pathways from pulp to peel was constructed during the postharvest storage of mango fruit. The above findings provide a global insight into the molecular regulation mechanisms of postharvest mango quality and flavor changes.

The role of sulfur nutrition in plant response to metal(loid) stress: Facilitating biofortification and phytoremediation

Metal(loid)s contamination poses a serious threat to ecosystem biosafety and human health. Phytoremediation is a cost-effective and eco-friendly technology with good public acceptance, although the process does require a significant amount of time for success. To enhance the phytoremediation efficiency, numerous approaches have been explored, including soil amendments application with chelators to facilitate remediation. Sulfur (S), a macronutrient for plant growth, plays vital roles in several metabolic pathways that can actively affect metal(loid)s phytoextraction, as well as attenuate metal(loid) toxicity. In this review, different forms of S-amendments (fertilizers) on uptake and translocation in plants upon exposure to various metal(loid) are evaluated. Possible mechanisms for S application alleviating metal(loid) toxicity are documented at the physiological, biochemical and molecular levels. Furthermore, this review highlights the crosstalk between S-assimilation and other biomolecules, such as phytohormones, polyamines and nitric oxide, which are also important for metal(loid) stress tolerance. Given the effectiveness and potential of S amendments on phytoremediation, future studies should focus on optimizing phytoremediation efficiency in long-term field studies and on investigating the appropriate S dose to maximize the food safety and ecosystem health.

Identification of dynamic microRNA associated with systemic defence against Helicoverpa armigera infestation in Cajanus scarabaeoides

BACKGROUND

Helicoverpa armigera is a major insect pest of several crop plants, including pigeonpea. Resistant gene sources are not available in the cultivated gene pool, but resistance has been observed in its crop wild relative, Cajanus scarabaeoides. Gene regulatory mechanisms governing the systemic immune response of this plant to pod borer infestation have not yet been deciphered. MicroRNA (miRNA) profiles of H. armigera-infested and undamaged adjacent leaves of C. scarabaeoides were compared to gain an insight into the plant-insect interactions and to identify dynamic miRNA molecules potentially acting as mediators of systemic defence responses.

RESULTS

A total of 211 conserved, temporally dynamic miRNA were identified in the unfed adjacent leaves, out of which 98 were found to be differentially expressed in comparison to control leaves. On further analysis, most of the miRNA detected in the adjacent leaves was found to target genes involved in the defence pathways and plant immune response. An overlap of the differentially expressing miRNAs was observed between insect-fed and adjacent unfed leaves, indicating the transmission of signal from the site of infestation to the undamaged parts of the plant, indicative of induction of a systemic defence response.

CONCLUSION

The miRNA response in the unfed leaves had the signatures of induced changes in metabolism and signal transduction for induction of defence pathway genes. This study reveals the participation of miRNAs in imparting pod borer resistance and mounting a systemic defence response against pod borer infestation in C. scarabaeoides. © 2022 Society of Chemical Industry.

Ethylene-nitrogen synergism induces tolerance to copper stress by modulating antioxidant system and nitrogen metabolism and improves photosynthetic capacity in mustard

This study aimed to test the efficiency of ethylene (Eth; 200 µL L^-1 ethephon) in presence or absence of nitrogen (N; 80 mg N kg^-1 soil) in protecting photosynthetic apparatus from copper (Cu; 100 mg Cu kg^-1 soil) stress in mustard (Brassica juncea L.) and to elucidate the physio-biochemical modulation for Eth plus N-induced Cu tolerance. Elevated Cu-accrued reductions in photosynthesis and growth were accompanied by significantly higher Cu accumulation in leaves and oxidative stress with reduced assimilation of N and sulfur (S). Ethylene in coordination with N considerably reduced Cu accumulation, lowered lipid peroxidation, lignin accumulation, and contents of reactive oxygen species (hydrogen peroxide, H₂O₂, and superoxide anion, O₂^•-), and mitigated the negative effect of Cu on N and S assimilation, accumulation of non-protein thiols and phytochelatins, enzymatic, and non-enzymatic antioxidants (activity of ascorbate peroxidase, APX, and glutathione reductase, GR; content of reduced glutathione, GSH, and ascorbate, AsA), cell viability, photosynthesis, and growth. Overall, the effect of ethylene-nitrogen synergism was evident on prominently mitigating Cu stress and protecting photosynthesis. The approach of supplementing ethylene with N may be used as a potential tool to restrain Cu stress, and protect photosynthesis and growth of mustard plants.

Potential Roles of 1-Aminocyclopropane-1-carboxylic Acid Synthase Genes in the Response of Gossypium Species to Abiotic Stress by Genome-Wide Identification and Expression Analysis

Ethylene plays a pivotal role in plant stress resistance and 1-aminocyclopropane-1-carboxylic acid synthase (ACS) is the rate-limiting enzyme in ethylene biosynthesis. Upland cotton (Gossypium hirsutum L.) is the most important natural fiber crop, but the function of ACS in response to abiotic stress has rarely been reported in this plant. We identified 18 GaACS, 18 GrACS, and 35 GhACS genes in Gossypiumarboreum, Gossypium raimondii and Gossypiumhirsutum, respectively, that were classified as types I, II, III, or IV. Collinearity analysis showed that the GhACS genes were expanded from diploid cotton by the whole-genome-duplication. Multiple alignments showed that the C-terminals of the GhACS proteins were conserved, whereas the N-terminals of GhACS10 and GhACS12 were different from the N-terminals of AtACS10 and AtACS12, probably diverging during evolution. Most type II ACS genes were hardly expressed, whereas GhACS10/GhACS12 were expressed in many tissues and in response to abiotic stress; for example, they were highly and hardly expressed at the early stages of cold and heat exposure, respectively. The GhACS genes showed different expression profiles in response to cold, heat, drought, and salt stress by quantitative PCR analysis, which indicate the potential roles of them when encountering the various adverse conditions, and provide insights into GhACS functions in cotton’s adaptation to abiotic stress.

The Functional Interplay between Ethylene, Hydrogen Sulfide, and Sulfur in Plant Heat Stress Tolerance

Plants encounter several abiotic stresses, among which heat stress is gaining paramount attention because of the changing climatic conditions. Severe heat stress conspicuously reduces crop productivity through changes in metabolic processes and in growth and development. Ethylene and hydrogen sulfide (H2S) are signaling molecules involved in defense against heat stress through modulation of biomolecule synthesis, the antioxidant system, and post-translational modifications. Other compounds containing the essential mineral nutrient sulfur (S) also play pivotal roles in these defense mechanisms. As biosynthesis of ethylene and H2S is connected to the S-assimilation pathway, it is logical to consider the existence of a functional interplay between ethylene, H2S, and S in relation to heat stress tolerance. The present review focuses on the crosstalk between ethylene, H2S, and S to highlight their joint involvement in heat stress tolerance.

Ethylene Supplementation Combined with Split Application of Nitrogen and Sulfur Protects Salt-Inhibited Photosynthesis through Optimization of Proline Metabolism and Antioxidant System in Mustard (Brassica juncea L.)

In the present study, the potential of ethylene as ethephon (an ethylene source) was investigated individually and in combination with split doses of nitrogen (N) and sulfur (S) soil treatments for removal of the damaging effects of salt stress (100 mM NaCl) in mustard (Brassica juncea L.). Plants were grown with 50 mg N plus 50 mg S kg^-1 soil at sowing time and an equivalent dose at 20 days after sowing [N50 + S50]_{0d and 20d}. Ethephon at 200 μL L^‒1 was applied to combined split doses of N and S with or without NaCl. Plants subjected to NaCl showed a decrease in growth and photosynthetic characteristics as well as N and S assimilation, whereas proline metabolism and antioxidants increased. The application of ethephon to plants grown with split N and S doses significantly enhanced photosynthetic efficiency by increasing the assimilation of N and S, improving the concentration of proline and induction of the antioxidant system with or without NaCl. The regulation of ethylene and/or split forms of N and S application may be potential tools for not just overcoming salt stress effects in this species and in related Brassicaceae but also enhancing their photosynthesis and growth potential through increased nutrient assimilation.

Hydrogen peroxide potentiates defense system in presence of sulfur to protect chloroplast damage and photosynthesis of wheat under drought stress

The involvement of hydrogen peroxide (H₂ O₂ ) combined with sulfur (S) was studied in the protection of the photosynthetic performance of wheat (Triticum aestivum L.) under drought stress. The mechanisms of S-assimilation, the activity of antioxidants, glucose sensitivity, water and osmotic relations and abscisic acid (ABA) content were the focus. The combined application of 50 μM H₂ O₂ and 100 mg S kg^-1 soil (sulfur) resulted in a marked increase in S-assimilation and activity of antioxidant enzymes, with decreased glucose sensitivity and ABA content causing improvement in the structure and function of the photosynthetic apparatus under drought stress. The photosynthetic performance, pigment system (PS) II activity, and growth were improved conspicuously by H₂ O₂ in the presence of S, as H₂ O₂ induced S-assimilation capacity, the activity of antioxidant enzymes, and GSH synthesis under drought stress. Our study shows that H₂ O₂ is more effective in the reversal of drought stress in the presence of S through its influence on S-assimilation, glucose sensitivity, and antioxidant system. These results provide evidence for the effectiveness of H₂ O₂ in improving photosynthesis under drought stress in the presence of S.

Ethylene regulates sulfur acquisition by regulating the expression of sulfate transporter genes in oilseed rape

To manage nutrient deficiencies, plants develop both morphological and physiological responses. The studies on the regulation of these responses are limited; however, certain hormones and signaling components have been largely implicated. Several studies depicted ethylene as a regulator of the response of some nutrient deficiencies like iron, phosphorous and potassium. The present study focused on the response of sulfur in the presence and absence of ethylene. The experiments were performed in hydroponic nutrient media, using oilseed rape grown with or without sulfur deficiency and ethylene treatments for 10 days. The ACC oxidase and ACC synthase were observed significantly reduced in sulfur-deficient plants treated with ethylene compared to control. The biomass and photosynthetic parameters, including the expression of multicomplex thylakoidal proteins showed a significant increase in sulfur deficient plants supplemented with ethylene. The enzymes related to sulfur regulation such as sulfate adenyltransferase, glutamine synthetase and O-acetylserine (thiol)lyase also showed similar results as shown by the morphological data. The relative expression of the sulfur transporter genes BnSultr1, 1, BnSultr1, 2, BnSultr4,1, BnSultr 4,2, ATP sulfurylase and OASTL increased in sulfur-deficient plants, whereas their expression decreased when ethylene was given to the plants. Fe and S nutritional correlations are already known; therefore, Fe-transporters like IRT1 and FRO1 were also evaluated, and similar results as for the sulfur transporter genes were observed. The overall results indicated that ethylene regulates sulfur acquisition by regulating the expression of sulfur transporter genes in oilseed rape (Brassica napus).

Ethylene and Sulfur Coordinately Modulate the Antioxidant System and ABA Accumulation in Mustard Plants under Salt Stress

This study explored the interactive effect of ethephon (2-chloroethyl phosphonic acid; an ethylene source) and sulfur (S) in regulating the antioxidant system and ABA content and in maintaining stomatal responses, chloroplast structure, and photosynthetic performance of mustard plants (Brassica juncea L. Czern.) grown under 100 mM NaCl stress. The treatment of ethephon (200 µL L⁻¹) and S (200 mg S kg⁻¹ soil) together markedly improved the activity of enzymatic and non-enzymatic components of the ascorbate-glutathione (AsA-GSH) cycle, resulting in declined oxidative stress through lesser content of sodium (Na⁺) ion and hydrogen peroxide (H₂O₂) in salt-stressed plants_. These changes promoted the development of chloroplast thylakoids and photosynthetic performance under salt stress. Ethephon + S also reduced abscisic acid (ABA) accumulation in guard cell, leading to maximal stomatal conductance under salt stress. The inhibition of ethylene action by norbornadiene (NBD) in salt- plus non-stressed treated plants increased ABA and H₂O₂ contents, and reduced stomatal opening, suggesting the involvement of ethephon and S in regulating stomatal conductance. These findings suggest that ethephon and S modulate antioxidant system and ABA accumulation in guard cells, controlling stomatal conductance, and the structure and efficiency of the photosynthetic apparatus in plants under salt stress.

Interaction Between Sulfur and Iron in Plants

It is well known that S interacts with some macronutrients, such as N, P, and K, as well as with some micronutrients, such as Fe, Mo, Cu, Zn, and B. From our current understanding, such interactions could be related to the fact that: (i) S shares similar chemical properties with other elements (e.g., Mo and Se) determining competition for the acquisition/transport process (SULTR transporter family proteins); (ii) S-requiring metabolic processes need the presence of other nutrients or regulate plant responses to other nutritional deficiencies (S-containing metabolites are the precursor for the synthesis of ethylene and phytosiderophores); (iii) S directly interacts with other elements (e.g., Fe) by forming complexes and chemical bonds, such as Fe-S clusters; and (iv) S is a constituent of organic molecules, which play crucial roles in plants (glutathione, transporters, etc.). This review summarizes the current state of knowledge of the interplay between Fe and S in plants. It has been demonstrated that plant capability to take up and accumulate Fe strongly depends on S availability in the growth medium in both monocots and dicot plants. Moreover, providing S above the average nutritional need enhances the Fe content in wheat grains, this beneficial effect being particularly pronounced under severe Fe limitation. On the other hand, Fe shortage induces a significant increase in the demand for S, resulting in enhanced S uptake and assimilation rate, similar to what happens under S deficiency. The critical evaluation of the recent studies on the modulation of Fe/S interaction by integrating old and new insights gained on this topic will help to identify the main knowledge gaps. Indeed, it remains a challenge to determine how the interplay between S and Fe is regulated and how plants are able to sense environmental nutrient fluctuations and then to adapt their uptake, translocation, assimilation, and signaling. A better knowledge of the mechanisms of Fe/S interaction might considerably help in improving crop performance within a context of limited nutrient resources and a more sustainable agriculture.

Phytohormonal Roles in Plant Responses to Heavy Metal Stress: Implications for Using Macrophytes in Phytoremediation of Aquatic Ecosystems

Heavy metals can represent a threat to the health of aquatic ecosystems. Unlike organic chemicals, heavy metals cannot be eliminated by natural processes such as their degradation into less toxic compounds, and this creates unique challenges for their remediation from soil, water, and air. Phytoremediation, defined as the use of plants for the removal of environmental contaminants, has many benefits compared to other pollution-reducing methods. Phytoremediation is simple, efficient, cost-effective, and environmentally friendly because it can be carried out at the polluted site, which simplifies logistics and minimizes exposure to humans and wildlife. Macrophytes represent a unique tool to remediate diverse environmental media because they can accumulate heavy metals from contaminated sediment via roots, from water via submerged leaves, and from air via emergent shoots. In this review, a synopsis is presented about how plants, especially macrophytes, respond to heavy metal stress; and we propose potential roles that phytohormones can play in the alleviation of metal toxicity in the aquatic environment. We focus on the uptake, translocation, and accumulation mechanisms of heavy metals in organs of macrophytes and give examples of how phytohormones interact with plant defense systems under heavy metal exposure. We advocate for a more in-depth understanding of these processes to inform more effective metal remediation techniques from metal-polluted water bodies. Environ Toxicol Chem 2021;40:7-22. © 2020 SETAC.

The Versatile Roles of Sulfur-Containing Biomolecules in Plant Defense-A Road to Disease Resistance

Sulfur (S) is an essential plant macronutrient and the pivotal role of sulfur compounds in plant disease resistance has become obvious in recent decades. This review attempts to recapitulate results on the various functions of sulfur-containing defense compounds (SDCs) in plant defense responses to pathogens. These compounds include sulfur containing amino acids such as cysteine and methionine, the tripeptide glutathione, thionins and defensins, glucosinolates and phytoalexins and, last but not least, reactive sulfur species and hydrogen sulfide. SDCs play versatile roles both in pathogen perception and initiating signal transduction pathways that are interconnected with various defense processes regulated by plant hormones (salicylic acid, jasmonic acid and ethylene) and reactive oxygen species (ROS). Importantly, ROS-mediated reversible oxidation of cysteine residues on plant proteins have profound effects on protein functions like signal transduction of plant defense responses during pathogen infections. Indeed, the multifaceted plant defense responses initiated by SDCs should provide novel tools for plant breeding to endow crops with efficient defense responses to invading pathogens.

Secretome Analysis of the Banana Fusarium Wilt Fungi Foc R1 and Foc TR4 Reveals a New Effector OASTL Required for Full Pathogenicity of Foc TR4 in Banana

Banana Fusarium wilt (BFW), which is one of the most important banana diseases worldwide, is mainly caused by Fusarium oxysporum f. sp. cubense tropic race 4 (Foc TR4). In this study, we conducted secretome analysis of Foc R1 and Foc TR4 and discovered a total of 120 and 109 secretory proteins (SPs) from Foc R1 cultured alone or with banana roots, respectively, and 129 and 105 SPs respectively from Foc TR4 cultured under the same conditions. Foc R1 and Foc TR4 shared numerous SPs associated with hydrolase activity, oxidoreductase activity, and transferase activity. Furthermore, in culture with banana roots, Foc R1 and Foc TR4 secreted many novel SPs, of which approximately 90% (Foc R1; 57/66; Foc TR4; 50/55) were unconventional SPs without signal peptides. Comparative analysis of SPs in Foc R1 and Foc TR4 revealed that Foc TR4 not only generated more specific SPs but also had a higher proportion of SPs involved in various metabolic pathways, such as phenylalanine metabolism and cysteine and methionine metabolism. The cysteine biosynthesis enzyme O-acetylhomoserine (thiol)-lyase (OASTL) was the most abundant root inducible Foc TR4-specific SP. In addition, knockout of the OASTL gene did not affect growth of Foc TR4; but resulted in the loss of pathogenicity in banana 'Brazil'. We speculated that OASTL functions in banana by interfering with the biosynthesis of cysteine, which is the precursor of an enormous number of sulfur-containing defense compounds. Overall, our studies provide a basic understanding of the SPs in Foc R1 and Foc TR4; including a novel effector in Foc TR4.

Transcriptomic analyses reveal a systemic defense role of the uninfested adjacent leaf in tea plant (Camellia sinensis) attacked by tea geometrids (Ectropis obliqua)

To get a more detailed understanding of the interaction between tea plant (Camellia sinensis) and tea geometrids (Ectropis obliqua), transcriptomic profile in undamaged adjacent leaf (TGL) of tea geometrids fed local leaves (LL) was investigated for the first time. Here, approximately 245 million clean reads contained 39.39 Gb of sequence data were obtained from TGL. Further analysis revealed that systemic response was induced in TGL after tea geometrids feeding on LL, although the defense response was weaker than that in LL. The differentially expressed genes (DEGs) identification analysis showed little overlap of DEGs between TGL and LL. Comparative transcriptome analysis suggested that JA signal regulated resistant pathway was induced in LL; whereas primary metabolism pathway was activated in TGL in response to tea geometrids feeding. This study reveals a novel resistance mechanism of TGL to tea geometrids feeding.

Tuber transcriptome profiling of eight potato cultivars with different cold-induced sweetening responses to cold storage

Tubers are vegetative reproduction organs formed from underground extensions of the plant stem. Potato tubers are harvested and stored for months. Storage under cold temperatures of 2-4 °C is advantageous for supressing sprouting and diseases. However, development of reducing sugars can occur with cold storage through a process called cold-induced sweetening (CIS). CIS is undesirable as it leads to darkened color with fry processing. The purpose of the current study was to find differences in biological responses in eight cultivars with variation in CIS resistance. Transcriptome sequencing was done on tubers before and after cold storage and three approaches were taken for gene expression analysis: 1. Gene expression correlated with end-point glucose after cold storage, 2. Gene expression correlated with increased glucose after cold storage (after-before), and 3. Differential gene expression before and after cold storage. Cultivars with high CIS resistance (low glucose after cold) were found to increase expression of an invertase inhibitor gene and genes involved in DNA replication and repair after cold storage. The cultivars with low CIS resistance (high glucose after cold) showed increased expression of genes involved in abiotic stress response, gene expression, protein turnover and the mitochondria. There was a small number of genes with similar expression patterns for all cultivars including genes involved in cell wall strengthening and phospholipases. It is proposed that the pattern of gene expression is related to chilling-induced DNA damage repair and cold acclimation and that genetic variation in these processes are related to CIS.

Identification of Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) responsive miRNAs in banana root

The fungus, Fusarium oxysporum f. sp. cubense (Foc), is the causal agent of Fusarium wilt disease, which is the most serious disease affecting the whole banana industry. Although extensive studies have characterized many Foc-responsive genes in banana, the molecular mechanisms on microRNA level underlying both banana defense and Foc pathogenesis are not yet fully understood. In this study, we aimed to reveal the role of miRNA during banana-Foc TR4 interactions. Illumina sequencing was used to reveal the changes in small RNAome profiles in roots of Foc TR4-inoculated ‘Tianbaojiao’ banana (Musa acuminata cv. Tianbaojiao) in the early stages (i.e. 5 h, 10 h and 25 h post Foc TR4 inoculation, respectively). The expression of some differentially expressed (DE) miRNAs and their predicted target genes was studied by using quantitative real time PCR (qRT-PCR). Totally, 254 known miRNAs from 31 miRNA families and 28 novel miRNAs were identified. Differential expression analysis identified 84, 77 and 74 DE miRNAs at the three respective Foc TR4 infection time points compared with control healthy banana (CK). GO and KEGG analysis revealed that most of the predicted target genes of DE miRNAs (DET) were implicated in peroxisome, fatty acid metabolism, auxin-activated signaling pathway, sulfur metabolism, lignin metabolism and so on, and many known stress responsive genes were identified to be DETs. Moreover, expected inverse correlations were confirmed between some miRNA and their corresponding target genes by using qRT-PCR analysis. Our study revealed that miRNA play important regulatory roles during the banana-Foc TR4 interaction by regulating peroxidase, fatty acid metabolism, auxin signaling, sulfur metabolism, lignin metabolism related genes and many known stress responsive genes.

Interaction of sulfur with phytohormones and signaling molecules in conferring abiotic stress tolerance to plants

Sulfur (S) is a macronutrient for the plant, which has an immense role in basic plant processes and regulation of several metabolic pathways. It has also a major role in providing protection against adverse conditions. Sulfur-containing amino acids and metabolites maintain plant cell mechanisms to improve stress tolerance. It interacts with several biomolecules such as phytohormones, polyamines, nitric oxide (NO), and even with other plant nutrients, which can produce some derivatives those are essential for abiotic stress tolerance. Different S derivatives stimulate signaling cascades, for the upregulation of different cellular messengers such as abscisic acid, Ca²⁺, and NO. Sulfur is also known to interact with some essential plant nutrients by influencing their uptake and transport, hence, confers nutrient homeostasis efficiencies. This review focuses on how S is interacted with several signaling molecules like NO, glutathiones, phytohormones, hydrogen sulfide, polyamines, etc. This is a concise summary aimed at guiding the researchers to study S-related plant processes in the light of abiotic stress tolerance.

Higher Novel L-Cys Degradation Activity Results in Lower Organic-S and Biomass in Sarcocornia than the Related Saltwort, Salicornia

Salicornia and Sarcocornia are almost identical halophytes whose edible succulent shoots hold promise for commercial production in saline water. Enhanced sulfur nutrition may be beneficial to crops naturally grown on high sulfate. However, little is known about sulfate nutrition in halophytes. Here we show that Salicornia europaea (ecotype RN) exhibits a significant increase in biomass and organic-S accumulation in response to supplemental sulfate, whereas Sarcocornia fruticosa (ecotype VM) does not, instead exhibiting increased sulfate accumulation. We investigated the role of two pathways on organic-S and biomass accumulation in Salicornia and Sarcoconia: the sulfate reductive pathway that generates Cys and l-Cys desulfhydrase that degrades Cys to H₂S, NH₃, and pyruvate. The major function of O-acetyl-Ser-(thiol) lyase (OAS-TL; EC 2.5.1.47) is the formation of l-Cys, but our study shows that the OAS-TL A and OAS-TL B of both halophytes are enzymes that also degrade l-Cys to H₂S. This activity was significantly higher in Sarcocornia than in Salicornia, especially upon sulfate supplementation. The activity of the sulfate reductive pathway key enzyme, adenosine 5'-phosphosulfate reductase (APR, EC 1.8.99.2), was significantly higher in Salicornia than in Sarcocornia These results suggest that the low organic-S level in Sarcocornia is the result of high l-Cys degradation rate by OAS-TLs, whereas the greater organic-S and biomass accumulation in Salicornia is the result of higher APR activity and low l-Cys degradation rate, resulting in higher net Cys biosynthesis. These results present an initial road map for halophyte growers to attain better growth rates and nutritional value of Salicornia and Sarcocornia.

Genome-wide identification and expression profiling of EIL gene family in woody plant representative poplar (Populus trichocarpa)

This study aimed to improve current understanding on ethylene-insensitive 3-like (EIL) members, least explored in woody plants such as poplar (Populus trichocarpa Torr. & Grey). Herein, seven putative EIL members were identified in P. trichocarpa genome and were roughly annotated either as EIN3-like sequence associated with ethylene pathway or EIL3-like sequences related with sulfur (S)-pathway. Motif-distribution pattern of proteins also corroborated this annotation. They were distributed on six chromosomes (chr1, 3, 4 and 8-10), and were revealed to encode a protein of 509-662 residues with nuclear localization. The presence of ethylene insensitive 3 (EIN3; PF04873) domain (covering first 80-280 residues from N-terminus) was confirmed by Hidden Markov Model-based search. The first half of EIL proteins (∼80-280 residues including EIN3 domain) was substantially conserved. The second half (∼300-600 residues) was considerably diverged. Additionally, first half of proteins harbored acidic, proline-rich and glutamine-rich sites, and supported the essentiality of these regions in the transcriptional-activation and protein-function. Moreover, identified six segmental and one-tandem duplications demonstrated the negative or purifying selective nature of mutations. Furthermore, expression profile analysis indicated the possibility of a crosstalk between EIN3- and EIL3-like genes, and co-expression networks implicated their interactions with very diverse panels of biological molecules.

CO2 effects on the waterlogging response of 'Gisela 5' and 'Gisela 6' (Prunus cerasusxPrunus canescens) sweet cherry (Prunus avium) rootstocks

Climate change is submitting countries of the Mediterranean arc to periods of drought alternating with heavy rain and waterlogging. Eventual floods along with the rising CO₂ in the atmosphere present an unpredictable scenario that affects crop survival. The effect of both stresses combined has been studied in sweet cherry plants. 'Gisela 5' and 'Gisela 6' were evaluated as rootstocks of the sweet cherry cultivar 'Burlat'. Plants were placed in a controlled-climate chamber for 7days, then they were submitted to waterlogging for another 7days and the response to this stress and the subsequent recovery were studied (7 more days). The experiment was carried out at 400μmolmol^-1 CO₂ (ambient CO₂) and 800μmolmol^-1 CO₂, at 26°C, and plant water status and growth, net CO₂ assimilation, transpiration, stomatal conductance, water potential, chlorophyll fluorescence, relative water content, anions content, proline, lipid peroxidation, soluble sugars, and starch were measured. Differences in the response and in its intensity were detected in both rootstocks. Some parameters - such as photosynthesis, soluble sugars, starch, TBARS, and NO₃^- - varied depending on the CO₂ conditions and the waterlogging effect. Elevated CO₂ was able to increase photosynthesis and thereby help plants to overcome waterlogging.

Nitric oxide improves S-assimilation and GSH production to prevent inhibitory effects of cadmium stress on photosynthesis in mustard (Brassica juncea L.)

Nitric oxide (NO) is an important gaseous signalling molecule that participates in many developmental and physiological processes, including defense responses against toxic metals in plants. The role of NO in cadmium (Cd)-induced toxic effects on photosynthesis was examined in mustard (Brassica juncea L.) plants. Exposure of plants to 50 μM Cd significantly enhanced oxidative stress (H₂O₂ content and lipid peroxidation) and impaired plant growth and photosynthesis, chlorophyll fluorescence and reduced chlorophyll content and stomatal conductance. However, the exogenous application of 100 μM sodium nitroprusside (SNP, a donor of NO) reversed the effects of Cd through its stimulation of ROS-scavenging compounds (superoxide dismutase, ascorbate peroxidase, glutathione reductase and reduced glutathione). Exogenous SNP significantly increased plant growth, photosynthesis and chlorophyll content and diminished the accumulation of reactive oxygen species (H₂O₂) and TBARS content. It also reduced the effects of Cd on thylakoid membrane of the chloroplasts. Application of SNP together with 1.0 mM SO₄^2- showed better responses than SNP alone. The protective effect of NO was achieved through enhanced production of reduced glutathione (GSH). GSH biosynthesis in plants treated with buthionine sulfoximine (BSO), a GSH biosynthetic inhibitor, was not completely inhibited in presence of NO and S, suggesting that NO stimulated S-assimilation and GSH production of Cd exposed plants. This study concludes that NO counteracts Cd toxicity in B. juncea strongly by regulating S-assimilation and GSH production.

EIN3 interferes with the sulfur deficiency signaling in Arabidopsis thaliana through direct interaction with the SLIM1 transcription factor

Sulfur deficiency in plants leads to metabolic reprogramming through changes of gene expression. SLIM1 is so far the only characterized transcription factor associated strictly with sulfur deficiency stress in Arabidopsis thaliana. It belongs to the same protein family as EIN3, a major positive switch of ethylene signaling pathway. It binds to the specific cis sequence called UPE-box. Here we show that SLIM1 interacts with UPE-box as a homodimer. Interestingly, the same region of the protein is used for heterodimerization with EIN3; however, the heterodimer is not able to recognize UPE-box. Expression of several SLIM1-dependent genes is enhanced in sulfur deficiency grown Arabidopsis ein3-1 seedlings (with mutated EIN3 protein). This implies a possible regulatory mechanism of ethylene in sulfur metabolism through direct EIN3-SLIM1 interaction.

LcMKK, a MAPK kinase from Lycium chinense, confers cadmium tolerance in transgenic tobacco by transcriptional upregulation of ethylene responsive transcription factor gene

Cadmium (Cd) is a highly toxic element to plants. Ethylene is an important phytohormone in the regulation of plant growth, development and stress response. Mitogen-activated protein kinase (MAPK) activation has been observed in plants exposed to Cd stress and was suggested to be involved in ethylene biosynthesis. We hypothesized that there may be a link between MAPK cascades and ethylene signalling in Cd-stressed plants. To test this hypothesis, the expression of LcMKK, LchERF and LcGSH1 genes, endogenous ethylene accumulation, GSH content and Cd concentration in Lycium chinense with or without Cd stress treatment were studied. Our results showed that LcMKK gene expression can be induced by the treatment of Cd in L. chinense. The transgenic tobacco expressing 35S::LcMKK showed greater tolerance to Cd stress and enhanced expression of NtERF and NtGSH1 genes, indicating that LcMKK is associated with the enhanced expression level of ERF and GSH synthesis-related genes in tobacco. We also found that endogenous ethylene and GSH content can be induced by Cd stress in L. chinense, and inhibited by cotreatment with PD98059, an inhibitor of MAPK kinase. Evidences presented here suggest that under Cd stress, GSH accumulation occurred at least partially by enhanced LcMKK gene expression and the ethylene signal transduction pathways might be involved in this accumulation.

Ethylene and Metal Stress: Small Molecule, Big Impact

The phytohormone ethylene is known to mediate a diverse array of signaling processes during abiotic stress in plants. Whereas many reports have demonstrated enhanced ethylene production in metal-exposed plants, the underlying molecular mechanisms are only recently investigated. Increasing evidence supports a role for ethylene in the regulation of plant metal stress responses. Moreover, crosstalk appears to exist between ethylene and the cellular redox balance, nutrients and other phytohormones. This review highlights our current understanding of the key role ethylene plays during responses to metal exposure. Moreover, particular attention is paid to the integration of ethylene within the broad network of plant responses to metal stress.

Links Between Ethylene and Sulfur Nutrition-A Regulatory Interplay or Just Metabolite Association?

Multiple reports demonstrate associations between ethylene and sulfur metabolisms, however the details of these links have not yet been fully characterized; the links might be at the metabolic and the regulatory levels. First, sulfur-containing metabolite, methionine, is a precursor of ethylene and is a rate limiting metabolite for ethylene synthesis; the methionine cycle contributes to both sulfur and ethylene metabolism. On the other hand, ethylene is involved in the complex response networks to various stresses and it is known that S deficiency leads to photosynthesis and C metabolism disturbances that might be responsible for oxidative stress. In several plant species, ethylene increases during sulfur starvation and might serve signaling purposes to initiate the process of metabolism reprogramming during adjustment to sulfur deficit. An elevated level of ethylene might result from increased activity of enzymes involved in its synthesis. It has been demonstrated that the alleviation of cadmium stress in plants by application of S seems to be mediated by ethylene formation. On the other hand, the ethylene-insensitive Nicotiana attenuata plants are impaired in sulfur uptake, reduction and metabolism, and they invest their already limited S into methionine needed for synthesis of ethylene constitutively emitted in large amounts to the atmosphere. Regulatory links of EIN3 and SLIM1 (both from the same family of transcriptional factors) involved in the regulation of ethylene and sulfur pathway, respectively, is also quite probable as well as the reciprocal modulation of both pathways on the enzyme activity levels.

Gene Co-Expression Analysis Inferring the Crosstalk of Ethylene and Gibberellin in Modulating the Transcriptional Acclimation of Cassava Root Growth in Different Seasons

Cassava is a crop of hope for the 21st century. Great advantages of cassava over other crops are not only the capacity of carbohydrates, but it is also an easily grown crop with fast development. As a plant which is highly tolerant to a poor environment, cassava has been believed to own an effective acclimation process, an intelligent mechanism behind its survival and sustainability in a wide range of climates. Herein, we aimed to investigate the transcriptional regulation underlying the adaptive development of a cassava root to different seasonal cultivation climates. Gene co-expression analysis suggests that AP2-EREBP transcription factor (ERF1) orthologue (D142) played a pivotal role in regulating the cellular response to exposing to wet and dry seasons. The ERF shows crosstalk with gibberellin, via ent-Kaurene synthase (D106), in the transcriptional regulatory network that was proposed to modulate the downstream regulatory system through a distinct signaling mechanism. While sulfur assimilation is likely to be a signaling regulation for dry crop growth response, calmodulin-binding protein is responsible for regulation in the wet crop. With our initiative study, we hope that our findings will pave the way towards sustainability of cassava production under various kinds of stress considering the future global climate change.

Role of Ethylene and Its Cross Talk with Other Signaling Molecules in Plant Responses to Heavy Metal Stress

Excessive heavy metals (HMs) in agricultural lands cause toxicities to plants, resulting in declines in crop productivity. Recent advances in ethylene biology research have established that ethylene is not only responsible for many important physiological activities in plants but also plays a pivotal role in HM stress tolerance. The manipulation of ethylene in plants to cope with HM stress through various approaches targeting either ethylene biosynthesis or the ethylene signaling pathway has brought promising outcomes. This review covers ethylene production and signal transduction in plant responses to HM stress, cross talk between ethylene and other signaling molecules under adverse HM stress conditions, and approaches to modify ethylene action to improve HM tolerance. From our current understanding about ethylene and its regulatory activities, it is believed that the optimization of endogenous ethylene levels in plants under HM stress would pave the way for developing transgenic crops with improved HM tolerance.

Transcriptional and metabolic alternations rebalance wheat grain storage protein accumulation under variable nitrogen and sulfur supply

Wheat (Triticum aestivum L.) grain storage proteins (GSPs) are major determinants of flour end-use value. Biological and molecular mechanisms underlying the developmental and nutritional determination of GSP accumulation in cereals are as yet poorly understood. Here we timed the accumulation of GSPs during wheat grain maturation relative to changes in metabolite and transcript pools in different conditions of nitrogen (N) and sulfur (S) availability. We found that the N/S supply ratio modulated the duration of accumulation of S-rich GSPs and the rate of accumulation of S-poor GSPs. These changes are likely to be the result of distinct relationships between N and S allocation, depending on the S content of the GSP. Most developmental and nutritional modifications in GSP synthesis correlated with the abundance of structural gene transcripts. Changes in the expression of transport and metabolism genes altered the concentrations of several free amino acids under variable conditions of N and S supply, and these amino acids seem to be essential in determining GSP expression. The comprehensive data set generated and analyzed here provides insights that will be useful in adapting fertilizer use to variable N and S supply, or for breeding new cultivars with balanced and robust GSP composition.

Exogenous salicylic acid improves photosynthesis and growth through increase in ascorbate-glutathione metabolism and S assimilation in mustard under salt stress

Ascorbate (AsA)-glutathione (GSH) cycle metabolism has been regarded as the most important defense mechanism for the resistance of plants under stress. In this study the influence of salicylic acid (SA) was studied on ascorbate-glutathione pathway, S-assimilation, photosynthesis and growth of mustard (Brassica juncea L.) plants subjected to 100 mM NaCl. Treatment of SA (0.5 mM) alleviated the negative effects of salt stress and improved photosynthesis and growth through increase in enzymes of ascorbate-glutathione pathway which suggest that SA may participate in the redox balance under salt stress. The increase in leaf sulfur content through higher activity of ATP sulfurylase (ATPS) and serine acetyl transferase (SAT) by SA application was associated with the increased accumulation of glutathione (GSH) and lower levels of oxidative stress. These effects of SA were substantiated by the findings that application of SA-analog, 2,6, dichloro-isonicotinic acid (INA) and 1 mM GSH treatment produced similar results on rubisco, photosynthesis and growth of plants establishing that SA application alleviates the salt-induced decrease in photosynthesis mainly through inducing the enzyme activity of ascorbate-glutathione pathway and increased GSH production. Thus, SA/GSH could be a promising tool for alleviation of salt stress in mustard plants.

Transcriptional responses of Medicago truncatula upon sulfur deficiency stress and arbuscular mycorrhizal symbiosis

Sulfur plays an essential role in plants' growth and development and in their response to various abiotic and biotic stresses despite its leachability and its very low abundance in the only form that plant roots can uptake (sulfate). It is part of amino acids, glutathione (GSH), thiols of proteins and peptides, membrane sulfolipids, cell walls and secondary products, so reduced availability can drastically alter plant growth and development. The nutritional benefits of symbiotic interactions can help the plant in case of S deficiency. In particular the arbuscular mycorrhizal (AM) interaction improves N, P, and S plant nutrition, but the mechanisms behind these exchanges are not fully known yet. Although the transcriptional changes in the leguminous model plant Medicago truncatula have been already assessed in several biotic and/or abiotic conditions, S deficiency has not been considered so far. The aim of this work is to get a first overview on S-deficiency responses in the leaf and root tissues of plants interacting with the AM fungus Rhizophagus irregularis. Several hundred genes displayed significantly different transcript accumulation levels. Annotation and GO ID association were used to identify biological processes and molecular functions affected by sulfur starvation. Beside the beneficial effects of AM interaction, plants were greatly affected by the nutritional status, showing various differences in their transcriptomic footprints. Several pathways in which S plays an important role appeared to be differentially affected according to mycorrhizal status, with a generally reduced responsiveness to S deficiency in mycorrhized plants.

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.

Ethylene production is associated with alleviation of cadmium-induced oxidative stress by sulfur in mustard types differing in ethylene sensitivity

We studied the response of ethylene-sensitive (Pusa Jai Kisan) and ethylene-insensitive (SS2) mustard (Brassica juncea) cultivars to 0, 0.5, 1.0 and 2.0 mM SO₄(2-), and the effect of 1.0 mM SO₄(2-) was studied in the amelioration of 50 µM cadmium (Cd). The Cd-induced oxidative stress and Cd accumulation were greater in SS2 than Pusa Jai Kisan, but sulfur (S) application alleviated Cd-induced oxidative stress more prominently in Pusa Jai Kisan by increasing S-metabolism and synthesis of reduced glutathione (GSH) and ethylene production; and promoted photosynthesis and plant dry mass under Cd stress. The ethylene-sensitive cultivar responded more to S treatment under Cd stress and showed increased activity of antioxidant system resulting in increased photosynthesis and growth. Cadmium treatment resulted in rapid increase in ethylene formation which adversely influenced photosynthesis and plant dry mass. However, S and ethephon application to Cd-treated plants lowered ethylene formation to optimal range responsible for maximal GSH synthesis and protection against Cd-induced oxidative stress. The similarity of the effectiveness of 1.0 mM SO₄(2-) with 200 µL L(-1) ethylene source as ethephon in alleviation of 50 µM Cd further verifies that differential alleviation of Cd toxicity in the two cultivars by S was dependent on ethylene production. The results suggest that ethylene production determines Cd stress alleviation by S via regulatory interaction with antioxidant metabolism. Thus, ethylene production and sensitivity bear a prominent role in alleviation of Cd stress by S and can be used as a criterion for developing Cd tolerant genotypes.

Transcriptome profiling of genes and pathways associated with arsenic toxicity and tolerance in Arabidopsis

BACKGROUND

Arsenic (As) is a toxic metalloid found ubiquitously in the environment and widely considered an acute poison and carcinogen. However, the molecular mechanisms of the plant response to As and ensuing tolerance have not been extensively characterized. Here, we report on transcriptional changes with As treatment in two Arabidopsis accessions, Col-0 and Ws-2.

RESULTS

The root elongation rate was greater for Col-0 than Ws-2 with As exposure. Accumulation of As was lower in the more tolerant accession Col-0 than in Ws-2. We compared the effect of As exposure on genome-wide gene expression in the two accessions by comparative microarray assay. The genes related to heat response and oxidative stresses were common to both accessions, which indicates conserved As stress-associated responses for the two accessions. Most of the specific response genes encoded heat shock proteins, heat shock factors, ubiquitin and aquaporin transporters. Genes coding for ethylene-signalling components were enriched in As-tolerant Col-0 with As exposure. A tolerance-associated gene candidate encoding Leucine-Rich Repeat receptor-like kinase VIII (LRR-RLK VIII) was selected for functional characterization. Genetic loss-of-function analysis of the LRR-RLK VIII gene revealed altered As sensitivity and the metal accumulation in roots.

CONCLUSIONS

Thus, ethylene-related pathways, maintenance of protein structure and LRR-RLK VIII-mediated signalling may be important mechanisms for toxicity and tolerance to As in the species. Here, we provide a comprehensive survey of global transcriptional regulation for As and identify stress- and tolerance-associated genes responding to As.