A GSHS-like gene from Lycium chinense maybe regulated by cadmium-induced endogenous salicylic acid and overexpression of this gene enhances tolerance to cadmium stress in Arabidopsis

From stress to resilience: Unraveling the molecular mechanisms of cadmium toxicity, detoxification and tolerance in plants

Soil contamination with cadmium (Cd) has become a global issue due to increasing human activities. Cd contamination poses threats to plant growth as well as jeopardizing food safety and human health through the accumulation of Cd in edible parts of plants. Unraveling the Cd toxicity mechanisms and responses of plants to Cd stress is critical for promoting plant growth and ensuring food safety in Cd-contaminated soils. Toxicological research on plant responses to heavy metal stress has extensively studied Cd, as it can disrupt multiple physiological processes. In addition to morpho-anatomical, hormonal, and biochemical responses, plants rapidly initiate transcriptional modifications to combat Cd stress-induced oxidative and genotoxic damage. Various families of transcription factors play crucial roles in triggering such responses. Moreover, epigenetic modifications have been identified as essential players in maintaining plant genome stability under genotoxic stress. Plants have developed several detoxification strategies to mitigate Cd-induced toxicity, such as cell-wall binding, complexation, vacuolar sequestration, efflux, and translocation. This review provides a comprehensive update on understanding of molecular mechanisms involved in Cd uptake, transportation, and detoxification, with a particular emphasis on the signaling pathways that involve transcriptional and epigenetic responses in plants. This review highlights the innovative strategies for enhancing Cd tolerance and explores their potential application in various crops. Furthermore, this review offers strategies for increasing Cd tolerance and limiting Cd bioavailability in edible parts of plants, thereby improving the safety of food crops.

Physiological and molecular mechanisms of medicinal plants in response to cadmium stress: Current status and future perspective

Medicinal plants have a wide range of uses worldwide. However, the quality of medicinal plants is affected by severe cadmium pollution. Cadmium can reduce photosynthetic capacity, lead to plant growth retardation and oxidative stress, and affect secondary metabolism. Medicinal plants have complex mechanisms to cope with cadmium stress. On the one hand, an antioxidant system can effectively scavenge excess reactive oxygen species produced by cadmium stress. On the other hand, cadmium chelates are formed by chelating peptides and then sequestered through vacuolar compartmentalization. Cadmium has no specific transporter in plants and is generally transferred to plant tissues through competition for the transporters of divalent metal ions, such as zinc, iron, and manganese. In recent years, progress has been achieved in exploring the physiological mechanisms by which medicinal plants responding to cadmium stress. The exogenous regulation of cadmium accumulation in medicinal plants has been studied, and the aim is reducing the toxicity of cadmium. However, research into molecular mechanisms is still lagging. In this paper, we review the physiological and molecular mechanisms and regulatory networks of medicinal plants exposed to cadmium, providing a reference for the study on the responses of medicinal plants to cadmium stress.

Reduction of lead toxicity effects and enhancing the glutathione reservoir in green beans through spraying sulfur and serine and glutamine amino acids

Acid rain is one of the influential environmental factors in transport of heavy metals, including lead from the atmosphere to the surface of the earth and growing plants. Such situation can not only damage the growing plants but can also toxify the food chain, and endanger human life. In order to reduce stress damage due to lead, on green bean plant, the effect of spraying the plants by sulfur, also amino acids including serine and glutamine, was evaluated. A factorial experiment based on randomized complete block design with three replications was carried out using the green bean Sunray cultivar in 2020. Treatments include foliar application of lead at two levels (0.0 and 1 mmol) as lead acetate, foliar application of liquid sulfur at two levels (0.0 and 2 per thousand), and foliar application of amino acids at four levels (0.0, serine at 200 mg/L, glutamine at 200 mg/L, and co-application of serine and glutamine at the same concentrations) at pre-flowering stage. The results showed that leaf foliar uptake of most of the employed treatments resulted in reduction of leaf area index, leaf, stem and pods dry weight, stem diameter and height, pod yield, photosynthetic pigments such as chlorophyll a, chlorophyll b, and carotenoids, and relative leaf water content. However, grain protein content, hydrogen peroxide, and glutathione antioxidant activity significantly increased. Spraying of sulfur solution and serine and glutamine were effective in reducing the negative effects of lead stress, as it reduced the amount of hydrogen peroxide and grain protein and increased the reservoir of glutathione. These treatments also, compared to the pure lead treatment, significantly reduced lead accumulation in the pod, as the edible organ of green beans. This study results showed that foliar application of sulfur along with amino acids serine and glutamine reduced the lead toxicity effects through improving the physiological functions, and thus can increase the final yield and consequently human access to healthier food (Fig. 1). Fig. 1 Graphical abstract.

Salicylic Acid, a Multifaceted Hormone, Combats Abiotic Stresses in Plants

In recent decades, many new and exciting findings have paved the way to the better understanding of plant responses in various environmental changes. Some major areas are focused on role of phytohormone during abiotic stresses. Salicylic acid (SA) is one such plant hormone that has been implicated in processes not limited to plant growth, development, and responses to environmental stress. This review summarizes the various roles and functions of SA in mitigating abiotic stresses to plants, including heating, chilling, salinity, metal toxicity, drought, ultraviolet radiation, etc. Consistent with its critical roles in plant abiotic tolerance, this review identifies the gaps in the literature with regard to the complex signalling network between SA and reactive oxygen species, ABA, Ca²⁺, and nitric oxide. Furthermore, the molecular mechanisms underlying signalling networks that control development and stress responses in plants and underscore prospects for future research on SA concerning abiotic-stressed plants are also discussed.

Salicylate and glutamate mediate different Cd accumulation and tolerance between Brassica napus and B. juncea

Most hyperaccumulator plants have little economic values, and therefore have not been widely used in Cd-contaminated soils. Rape species are Cd hyperaccumulators with high economic values. Black mustard seed (Brassica juncea) has a higher accumulation ability and a higher tolerance for Cd than oilseed rape (Brassica napus), but its biomass is relatively low and its geographical distribution is limited. However, it is unknown why B. juncea (Bj) is more tolerant to and accumulates more Cd than B. napus (Bn). Here, we found that the differences in Cd accumulation and tolerance between the two species is mainly because Bj plants have higher levels of salicylic acid and glutamic acid than Bn plants. Exogenous salicylate and glutamate treatments enhanced Cd accumulation (salicylate + glutamate co-treatment doubled Cd accumulation level in Bn seedlings) but reduced oxidative stresses by increasing glutathione biosynthesis and activating phytochelatin-based sequestration of Cd into vacuoles. Our results provide a new idea to simultaneously improve Cd accumulation and Cd tolerance in B. napus.

In vivo tracing of endogenous salicylic acids as the biomarkers for evaluating the toxicity of nano-TiO2 to plants

Currently, nano-titanium dioxide (nTiO₂) is considered an emerging environmental contaminant. Bottlenecked by the traditional destructive and lethal sampling methods, nTiO₂'s effect in living plants is poorly investigated. Here, in vivo tracing of endogenous salicylic acids at regular intervals was performed by using solid phase microextraction (SPME) technique for evaluating the effects of nTiO₂ on plants. By planting aloe in soil containing varying amounts of nTiO₂, the titanium (Ti) element accumulated in the leaves to concentrations and then reached the maximum of 1.1 ± 0.4 μg/g after nTiO₂ exceeding 0.1 g/kg. The levels of salicylic acid (SA) and acetylsalicylic acid (ASA) were up-regulated upon the exposure to nTiO₂, while were positively correlated to the contents of Ti. Moreover, the increased malondialdehyde, decreased total superoxide dismutase and fluctuated glutathione along with the addition of nTiO₂ demonstrated the oxidative stress caused by nTiO₂. Meanwhile, apparent growth indicators including leaf elongation, plant fresh weight and root development were influenced, which further confirmed the toxicity of nTiO₂ imparted on aloe. This study presents the possibility of using salicylic acids as biomarkers for revealing the toxicity of nTiO₂ on plants in addition to the other biomarkers and biomass data, and the in vivo SPME technique is powerful for their monitoring.

Phytohormones-induced senescence efficiently promotes the transport of cadmium from roots into shoots of plants: A novel strategy for strengthening of phytoremediation

Due to the long growth period of plants, phytoremediation is time costly. Improving the accumulation of cadmium (Cd) in shoots of plants will promote the efficiency of phytoremediation. In this study, two senescence-relative phytohormones, abscisic acid (ABA) and salicylic acid (SA), were applied to strengthening phytoremediation of Cd by tall fescue (Festuca arundinacea S.). Under hydroponic culture, phytohormones treatment increased the Cd content of shoots 11.4-fold over the control, reaching 316.3 mg/kg (dry weight). Phytohormones-induced senescence contributes to the transport of heavy metals, and HMA3 was found to play a key role in this process. Additionally, this strategy could strengthen the accumulation of Cu and Zn in tall fescue shoots. Moreover, in soil pot culture, the strategy increased shoot Cd contents 2.56-fold over the control in tall fescue, and 2.55-fold over the control in Indian mustard (Brassica juncea L.), indicating its comprehensive adaptability and potential use in the field. In summary, senescence-induced heavy metal transport is developed as a novel strategy to strengthen phytoremediation. The strategy could be applied at the end of phytoremediation with an additional short duration (7 days) with comprehensive adaptability, and markedly strengthen the phytoremediation in the field.

LcSABP2, a salicylic acid binding protein 2 gene from Lycium chinense, confers resistance to triclosan stress in Nicotiana tabacum

The triclosan (TCS) is one of the most commonly detected organic pollutants in the sewage sludge. TCS could induce phytotoxicity in plants. Salicylic acid (SA) is a phenolic compound capable of enhancing plant growth and development. It is well documented that abiotic stress tolerance could be enhanced by exogenous application of SA. However, the regulatory mechanisms for functions of endogenous SA in plants' responses to xenobiotics stress remains unclear. Our results indicated that TCS suppressed plant growth by restricting photosynthesis, decreasing chlorophyll contents and inducing over production of reactive oxygen species (ROS). Interestingly, SA or glutathione (GSH) application could significantly improve plant tolerance to TCS. Moreover, endogenous SA and the expression of a SA binding protein 2 (SABP2) gene were found to be elevated in tobacco under TCS treatment. The overexpression of LcSABP, a SABP2-like gene cloned from the leaves of Lycium chinense, markedly enhanced the SA content in the transgenic plants under TCS stress. The LcSABP-overexpressing plants presented higher photosynthesis rate, chlorophyll content, glutathione reductase (GR) and glutathione-S-transferase (GST) enzymes activities, GSH content and lower O2-•, H2O2 and malondialdehyde (MDA) content in comparison with WT tobacco with TCS treatment. One of the GSH synthesis-related gene, NtGSHS, also showed higher expression level in the transgenic tobacco in comparison with control plants with TCS stress treatment. These results indicated that SABP2 played a positive regulatory role in plant response to TCS stress via increasing the endogenous SA levels. The increased SA content might then increase the GSH content, probably through an increase in GR activity and GSHS gene expression, thus inducing the antioxidant and xenobiotics detoxification systems, which promoted TCS stress tolerance in tobacco plants.

Salicylic Acid Signals Plant Defence against Cadmium Toxicity

Salicylic acid (SA), as an enigmatic signalling molecule in plants, has been intensively studied to elucidate its role in defence against biotic and abiotic stresses. This review focuses on recent research on the role of the SA signalling pathway in regulating cadmium (Cd) tolerance in plants under various SA exposure methods, including pre-soaking, hydroponic exposure, and spraying. Pretreatment with appropriate levels of SA showed a mitigating effect on Cd damage, whereas an excessive dose of exogenous SA aggravated the toxic effects of Cd. SA signalling mechanisms are mainly associated with modification of reactive oxygen species (ROS) levels in plant tissues. Then, ROS, as second messengers, regulate a series of physiological and genetic adaptive responses, including remodelling cell wall construction, balancing the uptake of Cd and other ions, refining the antioxidant defence system, and regulating photosynthesis, glutathione synthesis and senescence. These findings together elucidate the expanding role of SA in phytotoxicology.

Analysis of potential strategies for cadmium stress tolerance revealed by transcriptome analysis of upland cotton

In recent years, heavy metal pollution has become a more serious global problem, and all countries are actively engaged in finding methods to remediate heavy metal-contaminated soil. We conducted transcriptome sequencing of the roots of cotton grown under three different cadmium concentrations, and analysed the potential strategies for coping with cadmium stress. Through Gene Ontology analysis, we found that most of the genes differentially regulated under cadmium stress were associated with catalytic activity and binding action, especially metal iron binding, and specific metabolic and cellular processes. The genes responsive to cadmium stress were mainly related to membrane and response to stimulus. The KEGG pathways enriched differentially expressed genes were associated with secondary metabolite production, Starch and sucrose metabolism, flavonoid biosynthesis, phenylalanina metalism and biosynthesis, in order to improve the activity of antioxidant system, repair systems and transport system and reduction of cadmium toxicity. There are three main mechanisms by which cotton responds to cadmium stress: thickening of physical barriers, oxidation resistance and detoxification complexation. Meanwhile, identified a potential cotton-specific stress response pathway involving brassinolide, and ethylene signaling pathways. Further investigation is needed to define the specific molecular mechanisms underlying cotton tolerance to cadmium stress. In this study potential coping strategies of cotton root under cadmium stress were revealed. Our findings can guide the selection of cotton breeds that absorb high levels of cadmium.

Salicylic Acid Alleviates Aluminum Toxicity in Soybean Roots through Modulation of Reactive Oxygen Species Metabolism

As an important signal molecule, salicylic acid (SA) improves plant tolerance to aluminum (Al) stress. The objective of this study was to investigate the effects of exogenous SA application on the dynamics of endogenous SA and reactive oxygen species in soybean (Glycine max L.) exposed to Al stress. The roots of soybean seedlings were exposed to a combination of AlCl₃ (30 μM) and SA (10 μM)/PAC (100 μM, paclobutrazol, SA biosynthesis inhibitor) for 3, 6, 9, and 12 h. Al stress induced an increase in endogenous SA concentration in a time-dependent manner, also verified by the up-regulated expression of GmNPR1, an SA-responsive gene. Al stress increased the activities of phenylalanine ammonia-lyase (PAL) and benzoic acid 2-hydroxylase (BA2H), and the contents of SA, O2- and malondialdehyde (MDA) in the root apex. The application of exogenous SA increased PAL and BA2H, and reduced O2- and MDA contents in soybean roots under Al stress. PAC inhibited the SA induced increase in BA2H activity. In addition, the SA application resulted in a rapid increase in hydrogen peroxide (H₂O₂) concentration under Al stress, followed by a sharp decrease. Compared with the plants exposed to Al alone, Al+SA plants possessed higher activities of superoxide dismutase, peroxidase, and ascorbate peroxidase, and lower catalase activity, indicating that SA alleviated Al-induced oxidative damage. These results suggested that PAL and BA2H were involved in Al-induced SA production and showed that SA alleviated the adverse effects of Al toxicity by modulating the cellular H₂O₂ level and the antioxidant enzyme activities in the soybean root apex.

ZnO nanoparticle effects on hormonal pools in Arabidopsis thaliana

At present, nanoparticles have been more and more used in a wide range of areas. However, very little is known about the mechanisms of their impact on plants, as both positive and negative effects have been reported. As plant interactions with the environment are mediated by plant hormones, complex phytohormone analysis has been performed in order to characterize the effect of ZnO nanoparticles (mean size 30nm, concentration range 0.16-100mgL^-1) on Arabidopsis thaliana plants. Taking into account that plant hormones exhibit high tissue-specificity as well as an intensive cross-talk in the regulation of growth and development as well as defense, plant responses were followed by determination of the content of five main phytohormones (cytokinins, auxins, abscisic acid, salicylic acid and jasmonic acid) in apices, leaves and roots. Increasing nanoparticle concentration was associated with gradually suppressed biosynthesis of the growth promoting hormones cytokinins and auxins in shoot apical meristems (apices). In contrast, cis-zeatin, a cytokinin associated with stress responses, was elevated by 280% and 590% upon exposure to nanoparticle concentrations 20 and 100mgL^-1, respectively, in roots. Higher ZnO nanoparticle doses resulted in up-regulation of the stress hormone abscisic acid, mainly in apices and leaves. In case of salicylic acid, stimulation was found in leaves and roots. The other stress hormone jasmonic acid (as well as its active metabolite jasmonate isoleucine) was suppressed at the presence of nanoparticles. The earliest response to nanoparticles, associated with down-regulation of growth as well as of cytokinins and auxins, was observed in apices. At higher dose, up-regulation of abscisic acid, was detected. This increase, together with elevation of the other stress hormone - salicylic acid, indicates that plants sense nanoparticles as severe stress. Gradual accumulation of cis-zeatin in roots may contribute to relatively higher stress resistance of this tissue.

GR1-like gene expression in Lycium chinense was regulated by cadmium-induced endogenous jasmonic acids accumulation

KEY MESSAGE

The G1-like gene from the Lycium chinense was cloned and transferred into N. tabacum. Evidence showed that endogenous JA accumulation was crucial to LcGR gene expression in cadmium-stressed L. chinense. Glutathione reductase (GR) plays a vital role in glutathione-ascorbate metabolism and is a key enzyme in maintaining the redox state in plants. Jasmonic acids (JA) are important hormones regulating protective responses against bacteria and mechanic damage in plants. At present, the relationship between the endogenous JA accumulation, the glutathione (GSH) content and GR gene expression in plants under cadmium (Cd) stress has not been elucidated. This study primarily aims to explore their interconnected relations. First, we isolated the GR1-like gene from Lycium chinense (LcGR). Real-time PCR showed that gene LcGR and allene oxide cyclase (LcAOC) (a JA synthesis gene) expression in L. chinense plants was significantly enhanced by CdCl2 and reduced by CdCl2 cotreatment with 12,13-epoxy-octadecenoic acid (EOA), a JA synthesis inhibitor. Meanwhile, the JA content in plants strongly increased under Cd stress and decreased under Cd + EOA treatment, which was in accordance with expression pattern of LcAOC. The function of gene LcGR was confirmed in vitro with E. coli expression system. The subcellular localization in chloroplasts of LcGR gene was proved in Nicotiana tabacum leaves with transient transfection system of Agrobacterium tumefaciens. Furthermore, the overexpression of gene LcGR in the transgenic tabacum led to great Cd-tolerance and higher GSH accumulation. Overall, the results showed that the endogenous JA accumulation in Cd-stressed plants affects the GR expression which is crucial to the GSH accumulation and GSH-dependent tolerance to cadmium in LcGR transformants.

Comparative transcriptomic analysis reveals the roles of ROS scavenging genes in response to cadmium in two pak choi cultivars

To identify key regulatory genes involved in ROS scavenging in response to cadmium (Cd) exposure in pak choi, eight cDNA libraries from Cd-treated and Cd-free roots of two cultivars, Baiyewuyueman (high Cd accumulator) and Kuishan’aijiaoheiye (low Cd accumulator), were firstly performed by RNA-sequencing. Totally 0.443 billion clean reads and 244,190 unigenes were obtained from eight transcriptome. About 797 and 1167 unigenes encoding ROS related proteins and transcription factors were identified. Of them, 11 and 16 ROS scavenging system related DEGs, and 29 and 15 transcription factors related DEGs were found in Baiyewuyueman and Kuishan’aijiaoheiye, respectively. Ten ROS-scavenging genes (Cu/Zn-SOD, GST1, PODs, TrxR2, PrxR, FER3 and NDPK) showed higher expression levels in Cd-exposed seedings of Baiyewuyueman than those of Kuishan’aijiaoheiye. Four genes (GPX, APX, GRX and GST3) specifically expressed in Cd-free roots of Kuishan’aijiaoheiye. For transcription factors, ERF12/13/22 and WRKY31 was up-regulated by Cd in Baiyewuyueman, while in Kuishan’aijiaoheiye, Cd induced down-regulations of bZIP, NAC and ZFP families. The results indicate that the two cultivars differed in the mechanism of ROS scavenging in response to Cd stress. Fe SOD1, POD A2/44/54/62 and GST1 may be responsible for the difference of Cd tolerance between Baiyewuyueman and Kuishan’aijiaoheiye.

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.

Alleviation of selenium toxicity in Brassica juncea L.: salicylic acid-mediated modulation in toxicity indicators, stress modulators, and sulfur-related gene transcripts

The present work reveals the response of different doses of selenium (Se) and alleviating effect of salicylic acid (SA) on Se-stressed Brassica juncea seedlings. Selenium, a micronutrient, is essential for both humans and animals but is toxic at higher doses. Its beneficial role for the survival of plants, however, is still debatable. On the other hand, SA, a phenolic compound, is known to have specific responses under environmental stresses. Experiments were conducted using leaves of hydroponically grown seedlings of Pusa bold (PB) variety of B. juncea, treated with different concentrations of Se (50, 150, 300 μM) for 24- and 96-h exposure times. Increasing Se concentrations inhibited growth and, caused lipid peroxidation, concomitantly increased stress modulators (proline, cysteine, SOD, CAT) along with sulfur-related gene transcripts (LAST, APS, APR, GR, OASL, MT-2, PCS) in Brassica seedlings. On the basis of the above studied parameters, maximum inhibition in growth was observed at 300 μM Se after 96-h exposure time. Further, co-application of SA along with 300 μM Se helped to mitigate Se stress, as shown by improved levels of growth parameters, toxicity indicators (chlorophyll, protein, MDA), stress modulators (proline, cysteine, SOD, and CAT), and expression of sulfur-related genes as compared to Se-treated seedlings alone. Altogether, this study revealed that Se + SA combinations improved seedling morphology and were effective in alleviation of Se stress in PB variety of B. juncea.

Effects of Cadmium Treatment on the Uptake and Translocation of Sulfate in Arabidopsis thaliana

Cadmium (Cd) is a highly toxic and non-essential element for plants, whereas phytochelatins and glutathione are low-molecular-weight sulfur compounds that function as chelators and play important roles in detoxification. Cadmium exposure is known to induce the expression of sulfur-assimilating enzymes and sulfate uptake by roots. However, the molecular mechanism underlying Cd-induced changes remains largely unknown. Accordingly, we analyzed the effects of Cd treatment on the uptake and translocation of sulfate and accumulation of thiols in Arabidopsis thaliana Both wild type (WT) and null mutant (sel1-10 and sel1-18) plants of the sulfate transporter SULTR1;2 exhibited growth inhibition when treated with CdCl₂ However, the mutant plants exhibited a lower growth rate and lower Cd accumulation. Cadmium treatment also upregulated the transcription of SULTR1;2 and sulfate uptake activity in WT plants, but not in mutant plants. In addition, the sulfate, phytochelatin and total sulfur contents were preferentially accumulated in the shoots of both WT and mutant plants treated with CdCl₂, and sulfur K-edge XANES spectra suggested that sulfate was the main compound responsible for the increased sulfur content in the shoots of CdCl₂-treated plants. Our results demonstrate that Cd-induced sulfate uptake depends on SULTR1;2 activity, and that CdCl₂ treatment greatly shifts the distribution of sulfate to shoots, increases the sulfate concentration of xylem sap and upregulates the expression of SULTRs involved in root-to-shoot sulfate transport. Therefore, we conclude that root-to-shoot sulfate transport is stimulated by Cd and suggest that the uptake and translocation of sulfate in CdCl₂-treated plants are enhanced by demand-driven regulatory networks.

Salicylic Acid and Sodium Salicylate Alleviate Cadmium Toxicity to Different Extents in Maize (Zea mays L.)

The role of salicylic acid in Cd tolerance has attracted more attention recently but no information is available on the efficiency of different forms of salicylic acid. The aim was thus to investigate whether both the acid and salt forms of salicylic acid provide protection against Cd stress and to compare their mode of action. Young maize plants were grown under controlled environmental conditions. One group of 10-day-old seedlings were treated with 0.5 mM SA or NaSA for 1 day then half of the pants were treated with 0.5 mM Cd for 1 day. Another group of seedlings was treated with 0.5 mM CdSO₄ for 1 day without pre-treatment with SA or NaSA, while a third group was treated simultaneously with Cd and either SA or NaSA. Both salicylic acid forms reduced the Cd accumulation in the roots. Treatment with the acidic form meliorated the Cd accumulation in the leaves, while Na-salicylate increased the phytochelatin level in the roots and the amount of salicylic acid in the leaves. Furthermore, increased antioxidant enzyme activity was mainly induced by the acid form, while glutathione-related redox changes were influenced mostly by the salt form. The acidic and salt forms of salicylic acid affected the two antioxidant systems in different ways, and the influence of these two forms on the distribution and detoxification of Cd also differed. The present results also draw attention to the fact that generalisations about the stress protective mechanisms induced by salicylic acid are misleading since different forms of SA may exert different effects on the plants via separate mechanisms.

Role of salicylic acid in resistance to cadmium stress in plants

We review and introduce the importance of salicylic acid in plants under cadmium stress, and provide insights into potential regulatory mechanisms for alleviating cadmium toxicity. Cadmium (Cd) is a widespread and potentially toxic environmental pollutant, originating mainly from rapid industrial processes, the application of fertilizers, manures and sewage sludge, and urban activities. It is easily taken up by plants, resulting in obvious toxicity symptoms, including growth retardation, leaf chlorosis, leaf and root necrosis, altered structures and ultrastructures, inhibition of photosynthesis, and cell death. Therefore, alleviating Cd toxicity in plants is a major aim of plant research. Salicylic acid (SA) is a ubiquitous plant phenolic compound that has been used in many plant species to alleviate Cd toxicity by regulating plant growth, reducing Cd uptake and distribution in plants, protecting membrane integrity and stability, scavenging reactive oxygen species and enhancing antioxidant defense system, improving photosynthetic capacity. Furthermore, SA functions as a signaling molecule involved in the expression of several important genes. Significant amounts of research have focused on understanding SA functions and signaling in plants under Cd stress, but several questions still remain unanswered. In this article, the influence of SA on Cd-induced stress in plants and the potential regulation mechanism for alleviating Cd toxicity are reviewed.

Glutathione homeostasis and Cd tolerance in the Arabidopsis sultr1;1-sultr1;2 double mutant with limiting sulfate supply

Cadmium sensitivity in sultr1;1 - sultr1;2 double mutant with limiting sulfate supply is attributed to the decreased glutathione content that affected oxidative defense but not phytochelatins' synthesis. In plants, glutathione (GSH) homeostasis plays pivotal role in cadmium (Cd) detoxification. GSH is synthesized by sulfur (S) assimilation pathway. Many studies have tried to investigate the role of GSH homeostasis on Cd tolerance using mutants; however, most of them have focused on the last few steps of S assimilation. Until now, mutant evidence that explored the relationship between GSH homeostasis on Cd tolerance and S absorption is rare. To further reveal the role of GSH homeostasis on Cd stress, the wild-type and a sultr1;1-sultr1;2 double mutant which had a defect in two distinct high-affinity sulfate transporters were used in this study. Growth parameters, biochemical or zymological indexes and S assimilation-related genes' expression were compared between the mutant and wild-type Arabidopsis plants. It was found that the mutations of SULTR1;1 and SULTR1;2 did not affect Cd accumulation. Compared to the wild-type, the double mutant was more sensitive to Cd under limited sulfate supply and suffered from stronger oxidative damage. More importantly, under the same condition, lower capacity of S assimilation resulted in decreased GSH content in mutant. Faced to the limited GSH accumulation, mutant seedlings consumed a large majority of GSH in pool for the synthesis of phytochelatins rather than participating in the antioxidative defense. Therefore, homeostasis of GSH, imbalance between antioxidative defense and severe oxidative damage led to hypersensitivity of double mutant to Cd under limited sulfate supply.

Transcriptome Analysis of Cadmium-Treated Roots in Maize (Zea mays L.)

Cadmium (Cd) is a heavy metal and is highly toxic to all plant species. However, the underlying molecular mechanism controlling the effects of auxin on the Cd stress response in maize is largely unknown. In this study, the transcriptome produced by maize 'Zheng 58' root responses to Cd stress was sequenced using Illumina sequencing technology. In our study, six RNA-seq libraries yielded a total of 244 million clean short reads and 30.37 Gb of sequence data. A total of 6342 differentially expressed genes (DEGs) were grouped into 908 Gene Ontology (GO) categories and 198 Kyoto Encyclopedia of Genes and Genomes terms. GO term enrichment analysis indicated that various auxin signaling pathway-related GO terms were significantly enriched in DEGs. Comparison of the transcript abundances for auxin biosynthesis, transport, and downstream response genes revealed a universal expression response under Cd treatment. Furthermore, our data showed that free indole-3-acetic acid (IAA) levels were significantly reduced; but IAA oxidase activity was up-regulated after Cd treatment in maize roots. The analysis of Cd activity in maize roots under different Cd and auxin conditions confirmed that auxin affected Cd accumulation in maize seedlings. These results will improve our understanding of the complex molecular mechanisms underlying the response to Cd stress in maize roots.