Root Sequestration Research Articles

Diethyl aminoethyl hexanoate ameliorates salt tolerance associated with ion transport, osmotic adjustment, and metabolite reprograming in white clover

BackgroundSoil salinization is a serious environmental hazard, limiting plant growth and production in different agro-ecological zones worldwide. Diethyl aminoethyl hexanoate (DA-6) as an essential plant growth regulator (PGR) exhibits a beneficial role in improving crop growth and stress tolerance. However, the DA-6-regulated effect and mechanism of salt tolerance in plants are still not fully understood. The objective of current study was to disclose salt tolerance induced by DA-6 in relation to changes in water and redox balance, photosynthetic function, ionic homeostasis, and organic metabolites reprogramming in white clover (Trifolium repens).ResultsA prolonged duration of salt stress caused water loss, impaired photosynthetic function, and oxidative injury to plants. However, foliar application of DA-6 significantly improved osmotic adjustment (OA), photochemical efficiency, and cell membrane stability under salt stress. In addition, high salinity induced massive accumulation of sodium (Na), but decreased accumulation of potassium (K) in leaves and roots of all plants. DA-6-treated plants demonstrated significantly higher transcript levels of genes involved in uptake and transport of Na and K such as VP1, HKT8, SOS1, NHX2, NHX6, and SKOR in leaves as well as VP1, HKT1, HKT8, H+-ATPase, TPK5, SOS1, NHX2, and SKOR in roots. Metabolomics analysis further illustrated that DA-6 primarily induced the accumulation of glucuronic acid, hexanoic acid, linolenic acid, arachidonic acid, inosose, erythrulose, galactopyranose, talopyranose, urea, 1-monopalmitin, glycerol monostearate, campesterol, stigmasterol, and alanine.ConclusionsThe DA-6 significantly up-regulated transcript levels of multiple genes associated with increased Na+ compartmentalization in vacuoles and Na+ sequestration in roots to reduce Na+ transport to photosynthetic organs, thereby maintaining Na+ homeostasis under salt stress. The accumulation of many organic metabolites induced by the DA-6 could be attributed to enhanced cell wall and membrane structural stability and functionality, OA, antioxidant defense, and downstream signal transduction in leaves under salt stress. The present study provides a deep insight about the synergistic role of DA-6 in salt tolerance of white clover.

Synergistic impact of Serendipita indica and Zhihengliuella sp. ISTPL4 on the mitigation of arsenic stress in rice.

Arsenic (As) is a highly toxic metal that interferes with plant growth and disrupts various biochemical and molecular processes in plants. In this study, the harmful effects of As on rice were mitigated using combined inoculation of a root endophyte Serendipita indica and an actinobacterium Zhihengliuella sp. ISTPL4. A randomized experiment was conducted, in which rice plants were grown under controlled conditions and As-stressed conditions. The control and treatment groups consisted of untreated and non-stressed plants (C1), treated and non-stressed plants (C2), stressed and untreated plants (T1), and stressed and treated plants (T2). Various phenotypic characteristics such as shoot length (SL), root length (RL), shoot fresh weight (SFW), root fresh weight (RFW), shoot dry weight (SDW), and root dry weight (RDW) and biochemical parameters such as chlorophyll content, protein content, and antioxidant enzymatic activities were evaluated. The activity of various antioxidant enzymes was increased in T2 followed by T1 plants. Furthermore, high concentrations of phytohormones such as ethylene (ET), gibberellic acid (GA), and cytokinin (CK) were found at 4.11 μmol mg-1, 2.53 μmol mg-1, and 3.62 μmol mg-1 of FW of plant, respectively. The results of AAS indicated an increased As accumulation in roots of T2 plants (131.5 mg kg-1) than in roots of T1 plants (120 mg kg-1). It showed that there was an increased As accumulation and sequestration in roots of microbial-treated plants (T2) than in uninoculated plants (T1). Our data suggest that this microbial combination can be used to reduce the toxic effects of As in plants by increasing the activity of antioxidant enzymes such as SOD, CAT, PAL, PPO and POD. Furthermore, rice plants can withstand As stress owing to the active synthesis of phytohormones in the presence of microbial combinations.

Phytohormones-mediated strategies for mitigation of heavy metals toxicity in plants focused on sustainable production.

In this manuscript, authors reviewed and explore the information on beneficial role of phytohormones to mitigate adverse effects of heavy metals toxicity in plants. Global farming systems are seriously threatened by heavy metals (HMs) toxicity, which can result in decreased crop yields, impaired food safety, and negative environmental effects. A rise in curiosity has been shown recently in creating sustainable methods to reduce HMs toxicity in plants and improve agricultural productivity. To accomplish this, phytohormones, which play a crucial role in controlling plant development and adaptations to stress, have emerged as intriguing possibilities. With a particular focus on environmentally friendly farming methods, the current review provides an overview of phytohormone-mediated strategies for reducing HMs toxicity in plants. Several physiological and biochemical activities, including metal uptake, translocation, detoxification, and stress tolerance, are mediated by phytohormones, such as melatonin, auxin, gibberellin, cytokinin, ethylene, abscisic acid, salicylic acid, and jasmonates. The current review offers thorough explanations of the ways in which phytohormones respond to HMs to help plants detoxify and strengthen their resilience to metal stress. It is crucial to explore the potential uses of phytohormones as long-term solutions for reducing the harmful effects of HMs in plants. These include accelerating phytoextraction, decreasing metal redistribution to edible plant portions, increasing plant tolerance to HMs by hormonal manipulation, and boosting metal sequestration in roots. These methods seek to increase plant resistance to HMs stress while supporting environmentally friendly agricultural output. In conclusion, phytohormones present potential ways to reduce the toxicity of HMs in plants, thus promoting sustainable agriculture.

Cell type-specific mapping of ion distribution in Arabidopsis thaliana roots

Cell type-specific mapping of element distribution is critical to fully understand how roots partition nutrients and toxic elements with aboveground parts. In this study, we developed a method that combines fluorescence-activated cell sorting (FACS) with inductively coupled plasma mass spectrometry (ICP-MS) to assess the ionome of different cell populations within Arabidopsis thaliana roots. The method reveals that most elements exhibit a radial concentration gradient increasing from the rhizodermis to inner cell layers, and detected previously unknown ionomic changes resulting from perturbed xylem loading processes. With this approach, we also identify a strong accumulation of manganese in trichoblasts of iron-deficient roots. We demonstrate that confining manganese sequestration in trichoblasts but not in endodermal cells efficiently retains manganese in roots, therefore preventing toxicity in shoots. These results indicate the existence of cell type-specific constraints for efficient metal sequestration in roots. Thus, our approach opens an avenue to investigate element compartmentation and transport pathways in plants.

Cadmium exposure is associated with increased transcript abundance of multiple heavy metal associated transporter genes in roots of hemp (Cannabis sativa L.).

Industrial hemp (Cannabis sativa L.) has demonstrated promise for phytoremediation due to an extensive root system, large biomass, and ability to survive under relatively high levels of heavy metals. However, little research has been conducted to determine the impact of heavy metal uptake in hemp grown for medicinal use. This study evaluated the potential for cadmium (Cd) uptake and its impact on growth, physiological responses, and transcript expression of metal transporter genes in a hemp variety grown for flower production. The cultivar 'Purple Tiger' was exposed to 0, 2.5, 10, and 25 mg·L-1 Cd in a greenhouse hydroponic study in two independent experiments. Plants exposed to 25 mg·L-1 Cd displayed stunted plant growth characteristics, reduced photochemical efficiency, and premature senescence suggesting Cd toxicity. At the two lower concentrations of Cd (2.5 and 10 mg·L-1 Cd), plant height, biomass, and photochemical efficiency were not affected, with chlorophyll content index (CCI) being slightly lower at 10 mg·L-1 Cd, compared to 2.5 mg·L-1 Cd. There were no consistent differences between the two experiments in total cannabidiol (CDB) and tetrahydrocannabinol (THC) concentrations in flower tissues at 2.5 and 10 mg·L-1 Cd, compared to the control treatment. Root tissue accumulated the highest amount of Cd compared to other tissues for all the Cd treatments, suggesting preferential root sequestration of this heavy metal in hemp. Transcript abundance analysis of heavy metal-associated (HMA) transporter genes suggested that all seven members of this gene family are expressed in hemp, albeit with higher expression in the roots than in the leaves. In roots, CsHMA3 was up-regulated at 45 and 68d after treatment (DAT), and CsHMA1, CsHMA4, and CsHMA5 were upregulated only under long term Cd stress at 68 DAT, at 10 mg·L-1 Cd. Results suggest that expression of multiple HMA transporter genes in the root tissue may be upregulated in hemp exposed to 10 mg·L-1 Cd in a nutrient solution. These transporters could be involved in Cd uptake in the roots via regulating its transport and sequestration, and xylem loading for long distance transport of Cd to shoot, leaf, and flower tissues.

The Cd sequestration effects of rice roots affected by different Si management in Cd-contaminated paddy soil

The application of exogenous silicon (Si) reportedly is one of the eco-friendly practices to mitigate cadmium (Cd) phytotoxicity and regulate the chemical behaviors of Cd in the soil-rice system. But the efficiency of Si on the Cd retention by rice root varies with the Si fertilizer management. The objective of this paper was to interpret the differences in Cd immobilization by rice roots and relevant mechanisms under different ways of Si application (T-Si, supplied at transplanting stage; TJ-Si, split at transplanting and jointing stage with the ratio of 50 % to 50 %; J-Si, supplied at jointing stage and CK, none of Si application) in Cd-contaminated paddy soils. The results showed that the Cd-retention capacity of rice root was increased by 0.60 % ~ 3.06 % under different Si management when compared to CK. The concentrations of monosilicic acid in soils and in apoplast and symplast of roots were increased significantly by Si application, while Cd concentrations in apoplast and symplast of root were decreased by 28.50 % (T-Si), 40.64 % (TJ-Si) and 30.26 % (J-Si), respectively. The distribution of Cd in rice cell wall was increased significantly by TJ-Si. The Cd concentrations of inert fractions (F3, F4 and F6) in root of TJ-Si were raised obviously. Si application downregulated the expression of OsIRT2 and OsNramp5 while upregulated OsHMA3, and the expression of OsHMA3 treated by TJ-Si was obviously higher than CK and J-Si. The distributions of the passive Cd in roots bound with thiol compounds (NPT, GSH and PCs) and polysaccharide components (pectin, hemicelluloses 1 and hemicellulose 2) were raised much more by TJ-Si than by T-Si and J-Si. On the whole, compared with T-Si and J-Si, TJ-Si could more easily replenish soil available Si and enhance Cd sequestration in roots as the result of the decrease of Cd transport factor in roots. This study unravels some mechanisms about different Si management on increasing Cd retention and decreasing Cd migration in rice roots, and TJ-Si is worthy of being recommended.

Dose-Effect Relationship of Water Salinity Levels on Osmotic Regulators, Nutrient Uptake, and Growth of Transplanting Vetiver [Vetiveria zizanioides (L.) Nash

Vetiver grass [Vetiveria zizanioides (L.) Nash] without seeds, suitable for growing on coastal saline land, has attracted attention because of oil extraction from its roots and industrial and agricultural use. In this study, a pot experiment with different NaCl contents was used to investigate the influence of water salinity levels on vetiver, salt tolerance, and the feasibility of transferring it to coastal saline regions. The results indicated that the fresh weight of roots and shoots increased initially and then gradually decreased with an increase in NaCl content, and the maximum was attributed to a water salinity of 0.3%. The vetiver can tolerate a maximum saline content of up to 2%. The promotion of vetiver growth under water salinity could be attributed to the acceleration of nutrient uptake-induced saline, including K, N, and Cl. The growth of vetiver was insignificantly inhibited with 0.5% water salinity (mild stress), significantly inhibited with 1.0% water salinity (moderate stress: biomass decrease), and severe inhibited with >1.5% water salinity (intense stress: biomass decrease). The salt tolerance of vetiver was due to osmotic regulation by reducing sugars under mild stress and of proline under intense stress, and Na+ sequestration in roots and the transformation of Cl− away from sensitive roots. The vetiver could be cultivated in slightly coastal saline soil (0.1–0.2% soil salinity) and even moderately saline coastal soil (0.2–0.4% soil salinity) under irrigation with low salt water during transplanting.

Adaptation of Arabidopsis halleri to extreme metal pollution through limited metal accumulation involves changes in cell wall composition and metal homeostasis.

Metallophytes constitute powerful models for the study of metal homeostasis, adaptation to extreme environments and the evolution of naturally selected traits. Arabidopsis halleri is a pseudometallophyte which shows constitutive zinc/cadmium (Zn/Cd) tolerance and Zn hyperaccumulation but high intraspecific variability in Cd accumulation. To examine the molecular basis of the variation in metal tolerance and accumulation, ionome, transcriptome and cell wall glycan array profiles were compared in two genetically close A.halleri populations from metalliferous and nonmetalliferous sites in Northern Italy. The metallicolous population displayed increased tolerance to and reduced hyperaccumulation of Zn, and limited accumulation of Cd, as well as altered metal homeostasis, compared to the nonmetallicolous population. This correlated well with the differential expression of transporter genes involved in trace metal entry and in Cd/Zn vacuolar sequestration in roots. Many cell wall-related genes were also more highly expressed in roots of the metallicolous population. Glycan array and histological staining analyses demonstrated that there were major differences between the two populations in terms of the accumulation of specific root pectin and hemicellulose epitopes. Our results support the idea that both specific cell wall components and regulation of transporter genes play a role in limiting accumulation of metals in A. halleri at contaminated sites.

Plantago subulata as indicator of potentially toxic elements in the substrate.

Plantago subulata is a facultative serpentinophyte, with predominantly ultramafic distribution in Serbia and Montenegro. The plant samples were collected from ultramafic and non-ultramafic substrate, including two abandoned mining sites, with the aim to assess the accumulation potential of this species. The samples were collected from 10 sites in Serbia and Montenegro and element concentrations in plants and soils were determined. Particularly high concentrations of Fe, Cu, Mn, Zn, and Cd were found in the soil and plant samples from an abandoned iron mining site, Mt. Kopaonik, Suvo Rudište. The concentrations of Ni, Cr, Fe, Co, and Cd were statistically different between soil and plant samples from ultramafic and non-ultramafic bedrock. Considering the fact that concentrations of Zn, Ni, and Cr in roots and shoots were positively correlated with their respective contents in the soil, i.e., the chemical composition of the plant and soil samples reflected the characteristics of the substrate, and for most of the elements analyzed, P. subulata acted as indicator species. For Cd and Pb, only the root concentrations were positively correlated with soil content, indicating exclusion and root sequestration as the main tolerance strategies for these elements. Although below the hyperaccumulation threshold, a strong accumulation capacity of P. subulata was found for Ni and Cu.

Influence of Biochar Particle Size and Concentration on Pb and As Availability in Contaminated Mining Soil and Phytoremediation Potential of Poplar Assessed in a Mesocosm Experiment

Metal(loid)s found at many sites as a result of human activities induce contamination of ecosystems, which threatens the environment and public health. Several approaches can be used in order to reduce such negative impacts of pollutants, and among them, phytoremediation has gained attractive attention. The objective of the present study was to assess the capacity of Populus euramericana, Dorskamp cultivar, for the phytoremediation of metal(loid)s on a technosol (former mining site) highly contaminated mainly with Pb and As, and amended with biochars having different particle sizes. In a 46-day mesocosm glasshouse pot experiment, technosol was mixed at three ratios (0, 2, or 5% w/w) with four different hardwood-derived biochars (with various particle sizes) in order to study the biochar application rate and particle size effects on soil pore water (SPW) characteristics, on poplar growth, and on metal(loid) distribution and concentrations in the plant organs. The results showed that all biochars tested had a significant impact on several SPW physico-chemical parameters. Especially, biochar additions reduced available Pb concentrations but with no effect on As. In such conditions, Populus growth in amended technosol increased whatever rate and particle size of biochar used. Metal(loid) concentrations and repartition in plant organs showed the following: (1) for Pb, a higher root concentration with low aerial part translocation, which depended on biochar used, and (2) for As, mainly a root sequestration. We identified biochar with the finest particle size and combined with P. euramericana, Dorskamp, was the most suitable as remediation tools for Pb stabilization in post-mining contaminated soils.

Sulfur application reduces cadmium uptake in edible parts of pakchoi (Brassica chinensis L.) by cadmium chelation and vacuolar sequestration.

Sulfur (S) application in pakchoi (Brassica chinensis L.) cultivation is vital for reducing cadmium (Cd) accumulation in the plants. However, the mechanism of S application on Cd uptake and translocation in pakchoi is unclear. In this study, a hydroponic experiment was performed to investigate the effects of S application on Cd accumulation in pakchoi at one Cd concentration (50μM, in comparison to the control condition, 0μM) and three S levels (0, 2, 4mM). The results showed that excessive S application (4mM) reduced Cd accumulation and alleviated pakchoi growth inhibition caused by Cd stress in shoots and roots. With increased S application, the proportion of Cd in the vacuolar fraction and the proportion of NaCl-extractable Cd increased in roots. Additionally, S application increased the content of glutathione (GSH) and phytochelatins (PCs). The reduced Cd uptake and accumulation in pakchoi shoots could have been due to increased Cd chelation and vacuolar sequestration in roots. In addition, sufficient S application (2mM) increased the expression of γ-glutamylcysteine synthetase (GSH1) and nicotinamide synthase (NAS) in roots, and excessive S application upregulated the expression of ATP sulfurylase (ATPS) and phytochelatin synthase (PCs). This study provides evidence for the mechanism of mitigating Cd toxicity in pakchoi and will be helpful for developing strategies to reduce Cd accumulation in the edible parts of pakchoi through S fertilizer application.

Na+ extrusion from the cytosol and tissue-specific Na+ sequestration in roots confer differential salt stress tolerance between durum and bread wheat.

The progress in plant breeding for salinity stress tolerance is handicapped by the lack of understanding of the specificity of salt stress signalling and adaptation at the cellular and tissue levels. In this study, we used electrophysiological, fluorescence imaging, and real-time quantitative PCR tools to elucidate the essentiality of the cytosolic Na+ extrusion in functionally different root zones (elongation, meristem, and mature) in a large number of bread and durum wheat accessions. We show that the difference in the root's ability for vacuolar Na+ sequestration in the mature zone may explain differential salinity stress tolerance between salt-sensitive durum and salt-tolerant bread wheat species. Bread wheat genotypes also had on average 30% higher capacity for net Na+ efflux from the root elongation zone, providing the first direct evidence for the essentiality of the root salt exclusion trait at the cellular level. At the same time, cytosolic Na+ accumulation in the root meristem was significantly higher in bread wheat, leading to the suggestion that this tissue may harbour a putative salt sensor. This hypothesis was then tested by investigating patterns of Na+ distribution and the relative expression level of several key genes related to Na+ transport in leaves in plants with intact roots and in those in which the root meristems were removed. We show that tampering with this sensing mechanism has resulted in a salt-sensitive phenotype, largely due to compromising the plant's ability to sequester Na+ in mesophyll cell vacuoles. The implications of these findings for plant breeding for salinity stress tolerance are discussed.

Silicon alleviates arsenic-induced toxicity in wheat through vacuolar sequestration and ROS scavenging

ABSTRACTArsenic (As) is a phytotoxic element causing health hazards. This work investigates whether and how silicon (Si) alleviates As toxicity in wheat. The addition of Si under As-stress significantly improved morphophysiological characteristics, total protein, and membrane stability compared to As-stressed plants, suggesting that Si does have critical roles in As detoxification in wheat. Analysis of arsenate reductase activity and phytosiderophore (PS) release reveals their no involvement in the Si-mediated alleviation of As in wheat. Furthermore, Si supplementation in As-stressed plants showed a significant increase of As in roots but not in shoots compared with the plants grown under As stress. Further, gene expression analysis of two chelating molecules, TaPCS1 (phytochelatin synthase) and TaMT1 (metallothionein synthase) showed significant induction due to Si application under As stress compared with As-stressed plants. It is consistent with the physiological observations and suggests that alleviation of As toxicity in rice might be associated with As sequestration in roots leading to reduced As translocation in shoots. Furthermore, increased catalase, peroxidase, and glutathione reductase activities in roots imply the active involvement of reactive oxygen species scavenging for protecting wheat plants from As-induced oxidative injury. The study provides mechanistic evidence on the beneficial effect of Si on As toxicity in wheat plants.

Sulfur supply reduces cadmium uptake and translocation in rice grains (Oryza sativa L.) by enhancing iron plaque formation, cadmium chelation and vacuolar sequestration

Sulfur (S) fertilizer application in rice (Oryza sativa L.) is crucial in determining rice grain productivity and quality. However, little information is available concerning the effect of S supply on cadmium (Cd) uptake and translocation in rice. In this study, both hydroponic and soil experiments were conducted to investigate the influence of S supply on Cd accumulation in rice under two Cd levels (0 and 50 μM), combined with three S concentrations (0, 2.64 and 5.28 mM). The moderate and excessive S supply (2.64 and 5.28 mM) tended to increase plant growth, root length, root and shoot dry weights of rice seedlings, and significantly decreased Cd concentrations in rice plants and grains in the absence or presence of Cd. The subcellular distribution and chemical forms of Cd in roots and shoots also varied with S supply levels. The decreased Cd uptake and translocation in rice grains could be ascribed to the enhanced formation of iron (Fe) plaque on the root surfaces and increased Cd chelation and vacuolar sequestration in roots, since Fe, Mn concentrations in Fe plaque, glutathione and phytochelatins contents, as well as phytochelatin synthase (OsPCS) and tonoplast heavy metal ATPase (OsHMA3) expressions in roots significantly increased with increased S supply. This work provides more insight into the mechanisms of Cd uptake and translocation in rice, and will be helpful for developing strategies to reduce rice grain Cd through S fertilizer application in Cd-contaminated soil.

Nickel hyperaccumulation mechanisms: a review on the current state of knowledge

Hyperaccumulator plants are unusual plants that accumulate particular metals or metalloids, such as nickel, zinc, cadmium and arsenic, in their living tissues to concentrations that are hundreds to thousands of times greater than what is normal for most plants. The hyperaccumulation phenomenon is rare (exhibited by less than 0.2% of all angiosperms), with most of the ~500 hyperaccumulator species known globally for nickel. This review highlights the contemporary understanding of nickel hyperaccumulation processes, which include root uptake and sequestration, xylem loading and transport, leaf compartmentation and phloem translocation processes. Hyperaccumulator plants have evolved highly efficient physiological mechanisms for taking up nickel in their roots followed by rapid translocation and sequestration into the aerial shoots. The uptake of nickel is mainly involved with low affinity transport systems, presumably from the ZIP family. The presence of high concentrations of histidine prevents nickel sequestration in roots. Nickel is efficiently loaded into the xylem, where it mainly presents as Ni2+. The leaf is the main storage organ, which sequestrates nickel in non-active sites, e.g. vacuoles and apoplast. Recent studies show that phloem translocates high levels of nickel, which has a strong impact on nickel accumulation in young growing tissues.

Role of chelant on Cu distribution and speciation in Lolium multiflorum by synchrotron techniques

Chelants are known to enhance metal translocation in plants; however, the underlying mechanisms are still not fully understood. This study aimed to elucidate the distribution and speciation of Cu in ryegrass (Lolium multiflorum) in both absence and presence of the biodegradable chelant [S,S′]-ethylenediamine disuccinic acid (EDDS). The results showed that EDDS increased the Cu translocation factor from root to shoot by 6–9 folds under CuEDDS in comparison with free Cu (50–250μM). Synchrotron-based microscopic X-ray fluorescence (μ-XRF) mapping revealed that EDDS alleviated Cu deposition in the root meristem of root apex and the junction of lateral root zone, and facilitated Cu transport to root stele for subsequent translocation upwards. X-ray absorption near edge structure (XANES) analysis found that free Cu was sequestered in plants as a mixture of Cu-organic ligands. In the EDDS treatment, Cu was primarily present as CuEDDS (49–67%) in plants with partial chemical transformation to Cu-histidine (21–36%) and Cu(I)-glutathione (0–24%). These results suggest that EDDS improves internal Cu mobility through forming CuEDDS, thus decreasing the root sequestration of Cu, and ultimately facilitating Cu transport to plant shoots.

Remediation of cadmium toxicity in field peas (Pisum sativum L.) through exogenous silicon

Cadmium (Cd) is an important phytotoxic element causing health hazards. This work investigates whether and how silicon (Si) influences the alleviation of Cd toxicity in field peas at biochemical and molecular level. The addition of Si in Cd-stressed plants noticeably increased growth and development as well as total protein and membrane stability of Cd-stressed plants, suggesting that Si does have critical roles in Cd detoxification in peas. Furthermore, Si supplementation in Cd-stressed plants showed simultaneous significant increase and decrease of Cd and Fe in roots and shoots, respectively, compared with Cd-stressed plants. At molecular level, GSH1 (phytochelatin precursor) and MTA (metallothionein) transcripts predominantly expressed in roots and strongly induced due to Si supplementation in Cd-stressed plants compared with Cd-free conditions, suggesting that these chelating agents may bind to Cd leading to vacuolar sequestration in roots. Furthermore, pea Fe transporter (RIT1) showed downregulation in shoots when plants were treated with Si along with Cd compared with Cd-treated conditions. It is consistent with the physiological observations and supports the conclusion that alleviation of Cd toxicity in pea plants might be associated with Cd sequestration in roots and reduced Cd translocation in shoots through the regulation of Fe transport. Furthermore, increased CAT, POD, SOD and GR activity along with elevated S-metabolites (cysteine, methionine, glutathione) implies the active involvement of ROS scavenging and plays, at least in part, to the Si-mediated alleviation of Cd toxicity in pea. The study provides first mechanistic evidence on the beneficial effect of Si on Cd toxicity in pea plants.

Remediation of Chromium Toxicity Through Exogenous Salicylic Acid in Rice (Oryza sativa L.)

This work investigates whether and how salicylic acid (SA) alleviates chromium (Cr) toxicity in rice. Addition of SA under Cr stress markedly increased growth parameters, total protein content, and membrane stability but reduced the concentration and translocation of Cr in shoots but not in roots, suggesting that SA does have critical roles in Cr detoxification associated with Cr sequestration in roots. Further, Fe along with the expression of two Fe transporters (OsIRT1, OsNRAMP1) showed no significant changes in roots due to SA supplementation under Cr stress, indicating that regulation of Fe uptake is not involved in Cr reduction in rice plants through SA. At molecular level, OsPCS1 (phytochelatin synthase) and OsMT1 (metallothionein) and OsHMA3 (P-type ATPase 3) transcripts significantly upregulated following SA supplementation under Cr stress, suggesting that these chelating agents may bind to Cr leading to elevated Cr retention in roots. Furthermore, increased CAT, POD, SOD, and GR leading to decreased H2O2 along with elevated metabolites (cysteine, methionine, glutathione, proline, ascorbic acid) in roots implies active involvement of ROS scavenging and plays partial role in SA-mediated alleviation of Cr toxicity in rice plants. These findings will be useful for bioremediation of Cr toxicity in rice and other crops.

Silicon ameliorates chromium toxicity through phytochelatin-mediated vacuolar sequestration in the roots of Oryza sativa (L.)

ABSTRACTHigh chromium (Cr) in rice causes reduced yield and health hazards. This work investigates how Si alleviates Cr toxicity in rice. Addition of Si under Cr stress restored the growth parameters, total protein content, and membrane stability along with reduced Cr content in shoots, confirming that Si plays critical roles in Cr detoxification in rice. However, Si supplementation under Cr stress caused no significant changes in root Cr content but decreased shoot Cr concentrations compared with Cr-stressed plants, indicating that alleviation of Cr toxicity might be associated with Cr sequestration in roots. Further, concentration of Fe and expression of Fe transporter (OsIRT1) showed no significant changes due to Si supplementation under Cr stress, implying that Fe regulation is not involved with Si-mediated mitigation of Cr toxicity in rice. Further, phytochelatin accumulation and OsPCS1 (phytochelatin synthase) transcripts strongly induced due to the dual treatment of Si and Cr compared with Cr-stressed plants, suggesting that phytochelatin might bind to Cr, which leads to vacuolar sequestration in roots. Furthermore, increased glutathione reductase activity in roots implies that active involvement of ROS scavenging partially ameliorates Cr toxicity in rice plants. The study illustrates first evidences on the effect of Si alleviating Cr toxicity in rice plants.

Zinc Isotope Fractionation in the Hyperaccumulator Noccaea caerulescens and the Nonaccumulating Plant Thlaspi arvense at Low and High Zn Supply.

On the basis of our previous field survey, we postulate that the pattern and degree of zinc (Zn) isotope fractionation in the Zn hyperaccumulator Noccaea caerulescens (J. & C. Presl) F. K. Mey may reflect a relationship between Zn bioavailability and plant uptake strategies. Here, we investigated Zn isotope discrimination during Zn uptake and translocation in N. caerulescens and in a nonaccumulator Thlaspi arvense L. with a contrasting Zn accumulation ability in response to low (Zn-L) and high (Zn-H) Zn supplies. The average isotope fractionations of the N. caerulescens plant as a whole, relative to solution (Δ(66)Znplant-solution), were -0.06 and -0.12‰ at Zn-L-C and Zn-H-C, respectively, indicative of the predominance of a high-affinity (e.g., ZIP transporter proteins) transport across the root cell membrane. For T. arvense, plants were more enriched in light isotopes under Zn-H-A (Δ(66)Znplant-solution = -0.26‰) than under Zn-L-A and N. caerulescens plants, implying that a low-affinity (e.g., ion channel) transport might begin to function in the nonaccumulating plants when external Zn supply increases. Within the root tissues of both species, the apoplast fractions retained up to 30% of Zn mass under Zn-H. Moreover, the highest δ(66)Zn (0.75‰-0.86‰) was found in tightly bound apoplastic Zn, pointing to the strong sequestration in roots (e.g., binding to high-affinity ligands/precipitation with phosphate) when plants suffer from high Zn stress. During translocation, the magnitude of isotope fractionation was significantly greater at Zn-H (Δ(66)Znroot-shoot = 0.79‰) than at Zn-L, indicating that fractionation mechanisms associated with root-shoot translocation might be identical to the two plant species. Hence, we clearly demonstrated that Zn isotope fractionation could provide insight into the internal sequestration mechanisms of roots when plants respond to low and high Zn supplies.