Net Cd2 Research Articles

Aluminum decreases cadmium accumulation by down-regulating the expression of cadmium-related genes in wheat

Aluminum (Al) and cadmium (Cd) often coexist in the acidic soils of southern China, but their interactive effects remain unclear. In this study, this was examined in Al-resistant (Atlas 66) and Al-sensitive (Scout 66) wheat cultivars. The results showed that Al significantly alleviated the root growth of Al-tolerant Atlas 66 under Cd stress but had no effect on the root growth of Al-sensitive Scout 66 or the shoot growth of both cultivars. However, Al substantially decreased Cd accumulation in the roots and shoots of Atlas 66 and Scout 66. This could be attributed to the decreased uptake of Cd by the roots rather than the changes in Cd distribution in various organs or the Cd binding and adsorption of the cell wall. Using non-invasive micro-test technology, we further confirmed that Al and Cd co-exposure significantly inhibited the net Cd2+ influx into the roots of Atlas 66 and Scout 66 compared to Cd-only stress. Furthermore, the higher Cd2+ influx into roots should be responsible for the high Cd accumulation in the roots and shoots of Atlas66 than in those of Scout66 under Al and Cd co-exposure, which may be due to the higher Al accumulation in the roots of Al-sensitive Scout 66 that resulted in the larger decrease of negative charges on root surfaces. Finally, the effects of Al on the expression of Cd-related genes responsible for Cd uptake and translocation in wheat roots were investigated. The results have suggested that Al significantly downregulated the expression of TaNramp5 in Atlas 66 and the expression of TaNramp5, TaIRT1 and TaHMA2 in Scout66.

Populus euphratica plant cadmium resistance 2 mediates Cd tolerance by root efflux of Cd ions in poplar.

Populus euphratica PePCR2 increases Cd resistance by functioning as a Cd extrusion pump and by mediating the expression of genes encoding other transporters. Cadmium (Cd) is a non-essential, toxic metal that negatively affects plant growth. Plant cadmium resistance (PCR) proteins play key roles in the response to heavy metal stress. In this study, we isolated the gene PePCR2 encoding a plant PCR from Populus euphratica. PePCR2 gene transcription was induced by Cd, and its transcript level peaked at 24h after exposure, at a level approximately 18-fold higher than that at 0h. The PePCR2 protein was localized to the plasma membrane. Compared with yeast cells harboring the empty vector, yeast cells expressing PePCR2 showed enhanced Cd tolerance and a lower Cd content. Compared with wild-type (WT) plants, poplar overexpressing PePCR2 showed higher Cd resistance. Net Cd2+ efflux measurements showed that Cd2+ efflux from the roots was 1.5 times higher in the PePCR2-overexpressing plants than in WT plants. Furthermore, compared with WT plants, the PePCR2-overexpressing plants showed increased transcript levels of ABCG29, HMA5, PDR2, YSL7, and ZIP1 and decreased transcript levels of NRAMP6, YSL3, and ZIP11 upon exposure to Cd. These data show that PePCR2 increased Cd resistance by acting as a Cd extrusion pump and/or by regulating other Cd2+ transporters to decrease Cd toxicity in the cytosol. The results of this study identify a novel plant gene with potential applications in Cd removal, and provide a theoretical basis for reducing Cd toxicity and protecting food safety.

Homolog of Human placenta-specific gene 8, PcPLAC8-10, enhances cadmium uptake by Populus roots

Cadmium (Cd) pollution of soil occurs worldwide. Phytoremediation is an effective approach for cleaning up Cd polluted soil. Fast growing Populus species with high Cd uptake capacities are desirable for phytoremediation. Thus, it is important to elucidate the molecular functions of genes involved in Cd uptake by poplars. In this study, PcPLAC8–10, a homolog of Human placenta-specific gene 8 (PLAC8) implicated in Cd transport was functionally characterized in Populus × canescens. PcPLAC8–10 was transcriptionally induced in Cd-treated roots and it encoded a plasma membrane-localized transporter. PcPLAC8–10 exhibited Cd uptake activity when expressed in yeast cells. No difference in growth was observed between wild type (WT) and PcPLAC8–10-overexpressing poplars. PcPLAC8–10-overexpressing poplars exhibited increases in net Cd2+ influxes by 192% and Cd accumulation by 57% in the roots. However, similar reductions in biomass were found in WT and transgenic poplars when exposed to Cd. The complete motif of CCXXXXCPC in PcPLAC8–10 was essential for its Cd transport activity. These results suggest that PcPLAC8–10 is a plasma membrane-localized transporter responsible for Cd uptake in the roots and the complete CCXXXXCPC motif of PcPLAC8–10 plays a key role in its Cd transport activity in poplars.

The alleviation mechanisms of cadmium toxicity in Broussonetia papyrifera by arbuscular mycorrhizal symbiosis varied with different levels of cadmium stress

The alleviation of cadmium (Cd) toxicity in Broussonetia papyrifera by arbuscular mycorrhizal (AM) fungi are still not completely elucidated. This study investigated the effects of Rhizophagus irregularis on physiological and biochemical characteristics, and molecular regulation in B. papyrifera under different levels of Cd (0, 30, 90 and 270 mg kg-1 Cd) stress. Results showed that (1) AM symbiosis improved the growth and photosynthesis, enhanced ROS levels as stress signaling and maintained ROS balance under low and medium Cd stress. (2) AM symbiosis regulated AsA-GSH cycle to mitigate ROS overproduction under high Cd stress. (3) AM fungus can chelate more Cd under high Cd stress, increasing soil pH and GRSP content. (4) AM plants can fix or chelate more Cd by P in leaves and reserve more P in stems under high Cd stress. (5) AM symbioses increased root net Cd2+ influx and uptake under medium Cd stress but inhibited under high Cd stress, with upregulation of genes related heavy metals (HMs) transport under medium Cd stress and inhibited the transcription of genes related HMs transport under high Cd stress. Therefore, the alleviation mechanisms of Cd toxicity in B. papyrifera by R. irregularis symbiosis depends on the levels of Cd stress.

Cadmium uptake and membrane transport in roots of hyperaccumulator Amaranthus hypochondriacus L.

Hyperaccumulator Amaranthus hypochondriacus L. has huge potential in the remediation of cadmium (Cd)-contaminated soils and is necessary to understand the mechanism of Cd uptake by the roots. In this study, the mechanism of Cd uptake into the root of A. hypochondriacus was investigated using the non-invasive micro-test technology (NMT) by analyzing the rate of Cd2+ fluxes at different regions of the root tip; also we assessed the impact of different channel blockers and inhibitors on the Cd accumulation in the roots, the real-time Cd2+ fluxes, and the distribution of Cd along the roots. The results showed that the Cd2+ influx was greater near the root tip (within 100 μm of the tip). All the inhibitors, ion-channel blockers, and metal cations had different degrees of inhibition on the absorption of Cd in the roots of A. hypochondriacus. The net Cd2+ flux in the roots was significantly decreased by the Ca2+ channel blockers lanthanum chloride (LaCl3) by up to 96% and verapamil by up to 93%; as for the K+ channel blocker tetraethylammonium (TEA), it also caused a 68%-reduction on the net Cd2+ flux in the roots. Therefore, we infer that the uptake by A. hypochondriacus roots is mainly through the Ca2+ channels. The Cd absorption mechanism appears to be related to the synthesis of plasma membrane P-type ATPase and phytochelatin (PC), which is reflected by the inhibition of Ca2+ upon addition of inorganic metal cations. In conclusion, access of Cd ions into the roots of A. hypochondriacus is achieved through various ion channels, with the most important being the Ca2+ channel. This study will further enhance the literature regarding Cd uptake and pathways of membrane transport in roots of Cd hyperaccumulators.

The effects of salinity and pH variation on hyperaccumulator Bidens pilosa L. accumulating cadmium with dynamic and real-time uptake of Cd2+ influx around its root apexes.

Bidens pilosa L. has been confirmed to be a potential Cd hyperaccumulator by some researchers, but the dynamic and real-time uptake of Cd2+ influx by B. pilosa root apexes was a conundrum up to now. The aim of our study was to investigate the effects of salinity and pH variations on the characteristics of Cd2+ influx around the root apexes of B. pilosa. The tested seedlings of B. pilosa were obtained by sand culture experiments in a greenhouse after 1month from germination, and the Cd2+ influxes from the root apex of B. pilosa under Cd treatments with different salinity and pH levels were determined with application of non-invasive micro-test technology (NMT). The results showed that Cd2+ influxes at 300μm from the root tips decreased under Cd treatments with 5mM and 10mM NaCl, as compared to Cd stress alone. However, Cd treatments with 2.5mM NaCl had little effect on the net Cd2+ influxes, as compared to Cd treatments alone. Importantly, Cd treatments at pH = 4.0 markedly increased Cd2+ influxes in roots, and Cd treatment at pH = 7.0 had no significant effect on the net Cd2+ influxes compared to Cd treatments at pH = 5.5. Results also showed that Cd treatments with 10mM NaCl significantly decreased concentrations of chlorophyll (Chl) a and b in leaves and root vigor of B. pilosa relative to Cd treatments alone, while there were no significant differences between Cd treatments with 2.5mM NaCl and Cd treatments alone. But root vigor was inhibited significantly under Cd treatments with 5mM and 10mM NaCl. A significant increase of root vigor was observed in Cd treatments at pH = 4.0, as compared to pH = 5.5. The Cd treatments with high and medium concentrations of NaCl inhibited the uptake of Cd by B. pilosa roots and affected the Chl and root vigor further. But the Cd treatments at pH = 4.0 could promote the Cd uptake and root vigor. Our results revealed the uptake mechanisms of B. pilosa as a potential phytoremediator under different salinity and pH levels combined with Cd contamination and provided a new idea for screening ideal hyperaccumulator and constructing evaluation system.

The OsNramp4 aluminum transporter is involved in cadmium accumulation in rice grains

Cadmium (Cd) toxicity affects many crops. Members of the OsNramp (Natural resistance-associated macrophage protein) gene family play important roles in the transport of divalent or trivalent cations in rice (Oryza sativa L.). This study explored a possible involvement of OsNramp4 in Cd2+ transport in rice. We employed the CRISPR/cas9 technique to obtain rice OsNramp4 knockout lines. Using NMT (Non-invasive Micro-test Technology) system for real-time measurement, it was found that the net Cd2+ flux was significantly lower than that of wild-type. The yeast strain expressing OsNramp4 grew poorly under Cd2+ stress, and accumulated more Cd2+ than the control strain, which enhanced the sensitivity of yeast to Cd2+. OsNramp4 was membrane-localized and mainly expressed in roots, but after tillering, its expression shifted to the nodes and glumes. Furthermore, the loss-of-function OsNramp4 mutation lowered the root cell sap Cd2+ content, resulting in a significant decrease in Cd2+ content in shoot and Cd2+ accumulation in grains. We characterized the OsNramp4, an Al3+ transporter with the ability to alter the cellular distribution of Cd2+ in rice and to reduce Cd2+ content in the grain. Our study highlights the complexity of ion uptake and transport in plants.

HvVPE3, a gene closely associated with Cd uptake and tolerance in barley

Cadmium (Cd) contamination in soils has become a serious issue of sustainable crop production and food safety. Here, HvVPE3, a γ-type VPE (vacuolar processing enzyme) gene, is firstly reported to regulate Cd uptake/tolerance. We isolated and functionally characterized HvVPE3 using genetic transformation coupled with physio-biochemical assay of Cd tolerance in barley. HvVPE3 was mainly expressed in leaves and being inducible by Cd stress. Subcellular localization verified that HvVPE3 is localized in the endoplasmic reticulum (ER). Silencing of HvVPE3 by RNA interfering (RNAi) significantly decreased Cd concentration in barley and improved tolerance to Cd stress, whereas HvVPE3 overexpression resulted in increased Cd concentration and hypersensitivity to Cd stress. Compared with the transformation-free wild-type (WT), HvVPE3-RNAi lines recorded significantly higher plant height and shoot biomass after two weeks 10 μM Cd exposure, but exhibited inhibited root net Cd2+ influx, suppressed Cd-induced increase in caspase-1, -8-like activities, and enhanced anti-oxidative activities such as catalase (CAT), peroxidase (POD), glutathione reductase (GR) and ascorbic acid oxidase (APX) thus mitigated Cd-induced oxidative stress and programmed cell death (PCD) to improve Cd tolerance in barley. Evolutionary bioinformatic confirms that VPE3s are highly conserved in green plants and may have evolved from the Order of Huperzia in Lycophytes. Our findings reveal a promising functional gene for breeding in developing barley varieties with low Cd accumulation and high Cd tolerance.

PcNRAMP1 Enhances Cadmium Uptake and Accumulation in Populus × canescens.

Poplars are proposed for the phytoremediation of heavy metal (HM) polluted soil. Characterization of genes involved in HM uptake and accumulation in poplars is crucial for improving the phytoremediation efficiency. Here, Natural Resistance-Associated Macrophage Protein 1 (NRAMP1) encoding a transporter involved in cadmium (Cd) uptake and transport was functionally characterized in Populus × canescens. Eight putative PcNRAMPs were identified in the poplar genome and most of them were primarily expressed in the roots. The expression of PcNRAMP1 was induced in Cd-exposed roots and it encoded a plasma membrane-localized protein. PcNRAMP1 showed transport activity for Cd2+ when expressed in yeast. The PcNRAMP1-overexpressed poplars enhanced net Cd2+ influxes by 39–52% in the roots and Cd accumulation by 25–29% in aerial parts compared to the wildtype (WT). However, Cd-induced biomass decreases were similar between the transgenics and WT. Further analysis displayed that the two amino acid residues of PcNRAMP1, i.e., M236 and P405, play pivotal roles in regulating its transport activity for Cd2+. These results suggest that PcNRAMP1 is a plasma membrane-localized transporter involved in Cd uptake and transporting Cd from the roots to aerial tissues, and that the conserved residues in PcNRAMP1 are essential for its Cd transport activity in poplars.

Pathways of cadmium fluxes in the root of the hyperaccumulator Celosia argentea Linn.

In order to study the mechanism of cadmium (Cd) uptake by the roots of Celosia argentea Linn. (Amaranthaceae), the effects of various inhibitors, ion channel blockers, and hydroponic conditions on Cd2+ fluxes in the roots were characterized using non-invasive micro-test technology (NMT). The net Cd2+ flux (72.5pmol∙cm-2∙s-1) in roots that had been pretreated with Mn was significantly higher than that in non-pretreated roots (58.1pmol∙cm-2∙s-1), indicating that Mn pretreatment enhanced Cd uptake by the roots. This finding may be explained by the fact that the addition of Mn significantly increased the expression of the transporter gene and thus promoted Cd uptake and transport. In addition, Mn pretreatment resulted in an increase in root growth, which may in turn promote root vigor. The uncoupler 2,4-dinitrophenol (DNP) caused a significant reduction in net Cd2+ fluxes in the roots, by 70.5% and 41.4% when exposed to Mn and Cd stress, respectively. In contrast, a P-type ATPase inhibitor (Na3VO4) had only a small effect on net Cd2+ fluxes to the plant roots, indicating that ATP has a relatively minor role in Cd uptake by roots. La3+ (a Ca channel inhibitor) had a more significant inhibitory effect on net Cd2+ fluxes than did TEA (a K channel inhibitor). Therefore, Cd uptake by plant roots may occur mainly through Ca channels rather than K channels. In summary, uptake of Cd by the roots of C. argentea appears to occur via several types of ion channels, and Mn can promote Cd uptake.

GABA-mediated inhibition of cadmium uptake and accumulation in apples

GABA, a four-carbon non-protein amino acid, plays an important role in animals and plants. We previously found GABA could alleviate alkali stress in apple seedlings. However, its physiological mechanism under heavy metal cadmium (Cd) stress need to be further studied. Thus, we explored its biological role in response to Cd stress. It was verified that 0.5 mM GABA could effectively alleviate Cd toxicity. Using NMT technique, we found that exogenous GABA could significantly reduce the net Cd2+ fluxes in apple roots, and Cd content was significantly lower than that in roots under Cd stress. Further analysis indicated exogenous GABA could significantly reduce the expression of genes related to the uptake and transport of Cd in apples under Cd stress. In addition, exogenous GABA could significantly increase the content of amino acids in apple roots under Cd stress. GAD is a key enzyme in GABA synthesis, we obtained transgenic apple roots of overexpression MdGAD1. Compared with the control, transgenic roots accumulated less Cd, maintained lower Cd uptake by roots, and lower expression of related transport genes. These results showed that GABA could effectively alleviate Cd toxicity in apple seedlings and provide a new perspective of GABA to alleviate Cd stress.

Unraveling the effects of arbuscular mycorrhizal fungi on cadmium uptake and detoxification mechanisms in perennial ryegrass (Lolium perenne)

Cadmium (Cd) is a major environmental pollutant and one of the most toxic metals in the environment. Arbuscular mycorrhizal fungi (AMF) assisted phytoremediation can be used to remove Cd from polluted soils but the role of AMF, which mediate in Cd accumulation and tolerance, remains poorly understood.Here we inoculated Lolium perenne with two different AMF species (Glomus etunicatum and Glomus mosseae). Mycorrhizal L. perenne and non-mycorrhizal controls were exposed to Cd stress and we tested the effects of AMF mycorrhization on Cd uptake and subsequent tolerance, as well as the underlying mechanisms.Mycorrhizal infection increased root Cd2+ uptake and we observed that net Cd2+ influx was coupled with net Ca2+ influx. The inactivation of Ca2+ transporter channels decreased Cd2+ uptake in non-inoculated roots to a greater extent than in inoculated roots, indicating that AMF activates additional ion transport channels. In consequence, inoculated plants exhibited higher Cd accumulation in both roots and shoots than non-inoculated controls. However, AMF-inoculated plants showed higher chlorophyll concentrations, photosynthesis, and growth under Cd, indicating lower Cd toxicity in AMF-inoculated plants, despite the increase in Cd uptake. We observed that AMF-inoculated favored the isolation of Cd within cell walls and vacuoles, and had higher concentrations of superoxide dismutase activity and glutathione concentration in roots than non-inoculated plants, consequently experiencing less stress upon Cd exposure.Our results highlight the potential and mechanism of AMF for enhancing phytoremediation of L. perenne in heavy metal contaminated environments.

TaWRKY70 positively regulates TaCAT5 enhanced Cd tolerance in transgenic Arabidopsis

The WRKY transcription factors (TFs) are involved in plant responses to multiple biotic and abiotic stressors; however, the role of WRKY TFs in the regulation of Cd stress in wheat remains unknown. In this study, we investigated the mechanism of the wheat TaWRKY70 TF in Cd stress. TaWRKY70 expression in Arabidopsis was used for the functional analysis. TaWRKY70 regulated Cd tolerance with Cd accumulating in roots but not in the leaf tissues of transgenic Arabidopsis. The net Cd2+ influx into Arabidopsis roots decreased when TaWRKY70 was expressed. The quantitative real-time-polymerase chain reaction (qRT-PCR) results showed that the expression levels of heavy metal ATPase (AtHMA3), natural resistance-associated macrophage protein (AtNRAMP5), yellow stripe1-like (AtYSL3), and iron transport protein (AtIRT1) decreased under Cd-stress condition. Electrolyte leakage, as well as malondialdehyde and hydrogen peroxide contents, were lower in transgenic Arabidopsis than those in the wild-type (WT), whereas antioxidant enzyme activities were higher. Electrophoretic mobility shift assay (EMSA), yeast one-hybrid (Y1H) assay, and transient transactivation assay confrmed that TaWRKY70 could directly bind to the TaCAT5 promoter. Our findings provide a novel understanding of the WRKY TFs involved in the response to heavy metal stress.

Interaction of Mn and Cd during their uptake in Celosia argentea differs between hydroponic and soil systems

Celosia argentea (Amaranthaceae) can accumulate both Cd and Mn. This study reveals the interactive effect and underlying mechanism of Cd and Mn during their uptake in this species. Plants were grown in hydroponic and soil media with different Cd and Mn treatments. The concentrations of Cd and Mn in plants and soil solutions were determined by ICP-MS. The real-time Cd2+ flux values into roots were measured with non-invasive micro-test technology. Manganese supply significantly decreased Cd bioaccumulation in C. argentea in hydroponic experiments. The mean net Cd2+ flux rates into roots decreased by 10.5% and 56.9% with the addition of 0.01 and 0.5 mM Mn2+, respectively. The Mn supply (1 mM) increased the Michaelis constant by 1.8-fold but did not significantly change the maximum uptake velocity of Cd. In soil culture experiments, Mn application significantly increased Cd bioaccumulation and Cd concentrations in soil solutions. The interactions of Cd and Mn in C. argentea differ between hydroponic and soil cultures. Mn inhibits Cd uptake in hydroponic experiments via competition in common transport systems. The enhancement effect of Mn on Cd uptake in soil systems is attributed to an increase of Cd in soil solutions by competitive adsorption.

Zinc effects on cadmium toxicity in two wheat varieties (Triticum aestivum L.) differing in grain cadmium accumulation

Presence of cadmium (Cd) in food poses serious risks to human health. Understanding the effects of zinc (Zn) on Cd absorption by crops could help provide a theoretical basis for the treatment with Zn on contaminated soils. In this study, two wheat varieties, differing in grain-Cd accumulation ability (L979, a Cd low-accumulation variety, and H27, a high-accumulation variety) were selected to investigate the effect of Zn addition on Cd toxicity. Cd was applied to nutrient solutions at 0 and 10 μM, and added Zn were 0, 50 and 100 μM. Zn supplements alleviated decreases in biomass induced by Cd toxicity for both varieties, and both varieties had different reduced concentrations of Cd in their shoots. Application of 50 μM Zn to H27 resulted in a 17% decrease in Cd concentrations. When treated with 100 μM Zn, only L979 showed a reduction in Cd concentration. The higher proportion of Cd in the soluble fraction was found in L979. In addition, ion-selective scanning at the root-surface indicated that Zn supplements reduced net root Cd2+ flux by 55% for L979, and 69% for H27. These mitigating effects of Zn in both varieties involved mechanisms related to photosynthesis, root growth, and antioxidant production. Additionally, both Zn available in the medium and absorbed in plant tissue causes antagonistic effects on Cd absorption for wheat. It seemed that vacuolar compartmentation could contribute Cd detoxification especially for low accumulation variety.

Ectopic Expression of Poplar ABC Transporter PtoABCG36 Confers Cd Tolerance in Arabidopsis thaliana.

Cadmium (Cd) is one of the most toxic heavy metals for plant growth in soil. ATP-binding cassette (ABC) transporters play important roles in biotic and abiotic stresses. However, few ABC transporters have been characterized in poplar. In this study, we isolated an ABC transporter gene PtoABCG36 from Populus tomentosa. The PtoABCG36 transcript can be detected in leaves, stems and roots, and the expression in the root was 3.8 and 2 times that in stems and leaves, respectively. The PtoABCG36 expression was induced and peaked at 12 h after exposure to Cd stress. Transient expression of PtoABCG36 in tobacco showed that PtoABCG36 is localized at the plasma membrane. When overexpressed in yeast and Arabidopsis, PtoABCG36 could decrease Cd accumulation and confer higher Cd tolerance in transgenic lines than in wild-type (WT) lines. Net Cd2+ efflux measurements showed a decreasing Cd uptake in transgenic Arabidopsis roots than WT. These results demonstrated that PtoABCG36 functions as a cadmium extrusion pump participating in enhancing tolerance to Cd through decreasing Cd content in plants, which provides a promising way for making heavy metal tolerant poplar by manipulating ABC transporters in cadmium polluted areas.

Aeration Increases Cadmium (Cd) Retention by Enhancing Iron Plaque Formation and Regulating Pectin Synthesis in the Roots of Rice (Oryza sativa) Seedlings

BackgroundAeration and water management increasing rhizosphere oxygen amount significantly promote rice (Oryza sativa) growth and yield, but the effect of root aeration on cadmium (Cd) toxicity and accumulation in rice seedlings under hydroponic culture remains unclear.ResultsResults showed that aeration promoted rice seedling growth and alleviated Cd toxicity. Transverse section discovered that Cd accelerated root mature and senescence while aeration delayed the mature and senescence of roots. Non-invasive Micro-test Technology (NMT) showed that aeration increased net O2 and Cd2+ influxes on the surface of roots while decreased net Cd2+ influx in xylem. Perls blue staining showed that aeration and Cd treatments increased iron plaque formation on the surface of roots. Results of metal concentration analysis showed that besides increasing Cd retention in iron plaque, aeration also increasing Cd retention in the cell wall of rice roots. Cell wall component analysis showed that aeration not only increased pectin content but also decreased pectin methylesterification degree (PMD) by increasing pectin methylesterase (PME) activity.ConclusionsAll of these results indicate that aeration not only delays root mature and senescence but also increases Cd retention in roots by enhancing iron plaque formation and regulating pectin synthesis in the roots of rice seedlings.

MhMAPK4 from Malus hupehensis Rehd. decreases cell death in tobacco roots by controlling Cd2+ uptake

Cadmium (Cd) induces cell death in plant roots. Mitogen-activated protein kinase (MAPK) plays a role in the regulation of cell death induced by Cd in plant roots. In this study, MhMAPK4 was isolated from the roots of Malus hupehensis. Subcellular localization showed that the MhMAPK4 protein was located in the cell membrane and cytoplasm and is a transmembrane protein that is characterized by hydrophily. The expression of MhMAPK4 in the roots of M. hupehensis was up-regulated by Cd sulfate and Cd chloride. Phenotypic comparison under Cd stress showed that the growth of wild-type (WT) tobacco was lower than the transgenic lines overexpressing MhMAPK4. The fresh weight and the root length of WT also was lower than that of the transgenic tobacco. The net Cd2+ influx in the tobacco roots was decreased by the overexpression of MhMAPK4, as was root Cd accumulation. The recovery time of the Cd2+ influx to stable state in the transgenic tobacco was also shorter than the WT. The expression of iron-regulated transporter 1 (NtIRT1) and natural resistance associated macrophage protein 5 (NtNRAMP5) was relatively low in the transgenic lines under Cd stress. Cell death and apoptosis in the tobacco roots was reduced following the overexpression of MhMAPK4. The activity of vacuolar processing enzyme (VPE) and the transcript level of VPE in the transgenic tobacco was lower than that of WT under Cd stress. In addition, the electrolyte leakage and malondialdehyde and hydrogen peroxide contents in the transgenic tobacco were lower than those of WT, whereas the antioxidant enzyme activity and expression were higher. These results suggest that MhMAPK4 regulates Cd accumulation by mediating Cd2+ uptake by the roots, and controls Cd-caused cell death by adjusting VPE activity.

Increasing ammonium nutrition as a strategy for inhibition of cadmium uptake and xylem transport in rice (Oryza sativa L.) exposed to cadmium stress

Nitrogen (N) speciation could influence the efficiency of Cd accumulation by plants grown in Cd-contaminated areas. However, limited information has been reported on how the application of N speciation to rice affects the Cd accumulation. Here, we investigated the physiological and genetic mechanisms involved in Cd uptake by roots, xylem translocation and subsequent Cd accumulation in rice (Oryza sativa L.) affected by different NO3−/NH4+ ratios under low-and-high Cd stress. Results showed that both N speciation and Cd stress affected plant growth, with high NH4+-N ratios treatments having higher tissue biomass, and high Cd treatment reducing tissue biomass. The Cd concentrations in shoots and roots were reduced with the increasing NH4+-N ratios (except for the full NO3−-N treatment), but did not affect the Cd translocation factor (TF). The total Cd accumulation of the whole plant were also reduced with the high NH4+-N ratios (NO3−/NH4+, 1:2 and 0:1) on the low Cd treatment due to the decline both in the Cd accumulation of shoots and roots. A scanning ion-selective electrode technique (SIET) showed that the net Cd2+ influxes at the root hair zone were inhibited with the increasing NH4+-N ratios in absence or pretreatment with different NO3−/NH4+ ratios. Additionally, the Cd concentrations in xylem sap were also displayed a decline trend with the increasing NH4+-N ratios under two dose Cd treatments. Furthermore, the gene expression related to Cd uptake (OsIRT1 and OsNRAPM5) and transport (OsHMA2) in roots showed the familiar tendencies with those of Cd uptake and transport at the physiological level treated with different NO3−/NH4+ ratios under two dose Cd treatments. It is concluded that increasing ammonium nutrition contributes to the inhibition of Cd uptake, xylem transport and subsequent accumulation in rice.

The pathway of transmembrane cadmium influx via calcium-permeable channels and its spatial characteristics along rice root.

To develop elite crops with low cadmium (Cd), a fundamental understanding of the mechanism of Cd uptake by crop roots is necessary. Here, a new mechanism for Cd2+ entry into rice root cells was investigated. The results showed that Cd2+ influx in rice roots exhibited spatially and temporally dynamic patterns. There was a clear longitudinal variation in Cd uptake along rice roots, with the root tip showing much higher Cd2+ influx and concentration than the root mature zone, which might be due to the much higher expression of the well-known Cd transporter genes OsIRT1, OsNRAMP1, OsNRAMP5, and OsZIP1 in the root tip. Both the net Cd2+ influx and the uptake of Cd in rice roots were highly inhibited by ion channel blockers Gd3+ and TEA+, supplementation of Ca2+ and K+, and the plasma membrane H+-ATPase inhibitor vanadate, with Gd3+ and Ca2+ showing the most inhibitory effects. Furthermore, Ca2+- or Gd3+-induced reduction in Cd2+ influx and Cd uptake did not coincide with the expression of Cd transporter genes, but with that of two Ca channel genes, OsAAN4 and OsGLR3.4. These results indicate that Cd transporters are in part responsible for Cd2+ entry into rice root, and provide a new perspective that the Ca channels OsAAN4 and OsGLR3.4 might play an important role in rice root Cd uptake.