Plant Cell Responses to Cadmium and Zinc. In: Nick P, Opatrny Z, editors. Applied Plant Cell Biology. Berlin: Springer Berlin Heidelberg; 2014. pp. 209-46. [DOI: 10.1007/978-3-642-41787-0_7]

Low concentrations of methyl jasmonate promote plant growth and mitigate Cd toxicity in Cosmos bipinnatus

Cadmium (Cd) is a biologically non-essential heavy metal, a major soil pollutant, and extremely harmful to plants. The phytohormone methyl jasmonate (MeJA) plays an important role in plant heavy-metal resistance. However, the understanding of the effects of MeJA supply level on alleviating Cd toxicity in plants is limited. Here, we investigated how MeJA regulated the development of physiological processes and cell wall modification in Cosmos bipinnatus. We found that low concentrations of MeJA increased the dry weight of seedlings under 120 µM Cd stress by reducing the transport of Cd from roots to shoots. Moreover, a threshold concentration of exogenous MeJA increased the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in plant roots, the concentration of Cd in the root cell wall, and the contents of pectin and hemicellulose 1 polysaccharides, through converting Cd into pectin-bound forms. These results suggested that MeJA mitigated Cd toxicity by modulating root cell wall polysaccharide and functional group composition, especially through pectin polysaccharides binding to Cd, with effects on Cd transport capacity, specific chemical forms of Cd, and homeostatic antioxidant systems in C. bipinnatus.

Silicon nanoparticles vs trace elements toxicity: Modus operandi and its omics bases

Phytotoxicity of trace elements (commonly misunderstood as 'heavy metals') includes impairment of functional groups of enzymes, photo-assembly, redox homeostasis, and nutrient status in higher plants. Silicon nanoparticles (SiNPs) can ameliorate trace element toxicity. We discuss SiNPs response against several essential (such as Cu, Ni, Mn, Mo, and Zn) and non-essential (including Cd, Pb, Hg, Al, Cr, Sb, Se, and As) trace elements. SiNPs hinder root uptake and transport of trace elements as the first line of defence. SiNPs charge plant antioxidant defence against trace elements-induced oxidative stress. The enrolment of SiNPs in gene expressions was also noticed on many occasions. These genes are associated with several anatomical and physiological phenomena, such as cell wall composition, photosynthesis, and metal uptake and transport. On this note, we dedicate the later sections of this review to support an enhanced understanding of SiNPs influence on the metabolomic, proteomic, and genomic profile of plants under trace elements toxicity.

Assessing lead and cadmium tolerance of Chenopodium ambrosioides during micropropagation: an in-depth qualitative and quantitative analysis

The tolerance of Chenopodium ambrosioides to some heavy metals under in vitro environment was thoroughly investigated. A micropropagation protocol was developed to facilitate the mass production of plants and to identify metals-tolerant species for potential use in the restoration of polluted areas. Nodal explants exhibited callus formation when treated with N6-benzyladenin (BA) (1.5 mg/l) and a combination of BA/α-naphthalene acetic acid (NAA) at concentrations of 1.5/1.0 mg/l on the Murashige and Skoog (MS) medium. The optimal shoot formation was achieved with the callus grown on a medium enriched with 1.5/1.0 mg/l BA/NAA, resulting in an impressive number (21.89) and length (11.79 cm) of shoots. The in vitro shoots were rooted using NAA (1.0 and 1.5 mg/l) and were acclimatized in pots with 71% survival rate. After standardizing micropropagation protocol, the in vitro shoots were subjected to various doses of lead nitrate (Pb(NO3)2 and cadmium chloride (CdCl2). Pb(NO3)2 and CdCl2 in the media let to a reduction in shoot multiplication, decreasing from 18.73 in the control group to 11.31 for Pb(NO3)2 and 13.89 for CdCl2 containing medium. However, Pb(NO3)2 and CdCl2 promoted shoot length from 5.61 in the control to 9.86 on Pb(NO3)2 and 12.51 on CdCl2 containing medium. In the case of Pb(NO3)2 treated shoots, the growth tolerance index (GTI) ranged from117.64% to 194.11%, whereas for CdCl2 treated shoots, the GTI ranged from 188.23% to 264.70%. Shoots treated with high level of Pb(NO3)2induced reddish-purple shoots, while a low level of Pb(NO3)2 induced shoots displayed both green and reddish-purple colors in the same explants. In CdCl2 treated culture, the toxic effects were narrow leaf lamina, elongated petiole and a dark reddish purple coloration. These findings highlight the remarkable potential of C. ambrosioides to maintain growth and organogenesis even in the presence Pb(NO3)2 and CdCl2 on the MS medium, indicating a high degree of metal tolerance.

In vitro and in vivo copper-treated Myrtus communis L.: terpene profiles and evidence for potential cultivation on metal-contaminated soils

The intensive application of agro-chemicals, and in particular of copper (Cu)-based compounds, causes increasing pollution of agricultural land, with serious risks for human health. Aromatic plants used for purposes other than food, can be considered for the exploitation and/or remediation of metal-polluted substrates, since contamination of the essential oils by these metals was not significant. Myrtle (Myrtus communis L.) is a Mediterranean evergreen shrub whose essential oil has many commercial applications. In this work, the effect of an excess of Cu in respect to control conditions was assessed on M. communis growth and foliar terpene composition. Metal accumulation in roots and shoots was also evaluated for the possible use of this species in phytoremediation. The amount of Cu applied in our experiments minimally affected the terpene profiles of in vitro grown plants, whereas no variations were detected in in vivo plants. The presence of the metal in the soil did not significantly impair plant growth, thus allowing its cultivation on polluted substrates. On the other hand, the amount of Cu in the plant was not enough to result in a significant reduction of Cu levels in the soil. Therefore, myrtle plants proved to be good candidates for the re-vegetation of Cu-contaminated lands.

Pea root responses under naproxen stress: changes in the formation of structural barriers in the primary root in context with changes of auxin and abscisic acid levels

Pharmaceuticals belong to pseudo-persistent pollutants because of constant entry into the environment and hazardous potential for non-target organisms, including plants, in which they can influence biochemical and physiological processes. Detailed analysis of results obtained by microscopic observations using fluorescent dyes (berberine hemisulphate, Fluorol Yellow 088), detection of phytohormone levels (radioimmunoassay, enzyme-linked immune sorbent assay) and thermogravimetric analysis of lignin content proved that the drug naproxen (NPX) can stimulate the formation of root structural barriers. In the primary root of plants treated with 0.5, 1, and 10 mg/L NPX, earlier Casparian strip formation and development of the whole endodermis circle closer to its apex were found after five days of cultivation (by 9-20% as compared to control) and after ten days from 0.1 mg/L NPX (by 8-63%). Suberin lamellae (SL) were deposited in endodermal cells significantly closer to the apex under 10 mg/L NPX by up to 75%. Structural barrier formation under NPX treatment can be influenced indirectly by auxin-supported cell division and differentiation caused by its eight-times higher level under 10 mg/L NPX and directly by stimulated SL deposition induced by abscisic acid (higher from 0.5 mg/L NPX), as proved by the higher proportion of cells with SL in the primary root base (by 8-44%). The earlier modification of endodermis in plant roots can help to limit the drug transfer and maintain the homeostasis of the plant.

Root anatomy, growth, and development of Typha domingensis Pers. (Typhaceae) and their relationship with cadmium absorption, accumulation, and tolerance

Typha domingensis Pers. is a plant that grows in marshy environments, where cadmium (Cd) accumulates. The root is the first organ that comes into contact with the metal. The aim of this study was to evaluate the effect of Cd on the roots of T. domingensis. The experiment was conducted in a greenhouse using different Cd concentrations: (1) 0 µM (control), (2) 10 µM, and (3) 50 µM, with 10 replicates for 90 days. The plants were placed in plastic containers containing 5 L of nutrient solution modified with the different Cd concentrations. At the end of the experiment, the roots were measured, sampled, fixed, and subjected to usual plant microtechniques. The slides were observed and photographed under light microscopy and analyzed in ImageJ software. To measure Cd absorption, atomic-absorption spectrometry was used. The data were subjected to analysis of variance and comparison of means by the Scott-Knott test at P < 0.05. When exposed to 50 µM of Cd, the roots accumulated 99.35% of the Cd. At this concentration, there was a reduction in the exodermis but there was an increase in the diameter of the cortical cells and in the proportion of aerenchyma in the cortex. There was an increase in the root cap, which guaranteed the protection of the primary meristems. Therefore, T. domingensis adjusts its root anatomy improving the Cd tolerance and shows potential for phytoremediation purposes.

Effect of Marginal-Quality Irrigation on Accumulation of some Heavy Metals (Mn, Pb, and Zn) in TypicTorripsamment Soils and Food Crops

Lack of active sorption sites in sandy soils renders metals added by irrigation water more labile and increases their soil-to-plant transfer. Thus, this study investigated the long-term impacts of irrigation using sewage effluents and contaminated groundwater on metal accumulations in TypicTorripsamment soils, and edible parts of food crops. Nine sites in El-Gabal El-Asfar farm, south-eastern to the Nile Delta of Egypt, were selected. At each site, irrigation water, soil (0–30 cm), and the crop’s edible part were sampled in triplicates and analyzed for Mn, Pb, and Zn. Results revealed significant (p < 0.05) differences in metal concentrations among water sources. Thus, constant irrigation caused significant spatial variations in total and available metal contents in soils. Total contents of Pb (in four sites) and Zn (in all sites) exceed the lithosphere range, while the available contents of the three metals exceeded the safe limits in all soils. The index of geo-accumulation indicated no Mn pollution but showed elevated pollution risks for Pb and Zn. The three metals showed high availability ratios, proving the effect of light soil texture. The multivariate statistical analysis indicated that Mn and Zn had similar geochemical behaviors in soils. Metal contents in all crop’s edible parts surpassed the safe limits. The bioaccumulation factor (BAF) was less than 1.0 for Mn and Zn but higher than 1.0 for Pb. The highest BAFs occurred in cabbage leaves, indicating the phytoextraction potential of this species. Sufficient water treatment and proper remediation techniques are recommended to alleviate metal accumulation in food crops and their transfer via the food chain.

Integrated analysis of miRNA-mRNA regulatory networks of potato (Solanum tuberosum L.) in response to cadmium stress

Cadmium (Cd) stress is a ubiquitous abiotic stress affecting plant growth worldwide and negatively impacting crop yield and food safety. Potato is the most important non-grain crop globally, but there is limited research available on the response of this crop to Cd stress. This study explored the coping mechanism for Cd stress in potato through analyses of miRNA and mRNA. Tissue culture seedlings (20-day-old) of potato variety 'Atlantic' were cultured for up to 48 h in liquid medium containing 5 mmol/L CdCl2, and phenotypic, physiological, and transcriptomic changes were observed at specific times. With the extension of Cd stress time, the potato leaves gradually wilted and curled, and root salicylic acid (SA), glutathione (GSH), and lignin contents and peroxidase (POD) activity increased, while indole-3-acetic acid (IAA) and zeatin (ZT) contents decreased. Using miRNA-seq, 161 existing miRNAs, 383 known miRNAs, and 7361 novel miRNAs were identified, and, 18 miRNAs were differentially expressed in response to Cd stress. Based on mRNA-seq, 7340 differentially expressed mRNAs (DEGs) were found. Through mRNA-miRNA integrated analysis, miRNA-target gene pairs consisting of 23 DEGs and 33 miRNAs were identified. Furthermore, "glutathione metabolism" "plant hormone signal transduction" and "phenylpropanoid biosynthesis" were established as crucial pathways in the Cd stress response of potato. Novel miRNAs novel-m3483-5p and novel-m2893-5p participate in these pathways through targeted regulation of cinnamic alcohol dehydrogenase (CAD; PG0005359) and alanine aminotransferase (POP; PG0024281), respectively. This study provides information that will help elucidate the complex mechanism of the Cd stress response in potato. Moreover, candidate miRNAs and mRNAs could yield new strategies for the development of Cd-tolerant potato breeding.

Non-glandular trichomes of sunflower are important in the absorption and translocation of foliar-applied Zn

Trichomes are potentially important for absorption of foliar fertilizers. A study has shown that the non-glandular trichromes (NGTs) of sunflower (Helianthus annuus) accumulated high concentrations of foliar-applied zinc (Zn); however, the mechanisms of Zn accumulation in the NGTs and the fate of this Zn are unclear. Here we investigated how foliar-applied Zn accumulates in the NGTs and the subsequent translocation of this Zn. Time-resolved synchrotron-based X-ray fluorescence microscopy and transcriptional analyses were used to probe the movement of Zn in the NGTs, with the cuticle composition of the NGTs examined using confocal Raman microscopy. The accumulation of Zn in the NGTs is both an initial preferential absorption process and a subsequent translocation process. This preferred absorption is likely because the NGT base has a higher hydrophilicity, whilst the subsequent translocation is due to the presence of plasmodesmata, Zn-chelating ligands, and Zn transporters in the NGTs. Furthermore, the Zn sequestered in the NGTs was eventually translocated out of the trichome once the leaf Zn concentration had decreased, suggesting that the NGTs are also important in maintaining leaf Zn homeostasis. This study demonstrates for the first time that trichomes have a key structural and functional role in the absorption and translocation of foliar-applied Zn.

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.

Structural modifications of plant organs and tissues by metals and metalloids in the environment: A review

At the dawn of the industrial revolution, the exorbitant use of heavy metals and toxic elements by mankind unfurls a powerful and complex web of hazard all around the world that significantly contributed to unprecedented trends in environmental degradation. Plants as sessile organisms, that cannot escape from the stress directly, have adapted to this environment via concurrent configurations of several traits. Among them the anatomy has been identified as much more advanced field of research that brought the explosion of interest among the expertise and its prodigious importance in stress physiology is unavoidable. In conjunction with various other disciplines, like physiology, biochemistry, genomics and metabolomics, the plant anatomy provides a large data sets that are paving the way towards a comprehensive and holistic understanding of plant growth, development, defense and productivity under heavy metal and toxic element stress. Present paper advances our recent knowledge about structural alterations of plant tissues induced by metals and metalloids, like antimony (Sb), arsenic (As), aluminium (Al), copper (Cu), cadmium (Cd), chromium (Cr), lead (Pb), manganese (Mn), mercury (Hg), nickel (Ni) and zinc (Zn) and points on essential role of plant anatomy and its understanding for plant growth and development in changing environment. Understanding of anatomical adaptations of various plant organs and tissues to heavy metals and metalloids could greatly contribute to integral and modern approach for investigation of plants in changing environmental conditions. These findings are necessary for understanding of the whole spectra of physiological and biochemical reactions in plants and to maintain the crop productivity worldwide. Moreover, our holistic perception regarding the processes underlying the plant responses to metal(loids) at anatomical level are needed for improving crop management and breeding techniques.

Alleviation mechanisms of metal(loid) stress in plants by silicon: a review

Silicon (Si), although not considered as an essential element for plants in general, can ameliorate the phytotoxicity induced by excess metal(loid)s whether non-essential (e.g. Cd, Pb, Cr, Al, As, and Sb) or essential (e.g. Cu, Ni, and Zn). The Si-enhanced resistance allowing plants to cope with this type of abiotic stress has been developed at multiple levels in plants. Restriction of root uptake and immobilization of metal(loid)s in the rhizosphere by Si is probably one of the first defence mechanism. Further, retention of elements in the root apoplasm might enhance the resistance and vigour of plants. At the cellular level, the formation of insoluble complexes between Si and metal(loid)s and their storage within cell walls help plants to decrease available element concentration and restrict symplasmic uptake. Moreover, Si influences the oxidative status of plants by modifying the activity of various antioxidants, improves membrane stability, and acts on gene expression, although its exact role in these processes is still not well understood. This review focuses on all currently known plant-based mechanisms related to Si supply and involved in amelioration of stress caused by excess metal(loid)s.

Enrichment of cadmium in rice (Oryza sativa L.) grown under different exogenous pollution sources

In order to unravel the cadmium (Cd) enrichment patterns in rice (Oryza sativa L.) grown under different exogenous exposure pathways, the pot experiment was conducted in a greenhouse. Cd was added to the soil-rice system via mixing soil with Cd-containing solution, irrigating the pots with Cd-containing water and leaf-spraying with Cd solution to simulate soil pollution (SPS), irrigation water pollution (IPS), and atmospheric deposit pollution sources (APS), respectively. No significant (p > 0.05) differences in plant height and rice grain yield were observed among all treatments including three different Cd pollution sources and control. The contents of Cd in rice plants significantly (p < 0.05) increased with increase in Cd concentrations in three pollution sources. The distribution pattern of Cd in the rice plant organs treated with SPS and IPS followed the order: roots > stems > leaves > husk > brown rice, while it was leaves > roots > stems > husk > brown rice treated with APS. At the same level of treatment, the highest concentration of Cd was observed in rice organs (except for middle and high concentrations treatment roots) grown under APS, followed by IPS and SPS, suggesting that the Cd bioavailability from different pollution sources followed the order of APS > IPS > SPS. It is concluded that the atmospheric pollution contributed more enrichment of rice with Cd. Therefore, in field environment, air deposits should also be analyzed for toxic metals during assessment of food chain contamination and health risk.

Comparative ecotoxicity of single and binary mixtures exposures of cadmium and zinc on growth and biomarkers of Lemna gibba

In the present study, single and mixture effects of cadmium (Cd) and zinc (Zn) on Lemna gibba were analyzed and compared using growth parameters, based on frond number and fresh weight, and biochemical parameters, such as pigment, protein content and activity of antioxidant enzymes. Plants were exposed for 7 days to these metals in nutrient solution. Single and mixture exposures affected plant growth and the biomarkers of the antioxidant response. Considering the growth parameters, Cd was found to be much more toxic than Zn. IC50-7d, based on growth rate calculated on frond number, were 17.8 and 76.73 mg/L, and on fresh weight were 1.08 and 76.93 mg/L, for Cd and Zn respectively. For Cd, LOEC values were obtained at 2.06 and 1.03 mg/L, for frond number and fresh weight respectively; while for Zn, at 20.1 and 74.6 mg/L. A high toxicity effect, considering the same response variables, was observed in plants exposed to the mixtures. Three fixed ratios, based on toxic units (TU) were assayed, ratio 1: 2/3 Cd-1/3 Zn, ratio 2: 1/2 Cd-1/2 Zn and ratio 3: 1/3 Cd-2/3 Zn. Ratio 3 (where Zn was added in higher proportion) was the less toxic. All concentrations of Ratio 1 and 2 significantly inhibited plant growth, showing a 100% inhibition of growth rate at the highest concentrations when based on frond number. Catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APOX; EC 1.11.1.11) and guaiacol peroxidase (GPOX; EC 1.11.1.7) activities in single metals assays were higher than controls. In mixture tests, the activity of APOX and GPOX was significantly stimulated in plants exposed to all evaluated combinations, while CAT was mainly stimulated in Ratio 3. It was observed that the activity of the enzymes was increased in the mixtures compared with similar concentrations evaluated individually. APOX activity was observed to fit the CA model and following a concentration-response pattern. The response of this antioxidant enzyme could serve as a sensitive stressor biomarker for Cd-Zn interactions. Frond number in Cd-Zn mixtures was not well predicted from dissolved metal concentration in solution using concentration addition (CA) as reference model, as results showed that toxicity was more than additive, with an average of ΣTU = 0.75. This synergistic effect was observed up to 50 mg Zn/L in the mixture, but when it was present in higher concentrations a less than additive effect was observed, indicating a protective effect of Zn. A synergistic and dose-ratio deviations from CA model were also observed.

Efficient antioxidant defence systems of spring barley in response to stress induced jointly by the cyst nematode parasitism and cadmium exposure

AIMS

This research aimed to establish how Hordeum vulgare responds to abiotic and biotic stress affecting in tandem.

METHODS

Plants were inoculated with Heterodera filipjevi and treated with cadmium (Cd) concentration (5 μM) that can occur in the cultivated soil. To verify the hypothesis about participation of increased antioxidative defence in H. vulgare under stress, biochemical and microscopic methods were implemented.

RESULTS

The amount of superoxide anions and hydrogen peroxide was diminished in plants that were both nematode-inoculated and cadmium-treated. Superoxide anions were rendered harmless by increased activity of superoxide dismutase, and H₂O₂ was scavenged via Foyer-Halliwell-Asada pathway. The unique enhanced antioxidant capacity of double stressed plants was also linked with the accumulation of S-nitrosoglutathione as nitrosoglutathione reductase activity was inhibited. Furthermore, stimulated activity of arginase in these plants could promote polyamine synthesis and indirectly enhance non-enzymatic antioxidant mechanism. Results indicate that different antioxidants operating together significantly restricted oxidation of lipids and proteins, thus the integrity of cell membranes and protein functions were maintained.

CONCLUSIONS

The ROS deactivation machinery in barley leaves showed an unusual response during stress induced by H. filipjevi infection and cadmium treatment. Plants could induce a multi-component model of stress response, to detoxify Cd ions and efficiently repair stress damage.

Root hair abundance impacts cadmium accumulation in Arabidopsis thaliana shoots

BACKGROUND AND AIMS

Root hairs increase the contact area of roots with soil and thereby enhance the capacity for solute uptake. The strict hair/non-hair pattern of Arabidopsis thaliana can change with nutrient deficiency or exposure to toxic elements, which modify root hair density. The effects of root hair density on cadmium (Cd) accumulation in shoots of arabidopsis genotypes with altered root hair development and patterning were studied.

METHODS

Arabidopsis mutants that are unable to develop root hairs (rhd6-1 and cpc/try) or produce hairy roots (wer/myb23) were compared with the ecotype Columbia (Col-0). Plants were cultivated on nutrient agar for 2 weeks with or without Cd. Cadmium was applied as Cd(NO3)2 at two concentrations, 10 and 100 µm. Shoot biomass, root characteristics (primary root length, lateral root number, lateral root length and root hair density) and Cd concentrations in shoots were assessed. Anatomical features (suberization of the endodermis and development of the xylem) that might influence Cd uptake and translocation were also examined.

KEY RESULTS

Cadmium inhibited plant growth and reduced root length and the number of lateral roots and root hairs per plant. Suberin lamellae in the root endodermis and xylem differentiation developed closer to the root apex in plants exposed to 100 µm Cd. The latter effect was genotype dependent. Shoot Cd accumulation was correlated with root hair abundance when plants were grown in the presence of 10 µm Cd, but not when grown in the presence of 100 µm Cd, in which treatment the development of suberin lamellae closer to the root tip appeared to restrict Cd accumulation in shoots.

CONCLUSIONS

Root hair density can have a large effect on Cd accumulation in shoots, suggesting that the symplasmic pathway might play a significant role in the uptake and accumulation of this toxic element.

Smelling the metal: Volatile organic compound emission under Zn excess in the mint Tetradenia riparia

This work investigated the effect of Zn excess on growth, metal accumulation and photosynthetic changes in Tetradenia riparia, in relation to possible variations in the composition of the plant volatilome. Experiments were carried out in hydroponics exposing plants to a range of Zn concentrations. Zinc excess negatively affected plant growth in a dose-dependent manner. The metal was accumulated proportionally to its concentration in the medium and preferentially allocated to roots. All the photosynthetic parameters and the concentration of some photosynthetic pigments were negatively affected by Zn, whereas the level of leaf total soluble sugars remained unchanged. Twenty-three different VOCs were identified in the plant volatilome. Each compound was emitted at a different level and intensity of emission was manifold increased by the presence of Zn in the growth medium. The Zn-induced compounds could represent both an adaptive response (f.i. methanol, acetylene, C₆-aldehydes, isoprene, terpenes) and a damage by-product (f.i. propanal, acetaldehyde, alkyl fragments) of the metal presence in the culture medium. Given that the Zn-mediated induction of those VOCs, considered protective, occurred even under a Zn-limited photosynthetic capacity, our work supports the hypothesis of an active role of such molecules in an adaptive plant response to trace metal stress.

Genome-Wide Identification of MicroRNAs in Response to Cadmium Stress in Oilseed Rape (Brassica napus L.) Using High-Throughput Sequencing

MicroRNAs (miRNAs) have important roles in regulating stress-response genes in plants. However, identification of miRNAs and the corresponding target genes that are induced in response to cadmium (Cd) stress in Brassica napus remains limited. In the current study, we sequenced three small-RNA libraries from B. napus after 0 days, 1 days, and 3 days of Cd treatment. In total, 44 known miRNAs (belonging to 27 families) and 103 novel miRNAs were identified. A comprehensive analysis of miRNA expression profiles found 39 differentially expressed miRNAs between control and Cd-treated plants; 13 differentially expressed miRNAs were confirmed by qRT-PCR. Characterization of the corresponding target genes indicated functions in processes including transcription factor regulation, biotic stress response, ion homeostasis, and secondary metabolism. Furthermore, we propose a hypothetical model of the Cd-response mechanism in B. napus. Combined with qRT-PCR confirmation, our data suggested that miRNAs were involved in the regulations of TFs, biotic stress defense, ion homeostasis and secondary metabolism synthesis to respond Cd stress in B. napus.

The composite water and solute transport of barley (Hordeum vulgare) roots: effect of suberized barriers

BACKGROUND AND AIMS

Roots have complex anatomical structures, and certain localized cell layers develop suberized apoplastic barriers. The size and tightness of these barriers depend on the growth conditions and on the age of the root. Such complex anatomical structures result in a composite water and solute transport in roots.

METHODS

Development of apoplastic barriers along barley seminal roots was detected using various staining methods, and the suberin amounts in the apical and basal zones were analysed using gas chromatography-mass spectometry (GC-MS). The hydraulic conductivity of roots ( Lp r ) and of cortical cells ( Lp c ) was measured using root and cell pressure probes.

KEY RESULTS

When grown in hydroponics, barley roots did not form an exodermis, even at their basal zones. However, they developed an endodermis. Endodermal Casparian bands first appeared as 'dots' as early as at 20 mm from the apex, whereas a patchy suberin lamellae appeared at 60 mm. The endodermal suberin accounted for the total suberin of the roots. The absolute amount in the basal zone was significantly higher than in the apical zone, which was inversely proportional to the Lp r . Comparison of Lp r and Lp c suggested that cell to cell pathways dominate for water transport in roots. However, the calculation of Lp r from Lp c showed that at least 26 % of water transport occurs through the apoplast. Roots had different solute permeabilities ( P sr ) and reflection coefficients ( σ sr ) for the solutes used. The σ sr was below unity for the solutes, which have virtually zero permeability for semi-permeable membranes.

CONCLUSIONS

Suberized endodermis significantly reduces Lp r of seminal roots. The water and solute transport across barley roots is composite in nature and they do not behave like ideal osmometers. The composite transport model should be extended by adding components arranged in series (cortex, endodermis) in addition to the currently included components arranged in parallel (apoplastic, cell to cell pathways).

Root morphology and seed and leaf ionomic traits in a Brassica napus L. diversity panel show wide phenotypic variation and are characteristic of crop habit

BACKGROUND

Mineral nutrient uptake and utilisation by plants are controlled by many traits relating to root morphology, ion transport, sequestration and translocation. The aims of this study were to determine the phenotypic diversity in root morphology and leaf and seed mineral composition of a polyploid crop species, Brassica napus L., and how these traits relate to crop habit. Traits were quantified in a diversity panel of up to 387 genotypes: 163 winter, 127 spring, and seven semiwinter oilseed rape (OSR) habits, 35 swede, 15 winter fodder, and 40 exotic/unspecified habits. Root traits of 14 d old seedlings were measured in a 'pouch and wick' system (n = ~24 replicates per genotype). The mineral composition of 3-6 rosette-stage leaves, and mature seeds, was determined on compost-grown plants from a designed experiment (n = 5) by inductively coupled plasma-mass spectrometry (ICP-MS).

RESULTS

Seed size explained a large proportion of the variation in root length. Winter OSR and fodder habits had longer primary and lateral roots than spring OSR habits, with generally lower mineral concentrations. A comparison of the ratios of elements in leaf and seed parts revealed differences in translocation processes between crop habits, including those likely to be associated with crop-selection for OSR seeds with lower sulphur-containing glucosinolates. Combining root, leaf and seed traits in a discriminant analysis provided the most accurate characterisation of crop habit, illustrating the interdependence of plant tissues.

CONCLUSIONS

High-throughput morphological and composition phenotyping reveals complex interrelationships between mineral acquisition and accumulation linked to genetic control within and between crop types (habits) in B. napus. Despite its recent genetic ancestry (<10 ky), root morphology, and leaf and seed composition traits could potentially be used in crop improvement, if suitable markers can be identified and if these correspond with suitable agronomy and quality traits.

Asymmetrical development of root endodermis and exodermis in reaction to abiotic stresses

Background and Aims In the present study, we show that development of endodermis and exodermis is sensitively regulated by water accessibility. As cadmium (Cd) is known to induce xeromorphic effects in plants, maize roots were exposed also to Cd to understand the developmental process of suberin lamella deposition in response to a local Cd source. Methods In a first experiment, maize roots were cultivated in vitro and unilaterally exposed to water-containing medium from one side and to air from the other. In a second experiment, the roots were placed between two agar medium layers with a strip of Cd-containing medium attached locally and unilaterally to the root surface. Key Results The development of suberin lamella (the second stage of exodermal and endodermal development) started asymmetrically, preferentially closer to the root tip on the side exposed to the air. In the root contact with Cd in a spatially limited area exposed to one side of the root, suberin lamella was preferentially developed in the contact region and additionally along the whole length of the root basipetally from the contact area. However, the development was unilateral and asymmetrical, facing the treated side. The same pattern occurred irrespective of the distance of Cd application from the root apex. Conclusions These developmental characteristics indicate a sensitive response of root endodermis and exodermis in the protection of vascular tissues against abiotic stresses.

Mixture toxicity and interactions of copper, nickel, cadmium, and zinc to barley at low effect levels: Something from nothing?

Metal contamination is mostly a mixture of different metals, and these multicomponent mixtures can produce significant mixture effects. The present study was set up to investigate the toxicity of multiple metal mixtures of Cu, Ni, Cd, and Zn to plants at metal doses individually causing low-level phytotoxic effects. Barley (Hordeum vulgare L.) root elongation toxicity tests were performed in resin-buffered nutrient solutions to control metal speciation. Treatments included single-metal concentrations and binary, ternary, and quaternary mixtures. Mixtures of different metals at free ion concentrations, each causing <10% inhibition of root elongation, yielded significant mixture effects, with inhibition reaching up to 50%. The independent action (IA) model predicted mixture toxicity statistically better than the concentration addition (CA) model, but some synergisms relative to the IA model were observed. These synergisms relative to IA were most pronounced in quaternary mixtures and when the dose-response curves had steep slopes. Generally, antagonistic interactions relative to the CA model were observed. Increasing solution Zn concentrations shifted metal interactions (CA based) from additive or slightly synergistic at background Zn concentrations to antagonistic at higher Zn concentrations, suggesting a protective effect of Zn. Overall, the present study shows that the CA model can be used as a conservative model to predict metal mixture toxicity to barley. Environ Toxicol Chem 2016;35:2483-2492. © 2016 SETAC.

Silicon alleviates cadmium toxicity by enhanced photosynthetic rate and modified bundle sheath's cell chloroplasts ultrastructure in maize

Silicon was shown to alleviate the negative effects of various biotic and abiotic stresses on plant growth. Although the positive role of Si on toxic and heavy metal Cd has been already described, the mechanisms have been explained only partially and still remain unclear. In the present study we investigated the effect of Si on photosynthetic-related processes in maize exposed to two different levels of Cd via measurements of net photosynthetic rate (AN), chlorophyll a fluorescence and pigment analysis, as well as studies of leaf tissue anatomy and cell ultrastructure using bright-field and transmission electron microscopy. We found that Si actively alleviated the toxic syndromes of Cd by increasing AN, effective photochemical quantum yield of photosystem II (ϕPSII) and content of assimilation pigments, although did not decrease the concentration of Cd in leaf tissues. Cadmium did not affect the leaf anatomy and ultrastructure of leaf mesophyll's cell chloroplasts; however, Cd negatively affected thylakoid formation in chloroplasts of bundle sheath cells, and this was alleviated by Si. Improved thylakoid formation in bundle sheath's cell chloroplasts may contribute to Si-induced enhancement of photosynthesis and related increase in biomass production in C4 plant maize.

Cadmium translocation by contractile roots differs from that in regular, non-contractile roots

BACKGROUND AND AIMS

Contractile roots are known and studied mainly in connection with the process of shrinkage of their basal parts, which acts to pull the shoot of the plant deeper into the ground. Previous studies have shown that the specific structure of these roots results in more intensive water uptake at the base, which is in contrast to regular root types. The purpose of this study was to find out whether the basal parts of contractile roots are also more active in translocation of cadmium to the shoot.

METHODS

Plants of the South African ornamental species Tritonia gladiolaris were cultivated in vitro for 2 months, at which point they possessed well-developed contractile roots. They were then transferred to Petri dishes with horizontally separated compartments of agar containing 50 µmol Cd(NO3)2 in the region of the root base or the root apex. Seedlings of 4-d-old maize (Zea mays) plants, which do not possess contractile roots, were also transferred to similar Petri dishes. The concentrations of Cd in the leaves of the plants were compared after 10 d of cultivation. Anatomical analyses of Tritonia roots were performed using appropriately stained freehand cross-sections.

KEY RESULTS

The process of contraction required specific anatomical adaptation of the root base in Tritonia, with less lignified and less suberized tissues in comparison with the subapical part of the root. These unusual developmental characteristics were accompanied by more intensive translocation of Cd ions from the basal part of contractile roots to the leaves than from the apical-subapical root parts. The opposite effects were seen in the non-contractile roots of maize, with higher uptake and transport by the apical parts of the root and lower uptake and transport by the basal part.

CONCLUSIONS

The specific characteristics of contractile roots may have a significant impact on the uptake of ions, including toxic metals from the soil surface layers. This may be important for plant nutrition, for example in the uptake of nutrients from upper soil layers, which are richer in humus in otherwise nutrient-poor soils, and also has implications for the uptake of surface-soil pollutants.

Ionome and expression level of Si transporter genes (Lsi1, Lsi2, and Lsi6) affected by Zn and Si interaction in maize

Zinc (Zn) is an essential microelement involved in various plant physiological processes. However, in excess, Zn becomes toxic and represents serious problem for plants resulting in Zn toxicity symptoms and decreasing biomass production. The effect of high Zn and its combination with silicon (Si) on ionome and expression level of ZmLsi genes was investigated in maize (Zea mays, L; hybrid Novania). Plants were cultivated hydroponically in different treatments: control (C), Zn (800 μM ZnSO4 · 7H2O), Si5 (5 mM of sodium silicate solution), and Si5 + Zn (combination of Zn and Si treatments). Growth of plants cultivated for 10 days was significantly inhibited in the presence of high Zn concentration and also by Zn and Si interaction in plants. Based on principal component analysis (PCA) and mineral element concentration in tissues, root ionome was significantly altered in both Zn and Si5 + Zn treatments in comparison to control. Mineral elements Mn, Fe, Ca, P, Mg, Ni, Co, and K significantly decreased, and Se increased in Zn and Si5 + Zn treatments. Shoot ionome was less affected than root ionome. Concentration of shoot Cu, Mn, and P decreased, and Mo increased in Zn and Si5 + Zn treatments. The PCA also revealed that the responsibility for ionome changes is mainly due to Zn exposure and also, but less, by Si application to Zn stressed plants. Expression level of Lsi1 and Lsi2 genes for the Si influx and efflux transporters was downregulated in roots after Si supply and even more downregulated by Zinc alone and also by Zn and Si interaction. Expression level of shoot Lsi6 gene was differently regulated in the first and second leaf. These results indicate negative effect of high Zn alone and also in interaction with Si on Lsi gene expression level and together with ionomic data, it was shown that homeostatic network of mineral elements was disrupted and caused negative alterations in mineral nutrition of young maize plants.

Transcriptome-Wide Identification of miRNAs and Their Targets from Typha angustifolia by RNA-Seq and Their Response to Cadmium Stress

MicroRNAs (miRNAs) play important roles in plant responses to environmental stress. In this work, we used high-throughput sequencing to analyze transcriptome and small RNAs (sRNAs) in Typha angustifolia under cadmium (Cd) stress. 57,608,230 raw reads were obtained from deep sequencing of a pooled cDNA library. Sequence assembly and analysis yielded 102,473 unigenes. We subsequently sequenced two sRNA libraries from T. angustifolia with or without Cd exposure respectively. Based on transcriptome data of T. angustifolia, we catalogued and analyzed the sRNAs, resulting in the identification of 114 conserved miRNAs and 41 novel candidate miRNAs in both small RNA libraries. In silico analysis revealed 764 targets for 89 conserved miRNAs and 21 novel miRNAs. Statistical analysis on sequencing reads abundance and experimental validation revealed that 4 conserved and 6 novel miRNAs showed specific expression. Combined with function of target genes, these results suggested that miRNAs might play a role in plant Cd stress response. This study provided the first transcriptome-based analysis of miRNAs and their targets responsive to Cd stress in T. angustifolia, which provide a framework for further analysis of miRNAs and their role in regulating plant responses to Cd stress.

Minimising toxicity of cadmium in plants--role of plant growth regulators

A range of man-made activities promote the enrichment of world-wide agricultural soils with a myriad of chemical pollutants including cadmium (Cd). Owing to its significant toxic consequences in plants, Cd has been one of extensively studied metals. However, sustainable strategies for minimising Cd impacts in plants have been little explored. Plant growth regulators (PGRs) are known for their role in the regulation of numerous developmental processes. Among major PGRs, plant hormones (such as auxins, gibberellins, cytokinins, abscisic acid, jasmonic acid, ethylene and salicylic acid), nitric oxide (a gaseous signalling molecule), brassinosteroids (steroidal phytohormones) and polyamines (group of phytohormone-like aliphatic amine natural compounds with aliphatic nitrogen structure) have gained attention by agronomist and physiologist as a sustainable media to induce tolerance in abiotic-stressed plants. Considering recent literature, this paper: (a) overviews Cd status in soil and its toxicity in plants, (b) introduces major PGRs and overviews their signalling in Cd-exposed plants, (c) appraises mechanisms potentially involved in PGR-mediated enhanced plant tolerance to Cd and (d) highlights key aspects so far unexplored in the subject area.

Selenium inhibits root elongation by repressing the generation of endogenous hydrogen sulfide in Brassica rapa

Selenium (Se) has been becoming an emerging pollutant causing severe phytotoxicity, which the biochemical mechanism is rarely known. Although hydrogen sulfide (H2S) has been suggested as an important exogenous regulator modulating plant physiological adaptions in response to heavy metal stress, whether and how the endogenous H2S regulates Se-induce phytotoxicity remains unclear. In this work, a self-developed specific fluorescent probe (WSP-1) was applied to track endogenous H2S in situ in the roots of Brassica rapa under Se(IV) stress. Se(IV)-induced root growth stunt was closely correlated with the inhibition of endogenous H2S generation in root tips. Se(IV) stress dampened the expression of most LCD and DCD homologues in the roots of B. rapa. By using various specific fluorescent probes for bio-imaging root tips in situ, we found that the increase in endogenous H2S by the application of H2S donor NaHS could significantly alleviate Se(IV)-induced reactive oxygen species (ROS) over-accumulation, oxidative impairment, and cell death in root tips, which further resulted in the recovery of root growth under Se(IV) stress. However, dampening the endogenous H2S could block the alleviated effect of NaHS on Se(IV)-induced phytotoxicity. Finally, the increase in endogenous H2S resulted in the enhancement of glutathione (GSH) in Se(IV)-treated roots, which may share the similar molecular mechanism for the dominant role of H2S in removing ROS by activating GSH biosynthesis in mammals. Altogether, these data provide the first direct evidences confirming the pivotal role of endogenous H2S in modulating Se(IV)-induced phytotoxicity in roots.