PRE-ADAPTATION OF YORKSHIRE FOG, HOLCUS LANATUS L. (POACEAE) TO ARSENATE TOLERANCE

Crop suitability assessment in remediation of Zn contaminated soil

Zinc (Zn) is naturally present in soils and constitutes an essential micronutrient for plants. Mining, industrial, as well as various agricultural activities all contribute to increasing the Zn concentrations in soils to levels that are toxic for plants. The aim of this study was to evaluate the capacity of field crops to remove Zn from contaminated soils. The experimental design included 28 treatments, comprising seven field crops (Hordeum vulgare L., Ricinus communis L., Phaseolus vulgaris L., Brassica juncea Czem., Sorgum vulgare L., Spinacea oleracea L., Solanum lycopersicum L.) and four Zn levels (0, 500, 1000, 1500 mg kg^-1) applied to soils. The dry weight (DW) of the aboveground biomass of R. communis and S. lycopersicum increased significantly as the Zn concentration in the soil increased, whereas the DW significantly decreased in P. vulgaris, B. juncea and S. vulgare. Results indicated that S. oleracea was the most efficient in concentrating Zn in the aboveground tissues, followed in decreasing order by H. vulgare, S. lycopersicum, R. communis, S. vulgare, P. vulgaris, and B. juncea. H. vulgare resulted the most efficient in accumulating Zn both in fruit and in leaves and stems, whereas S. lycopersicum resulted the most efficient in accumulating Zn in roots. The BAF and TF values indicated that H. vulgare and S. oleracea resulted being suitable for Zn phytoextraction, whereas the remaining crops being suitable for Zn phytostabilization. These results highlight the phytoremediation potential of the seven analysed crops.

Trait-directed de novo population transcriptome dissects genetic regulation of a balanced polymorphism in phosphorus nutrition/arsenate tolerance in a wild grass, Holcus lanatus

The aim of this study was to characterize the transcriptome of a balanced polymorphism, under the regulation of a single gene, for phosphate fertilizer responsiveness/arsenate tolerance in wild grass Holcus lanatus genotypes screened from the same habitat. De novo transcriptome sequencing, RNAseq (RNA sequencing) and single nucleotide polymorphism (SNP) calling were conducted on RNA extracted from H. lanatus. Roche 454 sequencing data were assembled into c. 22,000 isotigs, and paired-end Illumina reads for phosphorus-starved (P-) and phosphorus-treated (P+) genovars of tolerant (T) and nontolerant (N) phenotypes were mapped to this reference transcriptome. Heatmaps of the gene expression data showed strong clustering of each P+/P- treated genovar, as well as clustering by N/T phenotype. Statistical analysis identified 87 isotigs to be significantly differentially expressed between N and T phenotypes and 258 between P+ and P- treated plants. SNPs and transcript expression that systematically differed between N and T phenotypes had regulatory function, namely proteases, kinases and ribonuclear RNA-binding protein and transposable elements. A single gene for arsenate tolerance led to distinct phenotype transcriptomes and SNP profiles, with large differences in upstream post-translational and post-transcriptional regulatory genes rather than in genes directly involved in P nutrition transport and metabolism per se.

Plants growing on contaminated and brownfield sites appropriate for use in Organisation for Economic Co-operation and Development terrestrial plant growth test

The Organisation for Economic Co-operation and Development (OECD) terrestrial plant test is often used for the ecological risk assessment of contaminated land. However, its origins in plant protection product testing mean that the species recommended in the OECD guidelines are unlikely to occur on contaminated land. Six alternative species were tested on contaminated soils from a former Zn smelter and a metal fragmentizer with elevated concentrations of Cd, Cu, Pb, and Zn. The response of the alternative species was compared with that of two species recommended by the OECD: Lolium perenne (perennial ryegrass) and Trifolium pratense (red clover). Urtica dioica (stinging nettle) and Poa annua (annual meadowgrass) had low emergence rates in the control soil and so may be considered unsuitable. Festuca rubra (Chewings fescue), Holcus lanatus (Yorkshire fog), Senecio vulgaris (common groundsel), and Verbascum thapsus (great mullein) offer good alternatives to the OECD species. In particular, H. lanatus and S. vulgaris were more sensitive to the soils with moderate concentrations of Cd, Cu, Pb, and Zn than the OECD species.

An arsenic-accumulating, hypertolerant brassica, Isatis capadocica

Isatis capadocica, a brassica collected from Iranian arsenic-contaminated mine spoils and control populations, was examined to determine arsenate tolerance, metabolism and accumulation. I. cappadocica exhibited arsenate hypertolerance in both mine and nonmine populations, actively growing at concentrations of > 1 mm arsenate in hydroponic solution. I. cappadocica had an ability to accumulate high concentrations of arsenic in its shoots, in excess of 100 mg kg(-1) DW, with a shoot : root transfer ratio of > 1. The ability to accumulate arsenic was exhibited in both hydroponics and contaminated soils. Tolerance in this species was not achieved through suppression of high-affinity phosphate/arsenate root transport, in contrast to other monocotyledons and dicotyledons. A high percentage (> 50%) of arsenic in the tissues was phytochelatin complexed; however, it is argued that this is a constitutive, rather than an adaptive, mechanism of tolerance.

Arsenite efflux is not enhanced in the arsenate-tolerant phenotype of Holcus lanatus

Arsenate tolerance in Holcus lanatus is achieved mainly through suppressed arsenate uptake. We recently showed that plant roots can rapidly efflux arsenite to the external medium. Here, we tested whether arsenite efflux is a component of the adaptive arsenate tolerance in H. lanatus. Tolerant and nontolerant phenotypes were exposed to different arsenate concentrations with or without phosphate for 24 h, and arsenic (As) speciation was determined in nutrient solutions, roots and xylem sap. At the same arsenate exposure concentration, the nontolerant phenotype took up more arsenate and effluxed more arsenite than the tolerant phenotype. However, arsenite efflux was proportional to arsenate uptake and was not enhanced in the tolerant phenotype. Within 2-24 h, most (80-100%) of the arsenate taken up was effluxed to the medium as arsenite. About 86-95% of the As in the roots and majority of the As in xylem sap (c. 66%) was present as arsenite, and there were no significant differences between phenotypes. Arsenite efflux is not adaptively enhanced in the tolerant phenotype H. lanatus, but it could be a basal tolerance mechanism to greatly decrease cellular As burden in both phenotypes. Tolerant and nontolerant phenotypes had a similar capacity to reduce arsenate in roots.

Role of mycorrhizal fungi and phosphorus in the arsenic tolerance of basin wildrye

Revegetation of arsenic (As)-rich mine spoils is often impeded by the lack of plant species tolerant of high As concentrations and low nutrient availability. Basin wildrye [Leymus cinereus (Scribner & Merr.) A. Löve] has been observed to establish naturally in soils with elevated As content and thus may be useful for the stabilization of As-contaminated soils. An experiment was conducted to evaluate how variable phosphorus (P) concentrations and inoculation with site-specific arbuscular mycorrhizal fungi influence As tolerance of basin wildrye. Basin wildrye was grown in sterile sand in the greenhouse for 16 weeks. Pots of sterile sand were amended to create one of four rates of As (0, 3, 15, or 50 mg As kg(-1)), two rates of P (3 or 15 mg P kg(-1)), and +/-mycorrhizal inoculation in a 2 x 4 x 2 factorial arrangement. After 16 weeks of growth, plants were harvested, shoots and roots thoroughly washed, and the tissue analyzed for total shoot biomass, total root and shoot As and P concentrations, and degree of mycorrhizal infection. Basin wildrye was found to be tolerant of high As concentrations allowing for vigorous plant growth at application levels of 3 or 15 mg As kg(-1). Arsenic was sequestered in the roots, with 30 to 50 times more As in the roots than shoots under low P conditions. Mycorrhizal infection did not confer As tolerance in basin wildrye nor did mycorrhizal fungi influence biomass production. Phosphorus concentrations of 15 mg kg(-1) effectively inhibited As accumulation in basin wildrye. Basin wildrye has the potential to be used for stabilization of As-rich soils while minimizing exposure to grazing animals following reclamation.

Variation in arsenic accumulation - hyperaccumulation in ferns and their allies: Rapid report

•  A range of fern species (45) and their allies, Equisetum (5) and Selaginella (2) species and Psilotum nudum were screened for their ability to hyperaccumulate arsenic, to develop a phylogenetic understanding of this phenomenon. A number of varieties (5) of a known arsenic hyperaccumulator Pteris cretica were additionally included in this study. •  This study is the first to report members of the Pteris genus that do not hyperaccumulate arsenic, Pteris straminea and tremula . •  A phylogenetic basis for arsenic accumulation in ferns was investigated. Some orders can accumulate more arsenic than others. Although members of the Equisetales and Blechnales did not hyperaccumulate arsenic, they still accumulated relatively high levels in their fronds, approaching 100 mg kg ^-1 when grown on a soil dosed with 100 mg kg ^-1 arsenic. •  Arsenic hyperaccumulation was identified as a phenomenon at the extreme range of fern arsenic accumulation. Ferns that exhibit arsenic hyperaccumulation arrived relatively late in terms of fern evolution, as this character is not exhibited by primitive ferns or their allies.

Mechanisms of arsenate tolerance in Cytisus striatus

•  Arsenate tolerance, uptake and arsenate-induced phytochelatin (PC) accumulation were compared at different phosphorus supply rates in two populations of the broom, Cytisus striatus , one from an arsenic-enriched gold mine and one from a nonmetalliferous site. •  After 7 d of exposure, arsenate tolerance was higher in the mine population. Arsenate uptake was phosphate-suppressible, and much lower in the mine plants. When compared at equal levels of stress, the mine plants and the nonmetallicolous plants exhibited similar arsenic accumulation, suggesting that reduced arsenate uptake is mainly responsible for superior tolerance. •  Arsenate-induced PC accumulation occurred in both plant types. The γ-glutamylcysteine synthetase inhibitor, L-buthioninesulfoximine, caused arsenate hypersensitivity in both plant types, suggesting that PC-based arsenic sequestration is essential for both normal and enhanced arsenate tolerance. Mine plants produced longer PCs than the nonmetallicolous plants, possibly due to a differential temporal pattern of arsenate accumulation. •  Our results are consistent with a similar mechanism underlying arsenate hypertolerance in C. striatus and grasses, that is reduced arsenate uptake through suppression of phosphate transporter activity.

Arbuscular mycorrhizal fungi confer enhanced arsenate resistance on Holcus lanatus

•  The role of arbuscular mycorrhizal fungi (AMF) in arsenate resistance in arbuscular mycorrhizal associations is investigated here for two Glomus spp. isolated from the arsenate-resistant grass Holcus lanatus. •  Glomus mosseae and Glomus caledonium were isolated from H. lanatus growing on an arsenic-contaminated mine-spoil soil. The arsenate resistance of spores was compared with nonmine isolates using a germination assay. Short-term arsenate influx into roots and long-term plant accumulation of arsenic by plants were also investigated in uninfected arsenate resistant and nonresistant plants and in plants infected with mine and nonmine AMF. •  Mine AMF isolates were arsenate resistant compared with nonmine isolates. Resistant and nonresistant G. mosseae both suppressed high-affinity arsenate/phosphate transport into the roots of both resistant and nonresistant H. lanatus. Resistant AMF colonization of resistant H. lanatus growing in contaminated mine spoil reduced arsenate uptake by the host. •  We conclude that AMF have evolved arsenate resistance, and conferred enhanced resistance on H. lanatus.

Arsenate resistance in the ericoid mycorrhizal fungus Hymenoscyphus ericae

•  Differential resistance to arsenate (AsO₄ ^3- ) is demonstrated here among populations of the ericoid mycorrhizal fungus Hymenoscyphus ericae isolated from Calluna vulgaris in natural heathland soils and soils contaminated with AsO₄ ^3- . •  Isolates (c. 25) of the fungus from each of two As and Cu mine sites, and a natural heathland site, were screened for AsO₄ ^3- and Cu²⁺ resistance by growing isolates in media containing a range of AsO₄ ^3- and Cu²⁺ concentrations. •  H. ericae populations from the mine sites demonstrated resistance to AsO₄ ^3- compared with the heathland population; the mine-site populations producing significant growth at the highest AsO₄ ^3- concentration (4.67 mol m^-3 ), whereas growth of the heathland population was almost completely inhibited. EC₅₀ values for mine-site isolates were estimated to be 5-41-times higher than the heathland population. All isolates produced identical responses to increasing Cu²⁺ concentrations, with no differences observed between mine-site and heathland isolates. •  Populations of H. ericae on the contaminated mine sites have developed adaptive resistance to AsO₄ ^3- . By contrast, Cu²⁺ resistance appears to be constitutive.

The genetics of metal tolerance in vascular plants

In many parts of the world soils are detrimental to plant growth owing to elevated levels of metal ions, caused either by natural processes or by the result of man's activities. Many plants have evolved ecotypes or varieties that are able to grow more-or-less normally on these soils. This paper reviews our knowledge of the genetics of this phenomenon. The nature of tolerance and the problems of its measurement are discussed. Tolerance is frequently measured by an index produced by comparing growth in a contaminated environment with growth in a control environment. It is argued that this measurement is inappropriate for many genetical studies, and that it is frequently more useful to use growth at a single critical level of metal as a measure of tolerance. Polygenic inheritance provides a null hypothesis that has to be tested in a genetical analysis. Examples of major genes for tolerance to aluminium, arsenic, boron, cadmium, copper and manganese are discussed. Even where major genes have been demonstrated, it is probable that other minor genes, 'modifiers', are present as well. Because of the nature of tolerance as a character, dominance and epistasis are likely to vary with the level of metal at which an analysis is performed. Tolerance is generally found to be dominant at some levels of the metal. Studies which have mapped tolerance genes, particularly to aluminium and salt, are discussed. The specificity of tolerance is a matter of some confusion. Some studies indicate that tolerances evolve independently to different metals, but others have suggested that tolerance to one metal may often confer a degree of tolerance to some other metals. Very little is known about the molecular genetics of tolerance, and the mechanisms of tolerance to most metals. The possible role of phytochelatins and metallothionein-like proteins in metal tolerance is discussed. The distribution of tolerance in natural populations suggests that tolerance is a disadvantage in uncontaminated environments, but how this 'cost' arises is not known. There is some evidence that the disadvantage to tolerance may be associated more with the modifiers of tolerance than with the primary tolerance gene. The study of the genetics of tolerance is of importance in planning breeding programmes to produce tolerant crops for use in areas where metal contamination is a limiting factor in productivity. It can also assist in understanding the mechanisms of tolerance, as exemplified by the study of the mechanism of arsenic tolerance in Holcus lanatus. Important areas for further research are discussed. Contents Summary 541 I. Introduction 542 II. Introduction 542 III. Transmission genetics of tolerance 544 IV. Specificity of tolerance 550 V. Molecular genetics of tolerance 552 VI. Ecological genetics of tolerance 553 VII. Conclusions 555 Acknowledgements 556 References 556.