Henk Schat

Molecular mechanisms of metal hyperaccumulation in plants

Metal hyperaccumulator plants accumulate and detoxify extraordinarily high concentrations of metal ions in their shoots. Metal hyperaccumulation is a fascinating phenomenon, which has interested scientists for over a century. Hyperaccumulators constitute an exceptional biological material for understanding mechanisms regulating plant metal homeostasis as well as plant adaptation to extreme metallic environments.Our understanding of metal hyperaccumulation physiology has recently increased as a result of the development of molecular tools. This review presents key aspects of our current understanding of plant metal – in particular cadmium (Cd),nickel (Ni) and zinc (Zn) – hyperaccumulation.

Hyperaccumulators of metal and metalloid trace elements: Facts and fiction

BackgroundPlants that accumulate metal and metalloid trace elements to extraordinarily high concentrations in their living biomass have inspired much research worldwide during the last decades. Hyperaccumulators have been recorded and experimentally confirmed for elements such as nickel, zinc, cadmium, manganese, arsenic and selenium. However, to date, hyperaccumulation of lead, copper, cobalt, chromium and thallium remain largely unconfirmed. Recent uses of the term in relation to rare-earth elements require critical evaluation.ScopeSince the mid-1970s the term ‘hyperaccumulator’ has been used millions of times by thousands of people, with varying degrees of precision, aptness and understanding that have not always corresponded with the views of the originators of the terminology and of the present authors. There is therefore a need to clarify the circumstances in which the term ‘hyperaccumulator’ is appropriate and to set out the conditions that should be met when the terms are used. We outline here the main considerations for establishing metal or metalloid hyperaccumulation status of plants, (re)define some of the terminology and note potential pitfalls.ConclusionsUnambiguous communication will require the international scientific community to adopt standard terminology and methods for confirming the reliability of analytical data in relation to metal and metalloid hyperaccumulators.

Mechanisms to cope with arsenic or cadmium excess in plants.

The metalloid arsenic and the heavy metal cadmium have no demonstrated biological function in plants. Both elements are highly toxic and of major concern with respect to their accumulation in soils, in the food-chain or in drinking water. Arsenate is taken up by phosphate transporters and rapidly reduced to arsenite, As(III). In reducing environments, As(III) is taken up by aquaporin nodulin 26-like intrinsic proteins. Cd(2+) enters the root via essential metal uptake systems. As(III) and Cd(2+) share some similarity between their toxicology and sequestration machineries. Recent progress in understanding the mechanisms of As and Cd uptake and detoxification is presented, including the elucidation of why rice takes up so much arsenic from soil and of mechanisms of As and Cd hypertolerance.

Large Expression Differences in Genes for Iron and Zinc Homeostasis, Stress Response, and Lignin Biosynthesis Distinguish Roots of Arabidopsis thaliana and the Related Metal Hyperaccumulator Thlaspi caerulescens

The micronutrient zinc has an essential role in physiological and metabolic processes in plants as a cofactor or structural element in 300 catalytic and noncatalytic proteins, but it is very toxic when available in elevated amounts. Plants tightly regulate their internal zinc concentrations in a process called zinc homeostasis. The exceptional zinc hyperaccumulator species Thlaspi caerulescens can accumulate up to 3% of zinc, but also high amounts of nickel and cadmium, without any sign of toxicity. This should have drastic effects on the zinc homeostasis mechanism. We examined in detail the transcription profiles of roots of Arabidopsis thaliana and T. caerulescens plants grown under deficient, sufficient, and excess supply of zinc. A total of 608 zinc-responsive genes with at least a 3-fold difference in expression level were detected in A. thaliana and 352 in T. caerulescens in response to changes in zinc supply. Only 14% of these genes were also zinc responsive in A. thaliana. When comparing A. thaliana with T. caerulescens at each zinc exposure, more than 2,200 genes were significantly differentially expressed (≥5-fold and false discovery rate < 0.05). While a large fraction of these genes are of yet unknown function, many genes with a different expression between A. thaliana and T. caerulescens appear to function in metal homeostasis, in abiotic stress response, and in lignin biosynthesis. The high expression of lignin biosynthesis genes corresponds to the deposition of lignin in the endodermis, of which there are two layers in T. caerulescens roots and only one in A. thaliana.

Genetic engineering in the improvement of plants for phytoremediation of metal polluted soils

Metal concentrations in soils are locally quite high, and are still increasing due to many human activities, leading to elevated risk for health and the environment. Phytoremediation may offer a viable solution to this problem, and the approach is gaining increasing interest. Improvement of plants by genetic engineering, i.e. by modifying characteristics like metal uptake, transport and accumulation as well as metal tolerance, opens up new possibilities for phytoremediation. So far, only a few cases have been reported where one or more of these characteristics have been successfully altered; e.g. mercuric ion reduction causing improved resistance and phytoextraction, and metallothionein causing enhanced cadmium tolerance. These, together with other approaches and potentially promising genes for transformation of target plants are discussed.

Genetic control of copper tolerance in Silene vulgaris

The genetic control of heavy metal tolerance in higher plants is poorly understood, possibly in part because of several inherent properties of tolerance tests and tolerance measures. In this study we compared different methods of testing for copper tolerance in Silene vulgaris. A new type of multiple concentration test has been used to analyse the genetic control of copper tolerance in this species. Provisional results indicate that the occurrence of any tolerance, relative to a non-tolerant reference population from uncontaminated soil, is governed by a single major gene. The level of tolerance, however, seems to be controlled by a number of modifiers, which are completely hypostatic to the major gene. This model agrees with that proposed for Mimulus guttatus by Macnair (1983).

Phytochelatins in Cadmium-Sensitive and Cadmium-Tolerant Silene vulgaris (Chain Length Distribution and Sulfide Incorporation)

In response to a range of Cd concentrations, the root tips of Cd-tolerant plants of Silene vulgaris exhibit a lower rate of PC production accompanied by a lower rate of longer chain PC synthesis than those of Cd-sensitive plants. At the same Cd exposure level, stable PC-Cd complexes are more rapidly formed in the roots of Cd-sensitive plants than in those of tolerant plants. At an equal PC concentration in the roots, the PC composition and the amount of sulfide incorporated per unit of PC-thiol is the same in both populations. Although these compounds might play some role in mechanisms that contribute to Cd detoxification, the ability to produce these compounds in greater amounts is not, itself, the mechanism that produces increased Cd tolerance in tolerant S. vulgaris plants.

IDENTICAL MAJOR GENE LOCI FOR HEAVY METAL TOLERANCES THAT HAVE INDEPENDENTLY EVOLVED IN DIFFERENT LOCAL POPULATIONS AND SUBSPECIES OF SILENE VULGARIS

Heavy metal tolerant Silene vulgaris plants, originating from different metalliferous sites in Germany and one in Ireland, were crossed to each other and to nontolerant plants from a nonmetalliferous site in The Netherlands. Analysis of the crosses suggested that there were two distinct major gene loci for zinc tolerance among a total of five tolerant populations. The tolerance loci for zinc, copper, and cadmium in the Irish plants were shown to be identical with those in the German populations. It is argued that the occurrence of common major genes for tolerance among different geographically isolated populations must have resulted from independent parallel evolution in local nontolerant ancestral populations. Each of the tolerances studied seems to be controlled by only a few specific major genes.

Plant metallothioneins – metal chelators with ROS scavenging activity?

Metallothioneins (MTs) are ubiquitous cysteine-rich proteins present in plants, animals, fungi and cyanobacteria. In plants, MTs are suggested to be involved in metal tolerance or homeostasis, as they are able to bind metal ions through the thiol groups of their cysteine residues. Recent reports show that MTs are also involved in the scavenging of reactive oxygen species (ROS). The interplay between these roles is not entirely clear. Plants have many MT isoforms with overlapping expression patterns, and no specific role for any of them has been assigned. This review is focused on recent findings on plant MTs.

Arabidopsis thaliana transcription factors bZIP19 and bZIP23 regulate the adaptation to zinc deficiency

Zinc is an essential micronutrient for all living organisms. When facing a shortage in zinc supply, plants adapt by enhancing the zinc uptake capacity. The molecular regulators controlling this adaptation are not known. We present the identification of two closely related members of the Arabidopsis thaliana basic-region leucine-zipper (bZIP) transcription factor gene family, bZIP19 and bZIP23, that regulate the adaptation to low zinc supply. They were identified, in a yeast-one-hybrid screening, to associate to promoter regions of the zinc deficiency-induced ZIP4 gene of the Zrt- and Irt-related protein (ZIP) family of metal transporters. Although mutation of only one of the bZIP genes hardly affects plants, we show that the bzip19 bzip23 double mutant is hypersensitive to zinc deficiency. Unlike the wild type, the bzip19 bzip23 mutant is unable to induce the expression of a small set of genes that constitutes the primary response to zinc deficiency, comprising additional ZIP metal transporter genes. This set of target genes is characterized by the presence of one or more copies of a 10-bp imperfect palindrome in their promoter region, to which both bZIP proteins can bind. The bZIP19 and bZIP23 transcription factors, their target genes, and the characteristic cis zinc deficiency response elements they can bind to are conserved in higher plants. These findings are a significant step forward to unravel the molecular mechanism of zinc homeostasis in plants, allowing the improvement of zinc bio-fortification to alleviate human nutrition problems and phytoremediation strategies to clean contaminated soils.