Zinc (Zn) is an essential component of thousands of proteins in plants, although it is toxic in excess. In this review, the dominant fluxes of Zn in the soil-root-shoot continuum are described, including Zn inputs to soils, the plant availability of soluble Zn(2+) at the root surface, and plant uptake and accumulation of Zn. Knowledge of these fluxes can inform agronomic and genetic strategies to address the widespread problem of Zn-limited crop growth. Substantial within-species genetic variation in Zn composition is being used to alleviate human dietary Zn deficiencies through biofortification. Intriguingly, a meta-analysis of data from an extensive literature survey indicates that a small proportion of the genetic variation in shoot Zn concentration can be attributed to evolutionary processes whose effects manifest above the family level. Remarkable insights into the evolutionary potential of plants to respond to elevated soil Zn have recently been made through detailed anatomical, physiological, chemical, genetic and molecular characterizations of the brassicaceous Zn hyperaccumulators Thlaspi caerulescens and Arabidopsis halleri.

Zinc in plants

Plants 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. Since 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. Unambiguous 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.

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Hyperaccumulators of metal and metalloid trace elements: Facts and fiction

During the history of life on Earth, tectonic and climatic change repeatedly generated large territories that were virtually devoid of life and exhibited harsh environmental conditions. The ability of a few specialist pioneer plants to colonize such hostile environments was thus of paramount ecological importance for the continuous maintenance of primary production over time. Yet, we know very little about how extreme traits evolve and function in plants. Recent breakthroughs have given first insights into the molecular basis underlying the complex extreme model trait of metal hyperaccumulation and associated metal hypertolerance. This review gives an introduction into the hyperaccumulator research field and its history; provides an overview of hyperaccumulator germplasm; describes the state of the art of our understanding of the physiological, molecular, and genetic basis underlying metal hyperaccumulation and its evolution; and highlights future research needs and opportunities.

Metal Hyperaccumulation in Plants

This article reviews the responses of plant roots to elevated rhizosphere cadmium (Cd) concentrations. Cadmium enters plants from the soil solution. It traverses the root through symplasmic or apoplasmic pathways before entering the xylem and being translocated to the shoot. Leaf Cd concentrations in excess of 5-10 μg g(-1) dry matter are toxic to most plants, and plants have evolved mechanisms to limit Cd translocation to the shoot. Cadmium movement through the root symplasm is thought to be restricted by the production of phytochelatins and the sequestration of Cd-chelates in vacuoles. Apoplasmic movement of Cd to the xylem can be restricted by the development of the exodermis, endodermis, and other extracellular barriers. Increasing rhizosphere Cd concentrations increase Cd accumulation in the plant, especially in the root. The presence of Cd in the rhizosphere inhibits root elongation and influences root anatomy. Cadmium concentrations are greater in the root apoplasm than in the root symplasm, and tissue Cd concentrations decrease from peripheral to inner root tissues. This article reviews current knowledge of the proteins involved in the transport of Cd across root cell membranes and its detoxification through sequestration in root vacuoles. It describes the development of apoplastic barriers to Cd movement to the xylem and highlights recent experiments indicating that their maturation is accelerated by high Cd concentrations in their immediate locality. It concludes that accelerated maturation of the endodermis in response to local Cd availability is of functional significance in protecting the shoot from excessive Cd loads.

Root responses to cadmium in the rhizosphere: a review

Contents

Summary 759

I. Hyperaccumulation: the phenomenon 759
II. Macroevolution of hyperaccumulation 760
III. Microevolution of hyperaccumulation: variation within hyperaccumulator species 760
IV. Genetic analysis of trace metal accumulation and tolerance 761
V. Mechanisms of trace metal accumulation 762
VI. General discussion and research perspectives 769

Acknowledgements 772


References 772



Summary
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.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2008.02748.x

Molecular mechanisms of metal hyperaccumulation in plants

Summary
• The ability of metallicolous and nonmetallicolous populations of Arabidopsis halleri to accumulate zinc (Zn), cadmium (Cd) and lead (Pb) is compared here in order to explore the extent and variability of this trait in wild A. halleri plants.
• Aerial plant parts and the soil around the harvested plants were collected and analysed for metal concentrations or total and extractable metal concentrations, respectively, for 20 metallicolous and 13 nonmetallicolous populations.
• Results show that metallicolous and nonmetallicolous populations have the same ability to accumulate Zn and Cd but that neither population type is able to accumulate Pb. Between populations within type, an homogenous accumulating response is observed for Zn, whereas the ability to accumulate Cd is variable.
• Zn and Cd accumulation to very high concentrations is a constitutive property of the species. The Zn and Cd hyperaccumulator trait of A. halleri from contaminated sites was confirmed. Interestingly, nonmetallicolous plants are Zn and Cd hyperaccumulators. The possibility of using A. halleri in phytoremediation is discussed.

Do Arabidopsis halleri from nonmetallicolous populations accumulate zinc and cadmium more effectively than those from metallicolous populations

We examined how population age affects the relative importance of clonal propagation and sexual reproduction in the aquatic macrophyte Sparganium errectum. Plants were collected from two newly established populations and four successionally mature populations. The comparison of life history traits associated with sexual reproduction and clonal multiplication was carried out in a controlled environment experiment. We found that several physiological trade-offs occurred between sexual reproduction and clonal propagation. Moreover, we found a higher investment in sexual reproduction in newly established populations than in older populations. These results suggest that clonal propagation is favoured at the population level while, because seeds produced by sexual reproduction are the only means for long distance dispersal, selection will favour sexual reproduction at the metapopulation level. This discrepancy between selective pressures at different spatial scales could favour the maintenance of a mixed sexual-asexual reproductive system in Sparganium erectum.

Patrick P. De Laguérie

Papers

Do Arabidopsis halleri from nonmetallicolous populations accumulate zinc and cadmium more effectively than those from metallicolous populations

Variation in sexual and asexual reproduction among young and old populations of the perennial macrophyte Sparganium erectum