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University of Pennsylvania, Philadelphia, PA, USA   |
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Metallophytes:
a unique biodiversity resource ‘owned’ by the mining industry AJM Baker1, SN Whiting1 & D Richards2 1The University of Melbourne, Australia 2Rio Tinto plc, London, UK Metallophytes are characteristic plants of mineralised areas that support soils heavily loaded with metals, so possessing a potential for mining. Through the ages, these plants have developed biological mechanisms allowing them to tolerate metal concentrations toxic to other plants. Some are adapted variants (ecotypes) of common species but others are species strictly endemic to their particular metal-rich area and are often of very restricted distribution. Their metal-specific adaptations and large diversity (species, families and life-forms) grant metallophytes a unique place in biodiversity and genetic resource conservation. Only the European plants are well known. Tropical and sub-tropical metallophyte taxonomy and ecology lie far behind despite the rising mining activity taking place in these latitudes. Traditionally, metallophytes have been used as geobotanical indicators as they can be used to delineate metalliferous substrates when prospecting. In the more recent drive towards sustainable development and responsible mine site closure by the industry, two further uses need to be promoted:
Abandoned mine sites are generally seen as a liability since they remain as scars on the landscape and well after their closure persist as a source of pollution through dust and chemical leakages. If a former mining area and surrounding tailings is naturally recolonised by vegetation, the unwanted legacy can become a resource base of unique genetic materials with unique physiological traits. The vegetation developing directly on mine soil is both likely to support metallophytes and represent the surviving species that covered the mineralised area prior to its development into a mine. The study of metallophytes and their colonisation, behaviour and evolution observable on former mine sites can enhance closure and rehabilitation strategies beyond simple greening of the sites. Food web implications of metal hyperaccumulation:
herbivores and their predators and pathogens Recent studies have explored some of the food web consequences of metal hyperaccumulation by plants. Hyperaccumulation poisons or deters some herbivores, whereas others are able to circumvent these plant elemental defences. Field surveys of Ni hyperaccumulators in California, New Caledonia and South Africa have identified ten insect taxa that feed on Ni hyperaccumulator tissues and themselves contain >500 µg Ni/g. The metal in these insects may defend them against their predators and pathogens. I describe tests of this hypothesis using the high-Ni insect, Melanotrichus boydi (Heteroptera: Miridae). Most experiments showed no defensive effects, but one spider species was negatively affected by a diet of M. boydi. I conclude that further research, especially using very high-Ni (up to 3500 µg Ni/g) insects recently discovered from South Africa, is warranted to more completely explore this phenomenon. Arabidopsis thaliana and Arabidopsis
halleri as models: molecular analysis of metal tolerance and metal
signal transduction With the aim of identifying and characterizing molecular determinants of metal homeostasis and tolerance we are using different model systems: Arabidopsis thaliana and A. halleri on the one hand, Schizosaccharomyces pombe as the cellular model on the other hand. A. halleri is a known Zn hyperaccumulator and is also more Cd tolerant than A. thaliana. We are mainly interested (i) in the contribution of known candidate genes to these differences and (ii) in elucidating regulatory processes within the plant metal homeostasis network. Using cDNA-AFLP, a large collection of metal-regulated A. halleri genes was established. Several putative signal transduction components have been studied extensively with regard to their regulation under different conditions in the two Arabidopsis species. A few of them, e.g. a transcription factor and a protein phosphatase 2C, have been analyzed further using A. thaliana insertion lines. Metal responses are being analyzed for any changes caused by inactivation of a particular gene at the transcriptional level using microarrays and at the metabolite level using LC-ESI-Q-TOF-MS. The fission yeast S. pombe serves as a suitable model for cells that express phytochelatin synthases. The main emphasis of our work with S. pombe is on the analysis of phytochelatin synthesis and the role of transporters belonging to the Cation Diffusion Facilitator family. Also, we are trying to use S. pombe mutants as a vehicle for the expression cloning of plant metal homeostasis factors. Heavy metal detoxification mechanisms in plants Our work is aimed at identifying mechanisms directly involved in the detoxification of heavy metals in plants. Such mechanisms may include exclusion, chelation, transport and sequestration (or combinations of these). Some work is aimed at examining the role of the heavy metal binding peptides, phytochelatins (PCs), in metal detoxification. To this end PC-deficient mutants of Arabidopsis have been identified and genes in the biosynthetic pathway have been isolated. Other, more recent, work is aimed at the analysis of the roles of a family of heavy metal transporting P-type ATPases in Arabidopsis. Arabidopsis has seven members of this sub-group of the P-type ATPases, in contrast to other eukaryotes (for which nearly complete genome sequence is available) which have only one or two. It is likely that some of these genes, as in other eukaryotes, are required for copper homeostasis (and possibly detoxification). Other members of this family in Arabidopsis may be directly involved in non-essential metal transport. This work is aimed at identifying the roles of these genes in plant responses to heavy metals. Gene discovery in aid of plant nutrition, human
health and environmental remediation Increasing the ability of plants to take up minerals could have a dramatic impact on both plant and human health. Furthermore, understanding the pathways by which metals accumulate in plants will enable the engineering of plants to exclude toxic metals or to extract toxic metals from the soil. We have employed the tools available in the model plant Arabidopsis to identify genes involved in metal homeostasis. We had previously identified IRT1, a root specific transporter that is expressed in the epidermis under iron deficiency and is responsible for uptake of iron from the soil. Like other iron transporters identified to date, IRT1 transports a variety of cations, including essential metals such as zinc and manganese as well as the toxic metal cadmium. We have introduced site-directed and random mutations to elucidate how IRT1 functions. While many of the constructs lack transporter activity, several have altered transport specificities. IRT1 belongs to the 15-member ZIP gene family in Arabidopsis. We are currently addressing the role of each of these ZIP proteins in the transport of divalent cations throughout the plant. We are also using functional genomics approaches to identify genes involved in mineral homeostasis. Nutrient profiling via inductively coupled plasma spectroscopy (ICP) is being used in a high throughput screen for mutant plants with abnormal elemental compositions. We are also screening yeast mutants via ICP to obtain functional predictions of plant orthologs. The role of membrane transporters and low-molecular-weight
chelators in metal homeostasis of hyperaccumulators and closely related
model plants Essential metal cations, like Zn2+ or Ni2+, are required by plants in low amounts, but are toxic when accumulated in excess. Thus tight regulation and coordination is required of metal uptake, movement in the plant, cellular trafficking and sequestration. Metal hyperaccumulator plants, like the Zn hyperaccumulator Arabidopsis halleri or the Ni hyperaccumulator Alyssum lesbiacum, are highly metal-tolerant and accumulate metals predominantly in above-ground tissues. We are using microarray chip hybridisation to compare the responses of A. thaliana and A. halleri to elevated zinc concentrations in the rooting medium. Furthermore, we are investigating the role of candidate genes, primarily of the cation diffusion facilitator (CDF) family of metal transport proteins, in zinc tolerance of A. halleri. In silico analysis of the A. thaliana genomic sequence revealed 12 to 13 putative gene products related to CDF proteins of other organisms. To study the presumed function of these genes in metal homeostasis we are using a functional genomics approach including expression analysis, protein localization, heterologous expression and reverse genetics. In addition to metal transport, metal chelation may play an important role not only in metal tolerance, but also in determining metal mobility for transport from roots into shoots. The Ni hyperaccumulator A. lesbiacum was previously found to respond to Ni exposure by a large and proportional increase in xylem histidine concentrations. In a non-tolerant non-accumulator species the addition of exogenous histidine was shown to increase Ni tolerance as well as Ni flux into the xylem (Krämer et al. 1996, Nature 379: 635-638). We have conducted physiological experiments to further understand the role of histidine in Ni hyperaccumulation. In a transgenic approach we have attempted to increase Ni tolerance by engineering A. thaliana to overproduce histidine.
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Illustrations: Heavy Metal Plant cartoon by Sam
Day. Arabidopsis thaliana - the model plant (Philip Rea). Micrograph
of the leaf surface of the Ni-hyperaccumulator Alyssum lesbiacum (Ute
Kraemer). Arabidopsis halleri growing at the bottom of a heap of minewaste
(Ute Kraemer) Last updated: February 18, 2003 |
