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An Extreme Plant Lifestyle: Metal Hyperaccumulation
David E. Salt, Department of Horticulture and Landscape Architecture, Purdue University, West Lafayette, IN, USA
If you think that prokaryotes have a monopoly on extreme life styles, then think again. Imagine having 20–30* percent of your tissue dry-ash weight composed of cadmium or arsenic. That is more cadmium than is found in a household Ni-Cd battery, and enough arsenic in 1 gram of dry ash to kill a person. Well, such extreme tissue levels of these toxic elements are normal in shoots of metal hyperaccumulating plants. Metal hyperaccumulators are a unique group of plants that in their native habitats hyperaccumulate various elements, depending on the species, including arsenic, cadmium, cobalt, manganese, nickel, selenium or zinc (Reeves and Baker, 2000). Metal hyperaccumulation has been identified in over 400 taxa of plants, and occurs even when the plants are growing on soils with unremarkable levels of the hyperaccumulated metals. Hyperaccumulation is an active process and appears to help the plants protect themselves against pathogens and insect herbivores. To achieve such feats of uptake and accumulation these plants not only contain a remarkable internal hypertolerance mechanism to resist the cytotoxic burden of the accumulated metals, but also a powerful scavenging mechanism for the efficient uptake of these potentially toxic elements from the soil. Recent studies of this extreme lifestyle are starting to reveal an intriguing story of plant adaptation and evolution. Laying bare a fascinating patchwork of molecular mechanisms that natural selection has stitched together—from such divergent players as iron-homeostasis and plant-pathogen response—to achieve a unique and perhaps extreme, adaptive survival solution.
Comparative transcriptional profiling of the nonaccumulator Arabidopsis thaliana and the zinc and cadmium hyperaccumulator Arabidopsis halleri (Becher et al. 2004; Weber et al. 2004) has opened a window into the global changes driving metal hyperaccumulation, shedding light on a set of coordinated gene expression differences that control uptake, accumulation, and tolerance. At the transcriptional level, zinc uptake and accumulation appear to be driven by overexpression of zinc transporters from the ZIP family, implicated in zinc influx into cells. Direct measurements of zinc transport into roots of the related zinc hyperaccumulator Thlaspi caerulescens reveal constitutively elevated rates of zinc influx that is correlated with overexpression of a ZIP-family member (Pence et al. 2000; Assunção et al. 2001). These zinc-influx transporters appear to work in conjunction with certain P-type ATP-dependent metal transporters and Nramp ion-transporters that are also constitutively overexpressed (Papoyan and Kochian 2004; Weber et al. 2004).
Long distance transport, from roots to shoots, is clearly a critical process for hyperaccumulation of metals in shoot tissues. Both the iron chelate nicotianamine and the free amino acid, histidine, have been implicated in chaperoning metals during this transport process. In A. halleri nicotianamine synthase (NAS), genes are overexpressed in both roots and shoots (Becher et al. 2004; Weber et al. 2004). The implication of such changes for metal hyperaccumulation is not clear, though ectopic overexpression of NAS1 in tobacco and A. thaliana confers enhanced Ni resistance, but no increase in Ni accumulation in shoots when plants are grown at nontoxic concentrations of Ni (Douchkov et al. 2005; Pianelli et al. 2005). It is intriguing to note that, a suite of genes known to be involved in iron homeostasis, are overexpressed in hyperaccumulators. These include NAS2 & 3, IRT1 and FRO2 in A. halleri and T. caerulescens (Lombi et al. 2002; Becher et al. 2004; Weber et al. 2004). Such observations argue that selective pressures have co-opted part of the iron-response mechanisms in hyperaccumulators to play an as yet unknown role in the metal hyperaccumulation process. Free histidine also makes a good chelate for certain metals, including Ni at physiological pHs, and the concentration of free histidine in roots of certain Alyssum species strongly correlates with Ni hyperaccumulation ability (Ingle et al. 2005). Further, overexpression of ATP-PRT1, a gene involved in histidine biosynthesis, appears to drive this histidine overaccumulation (Ingle et al. 2005). Ectopic overexpression of ATP-PRT1 in A. thaliana enhances the concentrations of free histidine and increases Ni resistance; though it appears not to increase Ni accumulation in shoots (Wycisk et al. 2004; Ingle et al. 2005).
Metal hyperaccumulators also require hypertolerance mechanisms to resist the potentially acute cytotoxic effects of the accumulated metals. Cadmium, nickel, and zinc are compartmentalized in the vacuole of various hyperaccumulator species (Kupper et al. 1999, 2001; Krämer et al. 2000; Ma et al. 2005), where they are bound by organic acids (Salt et al. 1999; Krämer et al. 2000; Ueno et al. 2005). Constitutive overexpression of the CDF-family member MTP1 has been implicated in this process (Assunção et al. 2001; Persans et al. 2001). Genes encoding MTP1 in A. halleri have also been shown to be linked with zinc tolerance in the segregating F2 population from a cross between A. halleri and the nonaccumulator Arabidopsis lyrata (Dräger et al. 2004). However, it is also possible that MTP1 plays a role in long-distance transport of zinc, with knockdown mutants of MTP1 in A. thaliana showing lower accumulation of foliar zinc, and ectopic overexpression in yeast suggesting that MTP1 can act in zinc efflux from cells (Kim et al. 2004; Desbrosses-Fonrouge et al. 2005). Chelation of metals by the thiol-rich peptides phytochelatins is involved in stress responses to many metals in nonaccumulator plants (Cobbett and Goldsbrough 2002). In contrast, phytochelatins appear to play no role in metal binding, tolerance, or accumulation in hyperaccumulator plants (Kramer et al. 1996, 2000; Salt et al. 1999; Ebbs et al. 2002; Schat et al. 2002; Ueno et al. 2005).
Hyperaccumulation of metals has the potential to cause extensive oxidative damage to plant tissues. To protect against such oxidative damage, nickel hyperaccumulators in the Thlaspi genus overaccumulate the antioxidant glutathione (Freeman et al. 2004). Such overaccumulation is driven by constitutive activation of the sulfur assimilation pathway, through allosteric activation of the bottleneck enzyme, serine acetyltransferase (Freeman et al. 2004). Intriguingly, glutathione accumulation in Thlaspi hyperaccumulators appears related to the constitutive overaccumulation of salicylic acid (Freeman et al. 2005). Such an observation suggests a direct interaction between metal hyperaccumulation and plant/pathogen interactions, given that salicylic acid is a key signaling molecule involved in plant pathogen responses. Such a connection starts to establish a molecular foundation for the observations that metal hyperaccumulation appears to deter pathogens (Boyd et al. 1994). Genome-wide profiling of gene expression also supports such a link, with various pathogen-responselike genes being overexpressed in the hyperaccumulator A. haller, including a PR1-like gene (Becher et al. 2004).
Both genome-wide transcriptional profiling and more focused physiological studies paint a multifaceted picture of metal hyperaccumulation being driven by changes in multiple genes and processes. Genetic studies reveal that metal tolerance and hyperaccumulation are independent genetic traits (Macnair et al. 1999; Assunção et al. 2003), helping us subdivide this complex array of genetic and biochemical changes. Much progress has been made in understanding the changes involved in the evolution of the hyperaccumulation trait. Unfortunately, little if anything is known about the higher-level regulatory changes in the genomes of hyperaccumulators that coordinate the complex suite of biochemical and physiological processes required for metal hyperaccumulation.
*20–30 percent dry-ash weight is equivalent to 2–3 percent dry weight or 20,000–30,000 µg g-1 dry weight.
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Dräger, D. B., Desbrosses-Fonrouge, A. G., Krach, C., Chardonnens, A. N., Meyer, R. C., Saumitou-Laprade, P., and Krämer, U. (2004). Two genes encoding Arabidopsis halleri MTP1 metal transport proteins co-segregate with zinc tolerance and account for high MTP1 transcript levels. Plant J. 39:425–439.
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Ma, J. F., Ueno, D., Zhao, F. J., and McGrath, S. P. (2005) Subcellular localisation of Cd and Zn in the leaves of a Cd-hyperaccumulating ecotype of Thlaspi caerulescens. Planta 220: 731–736.
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