Uptake and translocation of cationic nutrients play essential roles in physiological processes including plant growth, nutrition, signal transduction, and development. Approximately 5% of the Arabidopsis genome appears to encode membrane transport proteins. These proteins are classified in 46 unique families containing approximately 880 members. In addition, several hundred putative transporters have not yet been assigned to families. In this paper, we have analyzed the phylogenetic relationships of over 150 cation transport proteins. This analysis has focused on cation transporter gene families for which initial characterizations have been achieved for individual members, including potassium transporters and channels, sodium transporters, calcium antiporters, cyclic nucleotide-gated channels, cation diffusion facilitator proteins, natural resistance-associated macrophage proteins (NRAMP), and Zn-regulated transporter Fe-regulated transporterlike proteins. Phylogenetic trees of each family define the evolutionary relationships of the members to each other. These families contain numerous members, indicating diverse functions in vivo. Closely related isoforms and separate subfamilies exist within many of these gene families, indicating possible redundancies and specialized functions. To facilitate their further study, the PlantsT database (http://plantst.sdsc.edu) has been created that includes alignments of the analyzed cation transporters and their chromosomal locations.

Worldwide more than 400 plant species are now known that hyperaccumulate various trace metals (Cd, Co, Cu, Mn, Ni, and Zn), metalloids (As) and nonmetals (Se) in their shoots. Of these, almost one-quarter are Brassicaceae family members, including numerous Thlaspi species that hyperaccumulate Ni up to 3% of there shoot dry weight. We observed that concentrations of glutathione, Cys, and O-acetyl-L-serine (OAS), in shoot tissue, are strongly correlated with the ability to hyperaccumulate Ni in various Thlaspi hyperaccumulators collected from serpentine soils, including Thlaspi goesingense, T. oxyceras, and T. rosulare, and nonaccumulator relatives, including T. perfoliatum, T. arvense, and Arabidopsis thaliana. Further analysis of the Austrian Ni hyperaccumulator T. goesingense revealed that the high concentrations of OAS, Cys, and GSH observed in this hyperaccumulator coincide with constitutively high activity of both serine acetyltransferase (SAT) and glutathione reductase. SAT catalyzes the acetylation of L-Ser to produce OAS, which acts as both a key positive regulator of sulfur assimilation and forms the carbon skeleton for Cys biosynthesis. These changes in Cys and GSH metabolism also coincide with the ability of T. goesingense to both hyperaccumulate Ni and resist its damaging oxidative effects. Overproduction of T. goesingense SAT in the nonaccumulator Brassicaceae family member Arabidopsis was found to cause accumulation of OAS, Cys, and glutathione, mimicking the biochemical changes observed in the Ni hyperaccumulators. In these transgenic Arabidopsis, glutathione concentrations strongly correlate with increased resistance to both the growth inhibitory and oxidative stress induced effects of Ni. Taken together, such evidence supports our conclusion that elevated GSH concentrations, driven by constitutively elevated SAT activity, are involved in conferring tolerance to Ni-induced oxidative stress in Thlaspi Ni hyperaccumulators.

The ability of Thlaspi goesingense to hyperaccumulate Ni seems to be governed in part by enhanced accumulation of Ni within leaf vacuoles. We have characterized genes from T. goesingense encoding putative vacuolar metal ion transport proteins, termed metal tolerance proteins (TgMTPs). These proteins contain all of the features of cation-efflux family members, and evidence indicates they are derived from a single genomic sequence (TgMTP1) that gives rise to an unspliced (TgMTP1t1) and a spliced (TgMTP1t2) transcript. Heterologous expression of these transcripts in yeast lacking the TgMTP1 orthologues COT1 and ZRC1 complements the metal sensitivity of these yeast strains, suggesting that TgMTP1s are able to transport metal ions into the yeast vacuole in a manner similar to COT1 and ZRC1. The unspliced and spliced TgMTP1 variants differ within a histidine-rich putative metal-binding domain, and these sequence differences are reflected as alterations in the metal specificities of these metal ion transporters. When expressed in yeast, TgMTP1t1 confers the highest level of tolerance to Cd, Co, and Zn, whereas TgMTP1t2 confers the highest tolerance to Ni. TgMTP1 transcripts are highly expressed in T. goesingense compared with orthologues in the nonaccumulators Arabidopsis thaliana, Thlaspi arvense, and Brassica juncea. We propose that the high-level expression of TgMTP1 in T. goesingense accounts for the enhanced ability of this hyperaccumulator to accumulate metal ions within shoot vacuoles. T he genus Thlaspi contains numerous species that hyperaccumulate Ni. For example, field-collected specimens of Thlaspi goesingense Hálácsy from an ultramafic site in Redschlag, Austria, have been recorded with shoot Ni concentrations as high as 12,400 g͞g shoot dry biomass (1.2%) (1, 2). In the laboratory, we analyzed individuals from this T. goesingense population and confirmed their hyperaccumulator status (3). To determine the physiological basis of this Ni hyperaccumulation phenotype, we have investigated a number of physiological parameters in both the hyperaccumulator and the related nonaccumulator Thlaspi arvense. Production of Ni chelates in root exudates in the hyperaccumulator and nonaccumulator were found to be equivalent (4), as well as rates of Ni translocation to the shoots (3). However, the hyperaccumulator was found to be more Ni tolerant when compared with the nonaccumulator (3). Ni tolerance in the hyperaccumulator is related to its enhanced ability to compartmentalize Ni in shoot vacuoles (5) with 75% of the intracellular leaf Ni in the hyperaccumulator being localized to the vacuole (5). Furthermore, vacuoles from the hyperaccumulator contain approximately double the Ni of the nonaccumulator, even though protoplasts from each species contained equal amounts of Ni (5). We conclude that vacuolar compartmentalization of Ni in the hyperaccumulator plays a major role in Ni tolerance and hyperaccumulation, although little is known about its molecular mechanism (6). Here we present data characterizing the functional ...

SummaryTo avoid metal toxicity, organisms have evolved mechanisms including ef¯ux of metal ions from cells and sequestration into internal cellular compartments. Members of the ubiquitous cation diffusion facilitator (CDF) family are known to play an important role in these processes. Overexpression of the plant CDF family member metal tolerance protein 1 (MTP1) from the Ni/Zn hyperaccumulator Thlaspi goesingense (TgMTP1), in the Saccharomyces cerevisiae D zinc resistance conferring (zrc)1D cobalt transporter (cot)1 double mutant, suppressed the Zn sensitivity of this strain. T. goesingense was found to contain several allelic variants of TgMTP1, all of which confer similar resistance to Zn in Dzrc1Dcot1. Similarly, MTP1 from various hyperaccumulator and non-accumulator species also confer similar resistance to Zn. Dzrc1Dcot1 lacks the ability to accumulate Zn in the vacuole and has lower accumulation of Zn after either long-or short-term Zn exposure. Expression of TgMTP1 in Dzrc1Dcot1 leads to further lowering of Zn accumulation and an increase in Zn ef¯ux from the cells. Expression of TgMTP1 in a V-type ATPase-de®cient S. cerevisiae strain also confers increased Zn resistance. In vivo and in vitro immunological staining of hemagglutinin (HA)-tagged TgMTP1::HA reveals the protein to be localized in both the S. cerevisiae vacuolar and plasma membranes. Taken together, these data are consistent with MTP1 functioning to enhance plasma membrane Zn ef¯ux, acting to confer Zn resistance independent of the vacuole in S. cerevisiae. Transient expression in Arabidopsis thaliana protoplasts also reveals that TgMTP1::green¯uorescent protein (GFP) is localized at the plasma membrane, suggesting that TgMTP1 may also enhance Zn ef¯ux in plants.

In its natural habitat, Astragalus bisulcatus can accumulate up to 0.65% (w/w) selenium (Se) in its shoot dry weight. X-ray absorption spectroscopy has been used to examine the selenium biochemistry of A. bisulcatus. High concentrations of the nonprotein amino acid Se-methylseleno-cysteine (Cys) are present in young leaves of A. bisulcatus, but in more mature leaves, the Se-methylseleno-Cys concentration is lower, and selenate predominates. Seleno-Cys methyltransferase is the enzyme responsible for the biosynthesis of Se-methylseleno-Cys from seleno-Cys and S-methyl-methionine. Seleno-Cys methyltransferase is found to be expressed in A. bisulcatus leaves of all ages, and thus the biosynthesis of Se-methylselenoCys in older leaves is limited earlier in the metabolic pathway, probably by an inability to chemically reduce selenate. A comparative study of sulfur (S) and Se in A. bisulcatus using x-ray absorption spectroscopy indicates similar trends for oxidized and reduced Se and S species, but also indicates that the proportions of these differ significantly. These results also indicate that sulfate and selenate reduction are developmentally correlated, and they suggest important differences between S and Se biochemistries.Many selenium (Se) compounds are toxic to mammals at high concentrations, but Se is also an essential micronutrient, and low doses have been implicated in cancer prevention (Clark et al., 1996; Combs et al., 1997). Not all diets provide adequate Se, and an obvious and inexpensive way to provide Se may be to engineer food plants to accumulate higher levels of the element (Ip et al., 1994). The Se hyperaccumulator Astragalus species, such as Astragalus bisulcatus, may be an excellent source of genetic material from which to isolate genes to develop such plants. In the wild, A. bisulcatus can accumulate Se levels of up to 0.65% (w/w) dry weight in the shoots (Byers, 1936), predominantly as Se-methylseleno-Cys (Trelease et al., 1960), and similar results are readily obtained in plants grown hydroponically in the laboratory (Orser et al., 1999). Understanding Se uptake in A. bisulcatus might also allow the development of highly effective cultivars for phytoremediation (Salt et al., 1998).A critical step in the biotransformation of selenate is the initial two-electron reduction to selenite. Hyperaccumulating plants might achieve this in at least three different ways: by substituting selenate into the sulfate reduction pathway (reduction by ATP sulfurylase/adenyl sulfate (APS) reductase; Shrift, 1969;Setya et al., 1996), by substituting selenate into the nitrate uptake pathway (microbial nitrate reductases can reduce selenate; Sabaty et al., 2001), or by a specific selenate reductase. For nonhyperaccumulating plants, there is good evidence that selenate reduction occurs via substitution for sulfate in the ATP sulfurylase/APS reductase system, and that this is the ratelimiting step in selenate transformation (Shrift, 1969;Shaw and Anderson, 1974; Burnell, 1981; Pilon-Smits et al., 1999). In these spe...