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metal cadmium (Cd) by Se protein also binding Cd

(2008)

2.3 Se Uptake and Transport

Selenate is accumulated in plant cells against an electrochemical potential (or gradient) by active transport driven by ATP (ATPase). Selenate readily competes with the uptake of sulphate, and both anions appear to be taken-up by a number of sulphate transporters in the root plasma membrane (Abrams et al. 1990). The sulphate transporters modulate Se uptake in bacteria and yeasts, and at least two types of these transporters are also present in plants. The S/Se transporters described belong to two main classes (Fig. 10.1):

(a) Transporters that have high affinity for sulphate (HAST). This is likely to be the primary transporter involved in sulphate uptake from the soil, and is expressed mainly in roots with a Km for sulphate of 7-10 ^M. HAST is also considered to be involved in selenate uptake; and

(b) Transporters with a low affinity for sulphate (LAST). This secondary transporter is more likely to be involved in intercellular transport of sulphate, expressed in both the roots and shoots with a Km for sulphate of 100 ^M. LAST is also considered to be involved in selenate uptake (Clarkson and Luttge 1991; Smith et al. 1995; Cherest et al. 1997).

Selenite uptake on the other hand may not be mediated by membrane transporters, as hydroxylamine a respiratory inhibitor inhibits selenite uptake by only about 20%, however hydroxylamine inhibited selenate uptake by 80% (Arvy 1997). Abrams et al. (1990) showed that SeMet uptake by wheat seedlings was coupled to metabolism as evident by the inhibition of uptake by the metabolic inhibitor

Fig. 10.1 Selenate and sulphate uptake across the root cell membrane driven by ATP (ATPase). LAST is the low affinity sulphate transporter and HAST is the high affinity sulphate transporter (Modified from Sors et al. 2005b)

Fig. 10.1 Selenate and sulphate uptake across the root cell membrane driven by ATP (ATPase). LAST is the low affinity sulphate transporter and HAST is the high affinity sulphate transporter (Modified from Sors et al. 2005b)

dinitrophenol and anaerobic conditions. Se concentrations in xylem exudate in roots exceeded that in the external medium by 6-13 times when selenate was added. However when selenite was added Se concentrations in the xylem were always lower than the outside solution, and tends to confirm that membrane transporters may not be involved in selenite uptake (Smith et al. 1997).

Translocation of Se from the roots to the shoots is highly dependent on the form of Se supplied. Selenate is transported more readily than selenite or organic Se compounds. For example, more than 50% of Se was transported from the roots to the shoots within 3 hours when selenate was added. Whilst less than 10% Se was transported from the roots to the shoots when selenite or organic Se was added (Shrift and Ulrich 1976). The reason may be that selenite is more easily converted to organic Se than selenate, and selenate is more strongly retained in the roots after transportation from the soil to the root by HAST. As well, the other conclusion could be that only selenate is readily available in the roots for transportation to the leaves by LAST. The distribution of Se in plants also differs with the type of Se accumulating plant species under investigation:

(a) Se accumulators - Se is accumulated most in young leaves, early vegetative growth, during reproductive stages and seeds; while Se content in mature leaves is reduced greatly (Broyer et al. 1972; Sors et al. 2005a).

(b) Se non-accumulators - Se is often similar in seeds and grains, and in the roots; with lower amounts in the stem and leaves (Arvy 1997; Asher et al. 1977).

Apart from the form and concentration of Se being important, the concentration of sulphur present is important (see Sect. 2.4 below). Plants can also absorb volatile forms of Se from the atmosphere, via the leaf surface and stomata. The Se can quickly be translocated down, probably in the phloem and accumulates in the roots as inorganic selenite, selenoglutathione (SeGSH) and protein bound seleno-methionione (SeMet) (Terry et al. 2000).

2.4 Se Interaction with Other Salts

Sulphates compete with selenate for uptake. Sulphate salinity (i.e., Na2SO4) therefore drastically inhibits plant selenate uptake. However, not all Se type plant species are affected in the same way:

(a) Se accumulator plants - selenate is preferentially taken up over sulphate, and so plants can take up high amounts of Se despite the high sulphate salinity present; and

(b) Se non-accumulator plants - have high discrimination for sulphate, and sele-nate uptake can be significantly inhibited by increasing sulphate supply (Banuelos etal. 1995; Zayed et al. 1998).

On the other hand, chloride salinity (i.e., NaCl) has a much reduced effect on Se uptake, but generally there can be a small decrease in shoot accumulation of Se with increasing NaCl levels (Wu and Huang 1991; Bell et al. 1992); but this may well be more of an indirect effect of NaCl generally decreasing plant metabolism.

Se is often associated with minerals also containing heavy metals, especially Cu, Ag, Hg and U (Broadley et al. 2007) therefore it is not surprising to find interactions between Se and heavy metals. For example De Filippis (1979) demonstrated that selenite and cysteine decreased the sub-lethal effects of zinc and mercury, including organic mercury to the freshwater alga Chlorella. In a recent study there appeared to be an association between Se binding proteins and a decrease in cadmium (Cd) toxicity, these binding proteins are usually rich in sulphydryl groups which may well explain the observations in Chlorella (Dutilleul et al. 2008). In reclamation of uranium mines there was present a growing risk of toxic levels of Se being released as a secondary problem to uranium toxicity (Sharmasarkar and Vance 2002). Finally, in phytoremediation of sites from mercury and organomercurials, Bizily et al. (1999) demonstrated that volatilisation of Hg was important and was a process similar to Se volatilisation. The genes for Hg volatilisation have been cloned and transgenic plants have been successfully used in phytoremediation; this appears to be a system in many ways similar to what is being proposed for Se phytoremediation (Rugh 2001).

3 Biochemistry

3.1 Se as an Essential Element

There is some evidence that Se may be required for growth and development in algae, but the question of Se being an essential element (micronutrient) in higher plants remains unresolved (Yokata et al. 1988; Whanger 2002; Pilon-Smits et al. 2009). In Se accumulating plants, indications are that Se may be required for maximum growth potential, especially those endemic to seleniferous soils (Broyer et al. 1966; Broyer et al. 1972). Even in the best studied Se accumulating plant Astragalus pectinatus the results of additional Se application in experiments have had differing results (Shrift 1969; Stadtman 1990). It is fair to point out that other nutrients can complex the situation such as phosphates and sulphates, however the experiments so far have not used controls where residual Se is not present at all; and indeed such experiments may be near impossible to perform (Forshhammer and Boek 1991; Stadtman 1996). This is simply because there will always be trace amounts of Se in plants, coming from impurities in the nutrients used or even coming from the atmosphere.

An alternative approach to try to resolve essentiality was to try to detect Se incorporation into Se dependent enzymes, with an integral SeCys residue as present in animals and bacteria (see Sect. 3.3) (Axley et al. 1991). To conclude, the evidence so far from molecular studies available is quite strong that there is no clear evidence for essential selenoproteins in higher plants, but part of the machinery for the synthesis of selenoproteins may be present in plants (see Sect. 4.2) (Berry et al. 1991; Berry etal. 2001).

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