14.2 MOLECULAR/ADAPTIVE PHYSIOLOGY AND GENETICS OF METAL HYPERACCUMULATION IN PLANTS
Cloning and characterization of metallothionein (MT) gene families in plants has progressed considerably in the last decade (Table 14.1). MTs and phytochelatins in plants contain a high percentage of cysteine sulfhydryl groups, which bind and sequester heavy metal ions in very stable complexes. Phytochelatins bind Cu and Cd with high affinity and are induced by various metals [62,63]. Phytochelatins may play a role in plant Cd tolerance. Howden and Cobbett  have isolated Arabidopsis mutants with increased sensitivity to Cd while Cu tolerance was almost unchanged [65,66]. These cad1 mutants were deficient in PC synthesis and showed greatly reduced levels of PC synthase activity.
MTs not only bind to metals but also regulate intracellular concentrations and detoxify lethal concentrations of metals [59,67]. Various MT genes, such as mouse MTI; human MTIA (alpha domain); human MTII; Chinese hamster MTII; yeast CUP1; and pea PsMTA, have been transferred to Nicotiana sp., Brassica sp., or A. thaliana [20-23,68-77]. As a result, varying degrees of constitutively enhanced Cd tolerance have been achieved compared with the control. Metal uptake was not markedly altered; in some cases, no differences were present and, in others, maximally 70% less or 60% more Cd was taken up by the shoots or leaves.
Only one study has been reported on a transgenic plant generated with MT of plant origin. When pea (Pisum sativum) MT-like gene PsMTA was expressed in A. thaliana, more Cu (several-fold in some plants) accumulated in the roots of transformed plants than in those of controls . S. Karenlampi (Finland) and her associates have isolated an MT gene from metal-tolerant Silene vulgaris and transferred it into several metal-sensitive yeasts. Increases in Cd and Cu tolerance were observed in the modified yeasts. These studies suggest that the MT gene may be useful in improving metal tolerance of plants.
The hyperaccumulator Thlaspi caerulescens and the related nonaccumulator T. arvense differ in their transcriptional regulation of ZNT1 (zinc transporter 1) capable of conferring uptake of Cd2+ and Zn2+ . Expression of ZNT1 and root zinc uptake rates is elevated in T. caerulescens when compared to T. arvense. Zinc-mediated down-regulation of ZNT1 transcript levels in the hyperac-cumulator occurs at about 50-fold higher external metal concentrations compared to the nonhyper-accumulator. In several nickel hyperaccumulators, metal exposure elicits a large and dose-dependent increase in the concentrations of free histidine, which can act as a specific chelator able to detoxify Ni2+ and which enhances the rate of nickel translocation from the rooting medium into the xylem for transport into the shoot via the transpiration stream.
In the shoots of hyperaccumulating plants, metal detoxification is achieved by metal chelation and subcellular compartmentalization into the vacuole and the apoplast [79-83] and by sequestration within specific tissues, e.g., in the epidermis or in trichomes. The plant detoxification systems remain to be characterized at the molecular level. The generation and analysis of crosses between hyperaccumulators and related nonhyperaccumulators will be one key tool in identifying the genes responsible for the metal hyperaccumulator phenotype.
Based on a preliminary genetic analysis of a number of F2 progeny from crosses between the cadmium- and zinc-tolerant zinc hyperaccumulator Arabidopsis halleri ssp. halleri (L.) and the closely related, nontolerant nonaccumulator A. lyrata ssp. petraea (L.), it was postulated that only a small number of major genes were involved in zinc hyperaccumulation and zinc tolerance in A. halleri [34,84].
14.3 PLANT METALLOTHIONEIN GENES AND GENETIC ENGINEERING FOR PHYTOREMEDIATION OF TOXIC METALS
The development of a phytoremediation technology for some trace elements requires the transfer of genes into plants across species borders. The molecular basis of trace element detoxification and hyperaccumulation in plants has been increasingly investigated [20,85-87].
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