Metal Induced Responses in Plants

Soils with high concentrations of heavy metals can result from naturally high background levels or various anthropogenic activities. In plants, resistance to excess metals is achieved by avoidance (plants can restrict metal uptake) or tolerance (plants can cope with extreme internal metal concentrations). Thus, to survive in metal-polluted environments, plants have developed the two basic metal uptake and tolerance strategies of exclusion and accumulation (Baker 1981, 1987).

In metal excluders, low to moderate metal concentrations are maintained in the tissues over a wide range of total soil metal concentrations until critical concentrations cause damage to the exclusion mechanisms. Plants can either avoid metal uptake into their roots (resistance) or restrict metal transport to the shoots (tolerance) through several mechanisms:

• Enhanced mucus production and root cell-cap detachment, which provides a barrier to root metal uptake (Llugany et al. 2003).

• Induced cell death of root epidermal cells and production of border cells protecting deeper cells of the meristem and root elongation zone (Delisle et al. 2001).

• High affinity of some metals for the cell wall components (e.g. polygalacturonic acid) (Adriano 2001; Ernst et al. 1992).

• Secretion of strong metal chelators, such as organic acids (e.g. malic and citric acid) or phenolics (Barcelo and Poschenrieder 2002; Briat and Lebrun 1999; Hall 2002; Ernst et al. 1992; Salt 2001).

• High efficiency of metal sequestration in root cell walls and vacuoles, which consequentially restricts metal xylem loading (Lasat 2002; Hall 2002).

• Mycorrhizal fungi inhibit metal uptake by binding the metals to components of the mycelium (Joner and Leyval 1997; Joner et al. 2000).

Metal-accumulating plants achieve enhanced metal uptake via the roots accompanied by successful metal loading in the xylem and transport to the shoots, where the metals concentrate (Baker 1981; Shen et al. 1997). Extreme accumulation pheno-types (i.e. hyperaccumulating plant species) can take up more than 10,000 mg g-1 Mn or Zn, 1,000 mg g-1 Ni, Cu, Pb and Se, and 100 mg g-1 Cd, in contrast to normal physiological requirements (if any). These levels are far in excess of those found in most other species (Baker 1981, 1987; Baker et al. 2000; Reeves 2006; Reeves and Baker 2000). Approximately 420 plant species (less than 0.2% of all angiosperms) have this character (Baker and Whiting 2002). The physiological and biochemical mechanisms of metal transport, sequestration and detoxification involve:

• Preferential spreading of the roots towards areas with higher metal concentrations in the soil (Haines 2002; Whiting et al. 2000).

• Overexpression of metal transporters in roots (Lasat et al. 1996).

• Efficient, long-distance transport of metals in the form of stable complexes bound either to free histidine, nicotinamine, organic acids or S-ligands (Krämer et al. 1996; Küpper et al. 2004).

• Accumulation of metals away from the photosynthetically active tissues within the leaves, preferentially using epidermal and vascular tissues, and, to a lesser extent, palisade and spongy mesophyll (Bhatia et al. 2004; Cosio et al. 2005 Vogel-Mikus et al. 2008 a, b; Wojcik et al. 2005).

• High-capacity detoxification mechanisms, such as binding to organic acids, amino acids, phytochelatins and metallothioneins, and sequestration in the vacuoles that acts as a central storage for ions, thus maintaining low free metal concentrations in the symplast (Briat and Lebrun 1999; Clemens et al. 2002; Salt and Krämer 2000).

• Limited metal phloem loading and transport from the leaves to the seeds (Lasat et al. 1998; Vogel-Mikus et al. 2007; Wojcik et al. 2005).

A number of the hyperaccumulating plants belong to the Brassicaceae family, including the Alyssum, Thlaspi, Arabidopsis species and Brassica juncea (Reeves and Baker 2000). Among these, many studies that have investigated this metal hyperaccumulation have been carried out on the Zn/Cd/Ni hyperaccumulator

Thlaspi caerulescens J. & C. Presl, which has therefore been proposed as a model metal-hyperaccumulating plant species (Assungao et al. 2003).

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