*3.4.1.1. The cell wall*

The cell wall and membrane interface could be a site of metal tolerance since a signifi‐ cant amount of metals has been reported to be accumulated there. Divalent and trivalent metal cations can bind plant cell walls because of the presence of functional groups such as –COOH, -OH and –SH. Pectins are polymers that contain carboxyl groups which enable the binding of divalent and trivalent heavy metals ions. In enriched heavy metal environ‐ ments, some plants will increase the capacity of their cell wall to bind metals by increas‐ ing polysaccharides, such as pectins (Colzi *et al.,* 2011; Pelloux *et al.,* 2007). Konno *et al.* (2010; 2005) showed that the pectin in root cell walls was important in binding Cu in the fern, *Lygodium japonicum,* and in the moss, *Scopelophila cataractae*. The cell wall of *Minuar‐ tia verna sp. hercynica* growing on heavy metal contaminated medieval mine dumps has been found to have high concentrations of Fe, Cu, Zn and Pb (Solanki and Dhankhar 2011; Neumann *et al.,* 1997). On the other hand, Colzi *et al.* (2012) found that a copper tolerant *Silene paradoxa* population restricted the accumulation of Cu in roots, when exposed to high Cu, by decreasing their pectin concentration in the cell wall and increasing pectin methyl‐ ation thus preventing the binding of Cu.
