Alyssum nickel hyperaccumulation

Serpentine soils have high levels of heavy metals (Ni, Co, Cr), and low levels of important plant nutrients (P, Ca, N). Because of these inhospitable conditions, serpentine soils support a specialized flora, including such plant species as the nickel hyperaccumulator Alyssum bertolonii. In a study that examined bacterial communities at various distances from A. bertolonii roots in different serpentine areas, it was found that the bacterial communities possessed high genetic diversity... 

Mengoni, A., Grassi, E., Barzanti, R., Biondi, E. G., Gonnelli, C., Kim, C. K., and Bazzicalupo, M. (2004). Genetic diversity of bacterial communities of serpentine soil and of rhizosphere of the nickel-hyperaccumulator plant Alyssum bertolonii. Microb. Ecol. 48, 209-217. 

Nickel-tolerant or accumulator species of plants are likely to be found only on nickel-rich soils. Hyperaccumulator species usually grow on relatively infertile nickel-rich serpentine soils and contain more than 10,000.0 mg Ni/kg DW. Leaves from some genera of nickel hyperaccumulator plants, including Alyssum, Homalium, and Hybanthus, growing on soils derived from volcanic rocks, which are rich in ifickel, accumulate nickel to concentrations of 120,000.0 mg kg DW. Nickel is bound as a citrate complex in hyperaccumulator plants from New Caledonia however, nickel accumulator plants from other locations do not contain unusually high levels of citrate, and nickel is not present as a citrate complex but as a carboxylic acid complex. 

Kramer U, Grime GW, Smith JAC, Hawes CR, Baker AJM. Micro-PIXE as a technique for studying nickel localization in leaves of the hyperaccumulator plant Alyssum lesbiacum. Nucl Instr Methods B 1997 130 346-350. 

The nickel content of most natural vegetation is 0.05-5 ppm on a dry weight basis (NAS 1975). Near source areas, nickel on plants may be elevated because of direct foliar deposition. Some species of plants have the ability to hyperaccumulate nickel (Brooks 1980). The concentration in the leaves of Alyssum bertolonii contained 120 ppm nickel (12%). These plants are thought to produce organic ligands that complex with nickel. 

Fig. 4-1. Histogram showing nickel concentrations in the gznm Alyssum. There are two distinct populations including hyperaccumulators with > 1000 pg g l Ni in dried tissue.

Hyperaccumulation of Ni appears to be a strategy where genera such as Alyssum have been able to evolve a physiological tolerance to phytotoxic nickel-rich soils, and avoid competition from other species by flourishing in environments so hostile, that often Alyssum is the only coloniser of the area. 

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