Evolution of nickel hyper-accumulation in the plant genus Stackhousia (Celastraceae)
More than 320 species of flowering plants, representing a total of 15 families, are known to accumulate the element nickel (Ni) in their tissues to levels exceeding 0.1% dry weight (Reeves 2003), concentrations that would be toxic to most organisms. This intriguing phenomenon, known as Ni hyper-accumulation, represents a unique and extreme adaptation to the Ni-rich soils in which these plants typically grow. Hyper-accumulators of Ni are known from Ni-rich soils in Australasia, Southeast Asia, Africa, Europe, and North America (Reeves 2003). Recently, these Ni hyper-accumulators have become the subject of intensive systematic (reviewed in Reeves 2003), ecological (Martens and Boyd 1994), physiological (Bhatia et al. 2005a, Freeman et al. 2005), and genetic (reviewed in Pollard et al. 2002) study. This research has been motivated by 1) an interest in understanding the function and origin of an extreme plant-soil interaction, and 2) by the potential use of Ni hyper-accumulating plant species in the detoxification (phytoremediation) of Ni contaminated soils (Reeves 2003). Given the great phylogenetic diversity and geographic spread of Ni hyper-accumulating plants, the current interest in understanding the biology of the Ni hyper-accumulating strategy, and the potential for phytoremediation, it is important to gain a detailed understanding of how Ni hyper-accumulation has evolved in specific groups of plants. The Australasian plant genus Stackhousia contains one Ni hyper-accumulating species, Stackhousia tryonii, which is endemic to Ni-rich soils in central Queensland, Australia (Batianoff et al. 1990). The remaining 13 species of Stackhousia are not known to hyper-accumulate Ni, although there is some evidence that other species are capable of accumulating Ni to unusual levels (Batianoff et al. 1990). Although Stackhousia has never been the subject of phylogenetic investigation, there is a wealth of data available on the physiology of the Ni hyper-accumulator S. tryonii (Bhatia et al. 2004, 2005a, 2005b). My research on Stackhousia combines molecular phylogenetic data with tissue chemistry and soil data from wild plants in order to reveal how Ni hyper-accumulation has evolved in the genus.
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