Metal Hyperaccumulators: Mechanisms of Hyperaccumulation and Metal Tolerance

Several hyperaccumulator plant species especially the species in Brassicaceae have been extensively investigated for their metal accumulation and detoxification. For example, Arabidopsis halleri, Noccaea caerulescens (formerly Thlaspi caerulescens), and Brassica nigra have enhanced our understanding of the physiological, molecular, and genetic basis of metal hyperaccumulation and associated hypertolerance. A number of regulatory mechanisms have been developed by metal hyperaccumulator plants for their survival in metal-polluted environment. In the last decade, with the development of advanced technologies most of the information about heavy metal stress in plants has been obtained through genome sequence, transcriptome, metabolome, and proteome studies. For example, through such techniques, it has been possible to identify numerous putative genes involved in the response to metal stress. A number of membrane transporter gene families have been found in accumulator plants such as ZIP (ZRT, IRT-like protein), NRAMP (natural resistance-associated macrophage protein), YSL (Yellow-stripe-like transporter), NAS (nicotinamine synthase), SAMS (S-adenosyl-methionine synthetase), FER (ferritin Fe (III) binding), CDF (cation diffusion facilitator), HMA (heavy metal ATPase), and IREG (iron-regulated transporter) families which are predicted to be involved in the cellular uptake and transport in plants. HMAs are particularly interesting and according to many recent studies they have been shown a key player in the metal hyperaccumulation. In this regard, we have analyzed the gene expression data of model crop plants in Brassicaceae family by searching several databases available online. The publicly available online resources for these plants from websites such as http://www.ncbi.nih.gov, http://www.tigr.org, http://www.brassica.info, and related sites were searched to collect nucleotide sequences that encode heavy metal ATPases and transporter protein homologues. The criterion observed in this research is that the sequences of different metal-induced genes have functional and evolutionary similarities among species. Our hypothesis is that the functionally related sequences of the genes from different species or organisms will be having conserved pattern or motif which will be possibly related to hyperaccumulation of heavy metals. Here, I will overview these findings and highlight their contribution to the field of plant metal homeostasis, and will discuss the emerging avenues of -omics technologies and their impact in understanding the mechanisms of metal accumulation and tolerance.