The Five AhMTP1 Zinc Transporters Undergo Different Evolutionary Fates towards Adaptive Evolution to Zinc Tolerance in Arabidopsis halleri

Abstract

Gene duplication is a major mechanism facilitating adaptation to changing environments. From recent genomic analyses, the acquisition of zinc hypertolerance and hyperaccumulation characters discriminating Arabidopsis halleri from its zinc sensitive/non-accumulator closest relatives Arabidopsis lyrata and Arabidopsis thaliana was proposed to rely on duplication of genes controlling zinc transport or zinc tolerance. Metal Tolerance Protein 1 (MTP1) is one of these genes. It encodes a Zn2+/H+ antiporter involved in cytoplasmic zinc detoxification and thus in zinc tolerance. MTP1 was proposed to be triplicated in A. halleri, while it is present in single copy in A. thaliana and A. lyrata. Two of the three AhMTP1 paralogues were shown to co-segregate with zinc tolerance in a BC1 progeny from a cross between A. halleri and A. lyrata. In this work, the MTP1 family was characterized at both the genomic and functional levels in A. halleri. Five MTP1 paralogues were found to be present in A. halleri, AhMTP1-A1, -A2, -B, -C, and -D. Interestingly, one of the two newly identified AhMTP1 paralogues was not fixed at least in one A. halleri population. All MTP1s were expressed, but transcript accumulation of the paralogues co-segregating with zinc tolerance in the A. halleri X A. lyrata BC1 progeny was markedly higher than that of the other paralogues. All MTP1s displayed the ability to functionally complement a Saccharomyces cerevisiæ zinc hypersensitive mutant. However, the paralogue showing the least complementation of the yeast mutant phenotype was one of the paralogues co-segregating with zinc tolerance. From our results, the hypothesis that pentaplication of MTP1 could be a major basis of the zinc tolerance character in A. halleri is strongly counter-balanced by the fact that members of the MTP1 family are likely to experience different evolutionary fates, some of which not concurring to increase zinc tolerance.

CATION EXCHANGER1 Cosegregates with Cadmium Tolerance in the Metal Hyperaccumulator Arabidopsis halleri and Plays a Role in Limiting Oxidative Stress in Arabidopsis Spp.

“…Indeed, some of the genes involved in metal tolerance (e.g. HMA4 and MTP1) have multiple copies in the A. halleri genome (Talke et al, 2006;Hanikenne et al, 2008;Shazad et al, 2013). Our data suggest that AhCAX1 is a single-copy gene in A. halleri and that differential expression between species can be attributed to cis-or transregulation.…”

Elevated Nicotianamine Levels in Arabidopsis halleri Roots Play a Key Role in Zinc Hyperaccumulation

“…Third, higher activity of certain metal homeostasis genes in A. halleri appears to be at least partly due to gene copy number expansion. Examples are three tandem HMA4 copies (Hanikenne et al, 2008) and a total of up to five MTP1 paralogs present either in tandem or dispersed in the genome (Shahzad et al, 2010). Not all of these MTP1 paralogs contribute to higher transcript abundance in A. halleri (Drä ger et al, 2004;Shahzad et al, 2010) and thus have not all been detected by QTL analysis.…”

Functional analysis of metal tolerance proteins isolated from Zn/Cd hyperaccumulating ecotype and non-hyperaccumulating ecotype of Sedum alfredii Hance

“…MTP1 transporters from different species of plants have been found to localize to the tonoplast and sequestrate Zn 2+ into vacuoles; over-expression of MTP1 gene can enhance zinc tolerance [12,[16][17][18][19]. Recently, it has been found that there are five duplications of MTP1 gene in Zn/Cd hyperaccumulator A. halleri and this could be a major basis of the zinc tolerance [20]. in leaves up to a maximum of 9 g kg À1 DW and also can accumulate Zn 2+ in shoots up to 2% of dry weight [21][22][23].…”