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Noccaea caerulescens  ( Nc ) exhibits a very high constitutive expression of the heavy metal transporting ATPase,  HMA4 , as compared to the non-hyperaccumulator  Arabidopsis thaliana  ( At ), due to copy number expansion and altered  cis -regulation. We screened a BAC library for  HMA4  and found that  HMA4  is triplicated in the genome of a  N. caerulescens  accession from a former Zn mine near La Calamine (LC), Belgium. We amplified multiple  HMA4  promoter sequences from three calamine  N. caerulescens  accessions, and expressed  AtHMA4  and different  NcHMA4  cDNAs under  At  and  Nc   HMA4  promoters in the  A. thaliana  (Col)  hma2hma4  double mutant. Transgenic lines expressing  HMA4  under the  At  promoter were always fully complemented for root-to-shoot Zn translocation and developed normally at a 2-μM Zn supply, whereas the lines expressing  HMA4  under  Nc  promoters usually showed only slightly enhanced root to shoot Zn translocation rates in comparison with the double mutant, probably owing to ectopic expression in the roots, respectively. When expression of the Zn deficiency responsive marker gene  ZIP4  was tested, the transgenic lines expressing  AtHMA4  under an  NcHMA4-1-LC  promoter showed on average a 7-fold higher expression in the leaves, in comparison with the double  hma2hma4  mutant, showing that this construct aggravated, rather than alleviated the severity of foliar Zn deficiency in the mutant, possible owing to expression in the leaf mesophyll.

frontiers in plant science

Expression of HMA4 cDNAs of the zinc hyperaccumulator Noccaea caerulescens from endogenous NcHMA4 promoters does not complement the zinc-deficiency phenotype of the Arabidopsis thaliana hma2hma4 double mutant