Characterization of the Arabidopsis metallothionein gene family:: tissue-specific expression and induction during senescence and in response to copper

Expression and regulation of Arabidopsis metallothionein (MT) genes were investigated to examine the functions of MTs in plants. To examine the tissue-specific expression of MT genes, GUS reporter gene activity driven by promoters of MT1a , MT2a , MT2b and MT3 was analysed in transgenic plants. MT1a and MT2b are expressed in the phloem of all organs and are copper (Cu)-inducible; MT2a and MT3 , by contrast, are expressed predominantly in mesophyll cells and are also induced by Cu in young leaves and at root tips. Expression of MT genes is highly induced by Cu in trichomes and increases during senescence. Expression of MT4 genes is restricted to seeds. We propose that plant MTs have distinct functions in heavy metal homeostasis, especially for Cu: MT1a and MT2b are involved in the distribution of Cu via the phloem, while MT2a and MT3 chaperone excess metals in mesophyll cells and root tips. These functional capabilities may allow MTs to play a role in mobilization of metal ions from senescing leaves and the sequestration of excess metal ions in trichomes.