Isolation of a Vicia faba metallothionein-like gene: expression in foliar trichomes

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Abstract
Animal metallothioneins (MTs) are cysteine-rich, low-molecular-weight proteins that bind to heavy metals and are believed to play a role in their metabolism and detoxification. Genes encoding MT-like proteins have been isolated in a number of plants although their function remains to be elucidated. We describe the isolation and characterization of a bean cDNA encoding an MT-like protein. The bean gene, called MT, was isolated as a result of a differential screen for genes that are expressed in leaves but not in the most common cell type, the mesophyll cell. MT contained two regions with abundant cysteines and sequence comparison found that MT had greatest homology to MT-like subtype 2 from other plant species. Northern blot analysis demonstrated that MT was expressed in the leaf, stem and flower, at very low levels in roots and was not detectable in mesophyll protoplasts. MT transcript levels were not significantly affected by treatment with Cu, Zn or Cd. In the leaf, in situ hybridization studies demonstrated striking cell specificity with MT expression confined predominantly to trichomes. Possible explanations for the pronounced expression of MT in leaf trichomes are discussed.