ISOLATION OF A VICIA-FABA METALLOTHIONEIN-LIKE GENE - EXPRESSION IN FOLIAR TRICHOMES

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 h ave been isolated in a number of plants although their function remain s 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 expr essed in leaves but not in the most common cell type, the mesophyll ce ll. MT contained two regions with abundant cysteines and sequence comp arison found that MT had greatest homology to MT-like subtype 2 from o ther plant species. Northern blot analysis demonstrated that MT was ex pressed 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 lea f, in situ hybridization studies demonstrated striking cell specificit y with MT expression confined predominantly to trichomes. Possible exp lanations for the pronounced expression of MT in leaf trichomes are di scussed.