Genetic improvement of heavy metal tolerance in plants by transfer of the yeast metallothionein gene (CUP1)

Cauliflower (Brassica oleracea var. botrytis) tolerates treatment with 25 µM CdCl2 for eight days, but is killed by that with a 50 µM concentration. However, even 15 µM CdCl2 is toxic in the presence of 1 mM L-buthionine sulfoximine (BSO), suggesting the presence of a Cd-inducible phytochelatin and its involvement in Cd-tolerance in cauliflower. To develop heavy metal-tolerant transgenic plants, we ligated the structural gene of yeast metallothionein gene (CUP1) downstream of CaMV35S promoter and introduced the fused gene into cauliflower. A Cd-tolerant transgenic cauliflower was selected, which grew well in the presence of 400 µM or less Cd, whereas the non-transformed cauliflower tolerated only up to 25 µM Cd. The transgenic cauliflower accumulated more Cd, especially in the upper leaves, than the non-transformed plant.