CADMIUM PARTITIONING IN TRANSGENIC TOBACCO PLANTS EXPRESSING A MAMMALIAN METALLOTHIONEIN GENE

Among the plant species cultivated for human consumption, tobacco is o ne that accumulates cadmium to a significant degree. In order to reduc e the Cd levels in tobacco leaves, we have introduced into the tobacco genome a gene encoding a mammalian metallothionein, since these low-m olecular-weight cysteine-rich proteins fix Cd and other heavy metals. Here we describe the Cd accumulation characteristics observed during t wo years of greenhouse tests and one year of field trial of tobacco pl ants expressing a metallothionein gene. in-all three tests, leaf Cd le vels were markedly decreased. For instance, in the field trial, Cd lev els in the leaf lamina tissue of the transformed lint: were decreased by 73% compared to controls. The decrease in leaf Cd was correlated wi th an increase in Cd in-the roots and stems. The plants had normal gro wth characteristics, and the distribution of other ions was not affect ed by the expression of the metallothionein gene. Even though the tran sformation/expression vector used here can cause frequent post-transcr iptional gene silencing, comparison of hemizygous and homozygous indiv iduals of the plant line expressing the metallothionein gene gave litt le evidence for silencing.