Accumulation of metallothionein transcripts in response to iron, copper and zinc: Metallothionein and metal-chelate reductase

It has been proposed that plant metallothionein (MT) sequesters excess copper, and possibly zinc, thereby preventing adverse metal-protein interactions. These metals can accumulate either gratuitously in response to other nutritional deficiencies or in plants grown in either copper-or zinc-enriched medium. Data are presented which confirm that in pea roots grown in low available iron there is increased (i) copper accumulation, (ii) MT transcript abundance, (iii) ferric-chelate reductase activity and (iv) cupric-chelate reductase activity. It is also shown that in roots grown in iron supplemented medium MT transcripts accumulate in response to elevated exogenous zinc. However, contrary to expectations, depletion of exogenous copper below normal micronutrient levels also confers an increase in the abundance of MT transcripts. The hypothesis that the products of plant metallothionein genes could act as copper chaperones is discussed.