In seedlings of the heavy metal accumulator Brassica juncea Cu2+ differentially affects transcript amounts for gamma-glutamylcysteine synthetase (gamma-ECS) and metallothionein (MT2).

Glutathione (GSH) is the precursor of the phytochelatins (PC), which in plants and fungi are involved in heavy metal sequestration. The regulatory enzyme gamma-glutamylcysteine synthetase (gamma-ECS) catalyzes the first step in GSH biosynthesis. For the heavy metal accumulator Brassica juncea L. a partial gamma-ECS cDNA was cloned by RT-PCR. Treatment of suspension-cultured dark grown seedlings with micromolar concentrations of CuSO4 resulted in a strong increase of gamma-ECS mRNA in roots and shoots, concomitant with an increase of GSH and phytochelatins. A significant up-regulation of gamma-ECS mRNA was observed at 25 microM CuSO4 (shoot growth: -11%), whereas maximum up-regulation was obtained at 100 microM CuSO4 (shoot growth: -60%). Unexpectedly, metallothionein 2 (MT2) mRNA was decreased in response to the CuSO4 treatments. CdSO4 at a concentration of 50 microM caused a 72% reduction in shoot growth without affecting the amounts of gamma-ECS- and MT2 mRNAs. ZnSO4 at a concentration of 500 microM did not reduce growth but induced transient increases of gamma-ECS- and MT2 mRNAs. The implications of the results with respect to differential regulation of gamma-ECS and MT2 during heavy metal exposure are discussed.