In seedlings of the heavy metal accumulator Brassica juncea Cu²⁺ differentially affects transcript amounts for γ‐glutamylcysteine synthetase (γ‐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.