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 mu M CuSO4 (shoot growth: -11%), whereas maximum up-regulation was obtained at 100 mu M CuSO4 (shoot growth: -60%). Unexpectedly, metallothionein 2 (MT2) mRNA was decreased in response to the CuSO4 treatments. CdSO4 at a concentration of 50 mu M caused a 72% reduction in shoot growth without affecting the amounts of gamma-ECS- and MT2 mRNAs, ZnSO4 at a concentration of 500 mu M 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. (C) 1997 Federation of European Biochemical Societies.