Cytokinin and the regulation of a tobacco metallothionein‐like gene during copper stress

Abstract

Higher plant metallothioneins are suspected of reducing metal‐induced oxidative stress and binding copper and zinc cofactor metals for delivery to important apometalloproteins. A metallothionein‐like gene (MT‐L2) cDNA clone pCkn16A1 (Accession U35225) was cloned from heat‐shock‐induced Nicotiana plumbaginifolia containing the heat‐shock‐inducible isopentenyl transferase (ipt) gene (HS‐ipt). Ipt expression in plants leads to enhanced cytokinin biosynthesis. In mature leaves of non‐transformed N. plumbaginifolia and Nicotiana tabacum, copper stress caused a significant loss of MT‐L2 mRNA transcripts. Under non‐stressed conditions, HS‐ipt induced (N. plumbaginifolia) or light‐induced (SSU‐ipt) (N. tabacum) plants had higher MT‐L2 transcript levels than non‐transformed or transformed (GUS) controls. Unlike control plants, copper stress did not reduce MT‐L2 mRNA levels in the cytokinin accumulating transgenic plants. Enhanced cytokinin production also led to lower lipid peroxidation compared with controls under non‐stressed and copper‐stressed conditions. Greater MT‐L2 transcript levels and protection against oxidative events prior to or during copper stress could contribute to the observed eight‐fold accumulation of copper in mature leaves of ipt expressing plants compared to non‐transformed plants. Expression of this tobacco MT‐L2 mRNA may be modulated directly by cytokinin or indirectly as a consequence of cytokinin‐mediated antioxidant activity.