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

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.