Improving Zinc Content and Antioxidant Activity in Transgenic Tomato Plants with Expression of Mouse Metallothionein-I by mt-I Gene

Metallothioneins (MTs), as a family of low-molecular-weight, cysteine-rich, and metal-binding proteins, show potential for utilization in functional food. Tomato plants were transformed with gene constructs that contained mt-I encoding the mouse MT-I, similar in sense orientation with the constitutively active double 35S promoter from cauliflower mosaic virus. Three independent transformants, which had copies of the gene in their genomes, were obtained. In these transgenic lines, high-level expression of MT-I, high zinc content, and some antioxidant enzyme activities were detected in leaves. The average zinc content in transgenic tomato leaves was 32.7 mg/100 g FW, which about 1.6 times higher than that in wild-type. The superoxide dismutase activity was also higher (68.6, 66.9, and 66.1 U/g FW in the three transformants) than that in wild-type (57.4 U/g FW). In particular, the levels of superoxide free radical scanvenging in the three transformants were 14.2%, 14.6%, and 13.7%, respectively, which about 1.5 times higher than that in control (5.6%). Transgenic MT tomato may potentially be used as an antioxidant and for zinc supplementation.