Solanum tuberosum double transgenic expressing phosphoenolpyruvate carboxylase and NADP-malic enzyme display reduced electron requirement for CO 2 fixation

Abstract

The cDNA of the NADP-dependent malic enzyme gene Me2 from the C 3 plant Flaveria pringlei was used for expression in Escherichia coli and Solanum tuberosum. A chimeric GST-Me2 gene complemented a malic enzyme deficient E. coli mutant. Two potato lines were transformed with Me2-cDNA constructs, one line already overexpressing the phospho enolpyruvate carboxylase gene ( ppc) from Corynebacterium glutamicum. Both genes were under the control of the constitutive 35S CaMV promoter. Increased levels of malic enzyme (ME) were found in chloroplasts of transformants. Western blot analysis indicated that the ME transit sequence was cleaved. Expression of both genes led to a significantly reduced electron requirement for apparent CO 2 assimilation (e/A) at higher temperature. At low temperatures (15°C) 11 electrons per CO 2 assimilated (e/A) were measured in controls, single transformants ( ppc or Me2) and double transformants ( ppc and Me2). However, when leaf temperature was raised to 36°C electron requirement of the double transformants (15 e/A) was 65% of controls or single transformants (23 e/A). Thus, the temperature dependent increase in electron requirement was reduced in the double transformants suggesting a suppression in the oxygenation reaction of Rubisco and with it presumably in the rate of photorespiratory CO 2 release which is more marked at high light and high temperatures.