GENETIC MANIPULATION OF KEY PHOTOSYNTHETIC ENZYMES IN THE C-4 PLANT FLAVERIA-BIDENTIS

The NADP-malic enzyme type C-4 dicot Flaveria bidentis (L.) Kuntze was transformed with antisense and cosense gene constructs that resulted in specific decreases in single photosynthetic enzymes. The enzymes ta rgeted were ribulose-1,5-bisphosphate carboxylase/oxygenase [EC 4.1.1. 39] (Rubisco), pyruvate, Pi dikinase [EC 2.7.9.1] (PPDK) and NADP mala te dehydrogenase [EC 1.1.1.82] (NADP-MDH). These enzymes were chosen a s targets because they have low activity compared to photosynthetic ra tes (Rubisco), are subject to complex covalent regulation (NADP-MDH), or both (PPDK). T1 progeny of a number of lines of these transformants were examined for the effects of these gene constructs on enzyme leve ls and photosynthetic performance. Rubisco antisense transformants exp ressing between 15 and 100% of wild-type enzyme activity were obtained . Pyruvate, Pi dikinase antisense lines were obtained with 40-100% wil d-type levels. NADP malate dehydrogenase sense constructs caused a co- suppression of enzyme activity with some lines containing less than 2% of wild-type activity. Under saturating illumination, the control coe fficients for these enzymes were determined to be up to 0.7 for Rubisc o, 0.2-0.3 for PPDK and effectively zero for NADP-MDH. The implication s of these observations for the regulation of photosynthetic flux and metabolism in C-4 plants and the role of regulation by covalent modifi cation are discussed.