Significant accumulation of C-4-specific pyruvate,orthophosphate dikinase in a C-3 plant, rice

The C-4-Pdk gene encoding the C-4 enzyme pyruvate, orthophosphate dikinase (PPDK) of maize (Zea mays cv Golden Cross Bantam) was introduced into the C -3 plant, rice (Oryza sativa cv Kitaake). When the intact maize C-4-Pdk gon e, containing its own promoter and terminator sequences and exon/intron str ucture, was introduced, the PPDK activity in the leaves of some transgenic lines was greatly increased, in one line reaching 40-fold over that of wild -type plants. In a homozygous line, the PPDK protein accounted for 35% of t otal leaf-soluble protein or 160% of total leaf nitrogen. In contrast, intr oduction of a chimeric gene containing the full-length cDNA of the maize PP DK fused to the maize C-4-Pdk promoter or the rice Cab promoter only increa sed PPDK activity and protein level slightly. These observations suggest th at the intron(s) or the terminator sequence of the maize gene, or a combina tion of both, is necessary for high-level expression. In maize and transgen ic rice plants carrying the intact maize gene, the level of transcript in t he leaves per copy of the maize C-4-Pdk gone was comparable, and the maize gene was expressed in a similar organ-specific manner. These results sugges t that the maize C-4-Pdk gene behaves in a quantitatively and qualitatively similar way in maize and transgenic rice plants. The activity of the maize PPDK protein expressed in rice leaves was light/dark regulated as it is in maize. This is the first reported evidence for the presence of an endogeno us PPDK regulatory protein in a C-3 plant.