Carbon metabolism in transgenic rice plants that express phosphoenolpyruvate carboxylase and/or phosphoenolpyruvate carboxykinase

Transgenic rice plants expressing phosphoenolpyruvate carboxykinase (PCK) in the chloroplasts showed C4-like carbon flow [S. Suzuki, N. Murai, J.N. Burnell, M. Arai, Changes in photosynthetic carbon flow in transgenic rice plants that express C4-type phosphoenolpyruvate carboxykinase from Urochloa panicoides, Plant Physiol. 124 (2000) 163–172]. Here we report on the carbon metabolism of newly generated transgenic rice plants expressing either phosphoenolpyruvate carboxylase (PEPC) or both PCK and PEPC. The incorporation of 14CO2 into 4C compounds in PEPC transgenic rice was two- to three-fold that in control plants, and the incorporation in PEPC/PCK-transgenic rice was three-fold that in PEPC transgenic rice, but the enhanced level in PEPC/PCK-transgenic rice was almost the same as that in PCK-transgenic rice. Higher rates of incorporation of 14C into photoassimilates (sucrose and 3-PGA plus sugar phosphates) from l-[4-14C]malate were observed in PEPC, PCK and PEPC/PCK-transgenic rice, but the incorporation rates in PEPC transgenic rice were reduced to those found in control plants by pretreatment with NaN3. Levels of PEP, malate and aspartate in PEPC/PCK-transgenic rice were similar to those in PCK-transgenic rice but different in PEPC transgenic rice. PEPC/PCK-transgenic rice showed a low chlorophyll concentration and swollen thylakoid membranes although PCK and PEPC transgenic rice showed normal phenotypes. These results suggest that the overexpression of PEPC enhances the anaplerotic pathway rather than the initial carbon fixation of the C4-like photosynthetic pathway, and that elevated PEPC activity in combination with PCK activity contributes little to C4-like carbon flow.