Overexpression of C4PEPC caused O2-insensitive photosynthesis in transgenic rice plants

We have introduced an intact maize gene for phosphoenolpyruvate carboxylase (PEPC) into C3 plants, rice (Oryza saliva cv. Kitaake). Most transgenic rice plants showed high-level expression of the maize gene. PEPC was two to three times more active in the leaves of some transgenic plants than in maize leaves. In transformants, the sensitivity of photosynthesis to O-2 inhibition was reduced with the increased activity of the maize C4 PEPC. However, the alleviation of O-2 inhibition was not due to an increase in the partial direct fixation of atmospheric CO2 via the enhanced maize PEPC, but rather due to the reduced stimulation of photosynthesis with a subatmospheric O-2 level. Pi feeding to the leaves restored CO2 assimilation rate under the subatmospheric O-2 condition, and consequently, the O-2 inhibition in the transformants increased to a level comparable to that of the non-transformants. These results suggested that the O-2-insensitive photosynthesis in the PEPC transformants was caused by a Pi limitation of photosynthesis. The activities of two key enzymes for sucrose synthesis, SPS and FBPase, and the sucrose and starch content were reduced in the leaves of the transformants. On the other hand, the dark respiration rate and the malate content in the leaves increased in the transformants. These results indicated that enhanced PEPC activity led to a decrease in the availability of Pi in chloroplast via a reduction of the activities of the key enzymes responsible for Pi recycling and it also caused an increased consumption of the substrate (triose-phosphate) in respiration, but not in sucrose biosynthesis. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.