Redesigning C4 rice from limited C4 photosynthesis

The photosynthetic characteristics of four transgenic rice lines overexpressing maize phosphoenolpyruvate carboxylase (PEPC; line PC), pyruvate, orthophosphate dikinase (PPDK; line PK), PEPC + PPDK (line CK), and NADP-malic enzyme (NADP-ME; line ME) were investigated using outdoor-grown plants. Relative to untransformed wild-type (WT) rice, PC transgenic rice exhibited high PEPC activity (a 25-fold increase) and enhanced activity of carbonic anhydrase (more than a two fold increase). The PC transgenic plants also showed a higher CO2 uptake rate and a carboxylation efficiency, and slightly reduced CO2 compensation point. Furthermore, PC transgenic rice produced 22% more grains than WT plants. Labeling with 14 CO2 for 20 s showed more 14C distributed to C4 primary photosynthate aspartate and feeding with exogenous C4 primary products such as oxaloacetate (OAA), malate (MA), or phosphoenolpyruvate (PEP) showed an increment of photosynthetic rate in PC transgenic rice, suggesting that a limited C4 cycle exists in leaves of transgenic rice. Introduction of the maize PEPC gene could activate or induce activities of the key enzymes scavenging active oxygen, such as superoxide dismutase (SOD) and peroxidase (POD). The line JAAS45 manifested higher photosynthetic rates and photochemical efficiency of PS II (Fv/Fm). The value of δ13C in PC transgenic rice was similar to that in untransformed rice, demonstrating that transgenic rice is still a C3 plant. How can we redesign C4 rice from the limited C4 features of photosynthesis reached currently? In future work, introduction of the PEPC gene from a CAM plant into C4-enzyme transgenic rice could carry out higher photosynthesis day and night. Simultaneously, the enhancement of endogenous ATP in PK transgenic rice through genetic engineering would increase its operation of the C4 cycle. Most importantly, recent advanced techniques such as laser capture microdissection enable us to to study the mechanisms of cellular differentiation, for example, of bundle sheath cells. From the above suggestions, these techniques might shed light on a new green revolution in rice breeding.