Lessons from engineering a single-cell C4 photosynthetic pathway into rice

The transfer of C-4 plant traits into C-3 plants has long been a strategy for improving the photosynthetic performance of C-3 plants. The introduction of a pathway mimicking the C-4 photosynthetic pathway into the mesophyll cells of C-3 plants was only a realistic approach when transgenic technology was sufficiently well developed and widely adopted. Here an attempt to introduce a single-cell C-4-like pathway in which CO2 capture and release occur in the mesophyll cell, such as the one found in the aquatic plant Hydrilla verticillata (L.f.) Royle, into rice (Oryza sativa L.) is described. Four enzymes involved in this pathway were successfully overproduced in the transgenic rice leaves, and 12 different sets of transgenic rice that overproduce these enzymes independently or in combination were produced and analysed. Although none of these transformants has yet shown dramatic improvements in photosynthesis, these studies nonetheless have important implications for the evolution of C-4 photosynthetic genes and their metabolic regulation, and have shed light on the unique aspects of rice physiology and metabolism. This article summarizes the lessons learned during these attempts to engineer single-cell C-4 rice.