Chapter 18 Hurdles to Engineering Greater Photosynthetic Rates in Crop Plants: C4 Rice

It is now 20 years since a suggestion was first made of introducing a C4 photosynthetic pathway into rice plants to increase rice productivity. With the amount of arable land decreasing, the human population increasing and rice productivity plateauing there is an urgent need to significantly increase rice productivity. One way of increasing rice productivity is by introducing the C4 photosynthetic pathway into rice by genetic manipulation. Over the past decade, a number of attempts have been made to modify the photosynthetic pathways present in C3 plants by introducing genes encoding enzymes associated with C4 photosynthesis. These efforts have increased the expression of specific enzymes such as pyruvate, orthophosphate dikinase, phosphoenolpyruvate carboxylase and NADP-malic enzyme in a range of plants, notably rice, tobacco, and potato. Claims and counter claims have been made regarding the success of attempts to construct a C4 rice plant, but to date, no functional C4 pathway has been engineered into C3 species. Novel approaches are now required to introduce a functional C4 photosynthetic pathway into rice and this may require a change to the molecular architecture of photosynthetic tissue (introduction of Kranz anatomy), the coordinated co-expression of enzyme regulatory mechanisms and the expression of specific transporters in rice leaves.