C₄ rice engineering, beyond installing a C₄ cycle

C₄ photosynthesis in higher plants is carried out by two distinct cell types: mesophyll cells and bundle sheath cells, as a result highly concentrated carbon dioxide is released surrounding RuBisCo in chloroplasts of bundle sheath cells and the photosynthetic efficiency is significantly higher than that of C₃ plants. The evolution of the dual-cell C₄ cycle involved complex modifications to leaf anatomy and cell ultra-structures. These include an increase in leaf venation, the formation of Kranz anatomy, changes in chloroplast morphology in bundle sheath cells, and increases in the density of plasmodesmata at interfaces between the bundle sheath and mesophyll cells. It is predicted that cereals will be in severe worldwide shortage at the mid-term of this century. Rice is a staple food that feeds more than half of the world's population. If rice can be engineered to perform C₄ photosynthesis, it is estimated that rice yield will be increased by at least 50% due to enhanced photosynthesis. Thus, the Second Green Revolution has been launched on this principle by genetically installing C₄ photosynthesis into C₃ crops. The studies on molecular mechanisms underlying the changes in leaf morphoanatomy involved in C₄ photosynthesis have made great progress in recent years. As there are plenty of reviews discussing the installment of the C₄ cycle, we focus on the current progress and challenges posed to the research regarding leaf anatomy and cell ultra-structure modifications made towards the development of C₄ rice.