C2 rice outcompetes C3 and C4 rice

There were many great articles concerning cross-disciplinary research at the interface between plant biology, mathematics and computer science before the launch in silico Plants (isP). We are excited for isP to be home to these types of articles in the future.

Over 3.5 billion people depend on rice for survival and future production must increase to meet the demand of a growing world population in the face of climate change. Much research has been done to increase rice yields over the previous decades. One high-risk, high-reward approach has been undertaken by the C₄ Rice Project. They propose to improved photosynthetic efficiency in rice by engineering its current C₃-type photosynthetic machinery to include functional components of the C₄-type pathway. This pathway is beneficial in that it concentrates CO₂ near the site of Rubisco, thereby reducing the magnitude of energy-wasting photorespiration. Introduction of ‘C₄‘ traits into rice is predicted to increase photosynthetic efficiency by 50%, improve nitrogen use efficiency and double water use efficiency.

In their new paper, Bellasio and Farquhar use a leaf-level biochemical model to simulate the introduction of two types of carbon concentrating mechanisms, the ‘C₂ shuttle’ and the aforementioned C₄ complex, in rice. C₂ photosynthesis is a key intermediate step in the evolution of C₄ photosynthesis. The C₂ mechanism limits certain reactions of the photorespiratory cycle to the bundle sheath cells, thereby elevating CO₂ at the site of Rubisco. Their model was newly derived to allow a seamless transition between all photosynthetic types except CAM, and included electron transport, biochemical, stoichiometric, and stomatal submodels.

The authors found that when energy budgets were accounted for, C₄ photosynthesis was unfavorable at high CO₂ concentrations, low PPFD and low temperatures, thus “providing theoretical support for decades of ecophysiological observations.” Despite the relative operational simplicity, the engagement of a C₂ shuttle always increased assimilation rate, relative to C₃. The authors conclude that the development of C₂ rice should be pursued not C₄ due to the complexities and trade–offs of implementing a C₄ cycle.

The model used in this paper is coded in Excel and is freely available.