Exploring avenues to improve light use efficiency of C4 CO2-concentrating mechanisms in C3 background: A proof of concept by introducing various single-cell C4 pathways into rice (Oryza sativa spp. japonica)

The C3 crop rice, one of the major food crops, needs to increase its production by 50% by 2050. C4 crops yield more than C3 crops because of their higher photosynthetic rates, due to the operation of a CO2 concentrating mechanism (CCM) that ensures a high CO2 concentration around Rubisco, thus minimizing RuBP oxygenation and photorespiratory loss. Introducing the C4 pathway into rice is required to improve its yields. The CCM requires ATP to operate, and the CCM cycle in current crop-C4 type (like maize) accounts for >40% of the total ATP required for CO2 assimilation; in contrast, the non-domesticated, PEP-CK C4 type theoretically requires much less ATP and therefore is preferred to achieve high rice yields. However, the observed light-use efficiency is not higher in PEP-CK than in crop-C4 plants. This is probably because there is more CO2 leakage in PEP-CK than in crop-C4 plants as a result of differences in underlying biochemistry and associated Kranz anatomical leaf structures. To overcome this potential confounding effect of higher leakage, I aim to introduce different C4 pathways of biochemistry (including crop-C4 and PEP-CK types) into mesophyll cells of rice and evaluate the engineered single-cell C4 systems by comparing their phenotypes for light-use efficiency and other physiological characteristics. Creating single-cell C4 rice will also provide a lead for more ambitious future projects, to engineer PEP-CK C4 rice with the di-cellular Kranz structure that is predicted to increase rice yield by >50%.