Accumulation of the components of cyclic electron flow around photosystem I in C4 plants, with respect to the requirements for ATP

By concentrating CO2, C-4 photosynthesis can suppress photorespiration and achieve high photosynthetic efficiency, especially under conditions of high light, high temperature, and drought. To concentrate CO2, extra ATP is required, which would also require a change in photosynthetic electron transport in C-4 photosynthesis from that in C-3 photosynthesis. Several analyses have shown that the accumulation of the components of cyclic electron flow (CEF) around photosystem I, which generates the proton gradient across thylakoid membranes (Delta pH) and functions in ATP production without producing NADPH, is increased in various NAD-malic enzyme and NADP-malic enzyme C-4 plants, suggesting that CEF may be enhanced to satisfy the increased need for ATP in C-4 photosynthesis. However, in C-4 plants, the accumulation patterns of the components of two partially redundant pathways of CEF, NAD(P)H dehydrogenase-like complex and PROTON GRADIENT REGULATION5-PGR5-like1 complex, are not identical, suggesting that these pathways may play different roles in C-4 photosynthesis. Accompanying the increase in the amount of NDH, the expression of some genes which encode proteins involved in the assembly of NDH is also increased at the mRNA level in various C-4 plants, suggesting that this increase is needed to increase the accumulation of NDH. To better understand the relation between CEF and C-4 photosynthesis, a reverse genetic approach to generate C-4 transformants with respect to CEF will be necessary.