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

Ishikawa, Noriko, Takabayashi, Atsushi, Sato, Fumihiko, Endo, Tsuyoshi
Photosynthesis research 2016 v.129 no.3 pp. 261-277
C3 photosynthesis, C4 photosynthesis, C4 plants, NADP (coenzyme), adenosine triphosphate, carbon dioxide, drought, electron transfer, gene expression, genes, light intensity, messenger RNA, photorespiration, photosystem I, proteins, temperature, thylakoids
By concentrating CO₂, C₄ 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 C₄ photosynthesis from that in C₃ 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 (ΔpH) and functions in ATP production without producing NADPH, is increased in various NAD-malic enzyme and NADP-malic enzyme C₄ plants, suggesting that CEF may be enhanced to satisfy the increased need for ATP in C₄ photosynthesis. However, in C₄ 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 C₄ 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 C₄ plants, suggesting that this increase is needed to increase the accumulation of NDH. To better understand the relation between CEF and C₄ photosynthesis, a reverse genetic approach to generate C₄ transformants with respect to CEF will be necessary.