1. Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma, Nara 630-0101, Japan
2. Plant Research Group, Research Institute of Innovative Technology for the Earth, Kizu, Kyoto 619-0225, Japan
3. Graduate School of Biostudies, Kyoto University, Sakyoku, Kyoto 606-8502, Japan
4. Present addresses: CEA Cadarache, Department of Plant Ecophysiology and Microbiology, 13108 Saint-Paul-lez-Durance Cedex, France (Y.M.); Graduate School of Agriculture, Kyushu University, Higashiku, Fukuoka 812-8581, Japan (T.S.)

Correspondence to: Toshiharu Shikanai^1,4 Email: shikanai@agr.kyushu-u.ac.jp

Abstract

Photosynthesis provides at least two routes through which light energy can be used to generate a proton gradient across the thylakoid membrane of chloroplasts, which is subsequently used to synthesize ATP. In the first route, electrons released from water in photosystem II (PSII) are eventually transferred to NADP⁺ by way of photosystem I (PSI)¹. This linear electron flow is driven by two photochemical reactions that function in series. The cytochrome b₆f complex mediates electron transport between the two photosystems and generates the proton gradient (pH). In the second route, driven solely by PSI, electrons can be recycled from either reduced ferredoxin or NADPH to plastoquinone, and subsequently to the cytochrome b₆f complex^{2, 3, 4, 5}. Such cyclic flow generates pH and thus ATP without the accumulation of reduced species. Whereas linear flow from water to NADP⁺ is commonly used to explain the function of the light-dependent reactions of photosynthesis, the role of cyclic flow is less clear. In higher plants cyclic flow consists of two partially redundant pathways. Here we have constructed mutants in Arabidopsis thaliana in which both PSI cyclic pathways are impaired, and present evidence that cyclic flow is essential for efficient photosynthesis.

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