Vol.:(0123456789) 1 3 Photosynthesis Research https://doi.org/10.1007/s11120-018-0575-z ORIGINAL ARTICLE Genetic characterization of a flap1 null mutation in Arabidopsis npq4 and pgr5 plants suggests that the regulatory role of FLAP1 involves the control of proton homeostasis in chloroplasts Mai Duy Luu Trinh 1  · Ryoichi Sato 1,3  · Shinji Masuda 2 Received: 7 March 2018 / Accepted: 24 August 2018 © Springer Nature B.V. 2018 Abstract Precise control of the proton concentration gradient across thylakoid membranes (ΔpH) is essential for photosynthesis and its regulation because the gradient contributes to the generation of the proton motive force used for ATP synthesis and also for the fast and reversible induction of non-photochemical quenching (NPQ) to avoid photoinhibition and photodamage. However, the regulatory mechanism(s) controlling ΔpH in response to fuctuating light has not been fully elucidated. We previously described a new NPQ-regulatory chloroplastic protein, Fluctuating-Light-Acclimation Protein1 (FLAP1), which is important for plant growth and modulation of ΔpH under fuctuating light conditions. For this report, we further charac- terized FLAP1 activity by individually crossing an Arabidopsis flap1 mutant with npq4 and pgr5 plants; npq4 is defective in PsbS-dependent NPQ, and pgr5 is defective in induction of steady-state proton motive force (pmf) and energy-dependent quenching (qE). Both npq4 and npq4 flap1 exhibited similar NPQ kinetics and other photosynthetic parameters under con- stant or fuctuating actinic light. Conversely, pgr5 flap1 had recovered NPQ, photosystem II quantum yield and growth under fuctuating light, each of which was impaired in pgr5. Together with other data, we propose that FLAP1 activity controls proton homeostasis under steady-state photosynthesis to manipulate luminal acidifcation levels appropriately to balance photoprotection and photochemical processes. Keywords FLAP1 · Fluctuating light · Luminal acidifcation · Non-photochemical quenching · PGR5 · Photosynthesis Introduction In nature, plants adapt to diferent and fuctuating light envi- ronments, including sunfexes in forests, seasonal changes, and cloudy conditions. Consequently, plants have devel- oped sophisticated mechanisms to control photosynthetic processes under a variety of light conditions (Joliot and Johnson 2011). Reactive oxygen species (ROS) are gen- erated in large amounts when plants are exposed to unfa- vorable conditions, e.g., extreme temperature, drought, or fuctuating light, which can cause photo-oxidative damage of their cellular components (Niyogi 1999; Krieger-Liszkay 2005; Møller et al. 2007). When generated in photosystem II (PSII), singlet oxygen ( 1 O 2 ) and/or charge chlorophyll dimer radical (P680 + ) damage chloroplastic lipids, essential pigments, and D1 proteins (Niyogi 1999; Krieger-Liszkay 2005; Krieger-Liszkay et al. 2008). In PSI, ROS can damage iron–sulfur clusters (e.g., F A , F B , and F x ; Tiwari et al. 2016). The generation of ROS results in upregulation of genes involved in the photo-oxidative stress response (Krieger- Liszkay 2005), which helps plants acclimate to long-term exposure to high-intensity light (Niyogi 1999; Matsubara et al. 2016). The rate of photosynthetic electron transfer, involved in several short-term photo-protective mechanisms, is sensi- tive to environmental changes (Colombo et al. 2016); these Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11120-018-0575-z) contains supplementary material, which is available to authorized users. * Shinji Masuda shmasuda@bio.titech.ac.jp 1 Graduate School of Life Science and Technology, Tokyo Institute of Technology, Yokohama 226-8501, Japan 2 Center for Biological Resources & Informatics, Tokyo Institute of Technology, Yokohama 226-8501, Japan 3 Present Address: Division of Environmental Photobiology, National Institute for Basic Biology, Okazaki 444-8585, Japan