Poster: Photosynthesis

Abs # 330: Cyclic electron flow around photosystem I is essential for both induction of thermal dissipation and ATP synthesis during photosynthesis

Presenter:	Munekage, Yuri , y-muneka@bs.aist-nara.ac.jp
Authors	Munekage, Yuri  (A)   Hashimoto, Mihoko  (A)   Endo, Tsuyoshi  (B)   Meurer, Jorg  (C)   Tasaka, Masao  (A)   Shikanai, Toshiharu  (A)
Affiliations:	(A): Nara Institute of Science and Technology (B): Kyoto University (C): Ludwig Maximilians Universitat

Cyclic electron flow around photosystem I (PSI) was first discovered based on its coupling with ATP synthesis approximately 50 years ago. However, it has been still a matter of debate whether it contributes to photosynthesis. Two redundant pathways mediated by ferredoxin-plastoquinone reductase (FQR) and NAD(P)H dehydrogegase (NDH) function in PSI cyclic electron flow. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation) lacking the induction of thermal dissipation. The pgr5 defect is due to a single amino acid alteration in novel thylakoid protein concerning with FQR activity. We showed that FQR is essential for the acidification of the thylakoid lumen to induce thermal dissipation when availability of NADP⁺ is limited. In contrast, knock out mutant of tobacco ndhB did not exhibit significant effect on photosynthetic electron flow. To clarify the physiological function of both pathways, we isolated Arabidopsis crr1,2,3,4 (chlororespiratory reduction) mutants specifically affected in NDH activity. In Arabidopsis crr mutants, photosynthetic electron flow was unaffected, as was in tobacco ndhB disruptant. Surprisingly, double mutants (pgr5 crr1,2,3,4) lacking both activity of FQR and NDH exhibited drastic defect in growth and impaired activity of electron transport even under low light conditions. In double mutants, PSI was highly reduced even at low light intensity, indicating that ATP deprivation and over-reduction occur in the stroma. We conclude that PSI cyclic electron flow is essential for ATP synthesis to maintain the proper ATP/NADPH production ratio which is required in various metabolisms including CO₂ fixation.