Regulation of photosynthesis by thylakoid K+/H+ antiport

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The study of trans-thylakoid ion fluxes represents an emerging field in plant bioenergetics. Recent advances in identifying the molecular nature of some long sought-for thylakoid ion transporters, and characterizing their physiological function by analyzing plants devoid of these proteins, have revealed that these transport systems play important roles in regulating photosynthesis in response to light energy levels. Our previous contribution to the field includes the finding that, upon high to low light transition, thylakoid K+/H+ antiport via the K+ efflux antiporter KEA3 accelerates the downregulation of photoprotective light energy quenching. Thus, KEA3 activity increases the light efficiency of photosynthesis in low light. However, the exact role of KEA3 in this process is poorly understood. With the proposed project we seek to achieve a comprehensive characterization of the role and regulation of KEA3-mediated ion fluxes. The recent analysis of different KEA3 variants has indicated that thylakoid K+/H+ antiport activity is highly regulated by a mechanism involving the KEA3 C-terminus. Additionally, we have evidence for a proton safety valve function of KEA3 under conditions of very low lumenal pH. With the present research project we seek to approach the following two specific questions: (i) Which is the bottleneck in the photosynthetic process during high to low light transitions and how exactly does KEA3 counteract this limitation and (ii) which are the molecular mechanisms underlying KEA3 activity regulation? We will approach these questions by a comprehensive strategy combining complementary approaches, including spectroscopy, ion, metabolite and protein analyses, genetics and computational modeling.