Characterization of ion transport proteins involved in chloroplast function from land plants and algae

Abstract: Evolved around 2.4 billion years ago and even earlier, according to the new geochemical evidence, oxygenic photosynthesis is the crucial turning point in the history of our planet. Used by cyanobacteria, algae, and land plants, photosynthesis represents the most important process in which absorbed sunlight is converted into the chemical energy and stored for later use. From the moment photosynthesis appeared until today, photosynthetic organisms have developed new and improved regulatory mechanisms to be able to cope with changes in light intensity, nutrient availability, different temperatures, altitudes, etc. Availability of nutrients (e.g., K+, Cl–, Mg2+) varies in the environment where photosynthetic organisms live and with that also the internal concentration of ions within the organism’s different compartments. To overcome such changes in the chloroplast, organelle specialized in photosynthesis, several important processes and specialized proteins are required to coordinate ion transport across different membranes and manage ion homeostasis. This thesis answers a few important questions and reveals ion transporters and channels involved in chloroplast function from land plants and green algae. These transport proteins are either localized to the thylakoid membrane (Paper I, II, and III) or chloroplast inner envelope (Paper IV) of Arabidopsis thaliana and Chlamydomonas reinhardtii. Potassium and chloride fluxes across the thylakoid membrane are mediated by the K+ /H+ antiporter KEA3, the Cl− channel/transporter ClCe, and the voltage-dependent Cl− channel VCCN1. In Paper I, we show that they act independently from each other in regulation of the proton motive force (PMF), and adjust electron transport and non-photochemical quenching (NPQ) at different light intensities. Paper II reveals one more important role of ClCe, namely in state transitions, by regulation of the ATP synthase activity in conditions of low light. In Paper III, we show that the Chlamydomonas bestrophin-like transporter BST4 when introduced in Arabidopsis affects PMF, NPQ and ATP synthase activity by a mechanism involving bicarbonate transport and interaction with RuBisCo. Paper IV reveals that the envelope magnesium transporters MGR8, MGR9, and MGT10/MRS4 activate NPQ by a mechanism building the pH gradient across the thylakoid membrane. MGT10/MRS4 also play role in thylakoid ultrastructure. Taken together, these findings help us to better understand the ion transporters across the chloroplast inner envelope and thylakoid membrane and their role in regulation of photosynthesis by different mechanisms.