Eukaryotic photosynthesis depends on the H + and ion-gradients established between the sub-organellar chloroplast compartments, the thylakoid lumen and the stroma. This review focuses on the chloroplast, which harbors the pathway of arguably the most important biochemical process for life on earth, photosynthesis. Given the central role of mitochondria and chloroplasts for plant energy metabolism, these systems are urgently awaiting more targeted research activity. In recent decades, most research has focused on H +-gradient dependent transporters in endomembrane organelles, but knowledge on mitochondria and chloroplasts lags behind ( Bassil et al., 2012 Pittman, 2012). The established pH-gradients are critical to drive secondary active transport by for instance organellar ion/H + exchange. Although H +-pumping activities have been detected in several organellar membranes, the molecular identity of all proteins involved is not known. In the chloroplast thylakoid membrane or in mitochondria this is achieved by H +-transporting electron transfer chains while other organelles depend on ATP fueled H +-pumps (ATPases). The pH gradients have to be actively maintained. Only mitochondria, the chloroplast stroma and some peroxisomes offer alkaline reaction conditions with pH values equal to or higher than 8 ( Shen et al., 2013). The cytosol has to stay neutral (pH 7.2-7.4) to ensure proper biochemical reactions ( Schumacher, 2014). While the apoplast and the vacuole maintain fairly acidic pH levels generally between pH 5 and 7 ( Grignon and Sentenac, 1991 Martiniere et al., 2013 Shen et al., 2013). In plant cells, several compartments with different pH exist in parallel. Proton (H +) gradients across biomembranes represent strong driving forces vital for cellular and cell organellar function. In this mini review we summarize the current status in the field and the open questions that need to be addressed in order to understand how pH gradients are maintained, how this is interconnected with other transport processes and what this means for chloroplast function. The established pH gradients are important to drive uptake of essential ions and solutes, but not many transporters involved have been identified to date. Notably several (Na +,K +)/H + antiporter systems where identified, that play a role in pH gradient regulation, ion homeostasis, osmoregulation, or coupling of secondary active transport. More than 30 years ago it was already established that these proton fluxes are electrically counterbalanced by Mg 2+, K +, or Cl - fluxes, but only recently the first transport systems that regulate the pH gradient were identified. To maintain this pH gradient chloroplasts actively extrude protons. This implies that a pH gradient between the cytosol (pH 7) and the stroma (pH 8) is established upon illumination. As a result of proton pumping into the thylakoid lumen, an alkaline stromal pH develops, which is required for full activation of pH-dependent Calvin Benson cycle enzymes. Across thylakoid membranes, the light induced -proton gradient is essential for ATP synthesis. Proton gradients are fundamental to chloroplast function. 2Departamento de Bioquímica, Biología Celular y Molecular de Plantas, Estacion Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, Granada, Spain.1Plant Physiology, School of Biological Sciences, Washington State University, Pullman, WA, USA.Ricarda Höhner 1 Ali Aboukila 2 Hans-Henning Kunz 1* Kees Venema 2*
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