Photosynthetic Water Oxidation Requires Water Transport Across the Thylakoid Membrane: Are Aquaporins Involved?

Water supply is crucial for the development and growth of all living organisms. This is especially true for oxygenic photosynthetic organisms (cyanobacteria, algae and terrestrial plants), which use water as a substrate to produce molecular oxygen through the activity of the water-oxidizing photosystem II complex. The precise site of water oxidation is on the lumenal side of the thylakoid membrane, harboring this complex. How water molecules reach the thylakoid lumen to sustain oxygen production is a crucial question. To date, the mechanism of water transport across the thylakoid membrane is unknown. Within the cell, the most common mechanisms for water transport are free diffusion and facilitated diffusion, the latter being mediated by specialized channel proteins named aquaporins. In this review, the following questions are addressed: 1) Could free diffusion through the thylakoid membrane provide sufficient amounts of water for effective photosynthetic reaction? or 2) Are aquaporins involved in water transport across the thylakoid membrane? Biophysical studies and theoretical calculations support the second possibility. Moreover, several aquaporins have been found using mass spectrometry-based proteomics in plant chloroplast membranes. Validation of their chloroplast location and investigation of a potential role in photosynthesis should be the focus of future studies.