Aquaporin-deficient mutant of Synechocystis is sensitive to salt and high-light stress

•The AqpZ-null Synechocystis was used to study the responses to salt and light stress.•The AqpZ-deficient cells cannot regulate the cytoplasmic volume under salt stress.•PSII and PSI of the mutant are more sensitive to NaCl and to high light.•AqpZ participates in regulation of the PSI and PSII under salt and high-light stress.•AqpZ might be necessary for the repair of PSII and PSI after photodamage. Cyanobacterial aquaporins play an important role in the regulation of various physiological functions: cell volume control, osmotic stress responses, gas exchange. We employed the AqpZ-deficient mutant of Synechocystis to study the role of aquaporins in responses to salt (NaCl) and high light stress. Electron microscopy and paramagnetic resonance revealed that AqpZ-deficient cells are unable to efficiently regulate the cytoplasmic volume under salt stress. Both photosystems (PSII and, especially, PSI) of these cells are more sensitive to NaCl and to high light. Thus, AqpZ of Synechocystis participates in regulation of the photosynthetic activity of PSI and PSII under salt and high-light stress. Our results demonstrate that AqpZ might be necessary for the repair of PSII and PSI after photodamage.