Mitochondrial respiration is essential for photosynthesis-dependent ATP supply of the plant cytosol.

Plants rely on solar energy to synthesize ATP and NADPH for photosynthetic carbon fixation and all cellular need. Mitochondrial respiration is essential in plants, but this may be due to heterotrophic bottlenecks during plant development or because it is also necessary in photosynthetically active cells. In this study, we examined in vivo changes of cytosolic ATP concentration in response to light, employing a biosensing strategy in the moss Physcomitrium patens and revealing increased cytosolic ATP concentration caused by photosynthetic activity. Plants depleted of respiratory Complex I showed decreased cytosolic ATP accumulation, highlighting a critical role of mitochondrial respiration in light-dependent ATP supply of the cytosol. Consistently, targeting mitochondrial ATP production directly, through the construction of mutants deficient in mitochondrial ATPase (complex V), led to drastic growth reduction, despite only minor alterations in photosynthetic electron transport activity. Since P. patens is photoautotrophic throughout its development, we conclude that heterotrophic bottlenecks cannot account for the indispensable role of mitochondrial respiration in plants. Instead, our results support that mitochondrial respiration is essential for ATP provision to the cytosol in photosynthesizing cells. Mitochondrial respiration provides metabolic integration, ensuring supply of cytosolic ATP essential for supporting plant growth and development.