Hidden protein switch controls photosynthesis as light conditions change

Hidden protein switch controls photosynthesis as light conditions change Andrew Zinin Lead Editor Scientists have discovered a previously unknown regulatory mechanism in plant photosynthesis in the unicellular green alga Chlamydomonas reinhardtii. It helps plants adapt to changes in light conditions. The results, published in the journal Nature Plants, show how a crucial protein interaction at the interface between photosystems I and II controls the photosynthetic machinery. The team led by Professor Michael Hippler and Dr. Felix Buchert from the Institute of Plant Biology and Biotechnology at the University of Münster investigated the interaction between the cytochrome b6f complex, which is important for photosynthesis in chloroplasts, and the protein kinase STT7—a central step in the process of so-called state transitions. These enable plants to balance the light-harvesting capacities of both photosystems so chemical energy for growth can be efficiently provided even under dynamic sunlight conditions. This acclimation is preceded by STT7-dependent phosphorylation of mobile light-harvesting complexes, which can be reversed by other phosphatases. Previous research has established that the cytochrome b6f complex activates the protein kinase STT7. The current investigation shows that the kinase activity is regulated via a feedback loop: STT7 phosphorylates a subunit of the cytochrome b6f complex called PetD at a specific position, threonine 4, which subsequently leads to the deactivation of the kinase. This mechanism prevents overactivation and ensures an acclimated response to changes in light. Photosynthesis takes place in the chloroplasts—the power plants of plant cells. There are two photochemical reaction centers, photosystems I and II, that work optimally under light of different wavelengths. The absorption of light energy into the two photosystems enables electron transport within the molecular photosynthetic machine. It thus drives the conversion of light energy into chemical energy. For the study, the researchers combined genetic methods with measurements of photosynthetic performance and biochemical analyses. They showed that various disruptions of the so-called N-terminal region of PetD can impair both the function of the cytochrome b6f complex and the activation of STT7—an indication of its central role in this process. In addition to the group from Münster, Germany, researchers from Stanford University and Sorbonne Université in France were involved. Publication details Afifa Zaeem et al, The amino terminus of PetD is essential for cytochrome b6 f function and the negative feedback control of STT7 kinase, Nature Plants (2026). DOI: 10.1038/s41477-026-02310-y Journal information: Nature Plants Provided by University of Münster