Cyt b ₆ /f Complex Fine-Tunes PSI Stability and Photosynthetic Capacity Under Fluctuating Light

Plants encounter dynamic light fluctuations in natural habitats, requiring robust mechanisms to protect Photosystem I (PSI) against photoinhibition. The cytochrome (Cyt) b ₆ /f complex plays a pivotal role in regulating electron flow and ensuring PSI stability under fluctuating light. However, its precise role in balancing PSI protection and photosynthetic capacity remains unclear. In this study, transgenic tobacco ( Nicotiana tabacum L. ‘W38’) with reduced Cyt b ₆ /f content was used to examine PSI tolerance to photoinhibition and photosynthetic performance under fluctuating light. Notably, the reduced Cyt b ₆ /f content conferred significant protection of PSI against photoinhibition induced by fluctuating light, albeit at the expense of the electron transport capacity. Remarkably, PSI stability was maintained even under extreme fluctuating light conditions in the plants with lowest Cyt b ₆ /f contents, highlighting a trade-off between photoprotection and photosynthetic capacity. These findings reveal that a reduction in Cyt b ₆ /f content enhances PSI protection while adjusting electron transport capacity, making plants more adaptable in environments dominated by low light and transient sunflecks. This study provides new insights into the adaptive strategies of plants under dynamic light environments and underscores the potential of targeting Cyt b ₆ /f for enhancing crop tolerance to fluctuating light conditions.