Drastic Reduction in Cytochrome b6/f Complex Confers Robust PSI Photoprotection Under Fluctuating Light at the Expense of Photosynthetic Capacity.

Plants in natural habitats frequently encounter fluctuating light (FL), which can lead to photoinhibition of Photosystem I (PSI), thereby limiting photosynthetic productivity. The cytochrome (Cyt) b₆/f complex plays a pivotal role in regulating photosynthetic electron flow and influencing PSI stability in plants. However, the precise impact of a substantial reduction in Cyt b₆/f content on PSI photoprotection under FL and the associated trade-offs with photosynthetic capacity remain to be elucidated. In this study, we investigated PSI tolerance and photosynthetic performance in transgenic tobacco (Nicotiana tabacum) lines with varying Cyt b₆/f levels, comparing wild-type (WT) plants to those with drastically reduced Cyt b₆/f content. Our results show that a marked reduction in Cyt b₆/f levels conferred substantial PSI photoprotection against FL-induced damage, even under extremely high light pulses. This enhanced PSI stability was attributed to the restricted electron flow towards PSI, which likely maintained P700 in a more oxidized state. However, this robust PSI protection in the plants with significantly reduced Cyt b₆/f levels came at a considerable cost to the overall photosynthetic capacity, as evidenced by reduced PSII efficiency (Y(II)), photochemical quenching (qL), and non-photochemical quenching (NPQ) under both steady-state and fluctuating light conditions. These findings reveal a critical trade-off between PSI photoprotection and photosynthetic productivity, which is strongly modulated by the abundance of the Cyt b₆/f complex. This trade-off offers key insights into plant adaptive strategies in dynamic light environments and highlights potential targets for improving crop productivity under natural light fluctuations.