High-light adaptation via ferredoxin-mediated tuning of the photosynthesis-photoprotection trade-off

Plants continuously adjust photosynthesis to balance growth and photoprotection under changing environmental conditions. Environmental fluctuations frequently impose a mismatch between energy production and CO₂ assimilation. How photochemical reactions are regulated to maintain performance under these conditions remains a central question in plant biology. We previously developed transplastomic tobacco (Fd1-OE plants) overexpressing ferredoxin displaying enhanced photoprotection and growth penalties with a variegated leaf phenotype under glasshouse conditions. Here, we investigate how these plants respond to different growth irradiances using physiological, ultrastructural, and photosynthetic analyses, including Pulse-Amplitude Modulation (PAM), gas exchange, and P700 absorbance measurements, and dynamic-light assays. Fd1-OE plants progressively recovered growth, leaf phenotype, and photosynthetic performance as growth irradiance increased, reaching near wild-type performance at 1400 μmol m^-2 s^-1. This enhanced adaptation to 'high-light' was associated with a larger fraction of open Photosystem II reaction centers and enhanced nonphotochemical quenching. Dynamic-light analyses further revealed faster plastoquinone (PQ) turnover, a more oxidized (PQ) pool and enhanced electron withdrawal downstream of Photosystem I. Our results indicate that ferredoxin overexpression redefines the balance between photochemistry and photoprotection. This adjustment shifts adaptation toward higher irradiance and enhances photosynthetic performance under changing light environments. Electron partitioning downstream of Photosystem I emerges as a promising target to improve photosynthetic resilience.