Photosystem rearrangements, photosynthetic efficiency, and plant growth in far red-enriched light

Arabidopsis plants were grown in white light (400-700 nm) or in white light supplemented with far-red (FR) light peaking at 730 nm. FR-enriched light induced the typical shade avoidance syndrome characterized by enhanced length of seedling hypocotyl and leaf petiole. FR supplementation also caused a noticeable decrease in the carotenoid and chlorophyll content that was attributable to a block of pigment accumulation during plant development. The carotenoid decrease resulted from a downregulation of their biosynthesis pathway rather than carotenoid degradation. The losses of photosynthetic pigments are part of structural and functional rearrangements of the photosynthetic apparatus. The plastoquinone pool was chronically more oxidized in plants acclimated to white + FR light compared to white light-grown plants. Growth in FR-enriched light was associated with a higher photochemical efficiency of PSII compared to growth in white light and with a substantial increase in root and shoot biomass production. Light distribution between the photosystems was modified in favor of PSII by an increase in the PSII/PSI ratio and an inhibition of state transitions. Neither LHCII abundance nor nonphotochemical energy dissipation in the PSII chlorophyll antennae were modified significantly by the addition of FR light. A PSI supercomplex, not previously observed in Arabidopsis, was specifically found in plants grown in FR-enriched light. This large PSI complex contains a supplementary Lhca1-4 dimer, leading to a total of 6 LHCI antennae instead of 4 in the canonical PSI. Through those photosystem rearrangements and the synergistic interaction with white light, FR light is photosynthetically active and can boost photosynthesis and plant growth.