PROTON GRADIENT REGULATION 5 enables efficient C4 photosynthesis under fluctuating light.

Area of Science:

• Plant Biology
• Photosynthesis Research
• Molecular Plant Physiology

Background:

• Proton Gradient Regulation 5 (PGR5) is crucial for proton motive force and photoprotection in C3 plants, likely via Photosystem I cyclic electron flow.
• PGR5 abundance increases during the C3 to C4 photosynthesis transition, suggesting a role in enhanced C4 function.

Purpose of the Study:

• To investigate the function of PGR5 in C4 photosynthesis and photoprotection using the model C4 monocot Setaria viridis.
• To understand how PGR5 contributes to energy regulation and light stress tolerance in C4 plants.

Main Methods:

• Generation of Setaria viridis lines with null pgr5 alleles.
• Assessment of proton motive force, photosynthetic control, non-photochemical quenching, Photosystem I activity, and growth under fluctuating light conditions.
• Comparison of C4 pgr5 mutants with C3 pgr5 mutants.

Main Results:

• Loss of PGR5 severely impairs proton motive force, photosynthetic control, and energy-dependent non-photochemical quenching in S. viridis, leading to Photosystem I damage under light stress.
• PGR5-deficient C4 plants show reduced growth and photosynthesis under fluctuating light but are more tolerant than C3 pgr5 mutants due to maintained photosynthetic control.
• A slower-relaxing, zeaxanthin-dependent non-photochemical quenching mechanism compensates for PGR5 loss, supporting survival at the cost of efficiency.

Conclusions:

• PGR5 is essential for efficient C4 photosynthesis under fluctuating light by regulating photosynthetic control and energy-dependent non-photochemical quenching.
• This study reveals the interplay of photoprotective mechanisms in safeguarding C4 photosynthesis, highlighting PGR5's critical role.