Oxygenic photosynthesis is sunlight-energized conversion of CO2 into 
carbohydrates using electrons extracted from water. It occurs in 
cyanobacteria and in their endosymbiotic evolutionary descendants, the 
chloroplasts of plants and algae, and enables the existence of most 
ecosystems on Earth. Electron transfer from water to ferredoxin produces 
NADPH and generates an electrochemical proton gradient across the 
thylakoid membrane, which is utilized to power the ATP synthase. In the 
stroma, the products of the light reactions are then used to assimilate CO2 
into sugar phosphates in the Calvin–Benson cycle. In natural growth 
conditions, plants experience fast and unpredictable fluctuations in light 
intensity and other environmental factors. This has necessitated evolution 
of intricate regulatory mechanisms to prevent damage to the photosynthetic 
machinery and to avoid energy-expensive futile reactions. An important 
way to control these mechanisms is through formation and cleavage of 
disulfide bridges in chloroplast proteins by thioredoxins. Indeed, plant 
chloroplasts contain a large variety of thioredoxin isoforms, as well as two 
distinct thioredoxin systems; one dependent on ferredoxin as reductant, the 
other on NADPH. 


In this thesis I have investigated the role of the NADPH-dependent 
chloroplast thioredoxin system (NTRC) in regulation of photosynthetic 
processes, as well as the coordination between the NTRC- and ferredoxindependent 
systems. I demonstrate that NTRC forms a crucial regulatory 
hub in chloroplasts that allows maintenance of redox balance between the 
photosynthetic electron transfer chain and stromal metabolism, particularly 
in low light conditions. This is achieved through regulation of the activities 
of the ATP synthase and enzymes of the Calvin–Benson cycle, as well as 
non-photochemical quenching, cyclic electron transfer around photosystem 
I via the NADH dehydrogenase-like complex, and reversible redistribution 
of excitation energy between the photosystems. I show that 
significant crosstalk exists between the thioredoxin systems, which allows 
dynamic control of photosynthetic processes and photoprotective 
mechanisms in fluctuating light conditions. Understanding these regulatory 
mechanisms of photosynthesis is of utmost importance in bioengineering 
projects aiming to maximize crop yields or biofuel production. Moreover, 
my results suggest that enhancement of chloroplast thioredoxin activity 
may provide a simple but effective tool for those purposes