Specificity, crosstalk and redundancy between the chloroplast thioredoxin systems in regulation of photosynthesis during the development of Arabidopsis rosettes (2 of 2)

Light is a substrate of photosynthetic reactions in plants but its intensity constantly fluctuates in the field. Optimization of photosynthetic production under fluctuating light conditions needs strict and rapid balancing of light reactions with chloroplast metabolism. Thioredoxins (TRX) are members of the regulatory network in plant chloroplast that balances the activities of photosynthesis reactions. TRX-dependent regulation of photosynthesis has proposed to mainly rely on TRXs activated by ferredoxin-dependent TRX reductase (FTR) in light. Chloroplast also have a NADPH-dependent TRX reductase (NTRC) which activity is independent of light. We have studied the impact and mutual relationship of the FTR and NTRC systems on regulation of photosynthesis from seedlings to mature Arabidopsis rosettes by mutants lacking or overexpressing a component of either FTR or NTRC system. Our experiments demonstrate that NTRC is an indispensable regulator of photosynthesis at dark-to-light and rapid light-intensity transitions as well as under low intensities limiting photosynthesis. Under these conditions, NTRC system balances light and carbon fixation reactions by regulating both electron transfer as well as carbon fixation reactions. Exposure of plants to growth or higher light intensities activates FTR system and diminishes the need of NTRC in regulation of photosynthesis.