Abstract
SUMMARYVarious regulatory mechanisms have evolved in plants to optimize photosynthetic activity under fluctuating light. Thioredoxins (TRX) are members of the regulatory network balancing activities of light and carbon fixation reactions in chloroplasts. We have studied the impact of two chloroplast TRX systems, the ferredoxin‐dependent TRX reductase (FTR) and the NADPH‐dependent TRX reductase C (NTRC) on regulation of photosynthesis by mutants lacking or overexpressing a component of either system. Plants were subjected to image‐based phenotyping and chlorophyll fluorescence measurements that allow long‐term monitoring of the development and photosynthetic activity of the rosettes, respectively. Our experiments demonstrate that NTRC and FTR systems respond differently to variation of light intensity. NTRC was an indispensable regulator of photosynthesis in young leaves, at light‐intensity transitions and under low light intensities limiting photosynthesis, whereas steady‐state exposure of plants to growth or higher light intensities diminished the need of NTRC in regulation of photosynthesis. In fluctuating light, overexpression of NTRC increased the quantum yield of Photosystem II (YII) at low light and stimulated the relaxation of non‐photochemical quenching (NPQ) after high light exposure, indicating that overexpression of NTRC improves leaf capacity to convert light energy to chemical energy under these conditions. Overexpression of chimeric protein (NTR‐TRXf) containing both the thioredoxin reductase and TRXf activity on an ntrc mutant background, did not completely recover either growth or steady‐state photosynthetic activity, whereas OE‐NTR‐TRXf plants exposed to fluctuating light regained the wild‐type level of Y(II) and NPQ.
Highlights
Photosynthesis comprises a series of highly regulated reactions that convert solar energy and CO2 to chemical energy of carbohydrates in chloroplasts
We show that the increased content of NADPH-dependent TRX reductase C (NTRC) improves the quantum yield of Photosystem II (PSII) and diminishes the dissipation of light energy as heat in Arabidopsis leaves at growth (GL) and lower light (LL) intensities
To investigate the specific roles of NTRC and TRXf in the regulation of chloroplast processes we constructed transgenic lines on an ntrc background overexpressing wildtype NTRC protein (OE-NTRC line), NTRC protein with an active NTR domain but an inactive TRX domain (NTRCSGPS) (OE-SGPS line), and a line with a chimeric protein, where the TRXf sequence was fused with the sequence of the NTR domain of NTRC (OE-NTR-TRXf line)
Summary
Photosynthesis comprises a series of highly regulated reactions that convert solar energy and CO2 to chemical energy of carbohydrates in chloroplasts. Photosynthetic light harvesting and energy conversion is regulated by multiphase network, including non-photochemical quenching (NPQ) of light energy in Photosystem II (PSII) antenna, regulation of linear and cyclic electron flow (CEF) and generation of proton motive force (pmf) in thylakoid membranes, regulation of the enzymes in carbon metabolism in stroma, as well as the control of generation and scavenging of ROS in chloroplasts (Tikkanen and Aro, 2014; Goldschmidt-Clermont and Bassi, 2015; Dietz et al, 2016; Ruban, 2016; Yamori and Shikanai, 2016; Armbruster et al, 2017). The ferredoxin-TRX (Fd-TRX) system is activated by light via reduction of Fd, and consists of the Fd-dependent
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