Two chloroplast thioredoxin systems differentially modulate photosynthesis in Arabidopsis depending on light intensity and leaf age.

Various 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.

Dual and dynamic intracellular localization of Arabidopsis thaliana SnRK1.1.

Sucrose non-fermenting 1 (SNF1)-related protein kinase 1.1 (SnRK1.1; also known as KIN10 or SnRK1α) has been identified as the catalytic subunit of the complex SnRK1, the Arabidopsis thaliana homologue of a central integrator of energy and stress signalling in eukaryotes dubbed AMPK/Snf1/SnRK1. A nuclear localization of SnRK1.1 has been previously described and is in line with its function as an integrator of energy and stress signals. Here, using two biological models (Nicotiana benthamiana and Arabidopsis thaliana), native regulatory sequences, different microscopy techniques, and manipulations of cellular energy status, it was found that SnRK1.1 is localized dynamically between the nucleus and endoplasmic reticulum (ER). This distribution was confirmed at a spatial and temporal level by co-localization studies with two different fluorescent ER markers, one of them being the SnRK1.1 phosphorylation target HMGR. The ER and nuclear localization displayed a dynamic behaviour in response to perturbations of the plastidic electron transport chain. These results suggest that an ER-associated SnRK1.1 fraction might be sensing the cellular energy status, being a point of crosstalk with other ER stress regulatory pathways.

Multilevel regulation of non-photochemical quenching and state transitions by chloroplast NADPH-dependent thioredoxin reductase.

In natural growth habitats, plants face constant, unpredictable changes in light conditions. To avoid damage to the photosynthetic apparatus on thylakoid membranes in chloroplasts, and to avoid wasteful reactions, it is crucial to maintain a redox balance both within the components of photosynthetic electron transfer chain and between the light reactions and stromal carbon metabolism under fluctuating light conditions. This requires coordinated function of the photoprotective and regulatory mechanisms, such as non-photochemical quenching (NPQ) and reversible redistribution of excitation energy between photosystem II (PSII) and photosystem I (PSI). In this paper, we show that the NADPH-dependent chloroplast thioredoxin system (NTRC) is involved in the control of the activation of these mechanisms. In plants with altered NTRC content, the strict correlation between lumenal pH and NPQ is partially lost. We propose that NTRC contributes to downregulation of a slow-relaxing constituent of NPQ, whose induction is independent of lumenal acidification. Additionally, overexpression of NTRC enhances the ability to adjust the excitation balance between PSII and PSI, and improves the ability to oxidize the electron transfer chain during changes in light conditions. Thiol regulation allows coupling of the electron transfer chain to the stromal redox state during these changes.

Interaction between plastid and mitochondrial retrograde signalling pathways during changes to plastid redox status.

Mitochondria and chloroplasts depend upon each other; photosynthesis provides substrates for mitochondrial respiration and mitochondrial metabolism is essential for sustaining photosynthetic carbon assimilation. In addition, mitochondrial respiration protects photosynthesis against photoinhibition by dissipating excess redox equivalents from the chloroplasts. Genetic defects in mitochondrial function result in an excessive reduction and energization of the chloroplast. Thus, it is clear that the activities of mitochondria and plastids need to be coordinated, but the manner by which the organelles communicate to coordinate their activities is unknown. The regulator of alternative oxidase (rao1) mutant was isolated as a mutant unable to induce AOX1a expression in response to the inhibitor of the mitochondrial cytochrome c reductase (complex III), antimycin A. RAO1 encodes the nuclear localized cyclin-dependent kinase E1 (CDKE1). Interestingly, the rao1 mutant demonstrates a genome uncoupled phenotype also in response to redox changes in the photosynthetic electron transport chain. Thus, CDKE1 was shown to regulate both LIGHT HARVESTING COMPLEX B (LHCB) and ALTERNATIVE OXIDASE 1 (AOX1a) expression in response to retrograde signals. Our results suggest that CDKE1 is a central nuclear component integrating mitochondrial and plastid retrograde signals and plays a role in regulating energy metabolism during the response to stress.