Abstract
Many enzymes involved in photosynthesis possess highly conserved cysteine residues that serve as redox switches in chloroplasts. These redox switches function to activate or deactivate enzymes during light-dark transitions and have the function of fine-tuning their activities according to the intensity of light. Accordingly, many studies on chloroplast redox regulation have been conducted under the hypothesis that "fine regulation of the activities of these enzymes is crucial for efficient photosynthesis." However, the impact of the regulatory system on plant metabolism is still unclear. To test this hypothesis, we here studied the impact of the ablation of a redox switch in chloroplast NADP-malate dehydrogenase (MDH). By genome editing, we generated a mutant plant whose MDH lacks one of its redox switches and is active even in dark conditions. Although NADPH consumption by MDH in the dark is expected to be harmful to plant growth, the mutant line did not show any phenotypic differences under standard long-day conditions. In contrast, the mutant line showed severe growth retardation under short-day or fluctuating light conditions. These results indicate that thiol-switch redox regulation of MDH activity is crucial for maintaining NADPH homeostasis in chloroplasts under these conditions.
Highlights
Many enzymes involved in photosynthesis possess highly conserved cysteine residues that serve as redox switches in chloroplasts
We have shown that the redox switch of NADPmalate dehydrogenase plays a crucial role in the optimal growth of plants under fluctuating light conditions and prolonged darkness
We developed mutant plants in which the thiol-based redox switch of the strictly regulated enzyme malate dehydrogenase (MDH) was deleted in the native genomic context by clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9)-mediated genome editing
Summary
Many enzymes involved in photosynthesis possess highly conserved cysteine residues that serve as redox switches in chloroplasts. The mutant line showed severe growth retardation under short-day or fluctuating light conditions These results indicate that thiol-switch redox regulation of MDH activity is crucial for maintaining NADPH homeostasis in chloroplasts under these conditions. Many photosynthetic enzymes possess conserved cysteine pairs, which serve as redox switches [1, 2] These enzymes are usually activated by the reduction of cysteines under photosynthetic conditions and are deactivated by oxidation in the dark [3]. They receive reducing equivalents from the redox-regulated proteins and transfer electrons to H2O2 via 2-Cys peroxiredoxins This redoxbased photosynthesis regulation system is considered to be one of the most important strategies for the acclimation of sessile plants to fluctuating light environment [9, 10]. We have shown that the redox switch of NADPmalate dehydrogenase plays a crucial role in the optimal growth of plants under fluctuating light conditions and prolonged darkness
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