Ferredoxin/thioredoxin system plays an important role in the chloroplastic NADP status of Arabidopsis

• Published in: The Plant journal : for cell and molecular biology Vol. 95; no. 6; pp. 947 - 960
• Main Authors: Hashida, Shin‐nosuke, Miyagi, Atsuko, Nishiyama, Maho, Yoshida, Keisuke, Hisabori, Toru, Kawai‐Yamada, Maki
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.09.2018
• Subjects: Amino acids; Arabidopsis; Calvin cycle; Catalysis; chloroplast; Chloroplasts; Cytosol; Dehydrogenase; Dehydrogenases; Ferredoxin; ferredoxin/thioredoxin reductase; Homeostasis; leaf reticulation; Light; Malate dehydrogenase; Metabolites; Mutants; NAD kinase; NADP; NADPH; Nitrate reduction; Photosynthesis; Point mutation; redox; Redox reactions; Reductase; Thioredoxin; NADPH; chloroplast; ferredoxin/thioredoxin reductase; leaf reticulation; NAD kinase; NADP; redox
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/tpj.14000

Summary NADP is a key electron carrier for a broad spectrum of redox reactions, including photosynthesis. Hence, chloroplastic NADP status, as represented by redox status (ratio of NADPH to NADP+) and pool size (sum of NADPH and NADP+), is critical for homeostasis in photosynthetic cells. However, the mechanisms and molecules that regulate NADP status in chloroplasts remain largely unknown. We have now characterized an Arabidopsis mutant with imbalanced NADP status (inap1), which exhibits a high NADPH/NADP+ ratio and large NADP pool size. inap1 is a point mutation in At2g04700, which encodes the catalytic subunit of ferredoxin/thioredoxin reductase. Upon illumination, inap1 demonstrated earlier increases in NADP pool size than the wild type did. The mutated enzyme was also found in vitro to inefficiently reduce m‐type thioredoxin, which activates Calvin cycle enzymes, and NADP‐dependent malate dehydrogenase to export reducing power to the cytosol. Accordingly, Calvin cycle metabolites and amino acids diminished in inap1 plants. In addition, inap1 plants barely activate NADP‐malate dehydrogenase, and have an altered redox balance between the chloroplast and cytosol, resulting in inefficient nitrate reduction. Finally, mutants deficient in m‐type thioredoxin exhibited similar light‐dependent NADP dynamics as inap1. Collectively, the data suggest that defects in ferredoxin/thioredoxin reductase and m‐type thioredoxin decrease the consumption of NADPH, leading to a high NADPH/NADP+ ratio and large NADP pool size. The data also suggest that the fate of NADPH is an important influence on NADP pool size. Significance statement Coordination of NADP+ photoreduction with NADP+ production by NAD kinase in chloroplasts modulates redox status and prevents over‐reduction under fluctuating light. Thus, the discovery of a mechanism associated with chloroplastic NADP status advances our understanding of how plants optimize photosynthesis in response to variable light.