Search

Linear electron transport in the thylakoid membrane drives photosynthetic NADPH and ATP production, while cyclic electron flow (CEF) around photosystem I only promotes the translocation of protons from stroma to thylakoid lumen. The chloroplast NADH dehydrogenase‐like complex (NDH) participates in one CEF route transferring electrons from ferredoxin back to the plastoquinone pool with concomitant proton pumping to the lumen. CEF has been proposed to balance the ratio of ATP/NADPH production and to control the redox poise particularly in fluctuating light conditions, but the mechanisms regulating the NDH complex remain unknown. We have investigated potential regulation of the CEF pathways by the chloroplast NADPH‐thioredoxin reductase (NTRC) in vivo by using an Arabidopsis knockout line of NTRC as well as lines overexpressing NTRC. Here, we present biochemical and biophysical evidence showing that NTRC stimulates the activity of NDH‐dependent CEF and is involved in the regulation of generation of proton motive force, thylakoid conductivity to protons, and redox balance between the thylakoid electron transfer chain and the stroma during changes in light conditions. Furthermore, protein–protein interaction assays suggest a putative thioredoxin‐target site in close proximity to the ferredoxin‐binding domain of NDH, thus providing a plausible mechanism for redox regulation of the NDH ferredoxin:plastoquinone oxidoreductase activity.

Despite being the primary source of energy in the biosphere, photosynthesis is a process that inevitably produces reactive oxygen species. Chloroplasts are a major source of hydrogen peroxide production in plant cells; therefore, different systems for peroxide reduction, such as ascorbate peroxidase and peroxiredoxins (Prxs), are found in this organelle. Most of the reducing power required for hydrogen peroxide reduction by these systems is provided by Fd reduced by the photosynthetic electron transport chain; hence, the function of these systems is highly dependent on light. Recently, it was described a novel plastidial enzyme, stated NTRC, formed by a thioredoxin reductase (NTR) domain at the N-terminus and a thioredoxin (Trx) domain at the C-terminus. NTRC is able to conjugate both NTR and Trx activities to efficiently reduce 2-Cys Prx using NADPH as a source of reducing power. Based on these results, it was proposed that NTRC is a new pathway to transfer reducing power to the chloroplast detoxification system, allowing the use of NADPH, besides reduced Fd, for such function. In this article, the most important features of NTRC are summarized and the implications of this novel activity in the context of chloroplast protection against oxidative damage are discussed.

Thioredoxins (TRXs) are protein oxidoreductases that control the structure and function of cellular proteins by cleavage of a disulphide bond between the side chains of two cysteine residues. Oxidized thioredoxins are reactivated by thioredoxin reductases (TR) and a TR-dependent reduction of TRXs is called a thioredoxin system. Thiol-based redox regulation is an especially important mechanism to control chloroplast proteins involved in biogenesis, in regulation of light harvesting and distribution of light energy between photosystems, in photosynthetic carbon fixation and other biosynthetic pathways, and in stress responses of plants. Of the two plant plastid thioredoxin systems, the ferredoxin-dependent system relays reducing equivalents from photosystem I via ferredoxin and ferredoxin-thioredoxin reductase (FTR) to chloroplast proteins, while NADPH-dependent thioredoxin reductase (NTRC) forms a complete thioredoxin system including both reductase and thioredoxin domains in a single polypeptide. Chloroplast thioredoxins transmit environmental light signals to biochemical reactions, which allows fine tuning of photosynthetic processes in response to changing environmental conditions. In this paper we focus on the recent reports on specificity and networking of chloroplast thioredoxin systems and evaluate the prospect of improving photosynthetic performance by modifying the activity of thiol regulators in plants. This article is part of the themed issue ‘Enhancing photosynthesis in crop plants: targets for improvement'.

Protein disulphide–dithiol interchange is a universal mechanism of redox regulation in which thioredoxins (Trxs) play an essential role. In heterotrophic organisms, and non-photosynthetic plant organs, NADPH provides the required reducing power in a reaction catalysed by NADPH-dependent thioredoxin reductase (NTR). It has been considered that chloroplasts constitute an exception because reducing equivalents for redox regulation in this organelle is provided by ferredoxin (Fd) reduced by the photosynthetic electron transport chain, not by NADPH. This view was modified by the discovery of a chloroplast-localised NTR, denoted NTRC, a bimodular enzyme formed by NTR and Trx domains with high affinity for NADPH. In this review, we will summarize the present knowledge of the biochemical properties of NTRC and discuss the implications of this enzyme on plastid redox regulation in plants. Ministerio de Ciencia e Innovación de España y Fondos FEDER de la Comisión Europea. BIO2010-15430 Junta de Andalucía. BIO-182 y CVI-5919

Photosynthesis is a process that inevitably produces reactive oxygen species, such as hydrogen peroxide, which is reduced by chloroplast-localized detoxification mechanisms one of which involves 2-Cys peroxiredoxins (2-Cys Prxs). Arabidopsis chloroplasts contain two very similar 2-Cys Prxs (denoted A and B). These enzymes are reduced by two pathways: NADPH thioredoxin reductase C (NTRC), which uses NADPH as source of reducing power; and plastidial thioredoxins (Trxs) coupled to photosynthetically reduced ferredoxin of which Trx x is the most efficient reductant in vitro. With the aim of establishing the functional relationship between NTRC, Trx x, and 2-Cys Prxs in vivo, an Arabidopsis Trx x knock-out mutant has been identified and a double mutant (denoted Δ2cp) with, This work was supported by grant BIO2007-60644 from Ministerio de Educacion y Ciencia, and grants P06-CVI01578 and BIO-182 from Junta de Andalucia (Spain). K-JD acknowledges support by the DFG (Di346). PP, KK, and MG were supported by pre-doctoral fellowships from Seville University, CSIC, and Junta de Andalucia, respectively.