Abstract
Photosynthesis includes a set of redox reactions that are the source of reducing power and energy for the assimilation of inorganic carbon, nitrogen and sulphur, thus generating organic compounds, and oxygen, which supports life on Earth. As sessile organisms, plants have to face continuous changes in environmental conditions and need to adjust the photosynthetic electron transport to prevent the accumulation of damaging oxygen by-products. The balance between photosynthetic cyclic and linear electron flows allows for the maintenance of a proper NADPH/ATP ratio that is adapted to the plant’s needs. In addition, different mechanisms to dissipate excess energy operate in plants to protect and optimise photosynthesis under adverse conditions. Recent reports show an important role of redox-based dithiol–disulphide interchanges, mediated both by classical and atypical chloroplast thioredoxins (TRXs), in the control of these photoprotective mechanisms. Moreover, membrane-anchored TRX-like proteins, such as HCF164, which transfer electrons from stromal TRXs to the thylakoid lumen, play a key role in the regulation of lumenal targets depending on the stromal redox poise. Interestingly, not all photoprotective players were reported to be under the control of TRXs. In this review, we discuss recent findings regarding the mechanisms that allow an appropriate electron flux to avoid the detrimental consequences of photosynthesis redox imbalances.
Highlights
Most of life on Earth is sustained by photochemical reactions
cyclic electron flow (CEF) depends on additional photosynthetic complexes: the NADH dehydrogenase-like (NDH) complex and/or the ferredoxin/PROTON GRADIENT REGULATION 5 (PGR5)/PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE 1 (PGRL1) complex
Unlike FD1 and FD2, FDC1 is not able to interact with FNR; it can interact with the CEF complexes NDH and PGR5/PGRL1 or with Fd-TRX reductase (FTR) [72]
Summary
Most of life on Earth is sustained by photochemical reactions. In general terms, in the so-called linear electron flow (LEF), photosynthetic light reactions involve three multi-protein complexes: photosystems (PS) II and I, and the cytochrome b6 f (Cyt b6 f ). TRXs are redox signalling proteins with an active site containing two Cys separated by two amino acid residues (CXXC) These redox proteins are prone to transfer electrons to target proteins and their enzymatic mechanism implies a dithiol/disulphide interchange [29]. The aim of this review is to unveil an integrative view of the different regulatory mechanisms rebalancing photosynthesis light reactions under unfavourable conditions In this regard, TRXs are key regulatory players receiving information about the redox state of the PETC, conveying back this information to orchestrate the whole photosynthetic process. Other mechanisms, with so far no reported TRX-dependent redox regulation, which participate in the balance of redox homeostasis in green tissues, will be discussed
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