Chloroplast Redox Regulatory Mechanisms in Plant Adaptation to Light and Darkness

Abstract

Light is probably the most important environmental stimulus for plant development. As sessile organisms, plants have developed regulatory mechanisms that allow the rapid adaptation of their metabolism to changes in light availability. Redox regulation based on disulfide-dithiol exchange constitutes a rapid and reversible post-translational modification, which affects protein conformation and activity. This regulatory mechanism was initially discovered in chloroplasts when it was identified that enzymes of the Calvin-Benson cycle (CBC) are reduced and active during the day and become rapidly inactivated by oxidation in the dark. At present, the large number of redox-sensitive proteins identified in chloroplasts extend redox regulation far beyond the CBC. The classic pathway of redox regulation in chloroplasts establishes that ferredoxin (Fdx) reduced by the photosynthetic electron transport chain fuels reducing equivalents to the large set of thioredoxins (Trxs) of this organelle via the activity of a Fdx-dependent Trx reductase (FTR), hence linking redox regulation to light. In addition, chloroplasts harbor an NADPH-dependent Trx reductase with a joint Trx domain, termed NTRC. The presence in chloroplasts of this NADPH-dependent redox system raises the question of the functional relationship between NTRC and the Fdx-FTR-Trx pathways. Here, we update the current knowledge of these two redox systems focusing on recent evidence showing their functional interrelationship through the action of the thiol-dependent peroxidase, 2-Cys peroxiredoxin (2-Cys Prx). The relevant role of 2-Cys Prxs in chloroplast redox homeostasis suggests that hydrogen peroxide may exert a key function to control the redox state of stromal enzymes. Indeed, recent reports have shown the participation of 2-Cys Prxs in enzyme oxidation in the dark, thus providing an explanation for the long-lasting question of photosynthesis deactivation during the light-dark transition.

Keywords: chloroplast; darkness; hydrogen peroxide; light; peroxiredoxin; photosynthesis; redox regulation; thioredoxin.

Figure 1

The classic view of redox regulatory pathways in heterotrophic organisms and plant chloroplasts. Regulation of enzyme activity based on the disulfide-dithiol exchange is a universal mechanism present in any type of organisms from bacteria and fungi to animals and plants. In heterotrophic organisms, this regulation is largely based in a two-component system. The disulfide reductase activity of Trx catalyzes the reduction of regulatory disulfides in target proteins (T), the required reducing power being provided by NADPH via the participation of NTR. In plant chloroplasts redox regulation is essential for the rapid adaptation of metabolism to ever changing light availability. Chloroplasts harbor a complex set of Trxs and Trx-like proteins, which rely on photo-reduced Fdx via the participation of FTR, thus linking redox regulation of enzyme activity to light. In addition, chloroplasts contain an additional redox system, NTRC, which relies on NADPH (see below, Figure 2).

Figure 2

NTRC allows the use of NADPH in chloroplast redox homeostasis. NTRC is an efficient reductant of 2-Cys Prxs and, thus, allows the use of reducing equivalents from NADPH to support the hydrogen peroxide scavenging activity of these thiol-dependent peroxidases. Though at lower efficiency, chloroplast Trxs are also able to reduce 2-Cys Prxs. Different studies have shown the participation of NTRC in the upstream photochemical reactions of photosynthesis and the redox regulation of the γ subunit of ATP synthase (γ), and of downstream targets including enzymes of the Calvin-Benson cycle, starch and chlorophyll biosynthesis. Moreover, NTRC might display NTR activity reducing chloroplast Trxs such as Trx f.

Figure 3

Low contents of 2-Cys Prxs exert a suppressor effect of the ntrc phenotype. (A) In chloroplasts from wild type plants, NTRC, which is the most efficient reductant of 2-Cys Prxs, maintains the redox state of these enzymes. Consequently, drainage of electron equivalents from the pool of Trxs is negligible and, thus, the redox state of Trxs is appropriate for the light-dependent reduction of redox-regulated targets. (B) In the ntrc mutant, the lack of the most efficient reductant of 2-Cys Prxs causes the imbalance of the redox state of these enzymes, which accumulate in oxidized form. Though Trxs transfer electrons to 2-Cys Prxs less efficiently than NTRC, the accumulation of the oxidized form of 2-Cys Prxs provokes a significant drainage of reducing equivalents from the pool of Trxs, thereby affecting the light-dependent reduction of downstream targets. (C) The suppressor effect is produced by decreased levels of 2-Cys Prxs. In the absence of NTRC, 2-Cys Prxs accumulate in oxidized form, however, as the amount of 2-Cys Prxs is low, the drainage of reducing equivalents from the pool of Trxs is also low, and hence the redox state of the pool of Trxs is appropriate for the reduction of downstream targets.

Figure 4

The NTRC-2-Cys Prx couple controls the reduction/oxidation balance of redox-regulated targets in the day and in the night. During the day, photo-reduced Fdx fuels reducing equivalents via FTR and Trxs for the reduction (activation) of redox-regulated enzymes. The redox state of the pool of Trxs is maintained by the couple NTRC-2-Cys Prxs, which relies on NADPH as source of reducing power. In the dark, the input of reducing equivalents via reduced Fdx ceases and Trxs mediate the oxidation of reduced targets transferring electrons through the activity of 2-Cys Prxs to hydrogen peroxide, which acts as final sink of electrons. The oxidant efficiency of different Trxs is presented as proposed by Yoshida et al. (2018) and Vaseghi et al. (2018).