Divergent Protein Redox Dynamics and Their Relationship with Electron Transport Efficiency during Photosynthesis Induction

Abstract

Various chloroplast proteins are activated/deactivated during the light/dark cycle via the redox regulation system. Although the photosynthetic electron transport chain provides reducing power to redox-sensitive proteins via the ferredoxin (Fd)/thioredoxin (Trx) pathway for their enzymatic activity control, how the redox states of individual proteins are linked to electron transport efficiency remains uncharacterized. Here we addressed this subject with a focus on the photosynthetic induction phase. We used Arabidopsis plants, in which the amount of Fd-Trx reductase (FTR), a core component in the Fd/Trx pathway, was genetically altered. Several chloroplast proteins showed different redox shift responses toward low- and high-light treatments. The light-dependent reduction of Calvin-Benson cycle enzymes fructose 1,6-bisphosphatase (FBPase) and sedoheptulose 1,7-bisphosphatase (SBPase) was partially impaired in the FTR-knockdown ftrb mutant. Simultaneous analyses of chlorophyll fluorescence and P700 absorbance change indicated that the induction of the electron transport reactions was delayed in the ftrb mutant. FTR overexpression also mildly affected the reduction patterns of FBPase and SBPase under high-light conditions, which were accompanied by the modification of electron transport properties. Accordingly, the redox states of FBPase and SBPase were linearly correlated with electron transport rates. In contrast, ATP synthase was highly reduced even when electron transport reactions were not fully induced. Furthermore, the redox response of proton gradient regulation 5-like photosynthetic phenotype1 (PGRL1; a protein involved in cyclic electron transport) did not correlate with electron transport rates. Our results provide insights into the working dynamics of the redox regulation system and their differential associations with photosynthetic electron transport efficiency.

Keywords: Arabidopsis thaliana; Ferredoxin–thioredoxin reductase; Photosynthetic electron transport; Redox regulation; Thioredoxin.

Fig. 1

Growth phenotypes of the ftrb mutant and 35S::FTRB plants under short-day conditions. (A) The wild-type plant (WT), ftrb mutant and 35S::FTRB plants were grown for 43 d. (B) Immunoblotting analysis of the FTR catalytic subunit (FTRc). The same amount of total leaf protein (except for the WT dilution series) was loaded onto each lane. As a loading control, the Rubisco large subunit was stained with Coomassie Brilliant Blue R-250 (CBB). (C) Quantification of the FTRc protein amount. Relative values to the WT level are shown. Each value represents the mean ± SD (three different samples). Asterisks denote a significant difference (*P < 0.05, **P < 0.01, Student’s t-test). (D) Fresh weight of aboveground tissues. Each value represents the mean ± SD (10 different samples). (E) Chlorophyll content in the leaf. Each value represents the mean ± SD (five different samples). (D and E) Different letters denote significant differences (P < 0.01, Tukey–Kramer multiple comparison test).

Fig. 2

Protein redox shift responses during dark to low light transitions. (A) The WT, ftrb mutant and 35S::FTRB plants were dark-adapted and then irradiated with low light (60 μmol photons m⁻² s⁻¹) for the indicated times. Finally, the redox states of CF₁-γ, FBPase, SBPase and PGRL1 were determined. As a loading control, the Rubisco large subunit was stained with CBB. Ox, oxidized form: Red, reduced form. (B) The reduction level was calculated as the ratio of the reduced form to the total. Each value represents the mean ± SD (three different samples).

Fig. 3

Protein redox shift responses during dark to high light transitions. (A) The WT, ftrb mutant and 35S::FTRB plants were dark adapted and then irradiated with high light (360 μmol photons m⁻² s⁻¹) for the indicated times. Finally, the redox states of CF₁-γ, FBPase, SBPase and PGRL1 were determined. As a loading control, the Rubisco large subunit was stained with CBB. Ox, oxidized form: Red, reduced form. (B) The reduction level was calculated as the ratio of the reduced form to the total. Each value represents the mean ± SD (three different samples).

Fig. 4

Photosynthetic electron transport dynamics during dark to light transitions. The WT, ftrb mutant and 35S::FTRB plants were dark-adapted and then irradiated with low (60 μmol photons m⁻² s⁻¹), middle (120 μmol photons m⁻² s⁻¹) or high light (360 μmol photons m⁻² s⁻¹) for the indicated times. During this period, several parameters, including ETR II, 1—qL, NPQ, ETR I, Y (ND) and Y (NA), were determined. Each value represents the mean ± SD (five different samples).

Fig. 5

Relationships between the electron transport efficiency and protein redox state during dark to low light (60 μmol photons m⁻² s⁻¹) or high light (360 μmol photons m⁻² s⁻¹) transitions. The reduction levels of CF₁-γ, FBPase, SBPase and PGRL1 (mean ± SD; n = 3) were plotted against ETR II and ETR I (mean ± SD; n = 5). For the data of FBPase and SBPase, the regression lines were shown with the R² values and slopes (× 10³ value).