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. 2025 Sep 30;177(5):e70483. doi: 10.1111/ppl.70483

Drastic Reduction in Cytochrome b6/f Complex Confers Robust PSI Photoprotection Under Fluctuating Light at the Expense of Photosynthetic Capacity

Masaru Kono 1,✉, Hiromasa Kodama 2, Keiichiro Tanigawa 2, Ichiro Terashima 3, Wataru Yamori 2,✉
PMCID: PMC12484396  PMID: 41028958

ABSTRACT

Plants in natural habitats frequently encounter fluctuating light (FL), which can lead to photoinhibition of Photosystem I (PSI), thereby limiting photosynthetic productivity. The cytochrome (Cyt) b 6 /f complex plays a pivotal role in regulating photosynthetic electron flow and influencing PSI stability in plants. However, the precise impact of a substantial reduction in Cyt b 6 /f content on PSI photoprotection under FL and the associated trade‐offs with photosynthetic capacity remain to be elucidated. In this study, we investigated PSI tolerance and photosynthetic performance in transgenic tobacco ( Nicotiana tabacum ) lines with varying Cyt b 6 /f levels, comparing wild‐type (WT) plants to those with drastically reduced Cyt b 6 /f content. Our results show that a marked reduction in Cyt b 6 /f levels conferred substantial PSI photoprotection against FL‐induced damage, even under extremely high light pulses. This enhanced PSI stability was attributed to the restricted electron flow towards PSI, which likely maintained P700 in a more oxidized state. However, this robust PSI protection in the plants with significantly reduced Cyt b 6 /f levels came at a considerable cost to the overall photosynthetic capacity, as evidenced by reduced PSII efficiency (Y(II)), photochemical quenching (qL), and non‐photochemical quenching (NPQ) under both steady‐state and fluctuating light conditions. These findings reveal a critical trade‐off between PSI photoprotection and photosynthetic productivity, which is strongly modulated by the abundance of the Cyt b 6 /f complex. This trade‐off offers key insights into plant adaptive strategies in dynamic light environments and highlights potential targets for improving crop productivity under natural light fluctuations.

Keywords: cytochrome b 6 /f complex, fluctuating light, photoprotection, photosynthesis, tobacco

1. Introduction

Plants inhabiting natural ecosystems are perpetually exposed to dynamic fluctuations in light intensity (Chazdon 1988; Pearcy 1983, 1990; Pearcy et al. 1994; Vierling and Wessman 2000). Such variable light conditions can impose significant stress on the photosynthetic machinery, with Photosystem I (PSI) being particularly vulnerable to photoinhibition, potentially impairing overall photosynthetic performance (Kono and Terashima 2014; Sejima et al. 2014; Tikkanen and Aro 2014). To cope with these challenges, plants have developed sophisticated protective strategies. These include the precise regulation of intersystem electron transport, dissipation of excess excitation energy via non‐photochemical quenching (NPQ), cyclic electron flow around PSI, and the maintenance of the PSI reaction center P700 in its oxidized, less damage‐prone state (P700+) (Kono et al. 2014, 2017; Miyake 2020; Suorsa et al. 2012; Yamori et al. 2015; Yamori, Makino, and Shikanai 2016; Yamori and Shikanai 2016).

The cytochrome (Cyt) b 6 /f complex occupies a central position in the photosynthetic electron transport chain. It not only mediates electron flow from Photosystem II (PSII) to PSI, but is also integral to the generation of the proton motive force (pmf) across the thylakoid membrane (Tikhonov 2014; Yamori, Kondo, et al. 2016). This pmf is crucial for ATP synthesis and the induction of NPQ (Miyake 2010; Strand et al. 2016, 2017). Consequently, variations in the content and activity of the Cyt b 6 /f complex are anticipated to have profound effects on both the capacity for photosynthesis and the efficacy of photoprotective responses. Previous research has indicated that adjustments in Cyt b 6 /f content may represent an adaptive strategy for plants to strike a balance between efficient light utilization and robust photoprotection (Schöttler and Tóth 2014; Terashima et al. 2021; Yamori et al. 2011).

While the regulatory role of the Cyt b 6 /f complex in “photosynthetic control” is well‐recognized (Tikhonov 2013), a detailed understanding of how incremental reductions in its content influence photosynthetic characteristics, and particularly how a substantial decrease impacts PSI stability under fluctuating light, remains an area requiring further investigation. It has been hypothesized that a reduction in Cyt b 6 /f could limit electron flow to PSI, thereby preventing its over‐reduction and subsequent photodamage, especially during rapid transitions to high light.

This study investigated the effects of varying Cyt b 6 /f complex content on photosynthetic electron transport, proton motive force (pmf) generation, and PSI photoprotection under fluctuating light conditions in tobacco ( Nicotiana tabacum ). We utilized transgenic tobacco lines with differing levels of the Rieske FeS protein, a key subunit of the Cyt b 6 /f complex (Yamori et al. 2011). This approach allowed us to compare wild‐type (WT) plants with those exhibiting intermediate (“Middle”) and severely reduced (“Low”) amounts of the complex. We examined a range of photosynthetic parameters across these three groups to understand how stepwise reductions in the Cyt b 6 /f content affect the overall photosynthetic machinery.

Furthermore, focusing on the WT and “Low” plants, which represent the extremes in Cyt b 6 /f content in our study, we specifically assessed PSI tolerance to photoinhibition induced by fluctuating light. We hypothesized that a drastic reduction in Cyt b 6 /f content (“Low” plants) would confer significant PSI protection by restricting electron flux to PSI and promoting a more oxidized P700, albeit at the cost of reduced photosynthetic capacity. The “Middle” plants, while characterized for their photosynthetic parameters, were not subjected to the same photoinhibition treatments in this study, which prioritized elucidating the mechanisms in the more contrasting phenotypes.

By characterizing these transgenic lines, this study aimed to elucidate the critical role of Cyt b 6 /f complex abundance in defining the balance between photosynthetic performance and PSI stability in response to dynamic light environments. The findings are discussed in the context of the trade‐offs between photoprotection and photosynthetic capacity and in relation to adaptive strategies observed in other plant systems, such as shade‐tolerant species that naturally exhibit altered stoichiometry of photosynthetic complexes (e.g., Terashima et al. 2021). This study aims to provide deeper insights into the regulatory roles of the Cyt b 6 /f complex, which could help guide efforts to enhance crop resilience under variable and potentially stressful light conditions.

2. Materials and Methods

2.1. Plant Materials and Growth Conditions

Tobacco ( Nicotiana tabacum L. cv. W38) plants and several transformants of anti‐Rieske FeS tobacco that had reduced amounts of Cyt b 6 /f were grown in growth cabinets (Yamori et al. 2011). Plants were grown in sterilized soil under controlled environmental conditions: an 8‐h photoperiod with a photosynthetic photon flux density (PPFD) of 200 μmol m−2 s−1, air temperatures of 23°C, and relative humidity of 60%. Plants were grown in 5‐L pots in a 1:1 ratio of Metro‐Mix (Hyponex Japan) and Vermiculite GL (Nittai Co. Ltd.), irrigated two to three times weekly, and fertilized with Hyponex 6–10‐5 solution (Hyponex) diluted to 1:1000 strength every irrigation from 2 weeks after germination. For the experiments, fully expanded leaves (third to fifth leaves from the apex) were selected to standardize the measurements across replicates.

2.2. Analysis of Photosynthetic Components

Immediately after photosynthetic measurements, leaf discs were taken, immersed in liquid nitrogen, and stored at −80°C. The frozen leaf samples were ground in liquid nitrogen and homogenized in an extraction buffer (Yamori et al. 2011). The Rieske FeS content of the Cyt b 6 /f complex and δ‐subunit of ATP synthase was quantified by immunoblotting with anti‐Rieske FeS and anti‐ATP synthase (δ) antibodies (Agrisera). Chlorophylls were extracted in 80% (v/v) acetone and quantified (Porra et al. 1989). The extract of one wild‐type leaf was selected as a standard (100%) and included as a dilution series on the gels. The protein content of the other samples was determined using this standard.

2.3. Fluctuating Light Treatment

Leaves were exposed to fluctuating light to investigate PSI photoinhibition and photosynthetic responses (Kono et al. 2017; Sejima et al. 2014; Tsuyama and Kobayashi 2009). To measure PSI photoinhibition, fluctuating light consisted of alternating high light (2000, 3000, or 20,000 μmol m−2 s−1) and low light (30 μmol m−2 s−1) phases at intervals of 800 ms/10 s for a total duration of 120 min. For these light treatments, red light‐emitting diodes (LEDs; Excelitas/ELCOS GmbH) with a wavelength peak at 635 nm were used.

For measurements of photosynthetic responses, we used fluctuating light, in which high light at 800 μmol m−2 s−1 for 10 min and low light at 30 μmol m−2 s−1 for 15 min alternated.

2.4. Chlorophyll Fluorescence and 830 nm Absorbance Change Measurements

Chlorophyll fluorescence and absorption changes at 830 nm were measured simultaneously in intact leaves using a DUAL‐PAM‐100 (Walz) in ventilated room air. Saturation pulses (SPs) from red LEDs (7000 μmol m−2 s−1, 500 ms duration) were applied to determine the maximum chlorophyll fluorescence with closed PSII centers after dark treatment (Fm) and during illumination (Fm′). The maximum photochemical quantum yield of PSII (Fv/Fm) in the dark and the effective quantum yield of PSII (Y(II)) in actinic light were calculated as (Fm − Fo)/Fm and (Fm′ − Fs′)/Fm′, respectively (Genty et al. 1989; Kitajima and Butler 1975), where Fo is the minimal chlorophyll fluorescence in the dark and Fs' is the steady‐state chlorophyll fluorescence level in actinic light from red LEDs. Non‐photochemical quenching (NPQ) was calculated as (Fm − Fm′)/Fm′ (Bilger and Björkman 1990; Schreiber and Bilger 1987). The coefficient of photochemical quenching (qL), a measure of the fraction of open PSII reaction centers based on the “lake model” of PSII antenna pigment organization, was calculated as (Fm′ – Fs′)/(Fm′ – Fo′) • Fo′/Fs′ (Kramer et al. 2004). Fo′ is the minimal fluorescence yield in actinic light and was estimated using the formula of Oxborough and Baker (1997) as Fo/(Fv/Fm + Fo/Fm′).

With the Dual‐PAM‐100, P700+ was monitored as the absorption difference between 830 and 875 nm in the transmission mode. In analogy to the quantum yields of PSII, the quantum yields of PSI were determined using the saturation pulse method (Klughammer and Schreiber 1994). The maximum oxidizable P700, Pm, was determined by applying the SP in the presence of far‐red light at 25.6 W m−2 with a wavelength peak at 740 nm. The decrease in Pm is an indicator of PSI photoinhibition. Y(I), Y(ND), and Y(NA) were determined in the light. These add up to unity with the photochemical quantum yield (i.e., Y(I) + Y(ND) + Y(NA) = 1).

2.5. Measurement of the Electrochromic Absorbance Shift

Electrochromic shift of carotenoids (ECS or P515) measurements were performed using the DUAL‐PAM‐100 with the P515/535 module to assess the proton motive force (pmf) and its components (ΔΨ and ΔpH). ECS signals were recorded at 515 nm with reference wavelengths of 550 nm (Junge and Witt 1968; Klughammer et al. 2013). To determine the size of the pmf (Cruz et al. 2001; Sacksteder et al. 2000), dark‐interval relaxation kinetics (DIRK) transient analysis was conducted (for details, see Baker et al. 2007). The pmf amplitude was determined from the ECS decay kinetics (Cruz et al. 2001; Sacksteder et al. 2000). The contributions of ΔΨ (electric potential) and ΔpH (proton gradient) were inferred by analyzing the relaxation of the ECS signal during dark intervals. The ECS signals were normalized using a single‐turnover flash for 8 μs.

2.6. Statistical Analysis

All data are expressed as mean ± standard error of the mean (SE). Statistical analyses were conducted using R software (version 4.1.2). One‐way ANOVA was performed to compare differences between WT and transgenic plants, followed by Tukey's post hoc test to identify significant pairwise differences. Statistical significance was set at p < 0.05 for all analyses.

3. Results

3.1. Characterization of Cyt b 6 /f Complex Content in Transgenic Plants

To investigate the role of the Cyt b 6 /f complex in photosynthesis, transgenic tobacco plants with reduced levels of the Rieske FeS protein, a key subunit of Cyt b 6 /f, were generated (Figure 1A). The plants were classified into three groups based on the Rieske FeS content: wild type (WT; 100%), plants with intermediate Rieske FeS levels (43.6%–76.2%), and plants with low Rieske FeS levels (18.5%–42.2%) (Figure 1B). In contrast, the levels of several photosynthetic components, including the δ‐subunit of ATP synthase, Rubisco, and chlorophylls, were similar between the wild type and transgenic plants (Figure 1C–E). Therefore, we assume that alterations in photosynthetic properties are primarily the result of the reduction in the Cyt b 6 /f complex in transgenic plants. These results validate the transgenic lines as an effective model for studying the role of Cyt b 6 /f in photosynthetic regulation under varying light conditions.

FIGURE 1.

FIGURE 1

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Characterization of the Cyt b 6 /f complex content in transgenic plants. (A) Proteins in the leaf extract were separated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS‐PAGE) and immunodetected with specific antibodies. The extracted proteins for each sample were reloaded on gel using an equal leaf area basis; the series of dilutions for wild‐type plants are indicated. (B) Relative content of the Rieske FeS protein of Cyt b 6 /f complex, (C) Relative content of the δ‐subunit of ATP synthase, (D) Rubisco content (μmol m−2) and (E) Chlorophyll a + b content (mmol m−2) in wild‐type (WT) and transgenic plants categorized into “Middle” and “Low” groups based on Cyt b 6 /f content: The wild type (WT; approximately 100%), plants with intermediate Rieske FeS level (43.6% to 76.2%), and plants with low Rieske FeS level (18.5% to 42.2%). Data are presented as means ± SE (n = 3–5, *p < 0.05).

3.2. Proton Motive Force (pmf) and Its Components

The proton motive force (pmf) and its components, ΔpH and ΔΨ, were measured to assess the impact of Cyt b 6 /f reduction on energy conversion (Figure 2A–C; for detailed statistical analyses and raw ECS traces, see Figures S1 and S2, respectively). In WT plants, the highest pmf was observed at all PPFDs, followed by the “Middle” and “Low” groups (Figures 2A and S1A). The pattern of ΔΨ in WT plants, which decreased under high light, is consistent with several recent studies (Klughammer et al. 2013; Takagi et al. 2017; Takizawa et al. 2007). In our transgenic lines, both the ΔpH and ΔΨ components also tended to decrease as the Cyt b 6 /f content was reduced. Notably, ΔΨ was nearly absent in “Low” plants, making pmf in this group almost entirely dependent on ΔpH (Figures 2C and S1C). These findings indicate that reductions in Cyt b 6 /f content impair energy transduction efficiency, particularly by diminishing ΔΨ, a component of pmf.

FIGURE 2.

FIGURE 2

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Proton motive force (pmf) and its components in WT and transgenic plants. Light response curves of (A) pmf, (B) ΔpH, and (C) ΔΨ components in WT, “Middle,” and “Low” plants. Each parameter was normalized by a single‐turnover flash for 8 μs, with measurements conducted at varying light intensities (μmol m−2 s−1). Data are presented as means ± SE (n = 3).

3.3. Light Response Curves for PSII and PSI Parameters

The light response curves for PSII and PSI parameters revealed the extent to which Cyt b 6 /f reduction affected photosynthetic electron transport (Figure 3). For PSII, Y(II) (Figures 3A and S3A) and qL (Figures 3C and S3C) were stable in “Middle” plants but significantly reduced in “Low” plants, indicating impaired electron transport in the latter. Compared to WT, NPQ (Figures 3B and S3B) was already reduced by about half in “Middle” plants and showed a further significant decrease in “Low” plants.

FIGURE 3.

FIGURE 3

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Light response curves of PSII and PSI parameters. PSII parameters: (A) quantum yield of PSII (Y(II)), (B) non‐photochemical quenching (NPQ), and (C) the redox state of the plastoquinone pool (qL) in WT, “Middle,” and “Low” plants. PSI parameters: (D) quantum yield of PSI (Y(I)), (E) donor‐side limitation of PSI (Y(ND)), and (F) acceptor‐side limitation of PSI (Y(NA)). Data are presented as means ± SE (n = 3).

Y(I) in “Middle” plants were similar to those in WT, but those in “Low” plants were decreased severely (Figures 3D and S3D). In the “Low” group, Y(ND) increased substantially (Figures 3E and S3E), whereas Y(NA) declined (Figures 3F and S3F), indicating severe donor‐side limitations on PSI electron flow. These results demonstrate that moderate reductions in Cyt b 6 /f have minimal effects on electron flow in PSII and PSI, whereas severe reductions suppress both PSII and PSI electron transport rates and NPQ.

3.4. Photosynthetic Performance Under Fluctuating Light

PSII and PSI performance in dark‐treated leaves was evaluated under fluctuating light conditions, in which the high and low light phases alternated (Figure 4). Y(II) and qL in “Low” plants reached lower steady‐state levels during the high light phase and, in the low light phase, these were also lower in “Low” plants than in WT (Figure 4A,C). NPQ induction during high light was delayed and lower in “Low” plants, and its relaxation during low light was small (Figure 4B). These patterns indicate that “Low” plants have a lower capacity for electron transport and PSII photoprotection under dynamic light conditions. For PSI, the patterns observed under fluctuating light were consistent with those observed under constant light (Figure 4). These findings confirm that Cyt b 6 /f plays a critical role in regulating PSII performance under fluctuating light conditions.

FIGURE 4.

FIGURE 4

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Photosynthetic responses to fluctuating light in WT and “Low” plants. (A–C) PSII parameters: Y(II), NPQ, and qL under alternating high (800 μmol m−2 s−1, 10 min) and low (30 μmol m−2 s−1, 15 min) light phases in WT and “Low” plants. (D–F) PSI parameters: Y(I), Y(ND), and Y(NA) under the same fluctuating light conditions in WT and “Low” plants. Data are presented as means ± SE (n = 3).

3.5. PSI Photoinhibition Under Fluctuating Light

To assess photoinhibition, dark‐treated leaves were exposed to fluctuating light with high‐light pulses superimposed on a weak background light. PSI photoinhibition was evaluated by measuring Pm, and PSII photoinhibition was assessed using Fv/Fm (Figure 5). For PSI, WT plants showed no photoinhibition at 2000 μmol m−2 s−1 but showed significant reductions in Pm at 3000 μmol m−2 s−1. At 20,000 μmol m−2 s−1, Pm in WT plants declined to less than 20% of the initial levels. Remarkably, the “Low” plants exhibited complete resistance to PSI photoinhibition, maintaining stable Pm levels even at the highest light intensity (Figure 5A).

FIGURE 5.

FIGURE 5

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PSI and PSII photoinhibition under sunfleck‐like fluctuating light. (A) Relative Pm values (%) and (B) Relative Fv/Fm values (%) before and after exposure to fluctuating light with strong light pulses (800 ms) at 2000, 3000, or 20,000 μmol m−2 s−1, in WT and plants with low Rieske FeS levels. Data are presented as means ± SE (n = 3, *p < 0.05).

For PSII, Fv/Fm values remained stable at 2000 and 3000 μmol m−2 s−1 but decreased to ~50% in both WT and “Low” plants at 20,000 μmol m−2 s−1, indicating comparable susceptibility to photoinhibition (Figure 5B). These results highlight the selective role of Cyt b 6 /f in protecting PSI from photoinhibition under extreme light conditions, whereas PSII vulnerability appears to be less dependent on Cyt b 6 /f content against sunfleck‐type fluctuating light.

4. Discussion

This study provides new insights into how the modulation of the Cyt b 6 /f complex influences photosynthetic regulation and PSI stability under FL. Our results demonstrate that a substantial reduction in Cyt b 6 /f content in “Low” plants effectively protects PSI against FL‐induced photoinhibition, even under extreme light intensities (Figures 1 and 5), while compromising overall photosynthetic capacity (Figures 2, 3, 4). These findings highlight a key trade‐off between photoprotection and productivity, which is governed by the electron transport capacity mediated by Cyt b 6 /f.

The Cyt b 6 /f complex is well recognized as a central regulatory node in the photosynthetic electron transport chain, controlling the flow of electrons from PSII to PSI and contributing to the generation of the proton motive force (Tikhonov 2013; Yamori et al. 2011). Beyond its contribution to pmf formation, the rate‐limiting nature of plastoquinol oxidation at the Qo site of Cyt b 6 /f makes it a dynamic modulator of the PSI redox state, especially under fluctuating light. During the high‐light phases, the slowing of electron flow to PSI via photosynthetic control helps prevent over‐reduction of the PSI acceptor side, thus reducing the risk of photoinhibition (Kono and Terashima 2016; Miyake 2020; Yamamoto and Shikanai 2019; Degen and Johnson 2024). Our finding that PSI in “Low” plants remained fully functional even under extreme lightflecks of 20,000 μmol m−2 s−1 indicates that reduced Cyt b 6 /f content enforces a strong upstream bottleneck, which maintains P700 in an oxidized state and prevents the accumulation of reduced intermediates.

This mechanism is supported by recent observations in Alocasia odora, a deep‐shade species that naturally resists PSI photoinhibition under strong sunflecks (Terashima et al. 2021). In that study, a low Cyt b 6 /f per P700 ratio was observed, suggesting that limited electron input into PSI is a naturally selected strategy in shaded environments with frequent sunflecks. However, such ecological correlations alone cannot directly clarify the causal mechanisms. In contrast, our study employed a transgenic approach to isolate the effect of Cyt b 6 /f reduction, providing mechanistic evidence that restricting intersystem electron flow enhances PSI protection. Moreover, while A. odora maintained PSI robustness without compromising photosynthetic rates under moderate light, our “Low” plants exhibited clear limitations in PSII efficiency and NPQ regulation. This contrast illustrates a fundamental trade‐off: limiting the Cyt b 6 /f content confers PSI stability at the cost of reduced photosynthetic capacity.

Our study also characterized the photosynthetic parameters, except for the data for PSI photoinhibition under fluctuating light, in the plants with intermediate levels of Cyt b 6 /f reduction (“Middle” plants). These “Middle” plants generally exhibited intermediate photosynthetic responses, including pmf generation, Y(II), and Y(I) under steady‐state conditions, between WT and “Low” plants (Figures 2, 3, S1, and S3).

This suggests a dose‐dependent effect of Cyt b 6 /f content on several aspects of photosynthetic electron transport and energy conversion. While the absence of photoinhibition data for “Middle” plants prevents a direct assessment of a precise threshold for PSI protection, the remarkable resilience observed specifically in “Low” plants implies that a substantial, rather than moderate, reduction in Cyt b 6 /f content may be critical for conferring such robust PSI photoprotection under severe fluctuating light. This underscores the significance of the drastic reduction in Cyt b 6 /f as a key factor in the observed PSI stability, which is central to our findings with the “Low” plants.

It is also noteworthy that the ΔΨ component of the proton motive force was greatly diminished in “Low” plants, whereas ΔpH remained relatively active (Figure S2). While the shift in pmf components may contribute to NPQ regulation, our findings indicate that the primary mechanism of PSI protection lies in controlling the rate of electron flow rather than energy dissipation. This view is consistent with the understanding that Cyt b 6 /f not only contributes to pmf generation but also acts as a dynamic checkpoint that coordinates redox balance across the entire electron transport chain (Tikhonov 2013).

Together, our findings establish that the Cyt b 6 /f content plays a dual role in determining the balance between photoprotection and photosynthetic productivity. Under FL, particularly in shaded environments with transient high light, limiting Cyt b 6 /f content is a potentially adaptive strategy for stabilizing PSI and preventing irreversible damage. However, this comes at a physiological cost, manifested as lower PSII performance and impaired energy utilization. These results underscore the ecological and physiological relevance of Cyt b 6 /f regulation and offer new perspectives for improving photosynthetic resilience in crops under dynamic light conditions.

5. Conclusions

This study demonstrates that a drastic reduction in the Cyt b 6 /f complex content in transgenic tobacco (“Low” plants) confers remarkable protection to PSI against photoinhibition induced by fluctuating light, even under exceptionally high irradiances. The primary mechanism underlying this enhanced PSI stability is likely the restriction of electron flow from PSII to PSI, which helps maintain the PSI reaction center, P700, in a more oxidized and less vulnerable state. However, this significant gain in PSI photoprotection in “Low” plants is achieved at the expense of the overall photosynthetic capacity. The reduced Cyt b 6 /f content led to diminished PSII efficiency, a more reduced plastoquinone pool, and impaired non‐photochemical quenching (NPQ) responses, indicating a compromised ability to utilize light energy efficiently under both steady‐state and fluctuating light conditions. Our findings establish a critical trade‐off between robust PSI photoprotection and optimal photosynthetic productivity, a balance that is significantly dictated by the abundance of the Cyt b 6 /f complex. Therefore, the capacity of the Cyt b 6 /f complex to modulate intersystem electron transport is a key determinant of how plants navigate the challenges of dynamic light environments. Understanding the molecular basis of this trade‐off and the regulatory role of the Cyt b 6 /f complex offers valuable insights into plant adaptive strategies and provides a potential avenue for developing crops with enhanced resilience to variable and potentially damaging light conditions.

Author Contributions

M.K., I.T., and W.Y. conceived and designed the study. M.K., H.K., K.T., and W.Y. conducted experiments and performed data analysis. M.K., I.T., and W.Y. wrote the manuscript. All authors discussed the results and revised the manuscript.

Disclosure

The authors have nothing to report.

Supporting information

Figure S1: Results of Tukey's HSD test (p < 0.05) for the data of Figure 2. The values represent the mean ± SE. Different letters indicate significant differences among the Rieske FeS content.

Figure S2: Representative raw traces of the dark‐interval relaxation kinetics (DIRK) of the electrochromic shift (ECS) signal. The data shown are representative traces used for the partitioning analysis presented in Figure 2. Traces are shown for wild‐type (WT), “Middle,” and “Low” plants (columns, from left to right) under actinic light (AL) intensities of 1600, 800,135 and 30 μmol m−2 s−1 (rows, from top to bottom). The ECS signals were normalized to the amplitude induced by a single‐turnover saturating flash for 8 μs. Arrows indicate the point at which the AL was switched off.

Figure S3: Results of Tukey's HSD test (p < 0.05) for the data of Figure 3. The values represent the mean ± SE. Different letters indicate significant differences among the Rieske FeS content.

PPL-177-e70483-s001.pdf (235.1KB, pdf)

Kono, M. , Kodama H., Tanigawa K., Terashima I., and Yamori W.. 2025. “Drastic Reduction in Cytochrome b6/f Complex Confers Robust PSI Photoprotection Under Fluctuating Light at the Expense of Photosynthetic Capacity.” Physiologia Plantarum 177, no. 5: e70483. 10.1111/ppl.70483.

Handling Editor: A. Krieger‐Liszkay

Funding: This work was supported by grants from Japan Society for the Promotion of Science (24K09493 to M.K.; 18KK0170, 21H02171, and 24H02277 to W.Y.; 22H02640 to I.T.).

Masaru Kono and Hiromasa Kodama contributed equally to this work.

Contributor Information

Masaru Kono, Email: konom07@bs.s.u-tokyo.ac.jp.

Wataru Yamori, Email: yamori@g.ecc.u-tokyo.ac.jp.

Data Availability Statement

Supporting data can be requested by contacting the corresponding author.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Figure S1: Results of Tukey's HSD test (p < 0.05) for the data of Figure 2. The values represent the mean ± SE. Different letters indicate significant differences among the Rieske FeS content.

Figure S2: Representative raw traces of the dark‐interval relaxation kinetics (DIRK) of the electrochromic shift (ECS) signal. The data shown are representative traces used for the partitioning analysis presented in Figure 2. Traces are shown for wild‐type (WT), “Middle,” and “Low” plants (columns, from left to right) under actinic light (AL) intensities of 1600, 800,135 and 30 μmol m−2 s−1 (rows, from top to bottom). The ECS signals were normalized to the amplitude induced by a single‐turnover saturating flash for 8 μs. Arrows indicate the point at which the AL was switched off.

Figure S3: Results of Tukey's HSD test (p < 0.05) for the data of Figure 3. The values represent the mean ± SE. Different letters indicate significant differences among the Rieske FeS content.

PPL-177-e70483-s001.pdf (235.1KB, pdf)

Data Availability Statement

Supporting data can be requested by contacting the corresponding author.

Articles from Physiologia Plantarum are provided here courtesy of Wiley

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