Induction Of Non-photochemical Quenching Research Articles

Non-photochemical quenching may contribute to the dominance of the pale mat-forming lichen Cladonia stellaris over the sympatric melanic Cetraria islandica.

The mat-forming fruticose lichens Cladonia stellaris and Cetraria islandica frequently co-occur on soils in sun-exposed boreal, subarctic, and alpine ecosystems. While the dominant reindeer lichen Cladonia lacks a cortex but produces the light-reflecting pale pigment usnic acid on its surface, the common but patchier Cetraria has a firm cortex sealed by the light-absorbing pigment melanin. By measuring reflectance spectra, high-light tolerance, photosynthetic responses, and chlorophyll fluorescence in sympatric populations of these lichens differing in fungal pigments, we aimed to study how they cope with high light while hydrated. Specimens of the two species tolerated high light equally well but with different protective mechanisms. The mycobiont of the melanic species efficiently absorbed excess light, consistent with a lower need for its photobiont to protect itself by non-photochemical quenching (NPQ). By contrast, usnic acid screened light at 450-700nm by reflectance and absorbed shorter wavelengths. The ecorticate usnic species with less efficient fungal light screening exhibited a consistently lower light compensation point and higher CO2 uptake rates than the melanic lichen. In both species, steady state NPQ rapidly increased at increasing light with no signs of light saturation. To compensate for less internal shading causing light fluctuations with a larger amplitude, the usnic lichen photobiont adjusted to changing light by faster induction and faster relaxation of NPQ rapidly transforming excess excitation energy to less damaging heat. The high and flexible NPQ tracking fluctuations in solar radiation probably contributes to the strong dominance of the usnic mat-forming Cladonia in open lichen-dominated heaths.

CBSX2 is required for the efficient oxidation of chloroplast redox-regulated enzymes in darkness.

Thiol/disulfide-based redox regulation in plant chloroplasts is essential for controlling the activity of target proteins in response to light signals. One of the examples of such a role in chloroplasts is the activity of the chloroplast ATP synthase (CFoCF1), which is regulated by the redox state of the CF1γ subunit and involves two cysteines in its central domain. To investigate the mechanism underlying the oxidation of CF1γ and other chloroplast redox-regulated enzymes in the dark, we characterized the Arabidopsis cbsx2 mutant, which was isolated based on its altered NPQ (non-photochemical quenching) induction upon illumination. Whereas in dark-adapted WT plants CF1γ was completely oxidized, a small amount of CF1γ remained in the reduced state in cbsx2 under the same conditions. In this mutant, reduction of CF1γ was not affected in the light, but its oxidation was less efficient during a transition from light to darkness. The redox states of the Calvin cycle enzymes FBPase and SBPase in cbsx2 were similar to those of CF1γ during light/dark transitions. Affinity purification and subsequent analysis by mass spectrometry showed that the components of the ferredoxin-thioredoxin reductase/thioredoxin (FTR-Trx) and NADPH-dependent thioredoxin reductase (NTRC) systems as well as several 2-Cys peroxiredoxins (Prxs) can be co-purified with CBSX2. In addition to the thioredoxins, yeast two-hybrid analysis showed that CBSX2 also interacts with NTRC. Taken together, our results suggest that CBSX2 participates in the oxidation of the chloroplast redox-regulated enzymes in darkness, probably through regulation of the activity of chloroplast redox systems in vivo.

The role of the pigment-protein complex LHCBM1 in nonphotochemical quenching in Chlamydomonas reinhardtii.

Nonphotochemical quenching (NPQ) is the process that protects photosynthetic organisms from photodamage by dissipating the energy absorbed in excess as heat. In the model green alga Chlamydomonas reinhardtii, NPQ is abolished in the knock-out mutants of the pigment-protein complexes LHCSR3 and LHCBM1. However, while LHCSR3 is a pH sensor and switches to a quenched conformation at low pH, the role of LHCBM1 in NPQ has not been elucidated yet. In this work, we combined biochemical and physiological measurements to study short-term high-light acclimation of npq5, the mutant lacking LHCBM1. In low light in the absence of this complex, the antenna size of PSII was smaller than in its presence; this effect was marginal in high light (HL), implying that a reduction of the antenna was not responsible for the low NPQ. The mutant expressed LHCSR3 at the wild-type level in HL, indicating that the absence of this complex is also not the reason. Finally, NPQ remained low in the mutant even when the pH was artificially lowered to values that can switch LHCSR3 to the quenched conformation. We concluded that both LHCSR3 and LHCBM1 are required for the induction of NPQ and that LHCBM1 is the interacting partner of LHCSR3. This interaction can either enhance the quenching capacity of LHCSR3 or connect this complex with the PSII supercomplex.

Characterizing compensatory mechanisms in the absence of photoprotective qE in Chlamydomonas reinhardtii.

Rapid fluctuations in the quantity and quality of natural light expose photosynthetic organisms to conditions when the capacity to utilize absorbed quanta is insufficient. These conditions can result in the production of reactive oxygen species and photooxidative damage. Non-photochemical quenching (NPQ) and alternative electron transport are the two most prominent mechanisms which synergistically function to minimize the overreduction of photosystems. In the green alga Chlamydomonas reinhardtii, the stress-related light-harvesting complex (LHCSR) is a required component for the rapid induction and relaxation of NPQ in the light-harvesting antenna. Here, we use simultaneous chlorophyll fluorescence and oxygen exchange measurements to characterize the acclimation of the Chlamydomonas LHCSR-less mutant (npq4lhcsr1) to saturating light conditions. We demonstrate that, in the absence of NPQ, Chlamydomonas does not acclimate to sinusoidal light through increased light-dependent oxygen consumption. We also show that the npq4lhcsr1 mutant has an increased sink capacity downstream of PSI and this energy flow is likely facilitated by cyclic electron transport. Furthermore, we show that the timing of additions of mitochondrial inhibitors has a major influence on plastid/mitochondrial coupling experiments.

Photosystem II monomeric antenna CP26 plays a key role in nonphotochemical quenching in Chlamydomonas.

Thermal dissipation of excess excitation energy, called nonphotochemical quenching (NPQ), is 1 of the main photoprotective mechanisms in oxygenic photosynthetic organisms. Here, we investigated the function of the monomeric photosystem II (PSII) antenna protein CP26 in photoprotection and light harvesting in Chlamydomonas reinhardtii, a model organism for green algae. We used CRISPR/Cas9 genome editing and complementation to generate cp26 knockout mutants (named k6#) that did not negatively affect CP29 accumulation, which differed from previous cp26 mutants, allowing us to compare mutants specifically deprived of CP26, CP29, or both. The absence of CP26 partially affected PSII activity, causing reduced growth at low or medium light but not at high irradiances. However, the main phenotype observed in k6# mutants was a more than 70% reduction of NPQ compared to the wild type (Wt). This phenotype was fully rescued by genetic complementation and complemented strains accumulating different levels of CP26, demonstrating that ∼50% of CP26 content, compared to the Wt, was sufficient to restore the NPQ capacity. Our findings demonstrate a pivotal role for CP26 in NPQ induction, while CP29 is crucial for PSII activity. The genetic engineering of these 2 proteins could be a promising strategy to regulate the photosynthetic efficiency of microalgae under different light regimes.

HHL1 and SOQ1 synergistically regulate nonphotochemical quenching in Arabidopsis

Nonphotochemical quenching (NPQ) is an important photoprotective mechanism that quickly dissipates excess light energy as heat. NPQ can be induced in a few seconds to several hours; most studies of this process have focused on the rapid induction of NPQ. Recently, a new, slowly induced form of NPQ, called qH, was found during the discovery of the quenching inhibitor suppressor of quenching 1 (SOQ1). However, the specific mechanism of qH remains unclear. Here, we found that hypersensitive to high light 1 (HHL1)-a damage repair factor of photosystem II-interacts with SOQ1. The enhanced NPQ phenotype of the hhl1 mutant is similar to that of the soq1 mutant, which is not related to energy-dependent quenching or other known NPQ components. Furthermore, the hhl1 soq1 double mutant showed higher NPQ than the single mutants, but its pigment content and composition were similar to those of the wildtype. Overexpressing HHL1 decreased NPQ in hhl1 to below wildtype levels, whereas NPQ in hhl1 plants overexpressing SOQ1 was lower than that in hhl1 but higher than that in the wildtype. Moreover, we found that HHL1 promotes the SOQ1-mediated inhibition of plastidial lipoprotein through its von Willebrand factor type A domain. We propose that HHL1 and SOQ1 synergistically regulate NPQ.

High light stress under phosphorus limitation in summer may accelerate diatom shift from Skeletonema to Chaetoceros in an oligotrophic coastal area of Japan

In the Seto Inland Sea, the largest semi-enclosed sea in Japan, the most dominant diatom in the past, Skeletonema spp., has been replaced by another diatom Chaetoceros spp. since the 1980s, and this shift is often explained as the result of oligotrophication. Based on previous observations of a shift from Skeletonema spp. to Chaetoceros spp. under prolonged sunny conditions, the recent increase in solar insolation over the last 30 years might have also accelerated the replacement of Skeletonema by Chaetoceros, especially during the summer when nutrient levels are relatively low and solar insolation is high. In our experiments, culture strains of Skeletonema costatum and Chaetoceros lorenzianus under severely nitrogen-limited conditions exhibited less non-photochemical quenching (NPQ) under prolonged exposure (1 h) to high light (800 µmol-photons m-2 s-1) and a decrease in photochemical quenching (qP) which was especially notable in S. costatum. Conversely, marked increases in NPQ were observed under severely phosphorus-limited conditions, even under short time exposure (30 s) to high light, even though the increase in NPQ could not relieve the decrease in qP, which was more apparent in S. costatum. These trends in NPQ and qP were attributed to the limited nutrients because replenishment of the nutrients led to a decrease in NPQ and an increase in qP. Interestingly, this recovery was faster in C. lorenzianus than S. costatum. The results showed that phosphorus depletion caused severe photoinhibition especially in S. costatum, irrespective of active NPQ induction. Further, given the severe phosphorus-limited conditions in the Seto Inland Sea for an extended period, we conducted competition experiments using continuous coculture of both species to simulate the typical summer environment where severe phosphorus limitation and high light occur. The results showed that the shift from S. costatum to C. lorenzianus was accelerated by continuous exposure to high light, which could explain the recent shift in the dominant species in the summer in the study area.

Elucidating the photosynthetic responses in chlorophyll-deficient soybean (Glycine max, L.) leaf

Chlorophyll (Chl)-deficient plants can potentially increase global surface albedo of mono-cropping systems, and simultaneously maintain a similar photosynthetic efficiency by increasing light canopy penetration and thus lowering investment in pigments. However, some previous studies have shown that pale mutants might reduce productivity in field conditions. Such lower yields were suspected to be due to loss of photosynthetic efficiency at leaf level during light fluctuations as a consequence of reduced capacity and slower relaxation of non-photochemical quenching (NPQ) of Chl fluorescence. In this paper, we tested this hypothesis by comparing, CO2 assimilation (A), photosystem II (PSII) efficiency (ΦPSII), photochemical quenching and NPQ, electron transport rate (ETR) and fluorescence yield (Fyield) in a green soybean (Glycine max L.) cultivar (Eiko) and in a Chl-deficient (MinnGold) mutant under dynamically fluctuating light conditions. MinnGold had significantly slower induction of ETR and lower A and ETR than Eiko, but there was little difference in ΦPSII between the two genotypes, suggesting that the lower photosynthesis of MinnGold was mainly due to lower light energy absorption by a Chl-deficient leaf. The NPQ capacity was also smaller in MinnGold than in Eiko. As for the kinetics of the rapidly inducible component of NPQ, MinnGold showed slower induction, not relaxation, than Eiko. The combination of the effect of Chl-deficiency on lower photosynthesis, NPQ capacity and slower NPQ induction may explain the lower biomass accumulation of MinnGold in the field. Our physiological observations, combined with fluorescence kinetics, can serve as a basis to parameterize Chl content in modelling radiative transfer and photosynthesis for upscaling measures of plant and ecosystem productivity by a big leaf model.

Adaptions of photosynthesis in sun and shade in populations of some Afromontane lichens

AbstractPhotosynthetic organisms have evolved a great variety of mechanisms to optimize their use of sunlight. Some of the clearest examples of adaptations can be seen by comparing photosynthesis in different species and in different individuals of the same species that grow under high and low light levels. While the adaptations of sun and shade higher plants have been relatively well studied, much less information is available on the photobionts of lichenized Ascomycetes. An important adaptation that can protect photosynthetic organisms from the potentially harmful effects of excess light is non-photochemical quenching (NPQ); NPQ can dissipate unused light energy as heat. Here we used chlorophyll fluorescence to compare the induction and relaxation of NPQ and the induction of electron transport (rETR) in collections of the same lichen species from exposed and from more shaded locations. All species have trebouxioid photobionts and normally grow in more exposed microhabitats but can also be readily collected from more shaded locations. Shade forms display generally higher NPQ, presumably to protect lichens from occasional rapid increases in light that occur during sunflecks. Furthermore, the NPQ of shade forms relaxes quickly when light levels are reduced, presumably to ensure efficient photosynthesis after a sunfleck has passed. The maximal relative electron transport rate is lower in shade than sun collections, probably reflecting a downregulation of photosynthetic capacity to reduce energy costs. We also compared collections of pale and melanized thalli from three species of shade lichens with Symbiochloris as their photobiont. Interestingly, NPQ in melanized thalli from slightly more exposed microhabitats induced and relaxed in a way that resembled shade rather than sun forms of the trebouxioid lichens. This might suggest that in some locations melanization induced during a temporary period of high light may be excessive and could potentially reduce photosynthesis later in the growing season. Taken together, the results suggest that lichen photobionts can flexibly adjust the amount and type of NPQ, and their levels of rETR in response to light availability.

High non-photochemical quenching of VPZ transgenic potato plants limits CO2 assimilation under high light conditions and reduces tuber yield under fluctuating light.

Under natural conditions, photosynthesis has to be adjusted to fluctuating light intensities. Leaves exposed to high light dissipate excess light energy in form of heat at photosystem II (PSII) by a process called non-photochemical quenching (NPQ). Upon fast transition from light to shade, plants lose light energy by a relatively slow relaxation from photoprotection. Combined overexpression of violaxanthin de-epoxidase (VDE), PSII subunit S (PsbS) and zeaxanthin epoxidase (ZEP) in tobacco accelerates relaxation from photoprotection, and increases photosynthetic productivity. In Arabidopsis, expression of the same three genes (VPZ) resulted in a more rapid photoprotection but growth of the transgenic plants was impaired. Here we report on VPZ expressing potato plants grown under various light regimes. Similar to tobacco and Arabidopsis, induction and relaxation of NPQ was accelerated under all growth conditions tested, but did not cause an overall increased photosynthetic rate or growth of transgenic plants. Tuber yield of VPZ expressing plants was unaltered as compared to control plants under constant light conditions and even decreased under fluctuating light conditions. Under control conditions, levels of the phytohormone abscisic acid (ABA) were found to be elevated, indicating an increased violaxanthin availability in VPZ plants. However, the increased basal ABA levels did not improve drought tolerance of VPZ transgenic potato plants under greenhouse conditions. The failure to benefit from improved photoprotection is most likely caused by a reduced radiation use efficiency under high light conditions resulting from a too strong NPQ induction. Mitigating this negative effect in the future might help to improve photosynthetic performance in VPZ expressing potato plants.

Effects of enhanced UV-B radiation on photosynthetic performance and non-photochemical quenching process of intertidal red macroalgae Neoporphyra haitanensis

The effects of enhanced ultraviolet-B (UV-B, 280–320 nm) radiation reaching the Earth’s surface has become an important issue in terrestrial and marine ecosystems. Living in the land–marine boundary, intertidal macroalgae have various adaptive strategies to resist drastic environmental changes. In this study, we investigated the effects of enhanced UV-B radiation on the photosynthetic performance and photoprotective mechanisms of the red macroalgae Neoporphyra haitanensis . Our results showed that increasing UV-B radiation inhibited growth, damaged chloroplast structures and photosynthetic pigments, and reduced PSII activity and recovery ability in N. haitanensis . UV-B radiation significantly increased non-photochemical quenching (NPQ), indicating that NPQ is an effective photoprotection strategy in N. haitanensis . The transthylakoid proton gradient (ΔpH) uncoupler ammonium chloride (NH 4 Cl) significantly inhibited NPQ induction, showing that the induction of NPQ in N. haitanensis was dependent on ΔpH. Furthermore, violaxanthin and antheraxanthin, essential components of the xanthophyll cycle for modulating NPQ, were not detected in N. haitanensis , indicating the absence of the xanthophyll cycle in N. haitanensis . However, the increased NPQ under UV-B radiation was still associated with the accumulation of zeaxanthin. These findings will be useful for revealing adaptive strategies of intertidal macroalgae to cope with increasing UV-B radiation in the future. • Enhanced UV-B radiation significantly reduces photosynthetic performance of N. haitanensis. • NPQ can be used as effective strategy for photoprotection of N. haitanensis to resist UV-B radiation. • Although xanthophyll cycle is absent in N. haitanensis , NPQ is still regulated by the accumulation of zeaxanthin.

An underlying mechanism of qE deficiency in marine angiosperm Zostera marina.

Non-photochemical quenching (NPQ) of photosystem II (PSII) fluorescence is one of the most important protective mechanisms enabling the survival of phototropic organisms under high-light conditions. A low-efficiency NPQ, characterized by weak NPQ induction capacity and a low level of protective NPQ, was observed in the marine angiosperm Zostera marina, which inhabits the shallow water regions. Furthermore, chlorophyll fluorescence and Western blot analysis verified that the fast-inducted component of NPQ, i.e., the energy-dependent quenching (qE), was not present in this species. In contrast with the lack of PSII antenna quenching sites for qE induction in brown algae and the lack of functional XC in Ulvophyceae belonging to green algae, all the antenna proteins and the functional XC are present in Z. marina. A novel underlying mechanism was observed that the limited construction of the trans-thylakoid proton gradient (ΔpH) caused by photoinactivation of the oxygen evolving complex (OEC) did not induce protonation of PsbS, thus explaining the inability to form quenching sites for qE induction. Although the ΔpH established under light exposure activated violaxanthin (V) de-epoxidase enzyme to catalyze conversion of V via antheraxanthin (A) and then to zeaxanthin (Z), the quenching capacity of de-epoxidized pigment was weak in Z. marina. We suggest that the low-efficiency NPQ was conducive to efficiently utilize the limited electrons to perform photosynthesis, resisting the adverse effect of OEC photoinactivation on the photosynthetic rate.

Inhibition of non-photochemical quenching increases functional absorption cross-section of photosystem II as excitation from closed reaction centres is transferred to open centres, facilitating earlier light saturation of photosynthetic electron transport.

Induction of non-photochemical quenching (NPQ) of chlorophyll fluorescence in leaves affords photoprotection to the photosynthetic apparatus when, for whatever reason, photon capture in the antennae of photosystems exceeds their capacity to utilise this excitation in photochemistry and ultimately in CO2 assimilation. Here we augment traditional monitoring of NPQ using the fast time resolution, remote and relatively non-intrusive light induced fluorescence transient (LIFT) technique (Kolber et al . 2005 ; Osmond et al . 2017 ) that allows direct measurement of functional (σ'PSII ) and optical cross-sections (a 'PSII ) of PSII in situ , and calculates the half saturation light intensity for ETR (E k ). These parameters are obtained from the saturation and relaxation phases of fluorescence transients elicited by a sequence of 270, high intensity 1 μs flashlets at controlled time intervals over a period of 30 ms in the QA flash at intervals of a few seconds. We report that although σ'PSII undergoes large transient increases after transfer from dark to strong white light (WL) it declines little in steady-state as NPQ is induced in shade- and sun-grown spinach and Arabidopsis genotypes Col , OEpsbs , pgr 5bkg , stn 7 and stn 7/8. In contrast, σ'PSII increases by ~30% when induction of NPQ in spinach is inhibited by dithiothreitol and by inhibition of NPQ in Arabidopsis npq 1, npq 4 and pgr 5. We propose this increase in σ'PSII arises as some excitation from closed PSII reaction centres is transferred to open centres when excitation partitioning to photochemistry (Y II ) and NPQ (Y NP ) declines, and is indicated by an increased excitation dissipation from closed PSII centres (Y NO , including fluorescence emission). Although E k increases following dissipation of excitation as heat when NPQ is engaged, it declines when NPQ is inhibited. Evidently photochemistry becomes more easily light saturated when excitation is transferred from closed RCIIs to open centres with larger σ'PSII . The NPQ mutant pgr 5 is an exception; E k increases markedly in strong light as electron transport QA → PQ and PQ → PSI accelerate and the PQ pool becomes strongly reduced. These novel in situ observations are discussed in the context of contemporary evidence for functional and structural changes in the photosynthetic apparatus during induction of NPQ.

Photoprotection and antioxidative metabolism in Ulva lactuca exposed to coastal oceanic acidification scenarios in the presence of Irgarol

Anthropogenic changes such as ocean acidification, eutrophication, and the release of hazardous chemicals affect coastal environments and aquatic organisms. We investigated the effects of seawater pH (7.4 and 8.2) isolated and in combination with Irgarol on Ulva lactuca. Stress indicators such as membrane damage, lipid peroxidation, and hydrogen peroxide content were assessed. In addition, chlorophyll fluorescence and antioxidant enzyme activities were measured. The photosynthetic yield was affected by low pH in assays with and without Irgarol. However, the combination of low pH and Irgarol promoted photoinhibition, besides the induction of non-photochemical quenching (NPQ) and changes in photosynthetic pigment contents. The induction of NPQ was directly influenced by low pH. The membrane damage was increased in low pH with and without Irgarol exposure. Total soluble protein and carbohydrate contents decreased in low pH, and in presence of Irgarol. The H2O2 content and lipid peroxidation were not affected by low pH. In contrast, Irgarol exposure strongly increased lipid peroxidation in both pHs, suggesting a possible synergistic effect. To avoid the harmful effects of high H2O2, U. lactuca increased antioxidant enzyme activities in treatments under low pH and in presence of Irgarol. Our results indicate that U. lactuca is tolerant to low pH by inducing NPQ, changing pigment contents, and increasing antioxidant defenses. In contrast, these protective mechanisms could not avoid the harmful effects of the combination with Irgarol.

Chlamydomonas reinhardtii LHCSR1 and LHCSR3 proteins involved in photoprotective non-photochemical quenching have different quenching efficiency and different carotenoid affinity

Microalgae are unicellular photosynthetic organisms considered as potential alternative sources for biomass, biofuels or high value products. However, their limited biomass productivity represents a bottleneck that needs to be overcome to meet the applicative potential of these organisms. One of the domestication targets for improving their productivity is the proper balance between photoprotection and light conversion for carbon fixation. In the model organism for green algae, Chlamydomonas reinhardtii, a photoprotective mechanism inducing thermal dissipation of absorbed light energy, called Non-photochemical quenching (NPQ), is activated even at relatively low irradiances, resulting in reduced photosynthetic efficiency. Two pigment binding proteins, LHCSR1 and LHCSR3, were previously reported as the main actors during NPQ induction in C. reinhardtii. While previous work characterized in detail the functional properties of LHCSR3, few information is available for the LHCSR1 subunit. Here, we investigated in vitro the functional properties of LHCSR1 and LHCSR3 subunits: despite high sequence identity, the latter resulted as a stronger quencher compared to the former, explaining its predominant role observed in vivo. Pigment analysis, deconvolution of absorption spectra and structural models of LHCSR1 and LHCR3 suggest that different quenching efficiency is related to a different occupancy of L2 carotenoid binding site.

The coordination of OsbZIP72 and OsMYBS2 with reverse roles regulates the transcription of OsPsbS1 in rice.

Nonphotochemical quenching (NPQ), an intricate photoprotective process, plays fundamental roles in maintaining plant fitness. The PsbS protein is essential for the rapid induction of NPQ, and acts in a dose-dependent manner in leaves. However, little information is known on the transcriptional control of PsbS in land plants. Here we demonstrated that the expression of OsPsbS1 is directly upregulated by OsbZIP72 while repressed by OsMYBS2 in rice. We identified a new cis-element GACAGGTG in japonica OsPsbS1 promoter, to which OsbZIP72 could strongly bind and activate the expression of OsPsbS1. The new cis-element CTAATC confers specific binding for OsMYBS2 in japonica OsPsbS1 promoter. OsbZIP72 can be activated by SAPK1, and acts depending on the abscisic acid (ABA) signalling pathway. GF14A protein affects the repression activity of OsMYBS2 by regulating its nucleocytoplasmic shuttling, and Ser53 is necessary for OsMYBS2 to be retained in the cytoplasm. The inducibility of OsPsbS1 transcription under high light conditions in OsbZIP72 knockout lines was greatly impaired, while the repression of OsPsbS1 transcription under a low light environment in OsMYBS2 knockout lines was significantly alleviated. These results reveal cross-talk among NPQ processes, the ABA signalling pathway and abiotic stress signalling. The elaborate mechanisms may help enhance photoprotection and improve photosynthesis in rice.

Contrasting nonphotochemical quenching patterns under high light and darkness aligns with light niche occupancy in Arctic diatoms

AbstractOver the seasons, Arctic diatom species occupy shifting habitats defined by contrasting light climates, constrained by snow and ice cover dynamics interacting with extreme photoperiod and solar angle variations. How Arctic diatom photoadaptation strategies differ across their heterogeneous light niches remains a poorly documented but crucial missing link to anticipate Arctic Ocean responses to shrinking sea‐ice and increasing light. To address this question, we selected five Arctic diatom species with diverse life traits, representative of distinct light niches across the seasonal light environment continuum: from snow‐covered dimly lit bottom ice to summer stratified waters. We studied their photoacclimation plasticity to two growth light levels and the subsequent responses of their nonphotochemical quenching (NPQ) and xanthophyll cycle to both dark incubations and light shifts. We deciphered NPQ and xanthophyll cycle tuning in darkness and their light‐dependent induction kinetics, which aligned with species' light niche occupancy. In ice‐related species, NPQ was sustained in darkness and its induction was more reactive to moderate light shifts. Open‐water species triggered strong NPQ induction in darkness and reached higher maximal NPQ under high light. Marginal ice zone species showed strong adaptation to light fluctuations with a dark response fine‐tuned depending upon light history. We argue these traits are anchored in diverging photoadaption strategies fostering Arctic diatom success in their respective light niches.

High salt stress in the upper part of floating mats of Ulva prolifera, a species that causes green tides, enhances non-photochemical quenching.

Salt stress is a major abiotic stress factor that can induce many adverse effects on photosynthetic organisms. Plants and algae have developed several mechanisms that help them respond to adverse environments. Non-photochemical quenching (NPQ) is one of these mechanisms. The thalli of algae in the intertidal zone that are attached to rocks can be subjected to salt stress for a short period of time due to the rise and fall of the tide. Ulva prolifera causes green tides and can form floating mats when green tides occur and the upper part of the thalli is subjected to high salt stress for a long period of time. In this study, we compared the Ulvaprolifera photosynthetic activities and NPQ kinetics when it is subjected to different salinities over various periods of time. Thalli exposed to a salinity of 90 for 4d showed enhanced NPQ, and photosynthetic activities decreased from 60min after exposure up to 4d. This indicated that the induction of NPQ in Ulvaprolifera under salt stress was closely related to the stressing extent and stressing time. The enhanced NPQ in the treated samples exposed for 4d may explain why the upper layer ofthe floating mats formed by Ulvaprolifera thalli were able to survive inthe harsh environment. Further inhibitorexperiments demonstrated that theenhanced NPQ was xanthophyll cycle and transthylakoid proton gradient-dependent. However, photosystem II subunit S and light-harvesting complex stress-related protein didn't over accumulate and may not be responsible for the enhanced NPQ.

In vitro and in vivo investigation of chlorophyll binding sites involved in non-photochemical quenching in Chlamydomonas reinhardtii.

Non‐photochemical quenching (NPQ) of the light energy absorbed is one of the main photoprotective mechanisms evolved by oxygenic photosynthetic organisms to avoid photodamage, at a cost of reduced photosynthetic efficiency. Tuning of NPQ has been reported as a promising biotechnological strategy to increase productivity in both higher plants and unicellular microalgae. Engineering of NPQ induction requires the comprehension of its molecular mechanism(s), strongly debated in the last three decades with several different models proposed. In this work, the molecular details of NPQ induction was investigated at intramolecular level by in vitro and in vitro site‐specific mutagenesis on chlorophyll binding sites of the Light‐Harvesting Complex Stress‐Related 3 (LHCSR3) protein, the pigment binding complexes identified as the quencher during NPQ induction in the model organism for green algae Chlamydomonas reinhardtii. The results obtained demonstrate a correlation between the quenching activity of LHCSR3 variants in vitro and the NPQ phenotypes observed in vivo. In particular, multiple quenching sites in LHCSR3 cooperatively dissipating the excitation energy were revealed with a peculiar role of Chl 613, a chromophore located a close distance to carotenoid binding site L1.

Changes in the photosynthesis properties and photoprotection capacity in rice (Oryza sativa) grown under red, blue, or white light.

Non-photochemical quenching (NPQ) of the excited state of chlorophyll a is a major photoprotective mechanism plants utilize to survive under high light. Here, we report the impact of long-term light quality treatment on photosynthetic properties, especially NPQ in rice. We used three LED-based light regimes, i.e., red (648-672nm), blue (438-460nm), and "warm" white light (529-624nm), with the incident photon flux density of 300µmolphotonsm-2s-1, the difference in the absorbed photon flux densities by leaves grown under different light quality being less than 7%. Our results show that blue light, as compared to white light, induced a significant decrease in Fv/Fm, a decreased rate of reduction of P700+ after P700 was completely oxidized; furthermore, blue light also induced higher NPQ with an increased initial speed of NPQ induction, which corresponds to the qE component of NPQ, and a lower maximum quantum yield of PSII, i.e., Y(II). In contrast, rice grown under long-term red light showed decreased Y(II) and increased NPQ, but with no change in Fv/Fm. Furthermore, we found that rice grown under either blue or red light showed decreased transcript abundance of both catalase and ascorbate peroxidase, together with an increased H2O2 content, as compared to rice grown under white light. All these data suggest that even under a moderate incident light level, rice grown under blue or red light led to compromised antioxidant system, which contributed to decreased quantum yield of photosystem II and increased NPQ.