Plastidial transporters KEA1, -2, and -3 are essential for chloroplast osmoregulation, integrity, and pH regulation in Arabidopsis

Photoprotective strategies in the motile cryptophyte alga Rhodomonas salina-role of non-photochemical quenching, ions, photoinhibition, and cell motility

We explored photoprotective strategies in a cryptophyte alga Rhodomonas salina. This cryptophytic alga represents phototrophs where chlorophyll a/c antennas in thylakoids are combined with additional light-harvesting system formed by phycobiliproteins in the chloroplast lumen. The fastest response to excessive irradiation is induction of non-photochemical quenching (NPQ). The maximal NPQ appears already after 20 s of excessive irradiation. This initial phase of NPQ is sensitive to Ca²⁺ channel inhibitor (diltiazem) and disappears, also, in the presence of non-actin, an ionophore for monovalent cations. The prolonged exposure to high light of R. salina cells causes photoinhibition of photosystem II (PSII) that can be further enhanced when Ca²⁺ fluxes are inhibited by diltiazem. The light-induced reduction in PSII photochemical activity is smaller when compared with immotile diatom Phaeodactylum tricornutum. We explain this as a result of their different photoprotective strategies. Besides the protective role of NPQ, the motile R. salina also minimizes high light exposure by increased cell velocity by almost 25% percent (25% from 82 to 104 μm/s). We suggest that motility of algal cells might have a photoprotective role at high light because algal cell rotation around longitudinal axes changes continual irradiation to periodically fluctuating light.

Evidence for potassium transport activity of Arabidopsis KEA1-KEA6

Arabidopsis thaliana contains the putative K+ efflux transporters KEA1-KEA6, similar to KefB and KefC of Escherichia coli. KEA1-KEA3 are involved in the regulation of photosynthetic electron transport and chloroplast development. KEA4-KEA6 mediate pH regulation of the endomembrane network during salinity stress. However, the ion transport activities of KEA1-KEA6 have not been directly characterized. In this study, we used an E. coli expression system to examine KEA activity. KEA1-KEA3 and KEA5 showed bi-directional K+ transport activity, whereas KEA4 and KEA6 functioned as a K+ uptake system. The thylakoid membrane-localized Na+/H+ antiporter NhaS3 from the model cyanobacterium Synechocystis is the closest homolog of KEA3. Changing the putative Na+/H+ selective site of KEA3 (Gln-Asp) to that of NhaS3 (Asp-Asp) did not alter the ion selectivity without loss of K+ transport activity. The first residue in the conserved motif was not a determinant for K+ or Na+ selectivity. Deletion of the possible nucleotide-binding KTN domain from KEA3 lowered K+ transport activity, indicating that the KTN domain was important for this function. The KEA3-G422R mutation discovered in the Arabidopsis dpgr mutant increased K+ transport activity, consistent with the mutant phenotype. These results indicate that Arabidopsis KEA1-KEA6 act as K+ transport systems, and support the interpretation that KEA3 promotes dissipation of ΔpH in the thylakoid membrane.

Photosynthesis in Arabidopsis Is Unaffected by the Function of the Vacuolar K+ Channel TPK3

Photosynthesis is limited by the slow relaxation of nonphotochemical quenching, which primarily dissipates excess absorbed light energy as heat. Because the heat dissipation process is proportional to light-driven thylakoid lumen acidification, manipulating thylakoid ion and proton flux via transport proteins could improve photosynthesis. However, an important aspect of the current understanding of the thylakoid ion transportome is inaccurate. Using fluorescent protein fusions, we show that the Arabidopsis (Arabidopsis thaliana) two-pore K+ channel TPK3, which had been reported to mediate thylakoid K+ flux, localizes to the tonoplast, not the thylakoid. The localization of TPK3 outside of the thylakoids is further supported by the absence of TPK3 in isolated thylakoids as well as the inability of isolated chloroplasts to import TPK3 protein. In line with the subcellular localization of TPK3 in the vacuole, we observed that photosynthesis in the Arabidopsis null mutant tpk3-1, which carries a transfer DNA insertion in the first exon, remains unaffected. To gain a comprehensive understanding of how thylakoid ion flux impacts photosynthetic efficiency under dynamic growth light regimes, we performed long-term photosynthesis imaging of established and newly isolated transthylakoid K+- and Cl--flux mutants. Our results underpin the importance of the thylakoid ion transport proteins potassium cation efflux antiporter KEA3 and voltage-dependent chloride channel VCCN1 and suggest that the activity of yet unknown K+ channel(s), but not TPK3, is critical for optimal photosynthesis in dynamic light environments.

K+ and Cl- channels/transporters independently fine-tune photosynthesis in plants

In variable light environments, plants adjust light use in photosynthetic electron transport and photoprotective dissipation in the thylakoid membrane. In this respect, roles of the K⁺/H⁺ antiporter KEA3, the Cl^- channel/transporter CLCe and the voltage-dependent Cl^- channel VCCN1 have been unraveled in Arabidopsis thaliana. Here we report that they independently adjust photosynthesis on the basis of analyses using single and higher order loss-of-function mutants. In short experiments of photosynthetic response on transition from dark to low light, we reveal a sequential functioning of VCCN1 and CLCe in the activation of photoprotection and of KEA3 in its downregulation to a low steady state while adjusting the electron transport. On transition from low to high light, VCCN1 accelerates the activation of photoprotection, whereas KEA3 slows it down on transition from high to low light. Based on parallel electrochromic band shift measurements, the mechanism behind is that VCCN1 builds up a pH gradient across the thylakoid membrane, whereas KEA3 dissipates this gradient, which affects photoprotection. CLCe regulates photosynthesis by a pH-independent mechanism likely involving Cl^- homeostasis. Nevertheless, all genotypes grow well in alternating high and low light. Taken together, the three studied ion channels/transporters function independently in adjusting photosynthesis to the light environment.

Natural genetic variation of the photosynthetic induction response to fluctuating light environment

Field-grown plants experience fluctuating light intensity for periods extending from seconds to hours because of cloud movements and self-shading. When full light intensity returns after shading, the net CO₂ assimilation rate in leaves does not reach its maximum value immediately, but rises gradually over several minutes to approach a new steady state. This phenomenon has been termed photosynthetic induction, which substantially affects the efficiency of carbon fixation, and thus crop production. The significant natural variation of the speed of induction response exists among not only interspecies but also intraspecies. Recent advances in molecular analysis and high-throughput measurement techniques have revealed the genetic and eco-physiological basis of observed genetic variations in photosynthetic induction response. Here, we review the current understanding of the physiological and genetic mechanisms behind photosynthetic induction, and discusses routes to further advances.

The Potassium Transporter SlHAK10 Is Involved in Mycorrhizal Potassium Uptake

Most terrestrial plants form a root symbiosis with arbuscular mycorrhizal (AM) fungi, which receive fixed carbon from the plant and enhance the plant's uptake of mineral nutrients. AM symbiosis improves the phosphorous and nitrogen nutrition of host plants; however, little is known about the role of AM symbiosis in potassium (K+) nutrition. Here, we report that inoculation with the AM fungus Rhizophagus irregularis improved tomato (Solanum lycopersicum) plant growth and K+ acquisition and that K+ deficiency has a negative effect on root growth and AM colonization. Based on its homology to a Lotus japonicus AM-induced K+ transporter, we identified a mycorrhiza-specific tomato K+ transporter, SlHAK10 (Solanum lycopersicum High-affinity Potassium Transporter10), that was exclusively expressed in arbuscule-containing cells. SlHAK10 could restore a yeast K+ uptake-defective mutant in the low-affinity concentration range. Loss of function of SlHAK10 led to a significant decrease in mycorrhizal K+ uptake and AM colonization rate under low-K+ conditions but did not affect arbuscule development. Overexpressing SlHAK10 from the constitutive cauliflower mosaic virus 35S promoter or the AM-specific Solanum melongena Phosphate Transporter4 not only improved plant growth and K+ uptake but also increased AM colonization efficiency and soluble sugar content in roots supplied with low K+ Our results indicate that tomato plants have a SlHAK10-mediated mycorrhizal K+ uptake pathway and that improved plant K+ nutrition could increase carbohydrate accumulation in roots, which facilitates AM fungal colonization.

The genetic structure of flax illustrates environmental and anthropogenic selections that gave rise to its eco-geographical adaptation

Flax, one of the eight founder crops of agriculture, has been cultivated for its oil and/or fiber for millennia. Understanding genetic divergence and geographic origins of germplasm in line with their cultivation history and ecological adaptation are essential for conservation and breeding. Here we performed a genome-wide assessment based on more than 51,000 single nucleotide polymorphic sites defining 383 flax accessions from a core collection representing 37 flax growing countries. Population structure analysis resulted in a total of 12 populations that were pooled into four major groups: Temperate, South Asian, Abyssinian and Mediterranean. The vast majority (n = 335) belonged to the Temperate group that comprised eight populations including one dominated by fiber flax. Genetic variation between fiber and oil morphotypes was less pronounced than variation within morphotypes. The genetic variation among groups and populations was attributed in part to eco-geographic and anthropogenic factors. Genetic signatures indicated loci under strong selection by environmental factors such as day length. A high concentration of private haplotypes were observed in the South Asian, Mediterranean and Abyssinian populations despite their low genotype representation, hinting at the long history of the crop in these regions. The addition of genotypes from these three regions would enrich the core collection by capturing a wider genetic breadth for breeding and conservation.

Salt-Enhanced Reproductive Development of Suaeda salsa L. Coincided With Ion Transporter Gene Upregulation in Flowers and Increased Pollen K+ Content

Halophytes are adapted to saline environments and demonstrate optimal reproductive growth under high salinity. To gain insight into the salt tolerance mechanism and effects of salinity in the halophyte Suaeda salsa, the number of flowers and seeds, seed size, anther development, ion content, and flower transcript profiles, as well as the relative expression levels of genes involved in ion transport, were analyzed in S. salsa plants treated with 0 or 200 mM NaCl. The seed size, flower number, seed number per leaf axil, and anther fertility were all significantly increased by 200 mM NaCl treatment. The Na+ and Cl- contents in the leaves, stems, and pollen of NaCl-treated plants were all markedly higher, and the K+ content in the leaves and stems was significantly lower, than those in untreated control plants. By contrast, the K+ content in pollen grains did not decrease, but rather increased, upon NaCl treatment. Genes related to Na+, K+ and, Cl- transport, such as SOS1, KEA, AKT1, NHX1, and CHX, showed increased expression in the flowers of NaCl-treated plants. These results suggest that ionic homeostasis in reproductive organs, especially in pollen grains under salt-treated conditions, involves increased expression of ion transport-related genes.

Identification, characterization and expression profiling of cation-proton antiporter superfamily in Triticum aestivum L. and functional analysis of TaNHX4-B

The monovalent cation proton antiporter (CPA) superfamily comprises Na⁺/H⁺ exchanger (NHX), K⁺ efflux antiporter (KEA), and cation/H⁺ exchanger (CHX) family proteins, which play vital functions in plants. A total of 107 TaCPA proteins were identified in Triticum aestivum, and phylogenetically classified into 35 TaNHX, 24 TaKEA and 48 TaCHX proteins. These families had representatives derived from all three sub-genomes. TaKEA genes consisted of higher number of exons, followed by TaNHXs and TaCHXs. The occurrence of about 10 transmembrane regions and higher composition of helices and coils support their membrane-bound and hydrophobic nature. Diverse expression in various tissues and modulated expression under stress conditions suggested their role in development and in response to stress. Co-expression analyses revealed their complex interaction networks. Expression of TaNHX4-B.1 and TaNHX4-B.4 facilitated differential abiotic stress tolerance to Escherichia coli. Our study provides comprehensive information about CPA genes, which would be useful in their future functional characterization.

Golgi-localized cation/proton exchangers regulate ionic homeostasis and skotomorphogenesis in Arabidopsis

Multiple transporters and channels mediate cation transport across the plasma membrane and tonoplast to regulate ionic homeostasis in plant cells. However, much less is known about the molecular function of transporters that facilitate cation transport in other organelles such as Golgi. We report here that Arabidopsis KEA4, KEA5, and KEA6, members of cation/proton antiporters-2 (CPA2) superfamily were colocalized with the known Golgi marker, SYP32-mCherry. Although single kea4,5,6 mutants showed similar phenotype as the wild type under various conditions, kea4/5/6 triple mutants showed hypersensitivity to low pH, high K⁺ , and high Na⁺ and displayed growth defects in darkness, suggesting that these three KEA-type transporters function redundantly in controlling etiolated seedling growth and ion homeostasis. Detailed analysis indicated that the kea4/5/6 triple mutant exhibited cell wall biosynthesis defect during the rapid etiolated seedling growth and under high K⁺ /Na⁺ condition. The cell wall-derived pectin homogalacturonan (GalA)₃ partially suppressed the growth defects and ionic toxicity in the kea4/5/6 triple mutants when grown in the dark but not in the light conditions. Together, these data support the hypothesis that the Golgi-localized KEAs play key roles in the maintenance of ionic and pH homeostasis, thereby facilitating Golgi function in cell wall biosynthesis during rapid etiolated seedling growth and in coping with high K⁺ /Na⁺ stress.

Whole Irradiated Plant Leaves Showed Faster Photosynthetic Induction Than Individually Irradiated Leaves via Improved Stomatal Opening

Rapid photosynthetic induction is crucial for plants under fluctuating light conditions in a crop canopy as well as in an understory. Most previous studies have focused on photosynthetic induction responses in a single leaf, whereas the systemic responses of the whole plant have not been considered. In a natural environment, however, both single leaves and whole plants are exposed to sunlight, since the light environment is not uniform even within a given plant. In the present study, we examined whether there is any difference between the photosynthetic induction response of a leaf of a whole irradiated plant and an individually irradiated leaf in Arabidopsis thaliana to consider photosynthetic induction as the response of a whole plant. We used two methods, the visualization of photosynthesis and direct measurements of gas-exchange and Chl fluorescence, to demonstrate that whole irradiated plant promoted its photosynthetic induction via improved stomatal opening compared with individually irradiated leaf. Furthermore, using two Arabidopsis knockout mutants of abscisic acid transporter, abcg25 and abcg40, the present study suggests that abscisic acid could be involved in this systemic response for stomatal opening, allowing plants to optimize the use of light energy at minimal cost in plants in a dynamic light environment.

Fluctuating light experiments and semi-automated plant phenotyping enabled by self-built growth racks and simple upgrades to the IMAGING-PAM

BACKGROUND

Over the last years, several plant science labs have started to employ fluctuating growth light conditions to simulate natural light regimes more closely. Many plant mutants reveal quantifiable effects under fluctuating light despite being indistinguishable from wild-type plants under standard constant light. Moreover, many subtle plant phenotypes become intensified and thus can be studied in more detail. This observation has caused a paradigm shift within the photosynthesis research community and an increasing number of scientists are interested in using fluctuating light growth conditions. However, high installation costs for commercial controllable LED setups as well as costly phenotyping equipment can make it hard for small academic groups to compete in this emerging field.

RESULTS

We show a simple do-it-yourself approach to enable fluctuating light growth experiments. Our results using previously published fluctuating light sensitive mutants, stn7 and pgr5, confirm that our low-cost setup yields similar results as top-prized commercial growth regimes. Moreover, we show how we increased the throughput of our Walz IMAGING-PAM, also found in many other departments around the world. We have designed a Python and R-based open source toolkit that allows for semi-automated sample segmentation and data analysis thereby reducing the processing bottleneck of large experimental datasets. We provide detailed instructions on how to build and functionally test each setup.

CONCLUSIONS

With material costs well below USD$1000, it is possible to setup a fluctuating light rack including a constant light control shelf for comparison. This allows more scientists to perform experiments closer to natural light conditions and contribute to an emerging research field. A small addition to the IMAGING-PAM hardware not only increases sample throughput but also enables larger-scale plant phenotyping with automated data analysis.

Chloroplast-localized BICAT proteins shape stromal calcium signals and are required for efficient photosynthesis

The photosynthetic machinery of plants must be regulated to maximize the efficiency of light reactions and CO₂ fixation. Changes in free Ca²⁺ in the stroma of chloroplasts have been observed at the transition between light and darkness, and also in response to stress stimuli. Such Ca²⁺ dynamics have been proposed to regulate photosynthetic capacity. However, the molecular mechanisms of Ca²⁺ fluxes in the chloroplasts have been unknown. By employing a Ca²⁺ reporter-based approach, we identified two chloroplast-localized Ca²⁺ transporters in Arabidopsis thaliana, BICAT1 and BICAT2, that determine the amplitude of the darkness-induced Ca²⁺ signal in the chloroplast stroma. BICAT2 mediated Ca²⁺ uptake across the chloroplast envelope, and its knockout mutation strongly dampened the dark-induced [Ca²⁺ ]_stroma signal. Conversely, this Ca²⁺ transient was increased in knockout mutants of BICAT1, which transports Ca²⁺ into the thylakoid lumen. Knockout mutation of BICAT2 caused severe defects in chloroplast morphology, pigmentation and photosynthetic light reactions, rendering bicat2 mutants barely viable under autotrophic growth conditions, while bicat1 mutants were less affected. These results show that BICAT transporters play a role in chloroplast Ca²⁺ homeostasis. They are also involved in the regulation of photosynthesis and plant productivity. Further work will be required to reveal whether the effect on photosynthesis is a direct result of their role as Ca²⁺ transporters.

K+ Efflux Antiporters 4, 5, and 6 Mediate pH and K+ Homeostasis in Endomembrane Compartments

KEA4, KEA5, and KEA6 are members of the Arabidopsis (Arabidopsis thaliana) K⁺ efflux antiporter (KEA) family that share high sequence similarity but whose function remains unknown. Here, we show their gene expression pattern, subcellular localization, and physiological function in Arabidopsis. KEA4, KEA5, and KEA6 had similar tissue expression patterns, and the three KEA proteins localized to the Golgi, the trans-Golgi network, and the prevacuolar compartment/multivesicular bodies, suggesting overlapping roles of these proteins in the endomembrane system. Phenotypic analyses of single, double, and triple mutants confirmed functional redundancy. The triple mutant kea4 kea5 kea6 had small rosettes, short seedlings, and was sensitive to low K⁺ availability and to the sodicity imposed by high salinity. Also, the kea4 kea5 kea6 mutant plants had a reduced luminal pH in the Golgi, trans-Golgi network, prevacuolar compartment, and vacuole, in accordance with the K/H exchange activity of KEA proteins. Genetic analysis indicated that KEA4, KEA5, and KEA6 as well as endosomal Na⁺/H⁺exchanger5 (NHX5) and NHX6 acted coordinately to facilitate endosomal pH homeostasis and salt tolerance. Neither cancelling nor overexpressing the vacuolar antiporters NHX1 and NHX2 in the kea4 kea5 kea6 mutant background altered the salt-sensitive phenotype. The NHX1 and NHX2 proteins in the kea4 kea5 kea6 mutant background could not suppress the acidity of the endomembrane system but brought the vacuolar pH close to wild-type values. Together, these data signify that KEA4, KEA5, and KEA6 are endosomal K⁺ transporters functioning in maintaining pH and ion homeostasis in the endomembrane network.

Chloroplast Acetyltransferase NSI Is Required for State Transitions in Arabidopsis thaliana

The amount of light energy received by the photosynthetic reaction centers photosystem II (PSII) and photosystem I (PSI) is balanced through state transitions. Reversible phosphorylation of a light-harvesting antenna trimer (L-LHCII) orchestrates the association between L-LHCII and the photosystems, thus adjusting the amount of excitation energy received by the reaction centers. In this study, we identified the enzyme NUCLEAR SHUTTLE INTERACTING (NSI; AT1G32070) as an active lysine acetyltransferase in the chloroplasts of Arabidopsis thaliana Intriguingly, nsi knockout mutant plants were defective in state transitions, even though they had a similar LHCII phosphorylation pattern as the wild type. Accordingly, nsi plants were not able to accumulate the PSI-LHCII state transition complex, even though the LHCII docking site of PSI and the overall amounts of photosynthetic protein complexes remained unchanged. Instead, the nsi mutants showed a decreased Lys acetylation status of specific photosynthetic proteins including PSI, PSII, and LHCII subunits. Our work demonstrates that the chloroplast acetyltransferase NSI is needed for the dynamic reorganization of thylakoid protein complexes during photosynthetic state transitions.

Chloroplastic ATP synthase plays an important role in the regulation of proton motive force in fluctuating light

The proton motive force (pmf) across the thylakoid membranes plays a key role for photosynthesis in fluctuating light. However, the mechanisms underlying the regulation of pmf in fluctuating light are not well known. In this study, we aimed to identify the roles of chloroplastic ATP synthase and cyclic electron flow (CEF) around photosystem I (PSI) in the regulation of the pmf in fluctuating light. To do this, we measured chlorophyll fluorescence, P700 parameters, and the electrochromic shift signal in the fluctuating light alternating between 918 (high light) and 89 (low light) μmol photons m^-2 s^-1 every 5 min. We found that the activity of chloroplastic ATP synthase (g_H⁺), pmf, CEF activity, non-photochemical quenching (NPQ), and the P700 redox state changed rapidly in fluctuating light. During transition from low to high light, the decreased g_H⁺ and the stimulation of CEF both contributed to the rapid formation of pmf, activating NPQ and optimizing the redox state of P700 in PSI. During the low-light phases, g_H⁺ rapidly increased and the pmf declined sharply, leading to the relaxation of NPQ and down-regulation of photosynthetic control. These findings indicate that in fluctuating light the g_H⁺ and CEF are finely regulated to modulate the pmf formation, avoiding the over-accumulation of reactive intermediates and maximizing energy use efficiency.

Plant Endomembrane Dynamics: Studies of K+/H+ Antiporters Provide Insights on the Effects of pH and Ion Homeostasis

Plants remodel their cells through the dynamic endomembrane system. Intracellular pH is important for membrane trafficking, but the determinants of pH homeostasis are poorly defined in plants. Electrogenic proton (H⁺) pumps depend on counter-ion fluxes to establish transmembrane pH gradients at the plasma membrane and endomembranes. Vacuolar-type H⁺-ATPase-mediated acidification of the trans-Golgi network is crucial for secretion and membrane recycling. Pump and counter-ion fluxes are unlikely to fine-tune pH; rather, alkali cation/H⁺ antiporters, which can alter pH and/or cation homeostasis locally and transiently, are prime candidates. Plants have a large family of predicted cation/H⁺ exchangers (CHX) of obscure function, in addition to the well-studied K⁺(Na⁺)/H⁺ exchangers (NHX). Here, we review the regulation of cytosolic and vacuolar pH, highlighting the similarities and distinctions of NHX and CHX members. In planta, alkalinization of the trans-Golgi network or vacuole by NHXs promotes membrane trafficking, endocytosis, cell expansion, and growth. CHXs localize to endomembranes and/or the plasma membrane and contribute to male fertility, pollen tube guidance, pollen wall construction, stomatal opening, and, in soybean (Glycine max), tolerance to salt stress. Three-dimensional structural models and mutagenesis of Arabidopsis (Arabidopsis thaliana) genes have allowed us to infer that AtCHX17 and AtNHX1 share a global architecture and a translocation core like bacterial Na⁺/H⁺ antiporters. Yet, the presence of distinct residues suggests that some CHXs differ from NHXs in pH sensing and electrogenicity. How H⁺ pumps, counter-ion fluxes, and cation/H⁺ antiporters are linked with signaling and membrane trafficking to remodel membranes and cell walls awaits further investigation.

Ion and metabolite transport in the chloroplast of algae: lessons from land plants

Chloroplasts are endosymbiotic organelles and play crucial roles in energy supply and metabolism of eukaryotic photosynthetic organisms (algae and land plants). They harbor channels and transporters in the envelope and thylakoid membranes, mediating the exchange of ions and metabolites with the cytosol and the chloroplast stroma and between the different chloroplast subcompartments. In secondarily evolved algae, three or four envelope membranes surround the chloroplast, making more complex the exchange of ions and metabolites. Despite the importance of transport proteins for the optimal functioning of the chloroplast in algae, and that many land plant homologues have been predicted, experimental evidence and molecular characterization are missing in most cases. Here, we provide an overview of the current knowledge about ion and metabolite transport in the chloroplast from algae. The main aspects reviewed are localization and activity of the transport proteins from algae and/or of homologues from other organisms including land plants. Most chloroplast transporters were identified in the green alga Chlamydomonas reinhardtii, reside in the envelope and participate in carbon acquisition and metabolism. Only a few identified algal transporters are located in the thylakoid membrane and play role in ion transport. The presence of genes for putative transporters in green algae, red algae, diatoms, glaucophytes and cryptophytes is discussed, and roles in the chloroplast are suggested. A deep knowledge in this field is required because algae represent a potential source of biomass and valuable metabolites for industry, medicine and agriculture.

Hacking the thylakoid proton motive force for improved photosynthesis: modulating ion flux rates that control proton motive force partitioning into Δψ and ΔpH

There is considerable interest in improving plant productivity by altering the dynamic responses of photosynthesis in tune with natural conditions. This is exemplified by the 'energy-dependent' form of non-photochemical quenching (q_E), the formation and decay of which can be considerably slower than natural light fluctuations, limiting photochemical yield. In addition, we recently reported that rapidly fluctuating light can produce field recombination-induced photodamage (FRIP), where large spikes in electric field across the thylakoid membrane (Δψ) induce photosystem II recombination reactions that produce damaging singlet oxygen (¹O₂). Both q_E and FRIP are directly linked to the thylakoid proton motive force (pmf), and in particular, the slow kinetics of partitioning pmf into its ΔpH and Δψ components. Using a series of computational simulations, we explored the possibility of 'hacking' pmf partitioning as a target for improving photosynthesis. Under a range of illumination conditions, increasing the rate of counter-ion fluxes across the thylakoid membrane should lead to more rapid dissipation of Δψ and formation of ΔpH. This would result in increased rates for the formation and decay of q_E while resulting in a more rapid decline in the amplitudes of Δψ-spikes and decreasing ¹O₂ production. These results suggest that ion fluxes may be a viable target for plant breeding or engineering. However, these changes also induce transient, but substantial mismatches in the ATP : NADPH output ratio as well as in the osmotic balance between the lumen and stroma, either of which may explain why evolution has not already accelerated thylakoid ion fluxes. Overall, though the model is simplified, it recapitulates many of the responses seen in vivo, while spotlighting critical aspects of the complex interactions between pmf components and photosynthetic processes. By making the programme available, we hope to enable the community of photosynthesis researchers to further explore and test specific hypotheses.This article is part of the themed issue 'Enhancing photosynthesis in crop plants: targets for improvement'.

Potassium: A Vital Regulator of Plant Responses and Tolerance to Abiotic Stresses

Among the plant nutrients, potassium (K) is one of the vital elements required for plant growth and physiology. Potassium is not only a constituent of the plant structure but it also has a regulatory function in several biochemical processes related to protein synthesis, carbohydrate metabolism, and enzyme activation. Several physiological processes depend on K, such as stomatal regulation and photosynthesis. In recent decades, K was found to provide abiotic stress tolerance. Under salt stress, K helps to maintain ion homeostasis and to regulate the osmotic balance. Under drought stress conditions, K regulates stomatal opening and helps plants adapt to water deficits. Many reports support the notion that K enhances antioxidant defense in plants and therefore protects them from oxidative stress under various environmental adversities. In addition, this element provides some cellular signaling alone or in association with other signaling molecules and phytohormones. Although considerable progress has been made in understanding K-induced abiotic stress tolerance in plants, the exact molecular mechanisms of these protections are still under investigation. In this review, we summarized the recent literature on the biological functions of K, its uptake, its translocation, and its role in plant abiotic stress tolerance.

The Main Functions of Plastids

Plastids are semiautonomous organelles like mitochondria, and derive from a cyanobacterial ancestor that was engulfed by a host cell. During evolution, they have recruited proteins originating from the nuclear genome, and only parts of their ancestral metabolic properties were conserved and optimized to limit functional redundancy with other cell compartments. Furthermore, large disparities in metabolic functions exist among various types of plastids, and the characterization of their various metabolic properties is far from being accomplished. In this review, we provide an overview of the main functions, known to be achieved by plastids or shared by plastids and other compartments of the cell. In short, plastids appear at the heart of all main plant functions.

Genomic and Transcriptomic Compilation of Chloroplast Ionic Transporters of Physcomitrella patens. Study of NHAD Transporters in Na+ and K+ Homeostasis

K+ is widely used by plant cells, whereas Na+ can easily reach toxic levels during plant growth, which typically occurs in saline environments; however, the effects and functions in the chloroplast have been only roughly estimated. Traditionally, the occurrence of ionic fluxes across the chloroplast envelope or the thylakoid membranes has been mostly deduced from physiological measurements or from knowledge of chloroplast metabolism. However, many of the proteins involved in these fluxes have not yet been characterized. Based on genomic and RNA sequencing (RNA-seq) analyses, we present a comprehensive compilation of genes encoding putative ion transporters and channels expressed in the chloroplasts of the moss Physcomitrella patens, with a special emphasis on those related to Na+ and K+ fluxes. Based on the functional characterization of nhad mutants, we also discuss the putative role of NHAD transporters in Na+ homeostasis and osmoregulation of this organelle and the putative contribution of chloroplasts to salt tolerance in this moss. We demonstrate that NaCl does not affect the chloroplast functionality in Physcomitrella despite significantly modifying expression of ionic transporters and cellular morphology, specifically the chloroplast ultrastructure, revealing a high starch accumulation. Additionally, NHAD transporters apparently do not play any essential roles in salt tolerance.

Relative contributions of PGR5- and NDH-dependent photosystem I cyclic electron flow in the generation of a proton gradient in Arabidopsis chloroplasts

Respective contributions of PGR5- and NDH-dependent cyclic electron flows around photosystem I for generating the proton gradient across the thylakoid membrane are ~30 and ~5%. The proton concentration gradient across the thylakoid membrane (ΔpH) produced by photosynthetic electron transport is the driving force of ATP synthesis and non-photochemical quenching. Two types of electron transfer contribute to ΔpH formation: linear electron flow (LEF) and cyclic electron flow (CEF, divided into PGR5- and NDH-dependent pathways). However, the respective contributions of LEF and CEF to ΔpH formation are largely unknown. We employed fluorescence quenching analysis with the pH indicator 9-aminoacridine to directly monitor ΔpH formation in isolated chloroplasts of Arabidopsis mutants lacking PGR5- and/or NDH-dependent CEF. The results indicate that ΔpH formation is mostly due to LEF, with the contributions of PGR5- and NDH-dependent CEF estimated as only ~30 and ~5%, respectively.

Plasma membrane depolarization precedes photosynthesis damage and long-term leaf bleaching in (E)-chalcone-treated Arabidopsis shoots

The plant phenolic compound (E)-chalcone has been previously found to induce noticeable seedling size reduction and progressive de-greening (bleaching) in shoots of Arabidopsis thaliana seedlings. In this work, we demonstrate that this progressive de-greening occurring on Arabidopsis shoots after (E)-chalcone treatment, is directly linked to early plasma membrane depolarization and dramatic effects on chloroplasts structure and function. Later effects in chalcone-treated seedlings included ROS accumulation, pigment degradation, reduced photosynthetic activity, bleaching, and eventually cell death. De-greening and pigment degradation induced by (E)-chalcone were partially reversed when NaCl was added together with chalcone, which could be related to restoration of altered pH gradients. All these results suggest that rapid alteration of plasma membrane potential after chalcone treatment is a major component of the mode of action of (E)-chalcone on Arabidopsis metabolism.

Regulation of potassium transport and signaling in plants

As an essential macronutrient, potassium (K⁺) plays crucial roles in diverse physiological processes during plant growth and development. The K⁺ concentration in soils is relatively low and fluctuating. Plants are able to perceive external K⁺ changes and generate chemical and physical signals in plant cells. The signals can be transducted across the plasma membrane and into the cytosol, and eventually regulates the downstream targets, particularly K⁺ channels and transporters. As a result, K⁺ homeostasis in plant cells is modulated, which facilitates plant adaptation to K⁺ deficient conditions. This minireview focuses on the latest research progress in the diverse functions of K⁺ channels and transporters as well as their regulatory mechanisms in plant response to low-K⁺ stress.

FLUCTUATING-LIGHT-ACCLIMATION PROTEIN1, Conserved in Oxygenic Phototrophs, Regulates H+ Homeostasis and Non-Photochemical Quenching in Chloroplasts

Plants have mechanisms allowing them to acclimate to intense light conditions, which involves the dissipation of excess light energy. These mechanisms allow plants to perform photosynthesis efficiently and, therefore, must be accurately and precisely controlled. However, how plants dissipate excess light energy has yet to be fully elucidated. Herein we report the identification of a gene, which we named Fluctuating-Light-Acclimation Protein1 (FLAP1), that is conserved in oxygenic phototrophs. We show that Arabidopsis FLAP1 is associated with chloroplast thylakoid and envelope membranes and that the flap1 mutant shows delayed non-photochemical quenching (NPQ) relaxation during induction of photosynthesis at moderate light intensity. Under fluctuating light conditions, NPQ levels in the flap1 mutant were higher than those in the wild type during the high light period, and the mutant exhibited a pale-green phenotype. These findings suggest that FLAP1 is involved in NPQ control, which is important for an acclimation response to fluctuating light.

An update on the regulation of photosynthesis by thylakoid ion channels and transporters in Arabidopsis

In natural, variable environments, plants rapidly adjust photosynthesis for optimal balance between light absorption and utilization. There is increasing evidence suggesting that ion fluxes across the chloroplast thylakoid membrane play an important role in this regulation by affecting the proton motive force and consequently photosynthesis and thylakoid membrane ultrastructure. This article presents an update on the thylakoid ion channels and transporters characterized in Arabidopsis thaliana as being involved in these processes, as well as an outlook at the evolutionary conservation of their functions in other photosynthetic organisms. This is a contribution to shed light on the thylakoid network of ion fluxes and how they help plants to adjust photosynthesis in variable light environments.

Relaxation of cellular K+ gradients by valinomycin induces diatoxanthin accumulation in Cyclotella meneghiniana cells and alters FCPa fluorescence yield in vitro

Regulation of photosynthetic light harvesting involves all major thylakoid membrane complexes. One important factor is the proton motive force (pmf) driving ATP production. Its proton gradient (ΔpH) component regulates the high energy quenching. Potassium ions largely contribute to the formation of the electric field (ΔΨ). ΔΨ and ΔpH partially compensate each other to form pmf. Whilst in plants considerable progress has been made in analyzing the interplay of H⁺ and K⁺ gradients, in diatoms knowledge in this field is still scarce. We relaxed cellular K⁺ gradients by valinomycin in Cyclotella meneghiniana. We observed a slow decrease of PSII maximum quantum yield in the dark upon valinomycin addition correlating with diatoxanthin accumulation which we attribute to the breakdown of organellar K⁺ gradients (either plastid or mitochondria) which might compensate for the loss of the K⁺ gradient by adjustment of the thylakoid pH in a secondary step. This response is reversible when ΔpH is relaxed. Similarly, we found higher non-photochemical quenching (NPQ) caused by higher DT accumulation in the steady state in valinomycin-treated cells. In vitro fucoxanthin chlorophyll a (FCPa) antenna complexes in liposomes with natural lipid composition showed a decrease in fluorescence yield if a K⁺ gradient is built up. The effect reversed by relaxing the gradient. We interpret these fluorescence changes with surface charge dynamics and FCPa organization in the membrane rather than a direct influence of K⁺ gradients on FCPa complexes. Both experiments reveal that K⁺ gradients might contribute to fine tuning of light harvesting capacity in relation to pmf in diatoms.

Chloroplastic ATP synthase builds up a proton motive force preventing production of reactive oxygen species in photosystem I

Over-reduction of the photosynthetic electron transport (PET) chain should be avoided, because the accumulation of reducing electron carriers produces reactive oxygen species (ROS) within photosystem I (PSI) in thylakoid membranes and causes oxidative damage to chloroplasts. To prevent production of ROS in thylakoid membranes the H⁺ gradient (ΔpH) needs to be built up across the thylakoid membranes to suppress the over-reduction state of the PET chain. In this study, we aimed to identify the critical component that stimulates ΔpH formation under illumination in higher plants. To do this, we screened ethyl methane sulfonate (EMS)-treated Arabidopsis thaliana, in which the formation of ΔpH is impaired and the PET chain caused over-reduction under illumination. Subsequently, we isolated an allelic mutant that carries a missense mutation in the γ-subunit of chloroplastic CF₀ CF₁ -ATP synthase, named hope2. We found that hope2 suppressed the formation of ΔpH during photosynthesis because of the high H⁺ efflux activity from the lumenal to stromal side of the thylakoid membranes via CF₀ CF₁ -ATP synthase. Furthermore, PSI was in a more reduced state in hope2 than in wild-type (WT) plants, and hope2 was more vulnerable to PSI photoinhibition than WT under illumination. These results suggested that chloroplastic CF₀ CF₁ -ATP synthase adjusts the redox state of the PET chain, especially for PSI, by modulating H⁺ efflux activity across the thylakoid membranes. Our findings suggest the importance of the buildup of ΔpH depending on CF₀ CF₁ -ATP synthase to adjust the redox state of the reaction center chlorophyll P700 in PSI and to suppress the production of ROS in PSI during photosynthesis.

Impact of the ion transportome of chloroplasts on the optimization of photosynthesis

Ions play fundamental roles in all living cells, and their gradients are often essential to fuel transport, regulate enzyme activities, and transduce energy within cells. Regulation of their homeostasis is essential for cell metabolism. Recent results indicate that modulation of ion fluxes might also represent a useful strategy to regulate one of the most important physiological processes taking place in chloroplasts, photosynthesis. Photosynthesis is highly regulated, due to its unique role as a cellular engine for growth in the light. Controlling the balance between ATP and NADPH synthesis is a critical task, and availability of these molecules can limit the overall photosynthetic yield. Photosynthetic organisms optimize photosynthesis in low light, where excitation energy limits CO2 fixation, and minimize photo-oxidative damage in high light by dissipating excess photons. Despite extensive studies of these phenomena, the mechanism governing light utilization in plants is still poorly understood. In this review, we provide an update of the recently identified chloroplast-located ion channels and transporters whose function impacts photosynthetic efficiency in plants.

Chloroplast function and ion regulation in plants growing on saline soils: lessons from halophytes

Salt stress impacts multiple aspects of plant metabolism and physiology. For instance it inhibits photosynthesis through stomatal limitation, causes excessive accumulation of sodium and chloride in chloroplasts, and disturbs chloroplast potassium homeostasis. Most research on salt stress has focused primarily on cytosolic ion homeostasis with few studies of how salt stress affects chloroplast ion homeostasis. This review asks the question whether membrane-transport processes and ionic relations are differentially regulated between glycophyte and halophyte chloroplasts and whether this contributes to the superior salt tolerance of halophytes. The available literature indicates that halophytes can overcome stomatal limitation by switching to CO2 concentrating mechanisms and increasing the number of chloroplasts per cell under saline conditions. Furthermore, salt entry into the chloroplast stroma may be critical for grana formation and photosystem II activity in halophytes but not in glycophytes. Salt also inhibits some stromal enzymes (e.g. fructose-1,6-bisphosphatase) to a lesser extent in halophyte species. Halophytes accumulate more chloride in chloroplasts than glycophytes and appear to use sodium in functional roles. We propose the molecular identities of candidate transporters that move sodium, chloride and potassium across chloroplast membranes and discuss how their operation may regulate photochemistry and photosystem I and II activity in chloroplasts.

The regulation of the chloroplast proton motive force plays a key role for photosynthesis in fluctuating light

Plants use sunlight as their primary energy source. During photosynthesis, absorbed light energy generates reducing power by driving electron transfer reactions. These are coupled to the transfer of protons into the thylakoid lumen, generating a proton motive force (pmf) required for ATP synthesis. Sudden alterations in light availability have to be met by regulatory mechanisms to avoid the over-accumulation of reactive intermediates and maximize energy efficiency. Here, the acidification of the lumen, as an intermediate product of photosynthesis, plays an important role by regulating photosynthesis in response to excitation energy levels. Recent findings reveal pmf regulation and the modulation of its composition as key determinants for efficient photosynthesis, plant growth, and survival in fluctuating light environments.

Alternative electron transport pathways in photosynthesis: a confluence of regulation

Photosynthetic reactions proceed along a linear electron transfer chain linking water oxidation at photosystem II (PSII) to CO₂ reduction in the Calvin-Benson-Bassham cycle. Alternative pathways poise the electron carriers along the chain in response to changing light, temperature and CO₂ inputs, under prolonged hydration stress and during development. We describe recent literature that reports the physiological functions of new molecular players. Such highlights include the flavodiiron proteins and their important role in the green lineage. The parsing of the proton-motive force between ΔpH and Δψ, regulated in many different ways (cyclic electron flow, ATPsynthase conductivity, ion/H⁺ transporters), is comprehensively reported. This review focuses on an integrated description of alternative electron transfer pathways and how they contribute to photosynthetic productivity in the context of plant fitness to the environment.

In-Depth Genomic and Transcriptomic Analysis of Five K+ Transporter Gene Families in Soybean Confirm Their Differential Expression for Nodulation

Plants have evolved a sophisticated network of K⁺ transport systems to regulate growth and development. Limited K⁺ resources are now forcing us to investigate how plant demand can be satisfied. To answer this complex question, we must understand the genomic and transcriptomic portfolio of K⁺ transporters in plants. Here, we have identified 70 putative K⁺ transporter genes from soybean, including 29 HAK/KT/KUP genes, 16 genes encoding voltage-gated K⁺ channels, 9 TPK/KCO genes, 4 HKT genes, and 12 KEA genes. To clarify the molecular evolution of each family in soybean, we analyzed their phylogeny, mode of duplication, exon structures and splice sites, and paralogs. Additionally, ortholog clustering and syntenic analysis across five other dicots further explored the evolution of these gene families and indicated that the soybean data is suitable as a model for all other legumes. Available microarray data sets from Genevestigator about nodulation was evaluated and further confirmed with the RNA sequencing data available by a web server. For each family, expression models were designed based on Transcripts Per Kilobase Million (TPM) values; the outcomes indicated differential expression linked to nodulation and confirmed the genes' putative roles. In-depth studies such as ours provides the basis for understanding K⁺ inventories in all other plants.

Salinity Response in Chloroplasts: Insights from Gene Characterization

Salinity is a severe abiotic stress limiting agricultural yield and productivity. Plants have evolved various strategies to cope with salt stress. Chloroplasts are important photosynthesis organelles, which are sensitive to salinity. An understanding of molecular mechanisms in chloroplast tolerance to salinity is of great importance for genetic modification and plant breeding. Previous studies have characterized more than 53 salt-responsive genes encoding important chloroplast-localized proteins, which imply multiple vital pathways in chloroplasts in response to salt stress, such as thylakoid membrane organization, the modulation of photosystem II (PS II) activity, carbon dioxide (CO₂) assimilation, photorespiration, reactive oxygen species (ROS) scavenging, osmotic and ion homeostasis, abscisic acid (ABA) biosynthesis and signaling, and gene expression regulation, as well as protein synthesis and turnover. This review presents an overview of salt response in chloroplasts revealed by gene characterization efforts.

Plants contain small families of UPF0016 proteins including the PHOTOSYNTHESIS AFFECTED MUTANT71 transporter

PHOTOSYNTHESIS AFFECTED MUTANT71 (PAM71) is an integral thylakoid membrane protein that functions in manganese uptake into the lumen. Manganese is needed in the thylakoid lumen to build up the inorganic Mn₄CaO₅ cluster, the catalytic center for water oxidation, and is hence indispensable for oxygen evolution. A recent study revealed that PAM71 is well conserved in plants and shares homology to GCR1 DEPENDENT TRANSLATION FACTOR1 (GDT1) and TRANSMEMBRANE PROTEIN 165 (TMEM165) in Saccharomyces cerevisiae and Homo sapiens, respectively. In most eukaryotes only single members of this family, designated "Uncharacterized Protein Family 0016" (UPF0016), are present; however, plant genomes contain genes for several UPF0016 proteins. In Arabidopsis thaliana, this protein family comprises 5 members, which mainly differ in their N-terminal regions. PAM71 and its closest homolog PAM71-HL possess chloroplast transit peptides at their N-terminus. Two of the remaining 3 members are derived from a segmental chromosomal duplication event and lack an N-terminal extension. Thus, plants have evolved UPF0016 members residing in various compartments of the cell, whereas in non-plant eukaryotes just a Golgi localization occurs. The identification of PAM71 as a candidate Mn²⁺ transporter opens the question on the function of the remaining plant members. Here we resume briefly our current knowledge of UPF0016 members in Arabidopsis in comparison to their yeast and human UPF0016 members.

Fine-tuned regulation of the K+ /H+ antiporter KEA3 is required to optimize photosynthesis during induction

KEA3 is a thylakoid membrane localized K⁺ /H⁺ antiporter that regulates photosynthesis by modulating two components of proton motive force (pmf), the proton gradient (∆pH) and the electric potential (∆ψ). We identified a mutant allele of KEA3, disturbed proton gradient regulation (dpgr) based on its reduced non-photochemical quenching (NPQ) in artificial (CO₂ -free with low O₂ ) air. This phenotype was enhanced in the mutant backgrounds of PSI cyclic electron transport (pgr5 and crr2-1). In ambient air, reduced NPQ was observed during induction of photosynthesis in dpgr, the phenotype that was enhanced after overnight dark adaptation. In contrast, the knockout allele of kea3-1 exhibited a high-NPQ phenotype during steady state in ambient air. Consistent with this kea3-1 phenotype in ambient air, the membrane topology of KEA3 indicated a proton efflux from the thylakoid lumen to the stroma. The dpgr heterozygotes showed a semidominant and dominant phenotype in artificial and ambient air, respectively. In dpgr, the protein level of KEA3 was unaffected but the downregulation of its activity was probably disturbed. Our findings suggest that fine regulation of KEA3 activity is necessary for optimizing photosynthesis.

Contribution of Cyclic and Pseudo-cyclic Electron Transport to the Formation of Proton Motive Force in Chloroplasts

Photosynthetic electron transport is coupled to proton translocation across the thylakoid membrane, resulting in the formation of a trans-thylakoid proton gradient (ΔpH) and membrane potential (Δψ). Ion transporters and channels localized to the thylakoid membrane regulate the contribution of each component to the proton motive force (pmf). Although both ΔpH and Δψ contribute to ATP synthesis as pmf, only ΔpH downregulates photosynthetic electron transport via the acidification of the thylakoid lumen by inducing thermal dissipation of excessive absorbed light energy from photosystem II antennae and slowing down of the electron transport through the cytochrome b₆f complex. To optimize the tradeoff between efficient light energy utilization and protection of both photosystems against photodamage, plants have to regulate the pmf amplitude and its components, ΔpH and Δψ. Cyclic electron transport around photosystem I (PSI) is a major regulator of the pmf amplitude by generating pmf independently of the net production of NADPH by linear electron transport. Chloroplast ATP synthase relaxes pmf for ATP synthesis, and its activity should be finely tuned for maintaining the size of the pmf during steady-state photosynthesis. Pseudo-cyclic electron transport mediated by flavodiiron protein (Flv) forms a large electron sink, which is essential for PSI photoprotection in fluctuating light in cyanobacteria. Flv is conserved from cyanobacteria to gymnosperms but not in angiosperms. The Arabidopsis proton gradient regulation 5 (pgr5) mutant is defective in the main pathway of PSI cyclic electron transport. By introducing Physcomitrella patens genes encoding Flvs, the function of PSI cyclic electron transport was substituted by that of Flv-dependent pseudo-cyclic electron transport. In transgenic plants, the size of the pmf was complemented to the wild-type level but the contribution of ΔpH to the total pmf was lower than that in the wild type. In the pgr5 mutant, the size of the pmf was drastically lowered by the absence of PSI cyclic electron transport. In the mutant, ΔpH occupied the majority of pmf, suggesting the presence of a mechanism for the homeostasis of luminal pH in the light. To avoid damage to photosynthetic electron transport by periods of excess solar energy, plants employ an intricate regulatory network involving alternative electron transport pathways, ion transporters/channels, and pH-dependent mechanisms for downregulating photosynthetic electron transport.

Role of Ions in the Regulation of Light-Harvesting

Regulation of photosynthetic light harvesting in the thylakoids is one of the major key factors affecting the efficiency of photosynthesis. Thylakoid membrane is negatively charged and influences both the structure and the function of the primarily photosynthetic reactions through its electrical double layer (EDL). Further, there is a heterogeneous organization of soluble ions (K+, Mg2+, Cl-) attached to the thylakoid membrane that, together with fixed charges (negatively charged amino acids, lipids), provides an electrical field. The EDL is affected by the valence of the ions and interferes with the regulation of "state transitions," protein interactions, and excitation energy "spillover" from Photosystem II to Photosystem I. These effects are reflected in changes in the intensity of chlorophyll a fluorescence, which is also a measure of photoprotective non-photochemical quenching (NPQ) of the excited state of chlorophyll a. A triggering of NPQ proceeds via lumen acidification that is coupled to the export of positive counter-ions (Mg2+, K+) to the stroma or/and negative ions (e.g., Cl-) into the lumen. The effect of protons and anions in the lumen and of the cations (Mg2+, K+) in the stroma are, thus, functionally tightly interconnected. In this review, we discuss the consequences of the model of EDL, proposed by Barber (1980b) Biochim Biophys Acta 594:253-308) in light of light-harvesting regulation. Further, we explain differences between electrostatic screening and neutralization, and we emphasize the opposite effect of monovalent (K+) and divalent (Mg2+) ions on light-harvesting and on "screening" of the negative charges on the thylakoid membrane; this effect needs to be incorporated in all future models of photosynthetic regulation by ion channels and transporters.

MSL1 is a mechanosensitive ion channel that dissipates mitochondrial membrane potential and maintains redox homeostasis in mitochondria during abiotic stress

Mitochondria must maintain tight control over the electrochemical gradient across their inner membrane to allow ATP synthesis while maintaining a redox-balanced electron transport chain and avoiding excessive reactive oxygen species production. However, there is a scarcity of knowledge about the ion transporters in the inner mitochondrial membrane that contribute to control of membrane potential. We show that loss of MSL1, a member of a family of mechanosensitive ion channels related to the bacterial channel MscS, leads to increased membrane potential of Arabidopsis mitochondria under specific bioenergetic states. We demonstrate that MSL1 localises to the inner mitochondrial membrane. When expressed in Escherichia coli, MSL1 forms a stretch-activated ion channel with a slight preference for anions and provides protection against hypo-osmotic shock. In contrast, loss of MSL1 in Arabidopsis did not prevent swelling of isolated mitochondria in hypo-osmotic conditions. Instead, our data suggest that ion transport by MSL1 leads to dissipation of mitochondrial membrane potential when it becomes too high. The importance of MSL1 function was demonstrated by the observation of a higher oxidation state of the mitochondrial glutathione pool in msl1-1 mutants under moderate heat- and heavy-metal-stress. Furthermore, we show that MSL1 function is not directly implicated in mitochondrial membrane potential pulsing, but is complementary and appears to be important under similar conditions.

Each of the chloroplast potassium efflux antiporters affects photosynthesis and growth of fully developed Arabidopsis rosettes under short-day photoperiod

In Arabidopsis thaliana, the chloroplast harbors three potassium efflux antiporters (KEAs), namely KEA1 and KEA2 in the inner envelope and KEA3 in the thylakoid membrane. They may play redundant physiological roles as in our previous analyses of young developing Arabidopsis rosettes under long-day photoperiod (16 h light per day), chloroplast kea single mutants resembled the wild-type plants, whereas kea1kea2 and kea1kea2kea3 mutants were impaired in chloroplast development and photosynthesis resulting in stunted growth. Here, we aimed to study whether chloroplast KEAs play redundant roles in chloroplast function of older Arabidopsis plants with fully developed rosettes grown under short-day photoperiod (8 h light per day). Under these conditions, we found defects in photosynthesis and growth in the chloroplast kea single mutants, and most dramatic defects in the kea1kea2 double mutant. The mechanism behind these defects in the single mutants involves reduction in the electron transport rate (kea1 and kea3), and stomata conductance (kea1, kea2 and kea3), which in turn affect CO₂ fixation rates. The kea1kea2 mutant, in addition to these alterations, displayed reduced levels of photosynthetic machinery. Taken together, our data suggest that, in addition to the previously reported roles in chloroplast development in young rosettes, each chloroplast KEA affects photosynthesis and growth of Arabidopsis fully developed rosettes.

Limitations to photosynthesis by proton motive force-induced photosystem II photodamage

The thylakoid proton motive force (pmf) generated during photosynthesis is the essential driving force for ATP production; it is also a central regulator of light capture and electron transfer. We investigated the effects of elevated pmf on photosynthesis in a library of Arabidopsis thaliana mutants with altered rates of thylakoid lumen proton efflux, leading to a range of steady-state pmf extents. We observed the expected pmf-dependent alterations in photosynthetic regulation, but also strong effects on the rate of photosystem II (PSII) photodamage. Detailed analyses indicate this effect is related to an elevated electric field (Δψ) component of the pmf, rather than lumen acidification, which in vivo increased PSII charge recombination rates, producing singlet oxygen and subsequent photodamage. The effects are seen even in wild type plants, especially under fluctuating illumination, suggesting that Δψ-induced photodamage represents a previously unrecognized limiting factor for plant productivity under dynamic environmental conditions seen in the field.

Limitations to photosynthesis by proton motive force-induced photosystem II photodamage

The thylakoid proton motive force (pmf) generated during photosynthesis is the essential driving force for ATP production; it is also a central regulator of light capture and electron transfer. We investigated the effects of elevated pmf on photosynthesis in a library of Arabidopsis thaliana mutants with altered rates of thylakoid lumen proton efflux, leading to a range of steady-state pmf extents. We observed the expected pmf-dependent alterations in photosynthetic regulation, but also strong effects on the rate of photosystem II (PSII) photodamage. Detailed analyses indicate this effect is related to an elevated electric field (Δψ) component of the pmf, rather than lumen acidification, which in vivo increased PSII charge recombination rates, producing singlet oxygen and subsequent photodamage. The effects are seen even in wild type plants, especially under fluctuating illumination, suggesting that Δψ-induced photodamage represents a previously unrecognized limiting factor for plant productivity under dynamic environmental conditions seen in the field.

DOI:http://dx.doi.org/10.7554/eLife.16921.001

A bestrophin-like protein modulates the proton motive force across the thylakoid membrane in Arabidopsis

During photosynthesis, photosynthetic electron transport generates a proton motive force (pmf) across the thylakoid membrane, which is used for ATP biosynthesis via ATP synthase in the chloroplast. The pmf is composed of an electric potential (ΔΨ) and an osmotic component (ΔpH). Partitioning between these components in chloroplasts is strictly regulated in response to fluctuating environments. However, our knowledge of the molecular mechanisms that regulate pmf partitioning is limited. Here, we report a bestrophin-like protein (AtBest), which is critical for pmf partitioning. While the ΔpH component was slightly reduced in atbest, the ΔΨ component was much greater in this mutant than in the wild type, resulting in less efficient activation of nonphotochemical quenching (NPQ) upon both illumination and a shift from low light to high light. Although no visible phenotype was observed in the atbest mutant in the greenhouse, this mutant exhibited stronger photoinhibition than the wild type when grown in the field. AtBest belongs to the bestrophin family proteins, which are believed to function as chloride (Cl^- ) channels. Thus, our findings reveal an important Cl^- channel required for ion transport and homeostasis across the thylakoid membrane in higher plants. These processes are essential for fine-tuning photosynthesis under fluctuating environmental conditions.

Envelope K+/H+ Antiporters AtKEA1 and AtKEA2 Function in Plastid Development

It is well established that thylakoid membranes of chloroplasts convert light energy into chemical energy, yet the development of chloroplast and thylakoid membranes is poorly understood. Loss of function of the two envelope K(+)/H(+) antiporters AtKEA1 and AtKEA2 was shown previously to have negative effects on the efficiency of photosynthesis and plant growth; however, the molecular basis remained unclear. Here, we tested whether the previously described phenotypes of double mutant kea1kea2 plants are due in part to defects during early chloroplast development in Arabidopsis (Arabidopsis thaliana). We show that impaired growth and pigmentation is particularly evident in young expanding leaves of kea1kea2 mutants. In proliferating leaf zones, chloroplasts contain much lower amounts of photosynthetic complexes and chlorophyll. Strikingly, AtKEA1 and AtKEA2 proteins accumulate to high amounts in small and dividing plastids, where they are specifically localized to the two caps of the organelle separated by the fission plane. The unusually long amino-terminal domain of 550 residues that precedes the antiport domain appears to tether the full-length AtKEA2 protein to the two caps. Finally, we show that the double mutant contains 30% fewer chloroplasts per cell. Together, these results show that AtKEA1 and AtKEA2 transporters in specific microdomains of the inner envelope link local osmotic, ionic, and pH homeostasis to plastid division and thylakoid membrane formation.

Rapid hyperosmotic-induced Ca2+ responses in Arabidopsis thaliana exhibit sensory potentiation and involvement of plastidial KEA transporters

Plants experience hyperosmotic stress when faced with saline soils and possibly with drought stress, but it is currently unclear how plant roots perceive this stress in an environment of dynamic water availabilities. Hyperosmotic stress induces a rapid rise in intracellular Ca(2+) concentrations ([Ca(2+)]i) in plants, and this Ca(2+) response may reflect the activities of osmo-sensory components. Here, we find in the reference plant Arabidopsis thaliana that the rapid hyperosmotic-induced Ca(2+) response exhibited enhanced response magnitudes after preexposure to an intermediate hyperosmotic stress. We term this phenomenon "osmo-sensory potentiation." The initial sensing and potentiation occurred in intact plants as well as in roots. Having established a quantitative understanding of wild-type responses, we investigated effects of pharmacological inhibitors and candidate channel/transporter mutants. Quintuple mechano-sensitive channels of small conductance-like (MSL) plasma membrane-targeted channel mutants as well as double mid1-complementing activity (MCA) channel mutants did not affect the response. Interestingly, however, double mutations in the plastid K(+) exchange antiporter (KEA) transporters kea1kea2 and a single mutation that does not visibly affect chloroplast structure, kea3, impaired the rapid hyperosmotic-induced Ca(2+) responses. These mutations did not significantly affect sensory potentiation of the response. These findings suggest that plastids may play an important role in early steps mediating the response to hyperosmotic stimuli. Together, these findings demonstrate that the plant osmo-sensory components necessary to generate rapid osmotic-induced Ca(2+) responses remain responsive under varying osmolarities, endowing plants with the ability to perceive the dynamic intensities of water limitation imposed by osmotic stress.

A Putative Chloroplast-Localized Ca(2+)/H(+) Antiporter CCHA1 Is Involved in Calcium and pH Homeostasis and Required for PSII Function in Arabidopsis

Calcium is important for chloroplast, not only in its photosynthetic but also nonphotosynthetic functions. Multiple Ca(2+)/H(+) transporters and channels have been described and studied in the plasma membrane and organelle membranes of plant cells; however, the molecular identity and physiological roles of chloroplast Ca(2+)/H(+) antiporters have remained unknown. Here we report the identification and characterization of a member of the UPF0016 family, CCHA1 (a chloroplast-localized potential Ca(2+)/H(+) antiporter), in Arabidopsis thaliana. We observed that the ccha1 mutant plants developed pale green leaves and showed severely stunted growth along with impaired photosystem II (PSII) function. CCHA1 localizes to the chloroplasts, and the levels of the PSII core subunits and the oxygen-evolving complex were significantly decreased in the ccha1 mutants compared with the wild type. In high Ca(2+) concentrations, Arabidopsis CCHA1 partially rescued the growth defect of yeast gdt1Δ null mutant, which is defective in a Ca(2+)/H(+) antiporter. The ccha1 mutant plants also showed significant sensitivity to high concentrations of CaCl2 and MnCl2, as well as variation in pH. Taken these results together, we propose that CCHA1 might encode a putative chloroplast-localized Ca(2+)/H(+) antiporter with critical functions in the regulation of PSII and in chloroplast Ca(2+) and pH homeostasis in Arabidopsis.

Regulation and Levels of the Thylakoid K+/H+ Antiporter KEA3 Shape the Dynamic Response of Photosynthesis in Fluctuating Light

Crop canopies create environments of highly fluctuating light intensities. In such environments, photoprotective mechanisms and their relaxation kinetics have been hypothesized to limit photosynthetic efficiency and therefore crop yield potential. Here, we show that overexpression of the Arabidopsis thylakoid K+/H+ antiporter KEA3 accelerates the relaxation of photoprotective energy-dependent quenching after transitions from high to low light in Arabidopsis and tobacco. This, in turn, enhances PSII quantum efficiency in both organisms, supporting that in wild-type plants, residual light energy quenching following a high to low light transition represents a limitation to photosynthetic efficiency in fluctuating light. This finding underscores the potential of accelerating quenching relaxation as a building block for improving photosynthetic efficiency in the field. Additionally, by overexpressing natural KEA3 variants with modification to the C-terminus, we show that KEA3 activity is regulated by a mechanism involving its lumen-localized C-terminus, which lowers KEA3 activity in high light. This regulatory mechanism fine-tunes the balance between photoprotective energy dissipation in high light and maximum quantum yield in low light, likely to be critical for efficient photosynthesis in fluctuating light conditions.