Evidence for potassium transport activity of Arabidopsis KEA1-KEA6

Dissecting structure and function of the monovalent cation/H+ antiporters Mdm38 and Ylh47 in Saccharomyces cerevisiae

Saccharomyces cerevisiae Mdm38 and Ylh47 are homologs of the Ca2+/H+ antiporter Letm1, a candidate gene for seizures associated with Wolf-Hirschhorn syndrome in humans. Mdm38 is important for K+/H+ exchange across the inner mitochondrial membrane and contributes to membrane potential formation and mitochondrial protein translation. Ylh47 also localizes to the inner mitochondrial membrane. However, knowledge of the structures and detailed transport activities of Mdm38 and Ylh47 is limited. In this study, we conducted characterization of the ion transport activities and related structural properties of Mdm38 and Ylh47. Growth tests using Na+/H+ antiporter-deficient Escherichia coli strain TO114 showed that Mdm38 and Ylh47 had Na+ efflux activity. Measurement of transport activity across E. coli-inverted membranes showed that Mdm38 and Ylh47 had K+/H+, Na+/H+, and Li+/H+ antiport activity, but unlike Letm1, they lacked Ca2+/H+ antiport activity. Deletion of the ribosome-binding domain resulted in decreased Na+ efflux activity in Mdm38. Structural models of Mdm38 and Ylh47 identified a highly conserved glutamic acid in the pore-forming membrane-spanning region. Replacement of this glutamic acid with alanine, a non-polar amino acid, significantly impaired the ability of Mdm38 and Ylh47 to complement the salt sensitivity of E. coli TO114. These findings not only provide important insights into the structure and function of the Letm1-Mdm38-Ylh47 antiporter family but by revealing their distinctive properties also shed light on the physiological roles of these transporters in yeast and animals.

IMPORTANCE

The inner membrane of mitochondria contains numerous ion transporters, including those facilitating H+ transport by the electron transport chain and ATP synthase to maintain membrane potential. Letm1 in the inner membrane of mitochondria in animals functions as a Ca2+/H+ antiporter. However, this study reveals that homologous antiporters in mitochondria of yeast, Mdm38 and Ylh47, do not transport Ca2+ but instead are selective for K+ and Na+. Additionally, the identification of conserved amino acids crucial for antiporter activity further expanded our understanding of the structure and function of the Letm1-Mdm38-Ylh47 antiporter family.

Chloroplast ion homeostasis - what do we know and where should we go?

Plant yields heavily depend on proper macro- and micronutrient supply from the soil. In the leaf cells, nutrient ions fulfill specific roles in biochemical reactions, especially photosynthesis housed in the chloroplast. Here, a well-balanced ion homeostasis is maintained by a number of ion transport proteins embedded in the envelope and thylakoid membranes. Ten years ago, the first alkali metal transporters from the K+ EFFLUX ANTIPORTER family were discovered in the model plant Arabidopsis. Since then, our knowledge about the physiological importance of these carriers and their substrates has greatly expanded. New insights into the role of alkali ions in plastid gene expression and photoprotective mechanisms, both prerequisites for plant productivity in natural environments, were gained. The discovery of a Cl- channel in the thylakoid and several additional plastid alkali and alkali metal transport proteins have advanced the field further. Nevertheless, scientists still have long ways to go before a complete systemic understanding of the chloroplast's ion transportome will emerge. In this Tansley review, we highlight and discuss the achievements of the last decade. More importantly, we make recommendations on what areas to prioritize, so the field can reach the next milestones. One area, laid bare by our similarity-based comparisons among phototrophs is our lack of knowledge what ion transporters are used by cyanobacteria to buffer photosynthesis fluctuations.

Ion Changes and Signaling under Salt Stress in Wheat and Other Important Crops

High concentrations of sodium (Na+), chloride (Cl-), calcium (Ca2+), and sulphate (SO42-) are frequently found in saline soils. Crop plants cannot successfully develop and produce because salt stress impairs the uptake of Ca2+, potassium (K+), and water into plant cells. Different intracellular and extracellular ionic concentrations change with salinity, including those of Ca2+, K+, and protons. These cations serve as stress signaling molecules in addition to being essential for ionic homeostasis and nutrition. Maintaining an appropriate K+:Na+ ratio is one crucial plant mechanism for salt tolerance, which is a complicated trait. Another important mechanism is the ability for fast extrusion of Na+ from the cytosol. Ca2+ is established as a ubiquitous secondary messenger, which transmits various stress signals into metabolic alterations that cause adaptive responses. When plants are under stress, the cytosolic-free Ca2+ concentration can rise to 10 times or more from its resting level of 50-100 nanomolar. Reactive oxygen species (ROS) are linked to the Ca2+ alterations and are produced by stress. Depending on the type, frequency, and intensity of the stress, the cytosolic Ca2+ signals oscillate, are transient, or persist for a longer period and exhibit specific "signatures". Both the influx and efflux of Ca2+ affect the length and amplitude of the signal. According to several reports, under stress Ca2+ alterations can occur not only in the cytoplasm of the cell but also in the cell walls, nucleus, and other cell organelles and the Ca2+ waves propagate through the whole plant. Here, we will focus on how wheat and other important crops absorb Na+, K+, and Cl- when plants are under salt stress, as well as how Ca2+, K+, and pH cause intracellular signaling and homeostasis. Similar mechanisms in the model plant Arabidopsis will also be considered. Knowledge of these processes is important for understanding how plants react to salinity stress and for the development of tolerant crops.

Modulation of potassium transport to increase abiotic stress tolerance in plants

Potassium is the major cation responsible for the maintenance of the ionic environment in plant cells. Stable potassium homeostasis is indispensable for virtually all cellular functions, and, concomitantly, viability. Plants must cope with environmental changes such as salt or drought that can alter ionic homeostasis. Potassium fluxes are required to regulate the essential process of transpiration, so a constraint on potassium transport may also affect the plant's response to heat, cold, or oxidative stress. Sequencing data and functional analyses have defined the potassium channels and transporters present in the genomes of different species, so we know most of the proteins directly participating in potassium homeostasis. The still unanswered questions are how these proteins are regulated and the nature of potential cross-talk with other signaling pathways controlling growth, development, and stress responses. As we gain knowledge regarding the molecular mechanisms underlying regulation of potassium homeostasis in plants, we can take advantage of this information to increase the efficiency of potassium transport and generate plants with enhanced tolerance to abiotic stress through genetic engineering or new breeding techniques. Here, we review current knowledge of how modifying genes related to potassium homeostasis in plants affect abiotic stress tolerance at the whole plant level.

The Potassium Efflux System Kef: Bacterial Protection against Toxic Electrophilic Compounds

Kef couples the potassium efflux with proton influx in gram-negative bacteria. The resulting acidification of the cytosol efficiently prevents the killing of the bacteria by reactive electrophilic compounds. While other degradation pathways for electrophiles exist, Kef is a short-term response that is crucial for survival. It requires tight regulation since its activation comes with the burden of disturbed homeostasis. Electrophiles, entering the cell, react spontaneously or catalytically with glutathione, which is present at high concentrations in the cytosol. The resulting glutathione conjugates bind to the cytosolic regulatory domain of Kef and trigger activation while the binding of glutathione keeps the system closed. Furthermore, nucleotides can bind to this domain for stabilization or inhibition. The binding of an additional ancillary subunit, called KefF or KefG, to the cytosolic domain is required for full activation. The regulatory domain is termed K⁺ transport-nucleotide binding (KTN) or regulator of potassium conductance (RCK) domain, and it is also found in potassium uptake systems or channels in other oligomeric arrangements. Bacterial RosB-like transporters and K⁺ efflux antiporters (KEA) of plants are homologs of Kef but fulfill different functions. In summary, Kef provides an interesting and well-studied example of a highly regulated bacterial transport system.

Metabolic imprint induced by seed halo-priming promotes a differential physiological performance in two contrasting quinoa ecotypes

"Memory imprint" refers to the process when prior exposure to stress prepares the plant for subsequent stress episodes. Seed priming is a strategy to change the performance of seedlings to cope with stress; however, mechanisms associated with the metabolic response are fragmentary. Salinity is one of the major abiotic stresses that affect crop production in arid and semiarid areas. Chenopodium quinoa Willd. (Amaranthaceae) is a promising crop to sustain food security and possesses a wide genetic diversity of salinity tolerance. To elucidate if the metabolic memory induced by seed halo-priming (HP) differs among contrasting saline tolerance plants, seeds of two ecotypes of Quinoa (Socaire from Atacama Salar, and BO78 from Chilean Coastal/lowlands) were treated with a saline solution and then germinated and grown under different saline conditions. The seed HP showed a more positive impact on the sensitive ecotype during germination and promoted changes in the metabolomic profile in both ecotypes, including a reduction in carbohydrates (starch) and organic acids (citric and succinic acid), and an increase in antioxidants (ascorbic acid and α-tocopherol) and related metabolites. These changes were linked to a further reduced level of oxidative markers (methionine sulfoxide and malondialdehyde), allowing improvements in the energy use in photosystem II under saline conditions in the salt-sensitive ecotype. In view of these results, we conclude that seed HP prompts a "metabolic imprint" related to ROS scavenger at the thylakoid level, improving further the physiological performance of the most sensitive ecotype.

Functional and biotechnological cues of potassium homeostasis for stress tolerance and plant development

Potassium (K+) is indispensable for the regulation of a plethora of functions like plant metabolism, growth, development, and abiotic stress responses. K+ is associated with protein synthesis and entangled in the activation of scores of enzymes, stomatal regulation, and photosynthesis. It has multiple transporters and channels that assist in the uptake, efflux, transport within the cell as well as from soil to different tissues, and the grain filling sites. While it is implicated in ion homeostasis during salt stress, it acts as a modulator of stomatal movements during water deficit conditions. K+ is reported to abate the effects of chilling and photooxidative stresses. K+ has been found to ameliorate effectively the co-occurrence of drought and high-temperature stresses. Nutrient deficiency of K+ makes leaves necrotic, leads to diminished photosynthesis, and decreased assimilate utilization highlighting the role it plays in photosynthesis. Notably, K+ is associated with the detoxification of reactive oxygen species (ROS) when plants are exposed to diverse abiotic stress conditions. It is irrefutable now that K+ reduces the activity of NADPH oxidases and at the same time maintains electron transport activity, which helps in mitigating the oxidative stress. K+ as a macronutrient in plant growth, the role of K+ during abiotic stress and the protein phosphatases involved in K+ transport have been reviewed. This review presents a holistic view of the biological functions of K+, its uptake, translocation, signaling, and the critical roles it plays under abiotic stress conditions, plant growth, and development that are being unraveled in recent times.

Salt Tolerance Potential in Onion: Confirmation through Physiological and Biochemical Traits

Production of many crops, including onion, under salinity is lagging due to limited information on the physiological, biochemical and molecular mechanisms of salt stress tolerance in plants. Hence, the present study was conducted to identify salt-tolerant onion genotypes based on physiological and biochemical mechanisms associated with their differential responses. Thirty-six accessions were evaluated under control and salt stress conditions, and based on growth and bulb yield. Results revealed that plant height (6.07%), number of leaves per plant (3.07%), bulb diameter (11.38%), bulb yield per plant (31.24%), and total soluble solids (8.34%) were reduced significantly compared to control. Based on percent bulb yield reduction, seven varieties were classified as salt tolerant (with <20% yield reduction), seven as salt-sensitive (with >40% yield reduction) and the remaining as moderately tolerant (with 20 to 40% yield reduction). Finally, seven salt-tolerant and seven salt-sensitive accessions were selected for detailed study of their physiological and biochemical traits and their differential responses under salinity. High relative water content (RWC), membrane stability index (MSI), proline content (PRO), and better antioxidants such as super oxide dismutase (SOD), peroxidase (POX), catalase (CAT), and ascorbate peroxidase (APX) were observed in tolerant accessions, viz. POS35, NHRDF Red (L-28), GWO 1, POS36, NHRDF Red-4 (L-744), POS37, and POS38. Conversely, increased malondialdehyde (MDA) and hydrogen peroxide (H2O2) content, reduced activity of antioxidants, more membrane injury, and high Na+/K+ ratio were observed in sensitive accessions, viz. ALR, GJWO 3, Kalyanpur Red Round, NHRDF Red-3 (L-652), Agrifound White, and NHRDF (L-920). Stepwise regression analysis identified bulb diameter), plant height, APX, stomatal conductance (gS), POX, CAT, MDA, MSI, and bulb Na+/K+ ratio as predictor traits accounting for maximum variation in bulb yield under salinity. The identified seven salt-tolerant varieties can be used in future onion breeding programs for developing tolerant genotypes for salt-prone areas.

Genomic & structural diversity and functional role of potassium (K+) transport proteins in plants

Potassium (K⁺) is an essential macronutrient for plant growth and productivity. It is the most abundant cation in plants and is involved in various cellular processes. Variable K⁺ availability is sensed by plant roots, consequently K⁺ transport proteins are activated to optimize K⁺ uptake. In addition to K⁺ uptake and translocation these proteins are involved in other important physiological processes like transmembrane voltage regulation, polar auxin transport, maintenance of Na⁺/K⁺ ratio and stomata movement during abiotic stress responses. K⁺ transport proteins display tremendous genomic and structural diversity in plants. Their key structural features, such as transmembrane domains, N-terminal domains, C-terminal domains and loops determine their ability of K⁺ uptake and transport and thus, provide functional diversity. Most K⁺ transporters are regulated at transcriptional and post-translational levels. Genetic manipulation of key K⁺ transporters/channels could be a prominent strategy for improving K⁺ utilization efficiency (KUE) in plants. This review discusses the genomic and structural diversity of various K⁺ transport proteins in plants. Also, an update on the function of K⁺ transport proteins and their regulatory mechanism in response to variable K⁺ availability, in improving KUE, biotic and abiotic stresses is provided.

Flavodiiron proteins enhance the rate of CO2 assimilation in Arabidopsis under fluctuating light intensity

The proton concentration gradient (ΔpH) and membrane potential (Δψ) formed across the thylakoid membrane contribute to ATP synthesis in chloroplasts. Additionally, ΔpH downregulates photosynthetic electron transport via the acidification of the thylakoid lumen. K+ exchange antiporter 3 (KEA3) relaxes this downregulation by substituting ΔpH with Δψ in response to fluctuation of light intensity. In the Arabidopsis (Arabidopsis thaliana) line overexpressing KEA3 (KEA3ox), the rate of electron transport is elevated by accelerating the relaxation of ΔpH after a shift from high light (HL) to low light. However, the plant cannot control electron transport toward photosystem I (PSI), resulting in PSI photodamage. In this study, we crossed the KEA3ox line with the line (Flavodiiron [Flv]) expressing the Flv proteins of Physcomitrium patens. In the double transgenic line (Flv-KEA3ox), electrons overloading toward PSI were pumped out by Flv proteins. Consequently, photodamage of PSI was alleviated to the wild-type level. The rate of CO2 fixation was enhanced in Flv and Flv-KEA3ox lines during HL periods of fluctuating light, although CO2 fixation was unaffected in any transgenic lines in constant HL. Upregulation of CO2 fixation was accompanied by elevated stomatal conductance in fluctuating light. Consistent with the results of gas exchange experiments, the growth of Flv and Flv-KEA3ox plants was better than that of WT and KEA3ox plants under fluctuating light.

Ion transporters and their regulatory signal transduction mechanisms for salinity tolerance in plants

Soil salinity is one of the most serious threats to plant growth and productivity. Due to global climate change, burgeoning population and shrinking arable land, there is an urgent need to develop crops with minimum reduction in yield when cultivated in salt-affected areas. Salinity stress imposes osmotic stress as well as ion toxicity, which impairs major plant processes such as photosynthesis, cellular metabolism, and plant nutrition. One of the major effects of salinity stress in plants includes the disturbance of ion homeostasis in various tissues. In the present study, we aimed to review the regulation of uptake, transport, storage, efflux, influx, and accumulation of various ions in plants under salinity stress. We have summarized major research advancements towards understanding the ion homeostasis at both cellular and whole-plant level under salinity stress. We have also discussed various factors regulating the function of ion transporters and channels in maintaining ion homeostasis and ionic interactions under salt stress, including plant antioxidative defense, osmo-protection, and osmoregulation. We further elaborated on stress perception at extracellular and intracellular levels, which triggers downstream intracellular-signaling cascade, including secondary messenger molecules generation. Various signaling and signal transduction mechanisms under salinity stress and their role in improving ion homeostasis in plants are also discussed. Taken together, the present review focuses on recent advancements in understanding the regulation and function of different ion channels and transporters under salt stress, which may pave the way for crop improvement.

Exogenous sucrose influences KEA1 and KEA2 to regulate abscisic acid-mediated primary root growth in Arabidopsis

Arabidopsis K⁺-efflux antiporter (KEA)1 and KEA2 are chloroplast inner envelope membrane K⁺/H⁺ antiporters that play an important role in plastid development and seedling growth. However, the function of KEA1 and KEA2 during early seedling development is poorly understood. In this work, we found that in Arabidopsis, KEA1 and KEA2 mediated primary root growth by regulating photosynthesis and the ABA signaling pathway. Phenotypic analyses revealed that in the absence of sucrose, the primary root length of the kea1kea2 mutant was significantly shorter than that of the wild-type Columbia-0 (Col-0) plant. However, this phenotype could be remedied by the external application of sucrose. Meanwhile, HPLC-MS/MS results showed that in sucrose-free medium, ABA accumulation in the kea1kea2 mutant was considerably lower than that in Col-0. Transcriptome analysis revealed that many key genes involved in ABA signals were repressed in the kea1kea2 mutant. We concluded that KEA1 and KEA2 deficiency not only affected photosynthesis but was also involved in primary root growth likely through an ABA-dependent manner. This study confirmed the new function of KEA1 and KEA2 in affecting primary root growth.

Impaired photoprotection in Phaeodactylum tricornutum KEA3 mutants reveals the proton regulatory circuit of diatoms light acclimation

Diatoms are successful phytoplankton clades able to acclimate to changing environmental conditions, including e.g. variable light intensity. Diatoms are outstanding at dissipating light energy exceeding the maximum photosynthetic electron transfer (PET) capacity via the nonphotochemical quenching (NPQ) process. While the molecular effectors of NPQ as well as the involvement of the proton motive force (PMF) in its regulation are known, the regulators of the PET/PMF relationship remain unidentified in diatoms. We generated mutants of the H⁺ /K⁺ antiporter KEA3 in the model diatom Phaeodactylum tricornutum. Loss of KEA3 activity affects the PET/PMF coupling and NPQ responses at the onset of illumination, during transients and in steady-state conditions. Thus, this antiporter is a main regulator of the PET/PMF coupling. Consistent with this conclusion, a parsimonious model including only two free components, KEA3 and the diadinoxanthin de-epoxidase, describes most of the feedback loops between PET and NPQ. This simple regulatory system allows for efficient responses to fast (minutes) or slow (e.g. diel) changes in light environment, thanks to the presence of a regulatory calcium ion (Ca²⁺ )-binding domain in KEA3 modulating its activity. This circuit is likely tuned by the NPQ-effector proteins, LHCXs, providing diatoms with the required flexibility to thrive in different ocean provinces.

Proto Kranz-like leaf traits and cellular ionic regulation are associated with salinity tolerance in a halophytic wild rice

Species of wild rice (Oryza spp.) possess a wide range of stress tolerance traits that can be potentially utilized in breeding climate-resilient cultivated rice cultivars (Oryza sativa) thereby aiding global food security. In this study, we conducted a greenhouse trial to evaluate the salinity tolerance of six wild rice species, one cultivated rice cultivar (IR64) and one landrace (Pokkali) using a range of electrophysiological, imaging, and whole-plant physiological techniques. Three wild species (O. latifolia, O. officinalis and O. coarctata) were found to possess superior salinity stress tolerance. The underlying mechanisms, however, were strikingly different. Na+ accumulation in leaves of O. latifolia, O. officinalis and O. coarctata were significantly higher than the tolerant landrace, Pokkali. Na+ accumulation in mesophyll cells was only observed in O. coarctata, suggesting that O. officinalis and O. latifolia avoid Na+ accumulation in mesophyll by allocating Na+ to other parts of the leaf. The finding also suggests that O. coarctata might be able to employ Na+ as osmolyte without affecting its growth. Further study of Na+ allocation in leaves will be helpful to understand the mechanisms of Na+ accumulation in these species. In addition, O. coarctata showed Proto Kranz-like leaf anatomy (enlarged bundle sheath cells and lower numbers of mesophyll cells), and higher expression of C4-related genes (e.g., NADPME, PPDK) and was a clear outlier with respect to salinity tolerance among the studied wild and cultivated Oryza species. The unique phylogenetic relationship of O. coarctata with C4 grasses suggests the potential of this species for breeding rice with high photosynthetic rate under salinity stress in the future.

Chloroplast pH Homeostasis for the Regulation of Photosynthesis

The pH of various chloroplast compartments, such as the thylakoid lumen and stroma, is light-dependent. Light illumination induces electron transfer in the photosynthetic apparatus, coupled with proton translocation across the thylakoid membranes, resulting in acidification and alkalization of the thylakoid lumen and stroma, respectively. Luminal acidification is crucial for inducing regulatory mechanisms that protect photosystems against photodamage caused by the overproduction of reactive oxygen species (ROS). Stromal alkalization activates enzymes involved in the Calvin-Benson-Bassham (CBB) cycle. Moreover, proton translocation across the thylakoid membranes generates a proton gradient (ΔpH) and an electric potential (ΔΨ), both of which comprise the proton motive force (pmf) that drives ATP synthase. Then, the synthesized ATP is consumed in the CBB cycle and other chloroplast metabolic pathways. In the dark, the pH of both the chloroplast stroma and thylakoid lumen becomes neutral. Despite extensive studies of the above-mentioned processes, the molecular mechanisms of how chloroplast pH can be maintained at proper levels during the light phase for efficient activation of photosynthesis and other metabolic pathways and return to neutral levels during the dark phase remain largely unclear, especially in terms of the precise control of stromal pH. The transient increase and decrease in chloroplast pH upon dark-to-light and light-to-dark transitions have been considered as signals for controlling other biological processes in plant cells. Forward and reverse genetic screening approaches recently identified new plastid proteins involved in controlling ΔpH and ΔΨ across the thylakoid membranes and chloroplast proton/ion homeostasis. These proteins have been conserved during the evolution of oxygenic phototrophs and include putative photosynthetic protein complexes, proton transporters, and/or their regulators. Herein, we summarize the recently identified protein players that control chloroplast pH and influence photosynthetic efficiency in plants.

Genome-wide analysis of potassium transport genes in Gossypium raimondii suggest a role of GrHAK/KUP/KT8, GrAKT2.1 and GrAKT1.1 in response to abiotic stress

Potassium (K⁺) is an important macro-nutrient for plants, which comprises almost 10% of plant's dry mass. It plays a crucial role in the growth of plants as well as other important processes related to metabolism and stress tolerance. Plants have a complex and well-organized potassium distribution system (channels and transporters). Cotton is the most important economic crop, which is the primary source of natural fiber. Soil deficiency in K⁺ can negatively affect yield and fiber quality of cotton. However, potassium transport system in cotton is poorly studied. Current study identified 43 Potassium Transport System (PTS) genes in Gossypium raimondii genome. Based on conserved domains, transmembrane domains, and motif structures, these genes were classified as K⁺ transporters (2 HKTs, 7 KEAs, and 16 KUP/HAK/KTs) and K⁺ channels (11 Shakers and 7 TPKs/KCO). The phylogenetic comparison of GrPTS genes from Arabidopsis thaliana, Glycine max, Oryza sativa, Medicago truncatula and Cicer arietinum revealed variations in PTS gene conservation. Evolutionary analysis predicted that most GrPTS genes were segmentally duplicated. Gene structure analysis showed that the intron/exon organization of these genes was conserved in specific-family. Chromosomal localization demonstrated a random distribution of PTS genes across all the thirteen chromosomes except chromosome six. Many stress responsive cis-regulatory elements were predicted in promoter regions of GrPTS genes. The RNA-seq data analysis followed by qRT-PCR validation demonstrated that PTS genes potentially work in groups against environmental factors. Moreover, a transporter gene (GrHAK/KUP/KT8) and two channel genes (GrAKT2.1 and GrAKT1.1) are important candidate genes for plant stress response. These results provide useful information for further functional characterization of PTS genes with the breeding aim of stress-resistant cultivars.

Genome-Wide Identification and Expression Profiling of Potassium Transport-Related Genes in Vigna radiata under Abiotic Stresses

Potassium (K+) is one of the most important cations that plays a significant role in plants and constitutes up to 10% of plants' dry weight. Plants exhibit complex systems of transporters and channels for the distribution of K+ from soil to numerous parts of plants. In this study, we have identified 39 genes encoding putative K+ transport-related genes in Vigna radiata. Chromosomal mapping of these genes indicated an uneven distribution across eight out of 11 chromosomes. Comparative phylogenetic analysis of different plant species, i.e., V. radiata, Glycine max, Cicer arietinum, Oryza sativa, and Arabidopsis thaliana, showed their strong conservation in different plant species. Evolutionary analysis of these genes suggests that gene duplication is a major route of expansion for this family in V. radiata. Comprehensive promoter analysis identified several abiotic stresses related to cis-elements in the promoter regions of these genes, suggesting their role in abiotic stress tolerance. Our additional analyses indicated that abiotic stresses adversely affected the chlorophyll concentration, carotenoids, catalase, total soluble protein concentration, and the activities of superoxide and peroxidase in V. radiata. It also disturbs the ionic balance by decreasing the uptake of K+ content and increasing the uptake of Na+. Expression analysis from high-throughput sequencing data and quantitative real-time PCR experiments revealed that several K+ transport genes were expressed in different tissues (seed, flower, and pod) and in abiotic stress-responsive manners. A highly significant variation of expression was observed for VrHKT (1.1 and 1.2), VrKAT (1 and 2) VrAKT1.1, VrAKT2, VrSKOR, VrKEA5, VrTPK3, and VrKUP/HAK/KT (4, 5, and 8.1) in response to drought, heat or salinity stress. It reflected their potential roles in plant growth, development, or stress adaptations. The present study gives an in-depth understanding of K+ transport system genes in V. radiata and will serve as a basis for a functional analysis of these genes.

Two plastid POLLUX ion channel-like proteins are required for stress-triggered stromal Ca2+release

Two decades ago, large cation currents were discovered in the envelope membranes of Pisum sativum L. (pea) chloroplasts. The deduced K+-permeable channel was coined fast-activating chloroplast cation channel but its molecular identity remained elusive. To reveal candidates, we mined proteomic datasets of isolated pea envelopes. Our search uncovered distant members of the nuclear POLLUX ion channel family. Since pea is not amenable to molecular genetics, we used Arabidopsis thaliana to characterize the two gene homologs. Using several independent approaches, we show that both candidates localize to the chloroplast envelope membrane. The proteins, designated PLASTID ENVELOPE ION CHANNELS (PEC1/2), form oligomers with regulator of K+ conductance domains protruding into the intermembrane space. Heterologous expression of PEC1/2 rescues yeast mutants deficient in K+ uptake. Nuclear POLLUX ion channels cofunction with Ca2+ channels to generate Ca2+ signals, critical for establishing mycorrhizal symbiosis and root development. Chloroplasts also exhibit Ca2+ transients in the stroma, probably to relay abiotic and biotic cues between plastids and the nucleus via the cytosol. Our results show that pec1pec2 loss-of-function double mutants fail to trigger the characteristic stromal Ca2+ release observed in wild-type plants exposed to external stress stimuli. Besides this molecular abnormality, pec1pec2 double mutants do not show obvious phenotypes. Future studies of PEC proteins will help to decipher the plant's stress-related Ca2+ signaling network and the role of plastids. More importantly, the discovery of PECs in the envelope membrane is another critical step towards completing the chloroplast ion transport protein inventory.

Functional characterization of proton antiport regulation in the thylakoid membrane

During photosynthesis, energy is transiently stored as an electrochemical proton gradient across the thylakoid membrane. The resulting proton motive force (pmf) is composed of a membrane potential (ΔΨ) and a proton concentration gradient (ΔpH) and powers the synthesis of ATP. Light energy availability for photosynthesis can change very rapidly and frequently in nature. Thylakoid ion transport proteins buffer the effects that light fluctuations have on photosynthesis by adjusting pmf and its composition. Ion channel activities dissipate ΔΨ, thereby reducing charge recombinations within photosystem II. The dissipation of ΔΨ allows for increased accumulation of protons in the thylakoid lumen, generating the signal that activates feedback downregulation of photosynthesis. Proton export from the lumen via the thylakoid K+ exchange antiporter 3 (KEA3), instead, decreases the ΔpH fraction of the pmf and thereby reduces the regulatory feedback signal. Here, we reveal that the Arabidopsis (Arabidopsis thaliana) KEA3 protein homo-dimerizes via its C-terminal domain. This C-terminus has a regulatory function, which responds to light intensity transients. Plants carrying a C-terminus-less KEA3 variant show reduced feed-back downregulation of photosynthesis and suffer from increased photosystem damage under long-term high light stress. However, during photosynthetic induction in high light, KEA3 deregulation leads to an increase in carbon fixation rates. Together, the data reveal a trade-off between long-term photoprotection and a short-term boost in carbon fixation rates, which is under the control of the KEA3 C-terminus.

Two Is Company, but Four Is a Party-Challenges of Tetraploidization for Cell Wall Dynamics and Efficient Tip-Growth in Pollen

Some cells grow by an intricately coordinated process called tip-growth, which allows the formation of long tubular structures by a remarkable increase in cell surface-to-volume ratio and cell expansion across vast distances. On a broad evolutionary scale, tip-growth has been extraordinarily successful, as indicated by its recurrent 're-discovery' throughout evolutionary time in all major land plant taxa which allowed for the functional diversification of tip-growing cell types across gametophytic and sporophytic life-phases. All major land plant lineages have experienced (recurrent) polyploidization events and subsequent re-diploidization that may have positively contributed to plant adaptive evolutionary processes. How individual cells respond to genome-doubling on a shorter evolutionary scale has not been addressed as elaborately. Nevertheless, it is clear that when polyploids first form, they face numerous important challenges that must be overcome for lineages to persist. Evidence in the literature suggests that tip-growth is one of those processes. Here, I discuss the literature to present hypotheses about how polyploidization events may challenge efficient tip-growth and strategies which may overcome them: I first review the complex and multi-layered processes by which tip-growing cells maintain their cell wall integrity and steady growth. I will then discuss how they may be affected by the cellular changes that accompany genome-doubling. Finally, I will depict possible mechanisms polyploid plants may evolve to compensate for the effects caused by genome-doubling to regain diploid-like growth, particularly focusing on cell wall dynamics and the subcellular machinery they are controlled by.

Loss of inner-envelope K+/H+ exchangers impairs plastid rRNA maturation and gene expression

The inner-envelope K+ EFFLUX ANTIPORTERS (KEA) 1 and 2 are critical for chloroplast development, ion homeostasis, and photosynthesis. However, the mechanisms by which changes in ion flux across the envelope affect organelle biogenesis remained elusive. Chloroplast development requires intricate coordination between the nuclear genome and the plastome. Many mutants compromised in plastid gene expression (PGE) display a virescent phenotype, that is delayed greening. The phenotypic appearance of Arabidopsis thaliana kea1 kea2 double mutants fulfills this criterion, yet a link to PGE has not been explored. Here, we show that a simultaneous loss of KEA1 and KEA2 results in maturation defects of the plastid ribosomal RNAs. This may be caused by secondary structure changes of rRNA transcripts and concomitant reduced binding of RNA-processing proteins, which we documented in the presence of skewed ion homeostasis in kea1 kea2. Consequently, protein synthesis and steady-state levels of plastome-encoded proteins remain low in mutants. Disturbance in PGE and other signs of plastid malfunction activate GENOMES UNCOUPLED 1-dependent retrograde signaling in kea1 kea2, resulting in a dramatic downregulation of GOLDEN2-LIKE transcription factors to halt expression of photosynthesis-associated nuclear-encoded genes (PhANGs). PhANG suppression delays the development of fully photosynthesizing kea1 kea2 chloroplasts, probably to avoid progressing photo-oxidative damage. Overall, our results reveal that KEA1/KEA2 function impacts plastid development via effects on RNA-metabolism and PGE.

Effect of Salt Stress on Growth, Physiological Parameters, and Ionic Concentration of Water Dropwort (Oenanthe javanica) Cultivars

Salt stress is an important environmental limiting factor. Water dropwort (Oenanthe javanica) is an important vegetable in East Asia; however, its phenotypic and physiological response is poorly explored. For this purpose, 48 cultivars of water dropwort were grown hydroponically and treated with 0, 50, 100, and 200 mm NaCl for 14 days. Than their phenotypic responses were evaluated, afterward, physiological studies were carried out in selected sensitive and tolerant cultivars. In the present study, the potential tolerant (V11E0022) and sensitive (V11E0135) cultivars were selected by screening 48 cultivars based on their phenotype under four different levels of salt concentrations (0, 50, 100, and 200 mm). The results depicted that plant height, number of branches and leaves were less effected in V11E0022, and most severe reduction was observed in V11E0135 in comparison with others. Than the changes in biomass, ion contents, accumulation of reactive oxygen species, and activities of antioxidant enzymes and non-enzymatic antioxidants were determined in the leaves and roots of the selected cultivars. The potential tolerant cultivar (V11E0022) showed less reduction of water content and demonstrated low levels of Na+ uptake, malondialdehyde, and hydrogen peroxide (H2O2) in both leaves and roots. Moreover, the tolerant cultivar (V11E0022) showed high antioxidant activities of ascorbate peroxidase (APX), superoxide dismutase, peroxidase, catalase (CAT), reduced glutathione (GSH), and high accumulation of proline and soluble sugars compared to the sensitive cultivar (V11E0135). These results suggest the potential tolerance of V11E0022 cultivar against salt stress with low detrimental effects and a good antioxidant defense system. The observations also suggest good antioxidant capacity of water dropwort against salt stress. The findings of the present study also suggest that the number of branches and leaves, GSH, proline, soluble sugars, APX, and CAT could serve as the efficient markers for understanding the defense mechanisms of water dropwort under the conditions of salt stress.

Physiological and Biochemical Traits of Two Major Arabidopsis Accessions, Col-0 and Ws, Under Salinity

Salinity affects plant growth and development as shown with the glycophyte model plant, Arabidopsis thaliana (Arabidopsis). Two Arabidopsis accessions, Wassilewskija (Ws) and Columbia (Col-0), are widely used to generate mutants available from various Arabidopsis seed resources. However, these two ecotypes are known to be salt-sensitive with different degrees of tolerance. In our study, 3-week-old Col-0 and Ws plants were treated with and without 150 mM NaCl for 48, 72, or 96 h, and several physiological and biochemical traits were characterized on shoots to identify any specific traits in their tolerance to salinity. Before salt treatment was carried out, a different phenotype was observed between Col-0 and Ws, whose main inflorescence stem became elongated in contrast to Col-0, which only displayed rosette leaves. Our results showed that Col-0 and Ws were both affected by salt stress with limited growth associated with a reduction in nutrient uptake, a degradation of photosynthetic pigments, an increase in protein degradation, as well as showing changes in carbohydrate metabolism and cell wall composition. These traits were often more pronounced in Col-0 and occurred usually earlier than in Ws. Tandem Mass Tags quantitative proteomics data correlated well with the physiological and biochemical results. The Col-0 response to salt stress was specifically characterized by a greater accumulation of osmoprotectants such as anthocyanin, galactinol, and raffinose; a lower reactive oxygen detoxification capacity; and a transient reduction in galacturonic acid content. Pectin degradation was associated with an overaccumulation of the wall-associated kinase 1, WAK1, which plays a role in cell wall integrity (CWI) upon salt stress exposure. Under control conditions, Ws produced more antioxidant enzymes than Col-0. Fewer specific changes occurred in Ws in response to salt stress apart from a higher number of different fascilin-like arabinogalactan proteins and a greater abundance of expansin-like proteins, which could participate in CWI. Altogether, these data indicate that Col-0 and Ws trigger similar mechanisms to cope with salt stress, and specific changes are more likely related to the developmental stage than to their respective genetic background.

Functional characterization of multiple PAS domain-containing diguanylate cyclases in Synechocystis sp. PCC 6803

Bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP) is a second messenger known to control a variety of bacterial processes. The model cyanobacterium, Synechocystis sp. PCC 6803, has a score of genes encoding putative enzymes for c-di-GMP synthesis and degradation. However, most of them have not been functionally characterized. Here, we chose four genes in Synechocystis (dgcA-dgcD), which encode proteins with a GGDEF, diguanylate cyclase (DGC) catalytic domain and multiple Per-ARNT-Sim (PAS) conserved regulatory motifs, for detailed analysis. Purified DgcA, DgcB and DgcC were able to catalyze synthesis of c-di-GMP from two GTPs in vitro. DgcA had the highest activity, compared with DgcB and DgcC. DgcD did not show detectable activity. DgcA activity was specific for GTP and stimulated by the divalent cations, magnesium or manganese. Full activity of DgcA required the presence of the multiple PAS domains, probably because of their role in protein dimerization or stability. Synechocystis mutants carrying single deletions of dgcA-dgcD were not affected in their growth rate or biofilm production during salt stress, suggesting that there was functional redundancy in vivo. In contrast, overexpression of dgcA resulted in increased biofilm formation in the absence of salt stress. In this study, we characterize the enzymatic and physiological function of DgcA-DgcD, and propose that the PAS domains in DgcA function in maintaining the enzyme in its active form.

Loss of function of the chloroplast membrane K+/H+ antiporters AtKEA1 and AtKEA2 alters the ROS and NO metabolism but promotes drought stress resilience

Potassium (K⁺) exerts key physiological functions such as osmoregulation, stomatal movement, membrane transport, protein synthesis and photosynthesis among others. Previously, it was demonstrated in Arabidopsis thaliana that the loss of function of the chloroplast K⁺Efflux Antiporters KEA1 and KEA2, located in the inner envelope membrane, provokes inefficient photosynthesis. Therefore, the main goal of this study was to evaluate the potential impact of the loss of function of those cation transport systems in the metabolism of reactive oxygen and nitrogen species (ROS and RNS). Using 14-day-old seedlings from Arabidopsis double knock-out kea1kea2 mutants, ROS metabolism and NO content in roots and green cotyledons were studied at the biochemical level. The loss of function of AtKEA1 and AtKEA2 did not cause oxidative stress but it provoked an alteration of the ROS homeostasis affecting some ROS-generating enzymes. These included glycolate oxidase (GOX) and NADPH-dependent superoxide generation activity, enzymatic and non-enzymatic antioxidants and both NADP-isocitrate dehydrogenase and NADP-malic enzyme activities. NO content, analyzed by confocal laser scanning microscopy (CLSM), was negatively affected in both photosynthetic and non-photosynthetic organs in kea1kea2 mutant seedlings. Furthermore, the S-nitrosoglutathione reductase (GSNOR) protein expression and activity were downregulated in kea1kea2 mutants, whereas the tyrosine nitrated protein profile, analyzed by immunoblot, was unaffected but the relative expression of each immunoreactive band changed. Moreover, kea1kea2 mutants showed an increased photorespiratory pathway and stomata closure, thus promoting a higher resilience to drought stress. Data suggest that the chloroplast osmotic balance and integrity maintained by AtKEA1 and AtKEA2 are necessary to keep the balance of ROS/RNS metabolism. Moreover, these data open new questions about how endogenous NO generation might be affected by the K⁺/H⁺ transport located in the chloroplasts.

Silencing of GhKEA4 and GhKEA12 Revealed Their Potential Functions Under Salt and Potassium Stresses in Upland Cotton

The K⁺ efflux antiporter (KEA) mediates intracellular K⁺ and H⁺ homeostasis to improve salt tolerance in plants. However, the knowledge of KEA gene family in cotton is largely absent. In the present study, 8, 8, 15, and 16 putative KEA genes were identified in Gossypium arboreum, G. raimondii, G. hirsutum, and G. barbadense, respectively. These KEA genes were classified into three subfamilies, and members from the same subfamilies showed similar motif compositions and gene structure characteristics. Some hormone response elements and stress response elements were identified in the upstream 2000 bp sequence of GhKEAs. Transcriptome data showed that most of the GhKEAs were highly expressed in roots and stems. The quantificational real-time polymerase chain reaction (qRT-PCR) results showed that most of the GhKEAs responded to low potassium, salt and drought stresses. Virus-induced gene silencing (VIGS) experiments demonstrated that under salt stress, after silencing genes GhKEA4 and GhKEA12, the chlorophyll content, proline content, soluble sugar content, peroxidase (POD) activity and catalase (CAT) activity were significantly decreased, and the Na⁺/K⁺ ratio was extremely significantly increased in leaves, leading to greater salt sensitivity. Under high potassium stress, cotton plants silenced for the GhKEA4 could still maintain a more stable Na⁺ and K⁺ balance, and the activity of transporting potassium ions from roots into leaves was reduced silenced for GhKEA12. Under low potassium stress, silencing the GhKEA4 increased the activity of transporting potassium ions to shoots, and silencing the GhKEA12 increased the ability of absorbing potassium ions, but accumulated more Na⁺ in leaves. These results provided a basis for further studies on the biological roles of KEA genes in cotton development and adaptation to stress conditions.

Collaboration between NDH and KEA3 Allows Maximally Efficient Photosynthesis after a Long Dark Adaptation

In angiosperms, the NADH dehydrogenase-like (NDH) complex mediates cyclic electron transport around PSI (CET). K⁺ Efflux Antiporter3 (KEA3) is a putative thylakoid H⁺/K⁺ antiporter and allows an increase in membrane potential at the expense of the ∆pH component of the proton motive force. In this study, we discovered that the chlororespiratory reduction2-1 (crr2-1) mutation, which abolished NDH-dependent CET, enhanced the kea3-1 mutant phenotypes in Arabidopsis (Arabidopsis thaliana). The NDH complex pumps protons during CET, further enhancing ∆pH, but its physiological function has not been fully clarified. The observed effect only took place upon exposure to light of 110 µmol photons m^-2 s^-1 after overnight dark adaptation. We propose two distinct modes of NDH action. In the initial phase, within 1 min after the onset of actinic light, the NDH-dependent CET engages with KEA3 to enhance electron transport efficiency. In the subsequent phase, in which the ∆pH-dependent down-regulation of the electron transport is relaxed, the NDH complex engages with KEA3 to relax the large ∆pH formed during the initial phase. We observed a similar impact of the crr2-1 mutation in the genetic background of the PROTON GRADIENT REGULATION5 overexpression line, in which the size of ∆pH was enhanced. When photosynthesis was induced at 300 µmol photons m^-2 s^-1, the contribution of KEA3 was negligible in the initial phase and the ∆pH-dependent down-regulation was not relaxed in the second phase. In the crr2-1 kea3-1 double mutant, the induction of CO₂ fixation was delayed after overnight dark adaptation.

Phylogenetic Diversity and Physiological Roles of Plant Monovalent Cation/H+ Antiporters

The processes of plant nutrition, stress tolerance, plant growth, and development are strongly dependent on transport of mineral nutrients across cellular membranes. Plant membrane transporters are key components of these processes. Among various membrane transport proteins, the monovalent cation proton antiporter (CPA) superfamily mediates a broad range of physiological and developmental processes such as ion and pH homeostasis, development of reproductive organs, chloroplast operation, and plant adaptation to drought and salt stresses. CPA family includes plasma membrane-bound Na⁺/H⁺ exchanger (NhaP) and intracellular Na⁺/H⁺ exchanger NHE (NHX), K⁺ efflux antiporter (KEA), and cation/H⁺ exchanger (CHX) family proteins. In this review, we have completed the phylogenetic inventory of CPA transporters and undertaken a comprehensive evolutionary analysis of their development. Compared with previous studies, we have significantly extended the range of plant species, including green and red algae and Acrogymnospermae into phylogenetic analysis. Our data suggest that the multiplication and complexation of CPA isoforms during evolution is related to land colonisation by higher plants and associated with an increase of different tissue types and development of reproductive organs. The new data extended the number of clades for all groups of CPAs, including those for NhaP/SOS, NHE/NHX, KEA, and CHX. We also critically evaluate the latest findings on the biological role, physiological functions and regulation of CPA transporters in relation to their structure and phylogenetic position. In addition, the role of CPA members in plant tolerance to various abiotic stresses is summarized, and the future priority directions for CPA studies in plants are discussed.

The topology of plastid inner envelope potassium cation efflux antiporter KEA1 provides new insights into its regulatory features

The plastid potassium cation efflux antiporters (KEAs) are important for chloroplast function, development, and photosynthesis. To understand their regulation at the protein level is therefore of fundamental importance. Prior studies have focused on the regulatory K⁺ transport and NAD-binding (KTN) domain in the C-terminus of the thylakoid carrier KEA3 but the localization of this domain remains unclear. While all three plastid KEA members are highly conserved in their transmembrane region and the C-terminal KTN domain, only the inner envelope KEA family members KEA1 and KEA2 carry a long soluble N-terminus. Interestingly, this region is acetylated at lysine 168 by the stromal acetyltransferase enzyme NSI. If an odd number of transmembrane domains existed for inner envelope KEAs, as it was suggested for all three plastid KEA carriers, regulatory domains and consequently protein regulation would occur on opposing sides of the inner envelope. In this study we therefore set out to investigate the topology of inner envelope KEA proteins. Using a newly designed antibody specific to the envelope KEA1 N-terminus and transgenic Arabidopsis plants expressing a C-terminal KEA1-YFP fusion protein, we show that both, the N-terminal and C-terminal, regulatory domains of KEA1 reside in the chloroplast stroma and not in the intermembrane space. Considering the high homology between KEA1 and KEA2, we therefore reason that envelope KEAs must consist of an even number of transmembrane domains.

Diverse Physiological Functions of Cation Proton Antiporters across Bacteria and Plant Cells

Membrane intrinsic transport systems play an important role in maintaining ion and pH homeostasis and forming the proton motive force in the cytoplasm and cell organelles. In most organisms, cation/proton antiporters (CPAs) mediate the exchange of K⁺, Na⁺ and Ca²⁺ for H⁺ across the membrane in response to a variety of environmental stimuli. The tertiary structure of the ion selective filter and the regulatory domains of Escherichia coli CPAs have been determined and a molecular mechanism of cation exchange has been proposed. Due to symbiogenesis, CPAs localized in mitochondria and chloroplasts of eukaryotic cells resemble prokaryotic CPAs. CPAs primarily contribute to keeping cytoplasmic Na⁺ concentrations low and controlling pH, which promotes the detoxification of electrophiles and formation of proton motive force across the membrane. CPAs in cyanobacteria and chloroplasts are regulators of photosynthesis and are essential for adaptation to high light or osmotic stress. CPAs in organellar membranes and in the plasma membrane also participate in various intracellular signal transduction pathways. This review discusses recent advances in our understanding of the role of CPAs in cyanobacteria and plant cells.

H+ Transport by K+ EXCHANGE ANTIPORTER3 Promotes Photosynthesis and Growth in Chloroplast ATP Synthase Mutants

The composition of the thylakoid proton motive force (pmf) is regulated by thylakoid ion transport. Passive ion channels in the thylakoid membrane dissipate the membrane potential (Δψ) component to allow for a higher fraction of pmf stored as a proton concentration gradient (ΔpH). K⁺/H⁺ antiport across the thylakoid membrane via K+ EXCHANGE ANTIPORTER3 (KEA3) instead reduces the ΔpH fraction of the pmf. Thereby, KEA3 decreases nonphotochemical quenching (NPQ), thus allowing for higher light use efficiency, which is particularly important during transitions from high to low light. Here, we show that in the background of the Arabidopsis (Arabidopsis thaliana) chloroplast (cp)ATP synthase assembly mutant cgl160, with decreased cpATP synthase activity and increased pmf amplitude, KEA3 plays an important role for photosynthesis and plant growth under steady-state conditions. By comparing cgl160 single with cgl160 kea3 double mutants, we demonstrate that in the cgl160 background loss of KEA3 causes a strong growth penalty. This is due to a reduced photosynthetic capacity of cgl160 kea3 mutants, as these plants have a lower lumenal pH than cgl160 mutants, and thus show substantially increased pH-dependent NPQ and decreased electron transport through the cytochrome b ₆ f complex. Overexpression of KEA3 in the cgl160 background reduces pH-dependent NPQ and increases photosystem II efficiency. Taken together, our data provide evidence that under conditions where cpATP synthase activity is low, a KEA3-dependent reduction of ΔpH benefits photosynthesis and growth.

Modification of Activity of the Thylakoid H+/K+ Antiporter KEA3 Disturbs ∆pH-Dependent Regulation of Photosynthesis

The thylakoid K+ efflux antiporter 3 (KEA3) is required for regulating components of the proton motive force (pmf), proton concentration gradient (ΔpH), and membrane potential (Δψ). The Arabidopsis (Arabidopsis thaliana) disturbed proton gradient regulation mutant (dpgr) is a dominant allele of KEA3, conferring disturbed transport activity. Here, we show that overexpressing the DPGR-type KEA3 (DPGRox) retarded plant growth, whereas overexpressing the wild-type KEA3 (KEA3ox) did not. In KEA3ox lines, the contribution of Δψ to pmf was enhanced, but in DPGRox lines, the size of pmf was reduced. In DPGRox plants, proton conductivity of the thylakoid membrane (g H +) was elevated under high light, implying disturbed stoichiometry of H+/K+ antiport through DPGR-type KEA3 rather than simply enhanced activity. The ΔpH-dependent regulation consisting of thermal dissipation of excessively absorbed light energy and downregulation of cytochrome b 6 f complex activity was severely and mildly affected in DPGRox and KEA3ox plants, respectively. Consequently, photosystem I was sensitive to fluctuating light in both transgenic plants. Both photosystems were sensitive to constant high light and were slightly photodamaged even at standard growth light intensity in DPGRox plants. KEA3 regulates the components of pmf and optimizes the operation of ∆pH-dependent regulation of electron transport.