Control of volume and turgor in stomatal guard cells

A high-resolution model of gene expression during Gossypium hirsutum (cotton) fiber development

BACKGROUND

Cotton fiber development relies on complex and intricate biological processes to transform newly differentiated fiber initials into the mature, extravagantly elongated cellulosic cells that are the foundation of this economically important cash crop. Here we extend previous research into cotton fiber development by employing controlled conditions to minimize variability and utilizing time-series sampling and analyses to capture daily transcriptomic changes from early elongation through the early stages of secondary wall synthesis (6 to 24 days post anthesis; DPA).

RESULTS

A majority of genes are expressed in fiber, largely partitioned into two major coexpression modules that represent genes whose expression generally increases or decreases during development. Differential gene expression reveals a massive transcriptomic shift between 16 and 17 DPA, corresponding to the onset of the transition phase that leads to secondary wall synthesis. Subtle gene expression changes are captured by the daily sampling, which are discussed in the context of fiber development. Coexpression and gene regulatory networks are constructed and associated with phenotypic aspects of fiber development, including turgor and cellulose production. Key genes are considered in the broader context of plant secondary wall synthesis, noting their known and putative roles in cotton fiber development.

CONCLUSIONS

The analyses presented here highlight the importance of fine-scale temporal sampling on understanding developmental processes and offer insight into genes and regulatory networks that may be important in conferring the unique fiber phenotype.

Dual function of overexpressing plasma membrane H+-ATPase in balancing carbon-water use

Stomata respond slowly to changes in light when compared with photosynthesis, undermining plant water-use efficiency (WUE). We know much about stomatal mechanics, yet efforts to accelerate stomatal responsiveness have been limited despite the breadth of potential targets for manipulation. Here, we use mechanistic modeling to establish a hierarchy of putative targets affecting stomatal kinetics. Counterintuitively, modeling predicted that overexpressing plasma membrane H+-ATPases could speed stomata and enhance WUE under fluctuating light, even though overexpressed H+-ATPases is known to promote stomatal opening and reduce WUE in the steady state. Experiments validated the prediction, implicating an unexpected role of the H+-ATPases in improving WUE under fluctuating light. It suggests that H+-ATPases have a dual function, acting as a facilitator of carbon assimilation and water use, depending on the light conditions. These findings highlight the importance of integrating in silico modeling with experiments in future efforts toward enhancing stomatal function.

Identification and Characterization of Shaker Potassium Channel Gene Family and Response to Salt and Chilling Stress in Rice

Shaker potassium channel proteins are a class of voltage-gated ion channels responsible for K+ uptake and translocation, playing a crucial role in plant growth and salt tolerance. In this study, bioinformatic analysis was performed to identify the members within the Shaker gene family. Moreover, the expression patterns of rice Shaker(OsShaker) K+ channel genes were analyzed in different tissues and salt treatment by RT-qPCR. The results revealed that there were eight OsShaker K+ channel genes distributed on chromosomes 1, 2, 5, 6 and 7 in rice, and their promoters contained a variety of cis-regulatory elements, including hormone-responsive, light-responsive, and stress-responsive elements, etc. Most of the OsShaker K+ channel genes were expressed in all tissues of rice, but at different levels in different tissues. In addition, the expression of OsShaker K+ channel genes differed in the timing, organization and intensity of response to salt and chilling stress. In conclusion, our findings provide a reference for the understanding of OsShaker K+ channel genes, as well as their potential functions in response to salt and chilling stress in rice.

CO2 response screen in grass Brachypodium reveals the key role of a MAP kinase in CO2-triggered stomatal closure

Plants respond to increased CO2 concentrations through stomatal closure, which can contribute to increased water use efficiency. Grasses display faster stomatal responses than eudicots due to dumbbell-shaped guard cells flanked by subsidiary cells working in opposition. However, forward genetic screening for stomatal CO2 signal transduction mutants in grasses has yet to be reported. The grass model Brachypodium distachyon is closely related to agronomically important cereal crops, sharing largely collinear genomes. To gain insights into CO2 control mechanisms of stomatal movements in grasses, we developed an unbiased forward genetic screen with an EMS-mutagenized B. distachyon M5 generation population using infrared imaging to identify plants with altered leaf temperatures at elevated CO2. Among isolated mutants, a "chill1" mutant exhibited cooler leaf temperatures than wild-type Bd21-3 parent control plants after exposure to increased CO2. chill1 plants showed strongly impaired high CO2-induced stomatal closure despite retaining a robust abscisic acid-induced stomatal closing response. Through bulked segregant whole-genome sequencing analyses followed by analyses of further backcrossed F4 generation plants and generation and characterization of sodium azide and CRISPR-cas9 mutants, chill1 was mapped to a protein kinase, Mitogen-Activated Protein Kinase 5 (BdMPK5). The chill1 mutation impaired BdMPK5 protein-mediated CO2/HCO3- sensing together with the High Temperature 1 (HT1) Raf-like kinase in vitro. Furthermore, AlphaFold2-directed structural modeling predicted that the identified BdMPK5-D90N chill1 mutant residue is located at the interface of BdMPK5 with the BdHT1 Raf-like kinase. BdMPK5 is a key signaling component that mediates CO2-induced stomatal movements and is proposed to function as a component of the primary CO2 sensor in grasses.

Exploring aquaporin functions during changes in leaf water potential

Maintenance of optimal leaf tissue humidity is important for plant productivity and food security. Leaf humidity is influenced by soil and atmospheric water availability, by transpiration and by the coordination of water flux across cell membranes throughout the plant. Flux of water and solutes across plant cell membranes is influenced by the function of aquaporin proteins. Plants have numerous aquaporin proteins required for a multitude of physiological roles in various plant tissues and the membrane flux contribution of each aquaporin can be regulated by changes in protein abundance, gating, localisation, post-translational modifications, protein:protein interactions and aquaporin stoichiometry. Resolving which aquaporins are candidates for influencing leaf humidity and determining how their regulation impacts changes in leaf cell solute flux and leaf cavity humidity is challenging. This challenge involves resolving the dynamics of the cell membrane aquaporin abundance, aquaporin sub-cellular localisation and location-specific post-translational regulation of aquaporins in membranes of leaf cells during plant responses to changes in water availability and determining the influence of cell signalling on aquaporin permeability to a range of relevant solutes, as well as determining aquaporin influence on cell signalling. Here we review recent developments, current challenges and suggest open opportunities for assessing the role of aquaporins in leaf substomatal cavity humidity regulation.

WRKY Genes Improve Drought Tolerance in Arachis duranensis

WRKY transcription factor participates in plant growth and development and response to biotic and abiotic stresses. Arachis duranensis, a turfgrass, has high drought tolerance, yet little is known about AdWRKYs response to drought stress in A. duranensis. In this study, RNA-seq identified five AdWRKYs, including AdWRKY18, AdWRKY40, AdWRKY42, AdWRKY56, and AdWRKY64, which were upregulated under drought stress. Orthologous relationships between AdWRKYs and Arabidopsis WRKY were determined to predict the regulatory networks of the five AdWRKYs based on AtWRKYs. Additionally, protein-protein interactions were predicted using differentially expressed proteins from RNA-seq. The quantitative real-time PCR (qRT-PCR) results showed that AdWRKY40 was upregulated, while AdWRKY42, AdWRKY56, and AdWRKY64 were downregulated at different time-points under drought stress. The predicted regulatory networks showed that AdWRKY40 activates COR47, RD21, and RD29A expression under drought stress. Besides, AdWRKY56 regulated CesA8 under drought stress. Aradu.YIQ80 (NAC019) interacted with AdWRKY40, AdWRKY42, AdWRKY56, and AdWRKY64, while Aradu.Z5H58 (NAC055) interacted with AdWRKY42 and AdWRKY64 under drought stress. This study used Arabidopsis to assess AdWRKYs function and regulatory networks, providing a basis for understanding drought tolerance in A. duranensis.

Two trehalase isoforms, produced from a single transcript, regulate drought stress tolerance in Arabidopsis thaliana

Alternative translation initiation of the unique Arabidopsis trehalase gene allows for the production of two isoforms with different subcellular localization, providing enzyme access to both intra- and extra-cellular trehalose. The trehalose-hydrolyzing enzyme trehalase mediates drought stress tolerance in Arabidopsis thaliana by controlling ABA-induced stomatal closure. We now report the existence of two trehalase isoforms, produced from a single transcript by alternative translation initiation. The longer full-length N-glycosylated isoform (AtTRE1L) localizes in the plasma membrane with the catalytic domain in the apoplast. The shorter isoform (AtTRE1S) lacks the transmembrane domain and localizes in the cytoplasm and nucleus. The two isoforms can physically interact and this interaction affects localization of AtTRE1S. Consistent with their role in plant drought stress tolerance, both isoforms are activated by AtCPK10, a stress-induced calcium-dependent guard cell protein kinase. Transgenic plants expressing either isoform indicate that both can mediate ABA-induced stomatal closure in response to drought stress but that the short (cytoplasmic/nuclear) isoform, enriched in those conditions, is significantly more effective.

Identification and characterization of HAK/KUP/KT potassium transporter gene family in barley and their expression under abiotic stress

BACKGROUND

HAK/KUP/KT (High-affinity K⁺ transporters/K⁺ uptake permeases/K⁺ transporters) is the largest potassium transporter family in plants, and plays pivotal roles in K⁺ uptake and transport, as well as biotic and abiotic stress responses. However, our understanding of the gene family in barley (Hordeum vulgare L.) is quite limited.

RESULTS

In the present study, we identified 27 barley HAK/KUP/KT genes (hereafter called HvHAKs) through a genome-wide analysis. These HvHAKs were unevenly distributed on seven chromosomes, and could be phylogenetically classified into four clusters. All HvHAK protein sequences possessed the conserved motifs and domains. However, the substantial difference existed among HAK members in cis-acting elements and tissue expression patterns. Wheat had the most orthologous genes to barley HAKs, followed by Brachypodium distachyon, rice and maize. In addition, six barley HAK genes were selected to investigate their expression profiling in response to three abiotic stresses by qRT-PCR, and their expression levels were all up-regulated under salt, hyperosmotic and potassium deficiency treatments.

CONCLUSION

Twenty seven HAK genes (HvHAKs) were identified in barley, and they differ in tissue expression patterns and responses to salt stress, drought stress and potassium deficiency.

The quest for osmosensors in plants

Osmotic stress has severe effects on crop productivity. Since climate change is predicted to exacerbate this problem, the development of new crops that are tolerant to osmotic stresses, especially drought and salinity stress, is required. However, only limited success has been achieved to date, primarily because of the lack of a clear understanding of the mechanisms that facilitate osmosensing. Here, we discuss the potential mechanisms of osmosensing in plants. We highlight the roles of proteins such as receptor-like kinases, which sense stress-induced cell wall damage, mechanosensitive calcium channels, which initiate a calcium-induced stress response, and phospholipase C, a membrane-bound enzyme that is integral to osmotic stress perception. We also discuss the roles of aquaporins and membrane-bound histidine kinases, which could potentially detect changes in extracellular osmolarity in plants, as they do in prokaryotes and lower eukaryotes. These putative osmosensors have the potential to serve as master regulators of the osmotic stress response in plants and could prove to be useful targets for the selection of osmotic stress-tolerant crops.

Distinct leaf transcriptomic response of water deficient Eucalyptus grandis submitted to potassium and sodium fertilization

While potassium fertilization increases growth yield in Brazilian eucalyptus plantations, it could also increase water requirements, making trees more vulnerable to drought. Sodium fertilization, which has been shown to promote eucalyptus growth compared to K-deficient trees, could partially mitigate this adverse effect of potassium. However, little is known about the influence of K and Na fertilization on the tree metabolic response to water deficit. The aim of the present study was thus to analyze the transcriptome of leaves sampled from Eucalyptus grandis trees subjected to 37% rainfall reduction, and fertilized with potassium (K), sodium (Na), compared to control trees (C). The multifactorial experiment was set up in a field with a throughfall exclusion system. Transcriptomic analysis was performed on leaves from two-year-old trees, and data analyzed using multifactorial statistical analysis and weighted gene co-expression network analysis (WGCNA). Significant sets of genes were seen to respond to rainfall reduction, in interaction with K or Na fertilization, or to fertilization only (regardless of the water supply regime). The genes were involved in stress signaling, primary and secondary metabolism, secondary cell wall formation and photosynthetic activity. Our focus on key genes related to cation transporters and aquaporins highlighted specific regulation of ion homeostasis, and plant adjustment to water deficit. While water availability significantly affects the transcriptomic response of eucalyptus species, this study points out that the transcriptomic response is highly dependent on the fertilization regime. Our study is based on the first large-scale field trial in a tropical region, specifically designed to study the interaction between water availability and nutrition in eucalyptus. To our knowledge, this is the first global transcriptomic analysis to compare the influence of K and Na fertilization on tree adaptive traits in water deficit conditions.

The potassium channel FaTPK1 plays a critical role in fruit quality formation in strawberry (Fragaria × ananassa)

Potassium (K⁺), an abundant cation in plant cells, is important in fruit development and plant resistance. However, how cellular K⁺ is directed by potassium channels in fruit development and quality formation of strawberry (Fragaria × ananassa) is not yet fully clear. Here, a two‐pore K⁺ (TPK) channel gene in strawberry, FaTPK1, was cloned using reverse transcription–PCR. A green fluorescent protein subcellular localization analysis showed that FaTPK1 localized in the vacuole membrane. A transcription analysis indicated that the mRNA expression level of FaTPK1 increased rapidly and was maintained at a high level in ripened fruit, which was coupled with the fruit's red colour development, suggesting that FaTPK1 is related to fruit quality formation. The down‐ and up‐regulation of the FaTPK1mRNA expression levels using RNA interference and overexpression, respectively, inhibited and promoted fruit ripening, respectively, as demonstrated by consistent changes in firmness and the contents of soluble sugars, anthocyanin and abscisic acid, as well as the transcript levels of ripening‐regulated genes PG1 (polygalacturonase), GAL6 (beta‐galactosidase), XYL2 (D‐xylulose reductase), SUT1 (sucrose transporter), CHS (chalcone synthase) and CHI (chalcone flavanone isomerase). Additionally, the regulatory changes influenced fruit resistance to Botrytis cinerea. An isothermal calorimetry analysis showed that the Escherichia coli‐expressed FaTPK1 recombinant protein could bind K⁺ with a binding constant of 2.1 × 10^–3 m⁻¹ and a dissociation constant of 476 μm. Thus, the strawberry TPK1 is a ubiquitously expressed, tonoplast‐localized two‐pore potassium channel that plays important roles in fruit ripening and quality formation.

Fundamental structural and functional properties of Aquaporin ion channels found across the kingdoms of life

Aquaporin (AQP) channels in the major intrinsic protein (MIP) family are known to facilitate transmembrane water fluxes in prokaryotes and eukaryotes. Some classes of AQPs also conduct ions, glycerol, urea, CO₂ , nitric oxide, and other small solutes. Ion channel activity has been demonstrated for mammalian AQPs 0, 1, 6, Drosophila Big Brain (BIB), soybean nodulin 26, and rockcress AtPIP2;1. More classes are likely to be discovered. Newly identified blockers are providing essential tools for establishing physiological roles of some of the AQP dual water and ion channels. For example, the arylsulfonamide AqB011 which selectively blocks the central ion pore of mammalian AQP1 has been shown to impair migration of HT29 colon cancer cells. Traditional herbal medicines are sources of selective AQP1 inhibitors that also slow cancer cell migration. The finding that plant AtPIP2;1 expressed in root epidermal cells mediates an ion conductance regulated by calcium and protons provided insight into molecular mechanisms of environmental stress responses. Expression of lens MIP (AQP0) is essential for maintaining the structure, integrity and transparency of the lens, and Drosophila BIB contributes to neurogenic signalling pathways to control the developmental fate of fly neuroblast cells; however, the ion channel roles remain to be defined for MIP and BIB. A broader portfolio of pharmacological agents is needed to investigate diverse AQP ion channel functions in situ. Understanding the dual water and ion channel roles of AQPs could inform the development of novel agents for rational interventions in diverse challenges from agriculture to human health.

Divalent Cations Regulate the Ion Conductance Properties of Diverse Classes of Aquaporins

Aquaporins (AQPs) are known to facilitate water and solute fluxes across barrier membranes. An increasing number of AQPs are being found to serve as ion channels. Ion and water permeability of selected plant and animal AQPs (plant Arabidopsis thaliana AtPIP2;1, AtPIP2;2, AtPIP2;7, human Homo sapiens HsAQP1, rat Rattus norvegicus RnAQP4, RnAQP5, and fly Drosophilamelanogaster DmBIB) were expressed in Xenopus oocytes and examined in chelator-buffered salines to evaluate the effects of divalent cations (Ca²⁺, Mg²⁺, Ba²⁺ and Cd²⁺) on ionic conductances. AtPIP2;1, AtPIP2;2, HsAQP1 and DmBIB expressing oocytes had ionic conductances, and showed differential sensitivity to block by external Ca²⁺. The order of potency of inhibition by Ca²⁺ was AtPIP2;2 > AtPIP2;1 > DmBIB > HsAQP1. Blockage of the AQP cation channels by Ba²⁺ and Cd²⁺ caused voltage-sensitive outward rectification. The channels with the highest sensitivity to Ca²⁺ (AtPIP2;1 and AtPIP2;2) showed a distinctive relief of the Ca²⁺ block by co-application of excess Ba²⁺, suggesting that divalent ions act at the same site. Recognizing the regulatory role of divalent cations may enable the discovery of other classes of AQP ion channels, and facilitate the development of tools for modulating AQP ion channels. Modulators of AQPs have potential value for diverse applications including improving salinity tolerance in plants, controlling vector-borne diseases, and intervening in serious clinical conditions involving AQPs, such as cancer metastasis, cardiovascular or renal dysfunction.

Vacuolar convolution: possible mechanisms and role of phosphatidylinositol 3,5-bisphosphate

The central or lytic vacuole is the largest intracellular organelle in plant cells, but we know unacceptably little about the mechanisms regulating its function in vivo. The underlying reasons are related to difficulties in accessing this organelle without disrupting the cellular integrity and to the dynamic morphology of the vacuole, which lacks a defined structure. Among such morphological changes, vacuolar convolution is probably the most commonly observed event, reflected in the (reversible) transformation of a large central vacuole into a structure consisting of interconnected bubbles of a smaller size. Such behaviour is observed in plant cells subjected to hyperosmotic stress but also takes place in physiological conditions (e.g. during stomatal closure). Although vacuolar convolution is a relatively common phenomenon in plants, studies aimed at elucidating its execution mechanisms are rather scarce. In the present review, we analyse the available evidence on the participation of the cellular cytoskeleton and ion transporters in vacuolar morphology dynamics, putting special emphasis on the available evidence of the role played by phosphatidylinositol 3,5-bisphosphate in this process.

Transport and homeostasis of potassium and phosphate: limiting factors for sustainable crop production

Potassium (K) and phosphate (Pi) are both macronutrients essential for plant growth and crop production, but the unrenewable resources of phosphorus rock and potash have become limiting factors for food security. One critical measure to help solve this problem is to improve nutrient use efficiency (NUE) in plants by understanding and engineering genetic networks for ion uptake, translocation, and storage. Plants have evolved multiple systems to adapt to various nutrient conditions for growth and production. Within the NUE networks, transport proteins and their regulators are the primary players for maintaining nutrient homeostasis and could be utilized to engineer high NUE traits in crop plants. A large number of publications have detailed K+ and Pi transport proteins in plants over the past three decades. Meanwhile, the discovery and validation of their regulatory mechanisms are fast-track topics for research. Here, we provide an overview of K+ and Pi transport proteins and their regulatory mechanisms, which participate in the uptake, translocation, storage, and recycling of these nutrients in plants.

Non-selective cation channel activity of aquaporin AtPIP2;1 regulated by Ca2+ and pH

The aquaporin AtPIP2;1 is an abundant plasma membrane intrinsic protein in Arabidopsis thaliana that is implicated in stomatal closure, and is highly expressed in plasma membranes of root epidermal cells. When expressed in Xenopus laevis oocytes, AtPIP2;1 increased water permeability and induced a non-selective cation conductance mainly associated with Na⁺ . A mutation in the water pore, G103W, prevented both the ionic conductance and water permeability of PIP2;1. Co-expression of AtPIP2;1 with AtPIP1;2 increased water permeability but abolished the ionic conductance. AtPIP2;2 (93% identical to AtPIP2;1) similarly increased water permeability but not ionic conductance. The ionic conductance was inhibited by the application of extracellular Ca²⁺ and Cd²⁺ , with Ca²⁺ giving a biphasic dose-response with a prominent IC₅₀ of 0.32 mм comparable with a previous report of Ca²⁺ sensitivity of a non-selective cation channel (NSCC) in Arabidopsis root protoplasts. Low external pH also inhibited ionic conductance (IC₅₀ pH 6.8). Xenopus oocytes and Saccharomyces cerevisiae expressing AtPIP2;1 accumulated more Na⁺ than controls. Establishing whether AtPIP2;1 has dual ion and water permeability in planta will be important in understanding the roles of this aquaporin and if AtPIP2;1 is a candidate for a previously reported NSCC responsible for Ca²⁺ and pH sensitive Na⁺ entry into roots.

Overexpression of the potassium channel TPKb in small vacuoles confers osmotic and drought tolerance to rice

Potassium (K(+) ) is the most important cationic nutrient for all living organisms. Vacuolar two-pore K(+) (TPK) channels are important players in the regulation of cellular levels of K(+) but have not been characterised in rice. In order to assess the role of OsTPKb, a K(+) selective ion channel predominantly expressed in the tonoplast of small vacuoles, we generated overexpressing (OX) lines using a constitutive promoter and compared their phenotypes with control plants. Relative to control plants, OX lines showed better growth when exposed to low-K(+) or water stress conditions. K(+) uptake was greater in OX lines which may be driven by increased AKT1 and HAK1 activity. The enhanced K(+) uptake led to tissue K(+) levels that were raised in roots and shoots. Furthermore, energy dispersive X-ray (EDX) analyses showed a higher cytoplasm: vacuole K(+) ratio which is likely to contribute to the increased stress tolerance. In all, the data suggest that TPKb can alter the K(+) status of small vacuoles, which is important for general cellular K(+) homeostasis which, in turn, affects stress tolerance.

Tobacco guard cells fix CO2 by both Rubisco and PEPcase while sucrose acts as a substrate during light-induced stomatal opening

Transcriptomic and proteomic studies have improved our knowledge of guard cell function; however, metabolic changes in guard cells remain relatively poorly understood. Here we analysed metabolic changes in guard cell-enriched epidermal fragments from tobacco during light-induced stomatal opening. Increases in sucrose, glucose and fructose were observed during light-induced stomatal opening in the presence of sucrose in the medium while no changes in starch were observed, suggesting that the elevated fructose and glucose levels were a consequence of sucrose rather than starch breakdown. Conversely, reduction in sucrose was observed during light- plus potassium-induced stomatal opening. Concomitant with the decrease in sucrose, we observed an increase in the level as well as in the (13) C enrichment in metabolites of, or associated with, the tricarboxylic acid cycle following incubation of the guard cell-enriched preparations in (13) C-labelled bicarbonate. Collectively, the results obtained support the hypothesis that sucrose is catabolized within guard cells in order to provide carbon skeletons for organic acid production. Furthermore, they provide a qualitative demonstration that CO2 fixation occurs both via ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPcase). The combined data are discussed with respect to current models of guard cell metabolism and function.

Going beyond nutrition: regulation of potassium homoeostasis as a common denominator of plant adaptive responses to environment

Partially and fully completed plant genome sequencing projects in both lower and higher plants allow drawing a comprehensive picture of the molecular and structural diversities of plant potassium transporter genes and their encoded proteins. While the early focus of the research in this field was aimed on the structure-function studies and understanding of the molecular mechanisms underlying K(+) transport, availability of Arabidopsis thaliana mutant collections in combination with micro-array techniques have significantly advanced our understanding of K(+) channel physiology, providing novel insights into the transcriptional regulation of potassium homeostasis in plants. More recently, posttranslational regulation of potassium transport systems has moved into the center stage of potassium transport research. The current review is focused on the most exciting developments in this field. By summarizing recent work on potassium transporter regulation we show that potassium transport in general, and potassium channels in particular, represent important targets and are mediators of the cellular responses during different developmental stages in a plant's life cycle. We show that regulation of intracellular K(+) homeostasis is essential to mediate plant adaptive responses to a broad range of abiotic and biotic stresses including drought, salinity, and oxidative stress. We further link post-translational regulation of K(+) channels with programmed cell death and show that K(+) plays a critical role in controlling the latter process. Thus, is appears that K(+) is not just the essential nutrient required to support optimal plant growth and yield but is also an important signaling agent mediating a wide range of plant adaptive responses to environment.

Aquaporins: highly regulated channels controlling plant water relations

Plant growth and development are dependent on tight regulation of water movement. Water diffusion across cell membranes is facilitated by aquaporins that provide plants with the means to rapidly and reversibly modify water permeability. This is done by changing aquaporin density and activity in the membrane, including posttranslational modifications and protein interaction that act on their trafficking and gating. At the whole organ level aquaporins modify water conductance and gradients at key "gatekeeper" cell layers that impact on whole plant water flow and plant water potential. In this way they may act in concert with stomatal regulation to determine the degree of isohydry/anisohydry. Molecular, physiological, and biophysical approaches have demonstrated that variations in root and leaf hydraulic conductivity can be accounted for by aquaporins but this must be integrated with anatomical considerations. This Update integrates these data and emphasizes the central role played by aquaporins in regulating plant water relations.

Protein phosphorylation in stomatal movement

As research progresses on how guard cells perceive and transduce environmental cues to regulate stomatal movement, plant biologists are discovering key roles of protein phosphorylation. Early research efforts focused on characterization of ion channels and transporters in guard cell hormonal signaling. Subsequent genetic studies identified mutants of kinases and phosphatases that are defective in regulating guard cell ion channel activities, and recently proteins regulated by phosphorylation have been identified. Here we review the essential role of protein phosphorylation in ABA-induced stomatal closure and in blue light-induced stomatal opening. We also highlight evidence for the cross-talk between different pathways, which is mediated by protein phosphorylation.

Photosynthesis of Quercus suber is affected by atmospheric NH3 generated by multifunctional agrosystems

Montados are evergreen oak woodlands dominated by Quercus species, which are considered to be key to biodiversity conservation and ecosystem services. This ecosystem is often used for cattle breeding in most regions of the Iberian Peninsula, which causes plants to receive extra nitrogen as ammonia (NH(3)) through the atmosphere. The effect of this atmospheric NH(3) (NH(3atm)) on ecosystems is still under discussion. This study aimed to evaluate the effects of an NH(3atm) concentration gradient downwind of a cattle barn in a Montado area. Leaves from the selected Quercus suber L. trees along the gradient showed a clear influence of the NH(3) on δ(13)C, as a consequence of a strong limitation on the photosynthetic machinery by a reduction of both stomatal and mesophyll conductance. A detailed study of the impact of NH(3atm) on the photosynthetic performance of Q. suber trees is presented, and new mechanisms by which NH(3) affects photosynthesis at the leaf level are suggested.

Open or close the gate - stomata action under the control of phytohormones in drought stress conditions

Two highly specialized cells, the guard cells that surround the stomatal pore, are able to integrate environmental and endogenous signals in order to control the stomatal aperture and thereby the gas exchange. The uptake of CO2 is associated with a loss of water by leaves. Control of the size of the stomatal aperture optimizes the efficiency of water use through dynamic changes in the turgor of the guard cells. The opening and closing of stomata is regulated by the integration of environmental signals and endogenous hormonal stimuli. The various different factors to which the guard cells respond translates into the complexity of the network of signaling pathways that control stomatal movements. The perception of an abiotic stress triggers the activation of signal transduction cascades that interact with or are activated by phytohormones. Among these, abscisic acid (ABA), is the best-known stress hormone that closes the stomata, although other phytohormones, such as jasmonic acid, brassinosteroids, cytokinins, or ethylene are also involved in the stomatal response to stresses. As a part of the drought response, ABA may interact with jasmonic acid and nitric oxide in order to stimulate stomatal closure. In addition, the regulation of gene expression in response to ABA involves genes that are related to ethylene, cytokinins, and auxin signaling. In this paper, recent findings on phytohormone crosstalk, changes in signaling pathways including the expression of specific genes and their impact on modulating stress response through the closing or opening of stomata, together with the highlights of gaps that need to be elucidated in the signaling network of stomatal regulation, are reviewed.

A putative role for TIP and PIP aquaporins in dynamics of leaf hydraulic and stomatal conductances in grapevine under water stress and re-watering

We examined the role of aquaporins (AQPs) in regulating leaf hydraulic conductance (Kleaf ) in Vitis vinifera L. (cv Chardonnay) by studying effects of AQP inhibitors, and AQP gene expression during water stress (WS) and recovery (REC). Kleaf was measured after 3 h of petiole perfusion with different solutions and to introduce inhibitors. The addition of 0.1 mm HgCl2 to 15 mm KCl reduced Kleaf compared with perfusion in 15 mM KNO3 or KCl, and these solutions were used for leaves from control, WS and REC plants. Perfusion for 3 h did not significantly alter stomatal conductance (gs ) though expression of VvTIP1;1 was increased. WS decreased Kleaf by about 30% and was correlated with gs . The expression of VvTIP2;1 and VvPIP2;1 correlated with Kleaf , and VvTIP2;1 was highly correlated with gs . There was no association between the expression of particular AQPs during WS and REC and inhibition of Kleaf by HgCl2 ; however, HgCl2 treatment itself increased expression of VvPIP2;3 and decreased expression of VvPIP2;1. Inhibition by HgCl2 of Kleaf only at early stages of WS and then after REC suggested that apoplasmic pathways become more important during WS. This was confirmed using fluorescent dyes confined to apoplasm or preferentially accumulated in symplasm.

Silver ions disrupt K⁺ homeostasis and cellular integrity in intact barley (Hordeum vulgare L.) roots

The heavy metals silver, gold, and mercury can strongly inhibit aquaporin-mediated water flow across plant cell membranes, but critical examinations of their side effects are rare. Here, the short-lived radiotracer (42)K is used to demonstrate that these metals, especially silver, profoundly change potassium homeostasis in roots of intact barley (Hordeum vulgare L.) plants, by altering unidirectional K(+) fluxes. Doses as low as 5 μM AgNO(3) rapidly reduced K(+) influx to 5% that of controls, and brought about pronounced and immediate increases in K(+) efflux, while higher doses of Au(3+) and Hg(2+) were required to produce similar responses. Reduced influx and enhanced efflux of K(+) resulted in a net loss of >40% of root tissue K(+) during a 15 min application of 500 μM AgNO(3), comprising the entire cytosolic potassium pool and about a third of the vacuolar pool. Silver also brought about major losses of UV-absorbing compounds, total electrolytes, and NH(4)(+). Co-application, with silver, of the channel blockers Cs(+), TEA(+), or Ca(2+), did not affect the enhanced efflux, ruling out the involvement of outwardly rectifying ion channels. Taken together with an examination of propidium iodide staining under confocal microscopy, the results indicate that silver ions affect K(+) homeostasis by directly inhibiting K(+) influx at lower concentrations, and indirectly inhibiting K(+) influx and enhancing K(+) efflux, via membrane destruction, at higher concentrations. Ni(2+), Cd(2+), and Pb(2+), three heavy metals not generally known to affect aquaporins, did not enhance K(+) efflux or cause propidium iodide incorporation. The study reveals strong and previously unknown effects of major aquaporin inhibitors and recommends caution in their application.

Silencing of G proteins uncovers diversified plant responses when challenged by three elicitors in Nicotiana benthamiana

Signalling through heterotrimeric G protein composed of α-, β- and γ-subunits is essential in numerous physiological processes. Here we show that this prototypical G protein complex acts mechanistically by controlling elicitor sensitivity towards hypersensitive response (HR) and stomatal closure in Nicotiana benthamiana. Gα-, Gβ1-, and Gβ2-silenced plants were generated using virus-induced gene silencing. All silenced plants were treated with Xanthomonas oryzae harpin, Magnaporthe oryzae Nep1 and Phytophthora boehmeriae boehmerin, respectively. HR was dramatically impaired in Gα- and Gβ2-silenced plants treated with harpin, indicating that harpin-, rather than Nep1- or boehmerin-triggered HR, is Gα- and Gβ2-dependent. Moreover, all Gα-, Gβ1- and Gβ2-silenced plants significantly impaired elicitor-induced stomatal closure, elicitor-promoted nitric oxide (NO) production and active oxygen species accumulation in guard cells. To our knowledge, this is the first report of Gα and Gβ subunits involvement in stomatal closure in response to elicitors. Furthermore, silencing of Gα, Gβ1 and Gβ2 has an effect on the transcription of plant defence-related genes when challenged by three elicitors. In conclusion, silencing of G protein subunits results in many interesting plant cell responses, revealing that plant immunity systems employ both conserved and distinct G protein pathways to sense elicitors from distinct phytopathogens formed during plant-microbe evolution.

Calcium storage in plants and the implications for calcium biofortification

Calcium (Ca) is an essential nutrient for plants and animals, with key structural and signalling roles, and its deficiency in plants can result in poor biotic and abiotic stress tolerance, reduced crop quality and yield. Likewise, low Ca intake in humans has been linked to various diseases (e.g. rickets, osteoporosis, hypertension and colorectal cancer) which can threaten quality of life and have major economic costs. Biofortification of various food crops with Ca has been suggested as a good method to enhance human intake of Ca and is advocated as an economically and environmentally advantageous strategy. Efforts to enhance Ca content of crops via transgenic means have had promising results. Overall Ca content of transgenic plants has been increased but in some cases adverse affects on plant function have been observed. This suggests that a better understanding of how Ca ions (Ca(2+)) are stored and transported through plants is required to maximise the effectiveness of future approaches.

The role of vacuolar processing enzymes in plant immunity

Proteases play important roles in plant innate immunity. In this mini-review, we describe the current view on the role of a plant protease, vacuolar processing enzyme (VPE), and the first identified plant caspase-1-like protein, in plant immunity. In the past several years, VPEs were determined to play important roles in various types of cell death in plants. Early studies demonstrated the identification of VPE as a vacuolar hydrolytic protein responsible for maturation of vacuolar proteins. Later, Nicotiana benthamiana VPE was reported to mediate virus-induced hypersensitive response by regulating membrane collapse. The ortholog of VPE in Arabidopsis is also suggested to be involved in both mycotoxin-induced cell death and developmental cell death. However, the role of VPE in elicitor-signaling is still unclear. Our recent studies demonstrated the involvement of VPE in elicitor signal transduction to induce stomatal closure and defense responses, including defense gene expression and hypersensitive cell death.

The role of vacuolar processing enzyme (VPE) from Nicotiana benthamiana in the elicitor-triggered hypersensitive response and stomatal closure

Elicitors/pathogen-associated molecular patterns (PAMPs) trigger the plant immune system, leading to rapid programmed cell death (hypersensitive response, HR) and stomatal closure. Previous reports have shown that the vacuolar processing enzyme (VPE), a cysteine proteinase responsible for the maturation of vacuolar proteins, has caspase-1-like activity and mediates TMV- and mycotoxin-induced cell death. The role of VPE from Nicotiana benthamiana in the response to three elicitors: bacterial harpin, fungal Nep1, and oomycete boehmerin, is described here. Single-silenced (NbVPE1a or NbVPE1b) and dual-silenced (NbVPE1a/1b) N. benthamiana plants were produced by virus-induced gene silencing. Although NbVPE silencing does not affect H(2)O(2) accumulation triggered by boehmerin, harpin, or Nep1, the HR is absent in NbVPE1a- and NbVPE1a/1b-silenced plants treated with harpin alone. However, NbVPE-silenced plants develop a normal HR after boehmerin and Nep1 treatment. These results suggest that harpin-triggered HR is VPE-dependent. Surprisingly, all gene-silenced plants show significantly impaired elicitor-induced stomatal closure and elicitor-promoted nitric oxide (NO) production in guard cells. Dual-silenced plants show increased elicitor-triggered AOS production in guard cells. The accumulation of transcripts associated with defence and cell redox is modified by VPE silencing in elicitor signalling. Overall, these results indicate that VPE from N. benthamiana functions not only in elicitor-induced HR, but also in elicitor-induced stomatal closure, suggesting that VPE may be involved in elicitor-triggered immunity.

Stomatal action directly feeds back on leaf turgor: new insights into the regulation of the plant water status from non-invasive pressure probe measurements

Uptake of CO(2) by the leaf is associated with loss of water. Control of stomatal aperture by volume changes of guard cell pairs optimizes the efficiency of water use. Under water stress, the protein kinase OPEN STOMATA 1 (OST1) activates the guard-cell anion release channel SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1), and thereby triggers stomatal closure. Plants with mutated OST1 and SLAC1 are defective in guard-cell turgor regulation. To study the effect of stomatal movement on leaf turgor using intact leaves of Arabidopsis, we used a new pressure probe to monitor transpiration and turgor pressure simultaneously and non-invasively. This probe permits routine easy access to parameters related to water status and stomatal conductance under physiological conditions using the model plant Arabidopsis thaliana. Long-term leaf turgor pressure recordings over several weeks showed a drop in turgor during the day and recovery at night. Thus pressure changes directly correlated with the degree of plant transpiration. Leaf turgor of wild-type plants responded to CO(2), light, humidity, ozone and abscisic acid (ABA) in a guard cell-specific manner. Pressure probe measurements of mutants lacking OST1 and SLAC1 function indicated impairment in stomatal responses to light and humidity. In contrast to wild-type plants, leaves from well-watered ost1 plants exposed to a dry atmosphere wilted after light-induced stomatal opening. Experiments with open stomata mutants indicated that the hydraulic conductance of leaf stomata is higher than that of the root-shoot continuum. Thus leaf turgor appears to rely to a large extent on the anion channel activity of autonomously regulated stomatal guard cells.

Effects of fusicoccin on ion fluxes in guard cells

The pharmacology has been further investigated of the two transport systems mediating potassium (rubidium) (K(+)(Rb(+))) release from the guard cell vacuole, responsible, respectively, for the resting efflux and abscisic acid (ABA)-induced transient stimulation of efflux, and for the transient stimulation induced by hypotonic treatment. Here, the effects of fusicoccin and of butyrate-induced cytoplasmic acidification on (86)Rb efflux were measured in isolated guard cells of Commelina communis. Fusicoccin (10 microM) inhibited the resting efflux at the tonoplast and the ABA-induced transient, but had no effect on the hypotonic transient. All three processes were inhibited by cytoplasmic acidification. Fusicoccin did not inhibit efflux at the plasmalemma. As the hypotonic response is inhibited by cytoplasmic acidification but not by fusicoccin, the effect of fusicoccin on the resting efflux and ABA response must be direct, and not the result of fusicoccin-induced cytoplasmic acidification. The collected tonoplast efflux properties resemble those of TPC1 (two-pore channel) rather than TPK1 (two-pore K channel). The flux and TPC1 are both activated by Ca(2+), but inhibited by phenylarsine oxide and by cytoplasmic acidification. The flux is inhibited by fusicoccin. TPC1 is inhibited by 14-3-3 proteins and has the C-terminal sequence STSDT, a type III binding site for 14-3-3 proteins, of the kind involved in fusicoccin binding.

Dynamic coordination of cytoskeletal and cell wall systems during plant cell morphogenesis

Underlying the architectural complexity of plants are diverse cell types that, under the microscope, easily reveal relationships between cell structure and specialized functions. Much less obvious are the mechanisms by which the cellular growth machinery and mechanical properties of the cell interact to dictate cell shape. The recent combined use of mutants, genomic analyses, sophisticated spectroscopies, and live cell imaging is providing new insight into how cytoskeletal systems and cell wall biosynthetic activities are integrated during morphogenesis. The purpose of this review is to discuss the unique geometric properties and physical processes that regulate plant cell expansion, then to overlay on this mechanical system some of the recent discoveries about the protein machines and cellular polymers that regulate cell shape. In the end, we hope to make clear that there are many interesting opportunities to develop testable mathematical models that improve our understanding of how subcellular structures, protein motors, and extracellular polymers can exert effects at spatial scales that span cells, tissues, and organs.

Actin filaments modulate hypoosmotic-responsive K+ efflux channels in specialised cells of developing bean seed coats

In developing bean (Phaseolus vulgaris L.) seeds, nutrients move in the symplasm from sieve elements to ground-parenchyma cells where they are transported across the plasma membrane into the seed apoplasm. Release of nutrients to the seed apoplasm is related to the osmotic conditions of the apoplasm. A hypoosmotic solution, resulting from enhanced uptake of nutrients by cotyledons, stimulates nutrient release from seed coat to the apoplasm. We investigated hypoosmotic nutrient release by examining the ionic membrane currents that respond to hypoosmotic treatment in protoplasts derived from three important cell types that occur at the seed coat-cotyledonary boundary. A non-selective but predominantly K⁺ efflux current that displayed a distinct time-dependent inactivation was elicited by membrane depolarisation under hypoosmotic conditions only in ground-parenchyma protoplasts. Hypoosmotic treatment had little effect on whole-cell ionic currents in protoplasts derived from coat chlorenchyma cells and cotyledon dermal cells. The inactivating K⁺ efflux current was elicited under isosmotic conditions by treatment with cytochalasin D, which disrupts actin filaments. Hypoosmotic treatment and cytochalasin D failed to induce the K⁺ current in ground-parenchyma protoplasts in the presence of the actin stabiliser, phalloidin. The net efflux of K⁺ from intact seed coats was enhanced by hypoosmotic treatment and cytochalasin D, and the stimulation of K⁺ efflux induced by the hypoosmotic treatment and cytochalasin D was abolished by phalloidin. A bursting Cl^- channel previously described showed a similar pattern of responses. These results suggest that hypoosmotic-dependent KCl efflux from seed coats is mediated by the inactivating K⁺ outward current and bursting Cl^- channel, and that actin filaments act as components of the transduction process that is a function of cell volume.

Roles of ion channels and transporters in guard cell signal transduction

Stomatal complexes consist of pairs of guard cells and the pore they enclose. Reversible changes in guard cell volume alter the aperture of the pore and provide the major regulatory mechanism for control of gas exchange between the plant and the environment. Stomatal movement is facilitated by the activity of ion channels and ion transporters found in the plasma membrane and vacuolar membrane of guard cells. Progress in recent years has elucidated the molecular identities of many guard cell transport proteins, and described their modulation by various cellular signal transduction components during stomatal opening and closure prompted by environmental and endogenous stimuli.