The role of ion channels in light-dependent stomatal opening

Salt elicitation to enhance phytochemicals in durum wheat seedlings

BACKGROUND

Salt has been identified as an elicitor that can increase the accumulation of phytochemicals in seedlings during the germination process. However, the salinity level required to maximize the yield of phytochemicals, particularly phenolic compounds, needs further investigation for several plant species. To address this issue, we imposed increasing levels of salinity (NaCl solutions) on the sprouting substrate of Triticum durum (var. Platone) grains, at concentrations of 0, 50, 100, 150, 200, 250, and 300 mM (0_S, 50_S, 100_S, 150_S, 200_S, 250_S, and 300_S, respectively).

RESULTS

The highest NaCl doses (250_S and 300_S) significantly impacted germination performance and were excluded from further analysis. The seedlings harvested at 8 days after sowing exhibited different growth stages depending on the salinity level: wheatgrass for 0_S, early wheatgrass for 50_S, intermediate between sprout and wheatgrass for 100_S, sprout for 150_S, and very early sprout for 200_S. Furthermore, salinity induced the concentration of phenolic compounds (PhCs) in the seedlings' tissues (i.e., both roots and shoots) in a salinity-dependent manner. The highest values were observed at 200_S, with an increase of 187% of the total investigated PhCs in comparison with 0_S, averaged over shoots and roots. In particular, in 200_S, the accumulation of phenolic acids was up to fourfold higher in roots, and that of flavonoids was up to twofold higher in shoots.

CONCLUSION

Our findings suggest that the use of 200 mM NaCl applied to the sprouting substrate is excessive for producing edible sprouts but may be suitable for phytochemical extraction purposes. © 2023 Society of Chemical Industry.

Role of Foliar Biointerface Properties and Nanomaterial Chemistry in Controlling Cu Transfer into Wild-Type and Mutant Arabidopsis thaliana Leaf Tissue

Seven Arabidopsis thaliana mutants with differences in cuticle thickness and stomatal density were foliar exposed to 50 mg L^-1 Cu₃(PO₄)₂ nanosheets (NS), CuO NS, CuO nanoparticles, and CuSO₄. Three separate fractions of Cu (surface-attached, cuticle, interior leaf) were isolated from the leaf at 0.25, 2, 4, and 8 h. Cu transfer from the surface through the cuticle and into the leaf varied with mutant and particle type. The Cu content on the surface decreased significantly over 8 h but increased in the cuticle. Cu derived from the ionic form had the greatest cuticle concentration, suggesting greater difficulty in moving across this barrier and into the leaf. Leaf Cu in the increased-stomatal mutants was 8.5-44.9% greater than the decreased stomatal mutants, and abscisic acid to close the stomata decreased Cu in the leaf. This demonstrates the importance of nanomaterial entry through the stomata and enables the optimization of materials for nanoenabled agriculture.

Effects of Low-Level Artificial Light at Night on Kentucky Bluegrass and an Introduced Herbivore

Increasing evidence suggests that artificial light at night (ALAN) can negatively impact organisms. However, most studies examine the impacts of ALAN on a single species or under high levels of artificial light that are infrequent or unrealistic in urban environments. We currently have little information on how low levels of artificial light emanating from urban skyglow affect plants and their interactions with herbivores. We examined how short-term, low levels of ALAN affect grass and insects, including growth rate, photosynthesis, and stomatal conductance in grass, and foraging behavior and survival in crickets. We compared growth and leaf-level gas exchange of Kentucky Bluegrass (Poa pratensis) under low-levels of ALAN (0.3 lux) and starlight conditions (0.001 lux). Furthermore, each light treatment was divided into treatments with and without house crickets (Acheta domesticus). Without crickets present, bluegrass grown under ALAN for three weeks grew taller than plants grown under natural night light levels. In the fourth week when crickets were introduced, grass height decreased resulting in no measurable effects of light treatment. There were no measurable differences in grass physiology among treatments. Our results indicate that low levels of light resulting from skyglow affect plant growth initially. However, with herbivory, the effects of ALAN on grass may be inconsequential. Gaining an understanding of how ALAN affects plant-insect interactions is critical to predicting the ecological and evolutionary consequences of anthropogenic light pollution.

Evolution of the Subgroup 6 R2R3-MYB Genes and Their Contribution to Floral Color in the Perianth-Bearing Piperales

Flavonoids, carotenoids, betalains, and chlorophylls are the plant pigments responsible for floral color. Anthocyanins, a class of flavonoids, are largely responsible for the red, purple, pink, and blue colors. R2R3-MYB genes belonging to subgroup 6 (SG6) are the upstream regulatory factors of the anthocyanin biosynthetic pathway. The canonical members of these genes in Arabidopsis include AtMYB75, AtMYB90, AtMYB113, and AtMYB114. The Aristolochiaceae is an angiosperm lineage with diverse floral groundplans and perianth colors. Saruma henryi exhibits a biseriate perianth with green sepals and yellow petals. All other genera have sepals only, with colors ranging from green (in Lactoris) to a plethora of yellow to red and purple mixtures. Here, we isolated and reconstructed the SG6 R2R3-MYB gene lineage evolution in angiosperms with sampling emphasis in Aristolochiaceae. We found numerous species-specific duplications of this gene lineage in core eudicots and local duplications in Aristolochiaceae for Saruma and Asarum. Expression of SG6 R2R3-MYB genes examined in different developmental stages and plant organs of four Aristolochiaceae species, largely overlaps with red and purple pigments, suggesting a role in anthocyanin and flavonoid synthesis and accumulation. A directed RNA-seq analysis corroborated our RT-PCR analyses, by showing that these structural enzymes activate during perianth development in Aristolochia fimbriata and that the regulatory genes are expressed in correlation with color phenotype. Finally, the reconstruction of the flavonoid and anthocyanin metabolic pathways using predicted peptides from transcriptomic data show that all pivotal enzymes are present in the analyzed species. We conclude that the regulatory genes as well as the biosynthetic pathway are largely conserved across angiosperms. In addition, the Aristolochiaceae emerges as a remarkable group to study the genetic regulatory network for floral color, as their members exhibit an outstanding floral diversity with elaborate color patterns and the genetic complement for SG6 R2R3-MYB genes is simpler than in core eudicot model species.

Anesthetics and plants: no pain, no brain, and therefore no consciousness

Plants have a rich variety of interactions with their environment, including adaptive responses mediated by electrical signaling. This has prompted claims that information processing in plants is similar to that in animals and, hence, that plants are conscious, intelligent organisms. In several recent reports, the facts that general anesthetics cause plants to lose their sensory responses and behaviors have been taken as support for such beliefs. These lipophilic substances, however, alter multiple molecular, cellular, and systemic functions in almost every organism. In humans and other animals with complex brains, they eliminate the experience of pain and disrupt consciousness. The question therefore arises: do plants feel pain and have consciousness? In this review, we discuss what can be learned from the effects of anesthetics in plants. For this, we describe the mechanisms and structural prerequisites for pain sensations in animals and show that plants lack the neural anatomy and all behaviors that would indicate pain. By explaining the ubiquitous and diverse effects of anesthetics, we discuss whether these substances provide any empirical or logical evidence for "plant consciousness" and whether it makes sense to study the effects of anesthetics on plants for this purpose. In both cases, the answer is a resounding no.

Potassium Control of Plant Functions: Ecological and Agricultural Implications

Potassium, mostly as a cation (K+), together with calcium (Ca2+) are the most abundant inorganic chemicals in plant cellular media, but they are rarely discussed. K+ is not a component of molecular or macromolecular plant structures, thus it is more difficult to link it to concrete metabolic pathways than nitrogen or phosphorus. Over the last two decades, many studies have reported on the role of K+ in several physiological functions, including controlling cellular growth and wood formation, xylem-phloem water content and movement, nutrient and metabolite transport, and stress responses. In this paper, we present an overview of contemporary findings associating K+ with various plant functions, emphasizing plant-mediated responses to environmental abiotic and biotic shifts and stresses by controlling transmembrane potentials and water, nutrient, and metabolite transport. These essential roles of K+ account for its high concentrations in the most active plant organs, such as leaves, and are consistent with the increasing number of ecological and agricultural studies that report K+ as a key element in the function and structure of terrestrial ecosystems, crop production, and global food security. We synthesized these roles from an integrated perspective, considering the metabolic and physiological functions of individual plants and their complex roles in terrestrial ecosystem functions and food security within the current context of ongoing global change. Thus, we provide a bridge between studies of K+ at the plant and ecological levels to ultimately claim that K+ should be considered at least at a level similar to N and P in terrestrial ecological studies.

Integrative Analysis of the Metabolome and Transcriptome of Sorghum bicolor Reveals Dynamic Changes in Flavonoids Accumulation under Saline-Alkali Stress

With the perpetuation of soil salinization, it is imperative to improve the salt and alkaline tolerance of crops. Sorghum bicolor, a C4 crop, is often grown in semiarid areas due to its high tolerance of various abiotic stresses. Whether to improve the resistance of the sorghum itself or that of other crops, it is necessary to understand the response of sorghum under saline-alkali stress. An integrative analysis of the metabolome and transcriptome of sorghum under normal conditions and treatments of moderate and severe saline-alkali stress was performed. Among the different accumulated metabolites (DAMs) and differentially expressed genes (DEGs), flavonoid-related DAMs and DEGs were clearly changed. The level of flavonoids was increased under saline-alkali stress, and the change in flavonoids was dynamic as to whether total flavonoids or most flavonoid components accumulated more under moderate saline-alkali stress compared to severe stress. Some flavonoid metabolites were significantly correlated with the expression of flavonoid biosynthesis genes. MYB transcription factors may also contribute to the regulation of flavonoids levels. These findings present the dynamic changes and possible molecular mechanisms of flavonoids under different saline-alkali stresses and provide a foundation for future research and crop improvement.

Global transcriptome analysis uncovers the gene co-expression regulation network and key genes involved in grain development of wheat (Triticum aestivum L.)

Wheat grain development is a robust biological process that largely determines grain quality and yield. In this study, we investigated the grain transcriptome of winter wheat cv. Xiaoyan-6 at four developmental stages (5, 10, 15, and 20 days post-anthesis), using high-throughput RNA sequencing (RNA-Seq). We identified 427 grain-specific transcription factors (TFs) and 1653 differentially expressed TFs during grain development as well as a grain co-expression regulation network (GrainNet) of the TFs and their predicted co-expressed genes. Our study identified ten putative key TFs and the predicted regulatory genes of these TFs in wheat grain development of Xiaoyan-6. The analysis was given a firm basis through the study of additional wheat tissues, including root, stem, leaf, flag leaf, grain, spikes (from wheat plants at booting or heading stages) to provide a dataset of 92,478 high-confidence protein-coding genes that were mostly evenly distributed among subgenomes, but unevenly distributed across each of the chromosomes or each of the seven homeologous groups. Within this larger framework of the transcriptomes, we identified 4659 grain-specific genes (SEGs) and 26,500 differentially expressed genes (DEGs) throughout grain development stages tested. The SEGs identified mainly associate with regulation and signaling-related biological processes, while the DEGs mainly involve in cellular component organization or biogenesis and nutrient reservoir activity during grain development of Xiaoyan-6. This study establishes new targets for modifying genes related to grain development and yield, to fine-tune expression in different varieties and environments.

Identification of regions responsible for the function of the plant K+ channels KAT1 and AKT2 in Saccharomyces cerevisiae and Xenopus laevis oocytes

The Arabidopsis K⁺ channel KAT1 complements in K⁺-limited medium the growth of the K⁺ uptake defective Saccharomyces cerevisiae mutant strain CY162, while another K⁺ channel, AKT2, does not. To gain insight into the structural basis for this difference, we constructed 12 recombinant chimeric channels from these two genes. When expressed in CY162, only three of these chimeras fully rescued the growth of CY162 under K⁺-limited conditions. We conclude that the transmembrane core region of KAT1 is important for its activity in S. cerevisiae. This involves not only the pore region but also parts of its voltage-sensor domain.

Electrical Signaling, Photosynthesis and Systemic Acquired Acclimation

Electrical signaling in higher plants is required for the appropriate intracellular and intercellular communication, stress responses, growth and development. In this review, we have focus on recent findings regarding the electrical signaling, as a major regulator of the systemic acquired acclimation (SAA) and the systemic acquired resistance (SAR). The electric signaling on its own cannot confer the required specificity of information to trigger SAA and SAR, therefore, we have also discussed a number of other mechanisms and signaling systems that can operate in combination with electric signaling. We have emphasized the interrelation between ionic mechanism of electrical activity and regulation of photosynthesis, which is intrinsic to a proper induction of SAA and SAR. In a special way, we have summarized the role of non-photochemical quenching and its regulator PsbS. Further, redox status of the cell, calcium and hydraulic waves, hormonal circuits and stomatal aperture regulation have been considered as components of the signaling. Finally, a model of light-dependent mechanisms of electrical signaling propagation has been presented together with the systemic regulation of light-responsive genes encoding both, ion channels and proteins involved in regulation of their activity. Due to space limitations, we have not addressed many other important aspects of hormonal and ROS signaling, which were presented in a number of recent excellent reviews.

A Dominant Negative OsKAT2 Mutant Delays Light-Induced Stomatal Opening and Improves Drought Tolerance without Yield Penalty in Rice

Stomata are the main gateways for water and air transport between leaves and the environment. Inward-rectifying potassium channels regulate photo-induced stomatal opening. Rice contains three inward rectifying shaker-like potassium channel proteins, OsKAT1, OsKAT2, and OsKAT3. Among these, only OsKAT2 is specifically expressed in guard cells. Here, we investigated the functions of OsKAT2 in stomatal regulation using three dominant negative mutant proteins, OsKAT2(T235R), OsKAT2(T285A) and OsKAT2(T285D), which are altered in amino acids in the channel pore and at a phosphorylation site. Yeast complementation and patch clamp assays showed that all three mutant proteins lost channel activity. However, among plants overexpressing these mutant proteins, only plants overexpressing OsKAT2(T235R) showed significantly less water loss than the control. Moreover, overexpression of this mutant protein led to delayed photo-induced stomatal opening and increased drought tolerance. Our results indicate that OsKAT2 is an inward- rectifying shaker-like potassium channel that mainly functions in stomatal opening. Interestingly, overexpression of OsKAT2(T235R) did not cause serious defects in growth or yield in rice, suggesting that OsKAT2 is a potential target for engineering plants with improved drought tolerance without yield penalty.

Methylglyoxal: An Emerging Signaling Molecule in Plant Abiotic Stress Responses and Tolerance

The oxygenated short aldehyde methylglyoxal (MG) is produced in plants as a by-product of a number of metabolic reactions, including elimination of phosphate groups from glycolysis intermediates dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. MG is mostly detoxified by the combined actions of the enzymes glyoxalase I and glyoxalase II that together with glutathione make up the glyoxalase system. Under normal growth conditions, basal levels of MG remain low in plants; however, when plants are exposed to abiotic stress, MG can accumulate to much higher levels. Stress-induced MG functions as a toxic molecule, inhibiting different developmental processes, including seed germination, photosynthesis and root growth, whereas MG, at low levels, acts as an important signaling molecule, involved in regulating diverse events, such as cell proliferation and survival, control of the redox status of cells, and many other aspects of general metabolism and cellular homeostases. MG can modulate plant stress responses by regulating stomatal opening and closure, the production of reactive oxygen species, cytosolic calcium ion concentrations, the activation of inward rectifying potassium channels and the expression of many stress-responsive genes. MG appears to play important roles in signal transduction by transmitting and amplifying cellular signals and functions that promote adaptation of plants growing under adverse environmental conditions. Thus, MG is now considered as a potential biochemical marker for plant abiotic stress tolerance, and is receiving considerable attention by the scientific community. In this review, we will summarize recent findings regarding MG metabolism in plants under abiotic stress, and evaluate the concept of MG signaling. In addition, we will demonstrate the importance of giving consideration to MG metabolism and the glyoxalase system, when investigating plant adaptation and responses to various environmental stresses.

Transcriptome-wide characterization of candidate genes for improving the water use efficiency of energy crops grown on semiarid land

Understanding the genetic basis of water use efficiency (WUE) and its roles in plant adaptation to a drought environment is essential for the production of second-generation energy crops in water-deficit marginal land. In this study, RNA-Seq and WUE measurements were performed for 78 individuals of Miscanthus lutarioriparius grown in two common gardens, one located in warm and wet Central China near the native habitats of the species and the other located in the semiarid Loess Plateau, the domestication site of the energy crop. The field measurements showed that WUE of M. lutarioriparius in the semiarid location was significantly higher than that in the wet location. A matrix correlation analysis was conducted between gene expression levels and WUE to identify candidate genes involved in the improvement of WUE from the native to the domestication site. A total of 48 candidate genes were identified and assigned to functional categories, including photosynthesis, stomatal regulation, protein metabolism, and abiotic stress responses. Of these genes, nearly 73% were up-regulated in the semiarid site. It was also found that the relatively high expression variation of the WUE-related genes was affected to a larger extent by environment than by genetic variation. The study demonstrates that transcriptome-wide correlation between physiological phenotypes and expression levels offers an effective means for identifying candidate genes involved in the adaptation to environmental changes.

Removal of trimethylamine (fishy odor) by C₃ and CAM plants

From screening 23 plant species, it was found that Pterocarpus indicus (C3) and Sansevieria trifasciata (crassulacean acid metabolism (CAM)) were the most effective in polar gaseous trimethylamine (TMA) uptake, reaching up to 90% uptake of initial TMA (100 ppm) within 8 h, and could remove TMA at cycles 1-4 without affecting photosystem II (PSII) photochemistry. Up to 55 and 45% of TMA was taken up by S. trifasciata stomata and leaf epicuticular wax, respectively. During cycles 1-4, interestingly, S. trifasciata changed its stomata apertures, which was directly induced by gaseous TMA and light treatments. In contrast, for P. indicus the leaf epicuticular wax and stem were the major pathways of TMA removal, followed by stomata; these pathways accounted for 46, 46, and 8%, respectively, of TMA removal percentages. Fatty acids, particularly tetradecanoic (C14) acid and octadecanoic (C18) acid, were found to be the main cuticular wax components in both plants, and were associated with TMA removal ability. Moreover, the plants could degrade TMA via multiple metabolic pathways associated with carbon/nitrogen interactions. In CAM plants, one of the crucial pathways enabled 78% of TMA to be transformed directly to dimethylamine (DMA) and methylamine (MA), which differed from C3 plant pathways. Various metabolites were also produced for further detoxification and mineralization so that TMA was completely degraded by plants.

Profiling of rutin-mediated alleviation of cadmium-induced oxidative stress in Zygophyllum fabago

Zygophyllum fabago grows in arid, saline soil, or disturbed sites, such as former industrial or mining areas. This species is able to grow in coarse mineral substrates contaminated with heavy metals. To investigate the effects of the flavonoid rutin (Rtn) on certain heavy metal stress responses such as antioxidant defense systems and water status, seedlings were subjected to 100 and 200 μM CdCl2 treatment without or with 0.25 and 1 mM Rtn for 7 and 14 d (days). Cd stress decreased growth (RGR), water content (RWC), leaf osmotic potential (Ψ(Π)), and chlorophyll fluorescence, all of which could be partly alleviated by addition of Rtn. Activities of superoxide dismutase, peroxidase (POX), ascorbate peroxidase, and glutathione reductase increased within the first 7 d after exposure to Cd. However, failure of antioxidant defense in the scavenging of reactive oxygen species (ROS) was evidenced by an abnormal rise in superoxide anion radical ( O2(•-)) and hydrogen peroxide contents and a decline in hydroxyl radical (OH(•)) scavenging activity, resulting in enhancement of lipid peroxidation (TBARS) as a marker of Cd-induced oxidative stress. However, exogenously applied Rtn considerably improved the stress tolerance of plants via a reduction in Cd accumulation, modulation of POX activity, increase of proline (Pro) content, decrease in TBARS and ROS content and consequent lowering of oxidative damage of membrane. Overall, 0.25 and 1 mM Rtn could protect Z. fabago from the harmful effects of 100 μM Cd-induced oxidative stress throughout the experiment.

The role of antioxidant responses on the tolerance range of extreme halophyte Salsola crassa grown under toxic salt concentrations

Salsola crassa (Amaranthaceae) is an annual halophytic species and naturally grows in arid soils that are toxic to most plants. In order to study the effects of salinity on their antioxidant system and to determine the tolerance range against salt stress, S. crassa seeds were grown with different concentrations of NaCl (0, 250, 500, 750, 1000, 1250 and 1500mM) for short (15d) and long-term (30d). Results showed that growth (RGR), water content (RWC) and osmotic potential (ΨΠ) decreased and, proline content (Pro) increased at prolonged salt treatment. Unlike K(+) and Ca(2+) contents, S. crassa highly accumulated Na(+) and Cl(-) contents. Chlorophyll fluorescence (Fv/Fm) only decreased in response to 1500mM NaCl at 30d. No salt stimulation of superoxide anion radical (O2(•-)) content was observed in plants treated with the range of 0-500mM NaCl during the experimental period. NaCl increased superoxide dismutase (SOD) activity depending on intensities of Mn-SOD and Fe-SOD isozymes except in 1500mM NaCl-treated plants at 30d. In contrast to catalase (CAT), peroxidase (POX) activity increased throughout the experiment. Also, salinity caused an increase in glutathione reductase (GR) and glutathione peroxidase (GPX) and decreased in ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR) at 15d. Both total ascorbate (tAsA) and glutathione (tGlut) contents significantly increased in treated plants with 1000-1500mM NaCl at 15d. After 0-1000mM NaCl stress, H2O2 and TBARS contents were similar to control groups at 15d, which were consistent with the increased antioxidant activity (POX, GR and GPX). However, H2O2 content was more pronounced at 30d. Therefore, S. crassa exhibited inductions in lipid peroxidation (TBARS content) in response to extreme salt concentrations. These results suggest that S. crassa is tolerant to salt-induced damage at short-term treatments as well as extreme salt concentrations.

Analysis of gas exchange, stomatal behaviour and micronutrients uncovers dynamic response and adaptation of tomato plants to monochromatic light treatments

Light spectrum affects the yield and quality of greenhouse tomato, especially over a prolonged period of monochromatic light treatments. Physiological and chemical analysis was employed to investigate the influence of light spectral (blue, green and red) changes on growth, photosynthesis, stomatal behaviour, leaf pigment, and micronutrient levels. We found that plants are less affected under blue light treatment, which was evident by the maintenance of higher A, gs, Tr, and stomatal parameters and significantly lower VPD and Tleaf as compared to those plants grown in green and red light treatments. Green and red light treatments led to significantly larger increase in the accumulation of Fe, B, Zn, and Cu than blue light. Moreover, guard cell length, width, and volume all showed highly significant positive correlations to gs, Tr and negative links to VPD. There was negative impact of monochromatic lights-induced accumulation of Mn, Cu, and Zn on photosynthesis, leaf pigments and plant growth. Furthermore, most of the light-induced significant changes of the physiological traits were partially recovered at the end of experiment. A high degree of morphological and physiological plasticity to blue, green and red light treatments suggested that tomato plants may have developed mechanisms to adapt to the light treatments. Thus, understanding the optimization of light spectrum for photosynthesis and growth is one of the key components for greenhouse tomato production.

Genomics-assisted breeding in four major pulse crops of developing countries: present status and prospects

KEY MESSAGE

Given recent advances in pulse molecular biology, genomics-driven breeding has emerged as a promising approach to address the issues of limited genetic gain and low productivity in various pulse crops. The global population is continuously increasing and is expected to reach nine billion by 2050. This huge population pressure will lead to severe shortage of food, natural resources and arable land. Such an alarming situation is most likely to arise in developing countries due to increase in the proportion of people suffering from protein and micronutrient malnutrition. Pulses being a primary and affordable source of proteins and minerals play a key role in alleviating the protein calorie malnutrition, micronutrient deficiencies and other undernourishment-related issues. Additionally, pulses are a vital source of livelihood generation for millions of resource-poor farmers practising agriculture in the semi-arid and sub-tropical regions. Limited success achieved through conventional breeding so far in most of the pulse crops will not be enough to feed the ever increasing population. In this context, genomics-assisted breeding (GAB) holds promise in enhancing the genetic gains. Though pulses have long been considered as orphan crops, recent advances in the area of pulse genomics are noteworthy, e.g. discovery of genome-wide genetic markers, high-throughput genotyping and sequencing platforms, high-density genetic linkage/QTL maps and, more importantly, the availability of whole-genome sequence. With genome sequence in hand, there is a great scope to apply genome-wide methods for trait mapping using association studies and to choose desirable genotypes via genomic selection. It is anticipated that GAB will speed up the progress of genetic improvement of pulses, leading to the rapid development of cultivars with higher yield, enhanced stress tolerance and wider adaptability.

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.

Multiple Roles of the Plasma Membrane H(+)-ATPase and Its Regulation

The plasma membrane H(+)-ATPase is the pump that provides the driving force for transport of numerous solutes in plant cells, and plays an essential role for the growth and maintenance of cell homeostasis. Recent investigations using guard cells with respect to blue-light-induced stomatal opening uncovered the regulatory mechanisms of the H(+)-ATPase, and revealed that the phosphorylation status of penultimate threonine in the C-terminus of H(+)-ATPase is key step for the activity regulation. The same regulatory mechanisms for the H(+)-ATPase were evidenced in hypocotyl elongation in response to ABA and auxin, suggesting that the phosphorylation of the penultimate threonine is a common regulatory mechanism for the H(+)-ATPase. We also present the data that the activity of the H(+)-ATPase limits the plant growth. Typical structure of the H(+)-ATPase in the C-terminus was acquired in the transition of plants from water to the terrestrial land.

Extracellular ATP promotes stomatal opening of Arabidopsis thaliana through heterotrimeric G protein α subunit and reactive oxygen species

In recent years, adenosine tri-phosphate (ATP) has been reported to exist in apoplasts of plant cells as a signal molecule. Extracellular ATP (eATP) plays important roles in plant growth, development, and stress tolerance. Here, extracellular ATP was found to promote stomatal opening of Arabidopsis thaliana in light and darkness. ADP, GTP, and weakly hydrolyzable ATP analogs (ATPγS, Bz-ATP, and 2meATP) showed similar effects, whereas AMP and adenosine did not affect stomatal movement. Apyrase inhibited stomatal opening. ATP-promoted stomatal opening was blocked by an NADPH oxidase inhibitor (diphenylene iodonium) or deoxidizer (dithiothreitol), and was impaired in null mutant of NADPH oxidase (atrbohD/F). Added ATP triggered ROS generation in guard cells via NADPH oxidase. ATP also induced Ca(2+) influx and H(+) efflux in guard cells. In atrbohD/F, ATP-induced ion flux was strongly suppressed. In null mutants of the heterotrimeric G protein α subunit, ATP-promoted stomatal opening, cytoplasmic ROS generation, Ca(2+) influx, and H(+) efflux were all suppressed. These results indicated that eATP-promoted stomatal opening possibly involves the heterotrimeric G protein, ROS, cytosolic Ca(2+), and plasma membrane H(+)-ATPase.

Determining novel functions of Arabidopsis 14-3-3 proteins in central metabolic processes

Background

14-3-3 proteins are considered master regulators of many signal transduction cascades in eukaryotes. In plants, 14-3-3 proteins have major roles as regulators of nitrogen and carbon metabolism, conclusions based on the studies of a few specific 14-3-3 targets.

Results

In this study, extensive novel roles of 14-3-3 proteins in plant metabolism were determined through combining the parallel analyses of metabolites and enzyme activities in 14-3-3 overexpression and knockout plants with studies of protein-protein interactions. Decreases in the levels of sugars and nitrogen-containing-compounds and in the activities of known 14-3-3-interacting-enzymes were observed in 14-3-3 overexpression plants. Plants overexpressing 14-3-3 proteins also contained decreased levels of malate and citrate, which are intermediate compounds of the tricarboxylic acid (TCA) cycle. These modifications were related to the reduced activities of isocitrate dehydrogenase and malate dehydrogenase, which are key enzymes of TCA cycle. In addition, we demonstrated that 14-3-3 proteins interacted with one isocitrate dehydrogenase and two malate dehydrogenases. There were also changes in the levels of aromatic compounds and the activities of shikimate dehydrogenase, which participates in the biosynthesis of aromatic compounds.

Conclusion

Taken together, our findings indicate that 14-3-3 proteins play roles as crucial tuners of multiple primary metabolic processes including TCA cycle and the shikimate pathway.

Poplar maintains zinc homeostasis with heavy metal genes HMA4 and PCS1

Perennial woody species, such as poplar (Populus spp.) must acquire necessary heavy metals like zinc (Zn) while avoiding potential toxicity. Poplar contains genes with sequence homology to genes HMA4 and PCS1 from other species which are involved in heavy metal regulation. While basic genomic conservation exists, poplar does not have a hyperaccumulating phenotype. Poplar has a common indicator phenotype in which heavy metal accumulation is proportional to environmental concentrations but excesses are prevented. Phenotype is partly affected by regulation of HMA4 and PCS1 transcriptional abundance. Wild-type poplar down-regulates several transcripts in its Zn-interacting pathway at high Zn levels. Also, overexpressed PtHMA4 and PtPCS1 genes result in varying Zn phenotypes in poplar; specifically, there is a doubling of Zn accumulation in leaf tissues in an overexpressed PtPCS1 line. The genomic complement and regulation of poplar highlighted in this study supports a role of HMA4 and PCS1 in Zn regulation dictating its phenotype. These genes can be altered in poplar to change its interaction with Zn. However, other poplar genes in the surrounding pathway may maintain the phenotype by inhibiting drastic changes in heavy metal accumulation with a single gene transformation.

The role of the plasma membrane H+-ATPase in plant-microbe interactions

Plasma membrane (PM) H+-ATPases are the primary pumps responsible for the establishment of cellular membrane potential in plants. In addition to regulating basic aspects of plant cell function, these enzymes contribute to signaling events in response to diverse environmental stimuli. Here, we focus on the roles of the PM H+-ATPase during plant-pathogen interactions. PM H+-ATPases are dynamically regulated during plant immune responses and recent quantitative proteomics studies suggest complex spatial and temporal modulation of PM H+-ATPase activity during early pathogen recognition events. Additional data indicate that PM H+-ATPases cooperate with the plant immune signaling protein RIN4 to regulate stomatal apertures during bacterial invasion of leaf tissue. Furthermore, pathogens have evolved mechanisms to manipulate PM H+-ATPase activity during infection. Thus, these ubiquitous plant enzymes contribute to plant immune responses and are targeted by pathogens to increase plant susceptibility.

Transcriptome phase distribution analysis reveals diurnal regulated biological processes and key pathways in rice flag leaves and seedling leaves

Plant diurnal oscillation is a 24-hour period based variation. The correlation between diurnal genes and biological pathways was widely revealed by microarray analysis in different species. Rice (Oryza sativa) is the major food staple for about half of the world's population. The rice flag leaf is essential in providing photosynthates to the grain filling. However, there is still no comprehensive view about the diurnal transcriptome for rice leaves. In this study, we applied rice microarray to monitor the rhythmically expressed genes in rice seedling and flag leaves. We developed a new computational analysis approach and identified 6,266 (10.96%) diurnal probe sets in seedling leaves, 13,773 (24.08%) diurnal probe sets in flag leaves. About 65% of overall transcription factors were identified as flag leaf preferred. In seedling leaves, the peak of phase distribution was from 2:00am to 4:00am, whereas in flag leaves, the peak was from 8:00pm to 2:00am. The diurnal phase distribution analysis of gene ontology (GO) and cis-element enrichment indicated that, some important processes were waken by the light, such as photosynthesis and abiotic stimulus, while some genes related to the nuclear and ribosome involved processes were active mostly during the switch time of light to dark. The starch and sucrose metabolism pathway genes also showed diurnal phase. We conducted comparison analysis between Arabidopsis and rice leaf transcriptome throughout the diurnal cycle. In summary, our analysis approach is feasible for relatively unbiased identification of diurnal transcripts, efficiently detecting some special periodic patterns with non-sinusoidal periodic patterns. Compared to the rice flag leaves, the gene transcription levels of seedling leaves were relatively limited to the diurnal rhythm. Our comprehensive microarray analysis of seedling and flag leaves of rice provided an overview of the rice diurnal transcriptome and indicated some diurnal regulated biological processes and key functional pathways in rice.

Competition between uptake of ammonium and potassium in barley and Arabidopsis roots: molecular mechanisms and physiological consequences

Plants can use ammonium (NH4+) as the sole nitrogen source, but at high NH4+ concentrations in the root medium, particularly in combination with a low availability of K+, plants suffer from NH4+ toxicity. To understand the role of K+ transporters and non-selective cation channels in K+/NH4+ interactions better, growth, NH4+ and K+ accumulation and the specific fluxes of NH4+, K+, and H+ were examined in roots of barley (Hordeum vulgare L.) and Arabidopsis seedlings. Net fluxes of K+ and NH4+ were negatively correlated, as were their tissue concentrations, suggesting that there is direct competition during uptake. Pharmacological treatments with the K+ transport inhibitors tetraethyl ammonium (TEA+) and gadolinium (Gd3+) reduced NH4+ influx, and the addition of TEA+ alleviated the NH4+-induced depression of root growth in germinating Arabidopsis plants. Screening of a barley root cDNA library in a yeast mutant lacking all NH4+ and K+ uptake proteins through the deletion of MEP1-3 and TRK1 and TRK2 resulted in the cloning of the barley K+ transporter HvHKT2;1. Further analysis in yeast suggested that HvHKT2;1, AtAKT1, and AtHAK5 transported NH4+, and that K+ supplied at increasing concentrations competed with this NH4+ transport. On the other hand, uptake of K+ by AtHAK5, and to a lesser extent via HvHKT2;1 and AtAKT1, was inhibited by increasing concentrations of NH4+. Together, the results of this study show that plant K+ transporters and channels are able to transport NH4+. Unregulated NH4+ uptake via these transporters may contribute to NH4+ toxicity at low K+ levels, and may explain the alleviation of NH4+ toxicity by K+.

Phytochrome B is involved in mediating red light-induced stomatal opening in Arabidopsis thaliana

The stomatal pores of higher plants enable gaseous exchange into and out of leaves for photosynthesis and evaporation. Stomatal opening is induced by both blue and red lights. It is shown that blue light-induced stomatal opening is mediated by the blue light receptor phototropins (PHOT1 and PHOT2) and cryptochromes (CRY1 and CRY2). However, whether phytochrome B (phyB) is involved in red light regulation of stomatal opening remains largely unclear. Here, we report a positive role for Arabidopsis (Arabidopsis thaliana) phyB in the regulation of red light-induced stomatal opening. The phyB mutant stomata displayed a reduced red light response, whereas stomata of the phyB-overexpressing plants displayed a hypersensitive response to red light. In addition, stomata of the cry1 cry2 phyB, phot1 phot2 phyB, and cry1 phyA phyB triple mutant plants showed more reduced light response than those of the single or double mutant plants under white light, implying that phyB acts in concert with phyA, CRY, and PHOT in light regulation of stomatal opening. Stomata of phyB cop1 mutant opened less wide than those of the cop1 mutant, and stomata of the pif3 pif4 mutant opened wider than those of the wild-type, indicating that COP1, together with the PIFs (phytochrome interacting factors), may act downstream of PHYB in regulating stomatal opening. Furthermore, quantitative RT-PCR analysis showed that the expression of MYB60 was reduced in the cry1 cry2 and phyA phyB mutants under blue and red lights, respectively, but induced in the CRY1- and phyB-overexpressing plants. These results demonstrate that phyB and CRY might regulate stomatal opening, at least in part, by regulating MYB60 expression.

Cellular mechanisms of potassium transport in plants

Potassium (K(+)) is the most abundant ion in the plant cell and is required for a wide array of functions, ranging from the maintenance of electrical potential gradients across cell membranes, to the generation of turgor, to the activation of numerous enzymes. The majority of these functions depend more or less directly upon the activities and regulation of membrane-bound K(+) transport proteins, operating over a wide range of K(+) concentrations. Here, we review the physiological aspects of potassium transport systems in the plasma membrane, re-examining fundamental problems in the field such as the distinctions between high- and low-affinity transport systems, the interactions between K(+) and other ions such as NH(4)(+) and Na(+), the regulation of cellular K(+) pools, the generation of electrical potentials and the problems involved in measurement of unidirectional K(+) fluxes. We place these discussions in the context of recent discoveries in the molecular biology of K(+) acquisition and produce an overview of gene families encoding K(+) transporters.

Light and oxygen are not required for harpin-induced cell death

Nicotiana sylvestris leaves challenged by the bacterial elicitor harpin N(Ea) were used as a model system in which to determine the respective roles of light, oxygen, photosynthesis, and respiration in the programmed cell death response in plants. The appearance of cell death markers, such as membrane damage, nuclear fragmentation, and induction of the stress-responsive element Tnt1, was observed in all conditions. However, the cell death process was delayed in the dark compared with the light, despite a similar accumulation of superoxide and hydrogen peroxide in the chloroplasts. In contrast, harpin-induced cell death was accelerated under very low oxygen (<0.1% O(2)) compared with air. Oxygen deprivation impaired accumulation of chloroplastic reactive oxygen species (ROS) and the induction of cytosolic antioxidant genes in both the light and the dark. It also attenuates the collapse of photosynthetic capacity and the respiratory burst driven by mitochondrial alternative oxidase activity observed in air. Since alternative oxidase is known to limit overreduction of the respiratory chain, these results strongly suggest that mitochondrial ROS accumulate in leaves elicited under low oxygen. We conclude that the harpin-induced cell death does not require ROS accumulation in the apoplast or in the chloroplasts but that mitochondrial ROS could be important in the orchestration of the cell suicide program.

Pseudomonas syringae type III effector AvrRpt2 alters Arabidopsis thaliana auxin physiology

The Pseudomonas syringae type III effector AvrRpt2 promotes bacterial virulence on Arabidopsis thaliana plants lacking a functional RPS2 gene (rps2 mutant plants). To investigate the mechanisms underlying the virulence activity of AvrRpt2, we examined the phenotypes of transgenic A. thaliana rps2 seedlings constitutively expressing AvrRpt2. These seedlings exhibited phenotypes reminiscent of A. thaliana mutants with altered auxin physiology, including longer primary roots, increased number of lateral roots, and increased sensitivity to exogenous auxin. They also had increased levels of free indole acetic acid (IAA). The presence of AvrRpt2 also was correlated with a further increase in free IAA levels during infection with P. syringae pv. tomato strain DC3000 (PstDC3000). These results indicate that AvrRpt2 alters A. thaliana auxin physiology. Application of the auxin analog 1-naphthaleneacetic acid promoted disease symptom development in PstDC3000-infected plants, suggesting that elevated auxin levels within host tissue promote PstDC3000 virulence. Thus, AvrRpt2 may be among the virulence factors of P. syringae that modulate host auxin physiology to promote disease.

Phosphatidylinositol 4,5-bisphosphate is important for stomatal opening

Previously, we demonstrated that a protein that binds phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] inhibits both light-induced stomatal opening and ABA-induced stomatal closing. The latter effect is due to a reduction in free PtdIns(4,5)P(2), decreasing production of inositol 1,4,5-trisphosphate and phosphatidic acid by phospholipases C and D. However, it is less clear how PtdIns(4,5)P(2) modulates stomatal opening. We found that in response to white light irradiation, the PtdIns(4,5)P(2)-binding domain GFP:PLCdelta1PH translocated from the cytosol into the plasma membrane. This suggests that the level of PtdIns(4,5)P(2) increases at the plasma membrane upon illumination. Exogenously administered PtdIns(4,5)P(2) substituted for light stimuli, inducing stomatal opening and swelling of guard cell protoplasts. To identify PtdIns(4,5)P(2) targets we performed patch-clamp experiments, and found that anion channel activity was inhibited by PtdIns(4,5)P(2). Genetic analyses using an Arabidopsis PIP5K4 mutant further supported the role of PtdIns(4,5)P(2) in stomatal opening. The reduced stomatal opening movements exhibited by a mutant of Arabidopsis PIP5K4 (At3g56960) was countered by exogenous application of PtdIns(4,5)P(2). The phenotype of reduced stomatal opening in the pip5k4 mutant was recovered in lines complemented with the full-length PIP5K4. Together, these data suggest that PIP5K4 produces PtdIns(4,5)P(2) in irradiated guard cells, inhibiting anion channels to allow full stomatal opening.

Cryptochrome signaling in plants

Cryptochromes are blue light receptors that mediate various light-induced responses in plants and animals. They share sequence similarity to photolyases, flavoproteins that catalyze the repair of UV light-damaged DNA, but do not have photolyase activity. Arabidopsis cryptochromes work together with the red/far-red light receptor phytochromes to regulate various light responses, including the regulation of cell elongation and photoperiodic flowering, and are also found to act together with the blue light receptor phototropins to mediate blue light regulation of stomatal opening. The signaling mechanism of Arabidopsis cryptochromes is mediated through negative regulation of COP1 by direct CRY-COP1 interaction through CRY C-terminal domain. Arabidopsis CRY dimerized through its N-terminal domain and dimerization of CRY is required for light activation of the photoreceptor activity. Recently, significant progresses have been made in our understanding of cryptochrome functions in other dicots such as pea and tomato and lower plants including moss and fern. This review will focus on recent advances in functional and mechanism characterization of cryptochromes in plants. It is not intended to cover every aspect of the field; readers are referred to other review articles for historical perspectives and a more comprehensive understanding of this photoreceptor.

Potential Role of Annexin AnnAt1 from Arabidopsis thaliana in pH-Mediated Cellular Response to Environmental Stimuli

Plant annexins, Ca(2+)- and membrane-binding proteins, are probably implicated in the cellular response to stress resulting from acidification of cytosol. To understand how annexins can contribute to cellular ion homeostasis, we investigated the pH-induced changes in the structure and function of recombinant annexin AnnAt1 from Arabidopsis thaliana. The decrease of pH from 7.0 to 5.8 reduced the time of the formation of ion channels by AnnAt1 in artificial lipid membranes from 3.5 h to 15-20 min and increased their unitary conductance from 32 to 63 pS. These changes were accompanied by an increase in AnnAt1 hydrophobicity as revealed by hydrophobicity predictions, by an increase in fluorescence of 2-(p-toluidino)naphthalene-6-sulfonic acid (TNS) bound to AnnAt1 and fluorescence resonance energy transfer from AnnAt1 tryptophan residues to TNS. Concomitant lipid partition of AnnAt1 at acidic pH resulted in its partial protection from proteolytic digestion. Secondary structures of AnnAt1 determined by circular dichroism and infrared spectroscopy were also affected by lowering the pH from 7.2 to 5.2. These changes were characterized by an increase in beta-sheet content at the expense of alpha-helical structures, and were accompanied by reversible formation of AnnAt1 oligomers as probed by ultracentrifugation in a sucrose gradient. A further decrease of pH from 5.2 to 4.5 or lower led to the formation of irreversible aggregates and loss of AnnAt1 ionic conductance. Our findings suggest that AnnAt1 can sense changes of the pH milieu over the pH range from 7 to 5 and respond by changes in ion channel conductance, hydrophobicity, secondary structure of the protein and formation of oligomers. Further acidification irreversibly inactivated AnnAt1. We suggest that the pH-sensitive ion channel activity of AnnAt1 may play a role in intracellular ion homeostasis.

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.

Phototropin blue-light receptors

Phototropins are blue-light receptors controlling a range of responses that serve to optimize the photosynthetic efficiency of plants. These include phototropism, light-induced stomatal opening, and chloroplast movements in response to changes in light intensity. Since the isolation of the Arabidopsis PHOT1 gene in 1997, phototropins have been identified in ferns and mosses where their physiological functions appear to be conserved. Arabidopsis contains two phototropins, phot1 and phot2, that exhibit overlapping functions in addition to having unique physiological roles. Phototropins are light-activated serine/threonine protein kinases. Light sensing by the phototropins is mediated by a repeated motif at the N-terminal region of the protein known as the LOV domain. Photoexcitation of the LOV domain results in receptor autophosphorylation and an initiation of phototropin signaling. Here we summarize the photochemical and biochemical events underlying phototropin activation in addition to the current knowledge of the molecular mechanisms associated with photoreceptor signaling.

Ion channels meet auxin action

The regulation of cell division and elongation in plants is accomplished by the action of different phytohormones. Auxin as one of these growth regulators is known to stimulate cell elongation growth in the aerial parts of the plant. Here, auxin enhances cell enlargement by increasing the extensibility of the cell wall and by facilitating the uptake of osmolytes such as potassium ions into the cell. Starting in the late 1990s, the auxin regulation of ion channels mediating K+ import into the cell has been studied in great detail. In this article we will focus on the molecular mechanisms underlying the modulation of K+ transport by auxin and present a model to explain how the regulation of K+ channels is involved in auxin-induced cell elongation growth.

The effects of resource availability and environmental conditions on genetic rankings for carbon isotope discrimination during growth in tomato and rice

Carbon isotope discrimination (Δ) is frequently used as an index of leaf intercellular CO₂ concentration (c_i) and variation in photosynthetic water use efficiency. In this study, the stability of Δ was evaluated in greenhouse-grown tomato and rice with respect to variable growth conditions including temperature, nutrient availability, soil flooding (in rice), irradiance, and root constriction in small soil volumes. Δ exhibited several characteristics indicative of contrasting set-point behaviour among genotypes of both crops. These included generally small main environmental effects and lower observed levels of genotype-by-environment interaction across the diverse treatments than observed in associated measures of relative growth rate, photosynthetic rate, biomass allocation pattern, or specific leaf area. Growth irradiance stood out among environmental parameters tested as having consistently large main affects on Δ for all genotypes screened in both crops. We suggest that this may be related to contrasting mechanisms of stomatal aperture modulation associated with the different environmental variables. For temperature and nutrient availability, feedback processes directly linked to c_i and / or metabolite pools associated with c_i may have played the primary role in coordinating stomatal conductance and photosynthetic capacity. In contrast, light has a direct effect on stomatal aperture in addition to feedback mediated through c_i.

Sugars and circadian regulation make major contributions to the global regulation of diurnal gene expression in Arabidopsis

The diurnal cycle strongly influences many plant metabolic and physiological processes. Arabidopsis thaliana rosettes were harvested six times during 12-h-light/12-h-dark treatments to investigate changes in gene expression using ATH1 arrays. Diagnostic gene sets were identified from published or in-house expression profiles of the response to light, sugar, nitrogen, and water deficit in seedlings and 4 h of darkness or illumination at ambient or compensation point [CO(2)]. Many sugar-responsive genes showed large diurnal expression changes, whose timing matched that of the diurnal changes of sugars. A set of circadian-regulated genes also showed large diurnal changes in expression. Comparison of published results from a free-running cycle with the diurnal changes in Columbia-0 (Col-0) and the starchless phosphoglucomutase (pgm) mutant indicated that sugars modify the expression of up to half of the clock-regulated genes. Principle component analysis identified genes that make large contributions to diurnal changes and confirmed that sugar and circadian regulation are the major inputs in Col-0 but that sugars dominate the response in pgm. Most of the changes in pgm are triggered by low sugar levels during the night rather than high levels in the light, highlighting the importance of responses to low sugar in diurnal gene regulation. We identified a set of candidate regulatory genes that show robust responses to alterations in sugar levels and change markedly during the diurnal cycle.

Differential expression of K+ channels between guard cells and subsidiary cells within the maize stomatal complex

Grass stomata are characterized by dumbbell-shaped guard cells forming a complex with a pair of specialized epidermal cells, the subsidiary cells. Stomatal movement is accomplished by a reversible exchange of potassium and chloride between these two cell types. To gain insight into the molecular machinery involved in K+ transport within the stomatal complex of Zea mays, we determined the spatial and temporal expression pattern of potassium channels in the maize leaf. KZM2 and ZORK were isolated and identified as new members of the plant Shaker K+ channel family. Northern blot analysis identified fully developed leaves as the predominant site of KZM2 expression. Following enzymatic digestion and separation of leaf tissue into epidermal, mesophyll, and vascular fractions, KZM2 and ZORK transcripts were localized in the epidermis. Using a collection of individually isolated guard cell or subsidiary cell protoplasts, ZORK transcripts were found in both cell types while KZM2 was exclusively expressed in the guard cell population. The previously identified K+ channel genes ZMK1 and KZM1 were expressed in subsidiary cells and guard cells, respectively, whereas ZMK2 transcripts were not detected. These data indicate that the interaction between subsidiary cells and guard cells is based on overlapping as well as differential expression of K+ channels in the two cell types of the maize stomatal complex.

In the light of stomatal opening: new insights into 'the Watergate'

Stomata can be regarded as hydraulically driven valves in the leaf surface, which open to allow CO2 uptake and close to prevent excessive loss of water. Movement of these 'Watergates' is regulated by environmental conditions, such as light, CO2 and humidity. Guard cells can sense environmental conditions and function as motor cells within the stomatal complex. Stomatal movement results from the transport of K+ salts across the guard cell membranes. In this review, we discuss the biophysical principles and mechanisms of stomatal movement and relate these to ion transport at the plasma membrane and vacuolar membrane. Studies with isolated guard cells, combined with recordings on single guard cells in intact plants, revealed that light stimulates stomatal opening via blue light-specific and photosynthetic-active radiation-dependent pathways. In addition, guard cells sense changes in air humidity and the water status of distant tissues via the stress hormone abscisic acid (ABA). Guard cells thus provide an excellent system to study cross-talk, as multiple signaling pathways induce both short- and long-term responses in these sensory cells.

From The Cover: A role for Arabidopsis cryptochromes and COP1 in the regulation of stomatal opening

Cryptochromes (CRY) are blue light photoreceptors that mediate various light-induced responses in plants and animals. Arabidopsis CRY (CRY1 and CRY2) functions through negatively regulating constitutive photomorphogenic (COP) 1, a repressor of photomorphogenesis. Water evaporation and photosynthesis are regulated by the stomatal pores in plants, which are closed in darkness but open in response to blue light. There is evidence only for the phototropin blue light receptors (PHOT1 and PHOT2) in mediating blue light regulation of stomatal opening. Here, we report a previously uncharacterized role for Arabidopsis CRY and COP1 in the regulation of stomatal opening. Stomata of the cry1 cry2 double mutant showed reduced blue light response, whereas those of the CRY1-overexpressing plants showed hypersensitive response to blue light. In addition, stomata of the phot1 phot2 double mutant responded to blue light, but those of the cry1 cry2 phot1 phot2 quadruple mutant hardly responded. Strikingly, stomata of the cop1 mutant were constitutively open in darkness and stomata of the cry1 cry2 cop1 and phot1 phot2 cop1 triple mutants were open as wide as those of the cop1 single mutant under blue light. These results indicate that CRY functions additively with PHOT in mediating blue light-induced stomatal opening and that COP1 is a repressor of stomatal opening and likely acts downstream of CRY and PHOT signaling pathways.

AKT2/3 subunits render guard cell K+ channels Ca2+ sensitive

Inward-rectifying K+ channels serve as a major pathway for Ca2+-sensitive K+ influx into guard cells. Arabidopsis thaliana guard cell inward-rectifying K+ channels are assembled from multiple K+ channel subunits. Following the recent isolation and characterization of an akt2/3-1 knockout mutant, we examined whether the AKT2/3 subunit carries the Ca2+ sensitivity of the guard cell inward rectifier. Quantification of RT-PCR products showed that despite the absence of AKT2 transcripts in guard cells of the knockout plant, expression levels of the other K+ channel subunits (KAT1, KAT2, AKT1, and AtKC1) remained largely unaffected. Patch-clamp experiments with guard cell protoplasts from wild type and akt2/3-1 mutant, however, revealed pronounced differences in Ca2+ sensitivity of the K+ inward rectifier. Wild-type channels were blocked by extracellular Ca2+ in a concentration- and voltage-dependent manner. Akt2/3-1 mutants lacked the voltage-dependent Ca2+ block, characteristic for the K+ inward rectifier. To confirm the akt2/3-1 phenotype, two independent knockout mutants, akt2-1 and akt2::En-1 were tested, demonstrating that the loss of AKT2/3 indeed affects the Ca2+ dependence of guard cell inward rectifier. In contrast to AKT2 knockout plants, AKT1, AtKC1, and KAT1 loss-of-function mutants retained Ca2+ block of the guard cell inward rectifier. When expressed in HEK293 cells, AKT2 channel displayed a pronounced susceptibility toward extracellular Ca2+, while the dominant guard cell K+ channel KAT2 was Ca2+ insensitive. Thus, we conclude that the AKT2/3 subunit constitutes the Ca2+ sensitivity of the guard cell K+ uptake channel.

A mechanosensitive anion channel in Arabidopsis thaliana mesophyll cells

Mechanosensitive ion channels are expected to play important roles in transducing mechanical stimuli into intracellular signals during the development and morphogenesis of higher plants. We have identified a novel mechanosensitive anion channel in the protoplast of Arabidopsis thaliana mesophyll cells by using the patch-clamp technique. The channel in the outside-out patches could be activated by positive pressure in the pipette while negative pressure had no effect. The amphipathic membrane crenator trinitrophenol, which is supposed to preferentially insert in the outer leaflet of the lipid bilayer of the plasma membrane, synergized with mechanical membrane stretch to activate the channel. These results suggest that the channel activation is mediated by a convex curvature of the plasma membrane. Therefore, activation of this channel may play an important role when cell volume is increasing during cell growth or hypo-osmotic challenge, which is accompanied by membrane stretch with increasingly convex curvature.

Water ascent in tall trees: does evolution of land plants rely on a highly metastable state?

The Cohesion Theory considers plant xylem as a 'vulnerable pipeline' isolated from the osmotically connected tissue cells, phloem and mycorrhizas living in symbiosis with plant roots. It is believed that water is pulled exclusively by transpiration-induced negative pressure gradients of several megapascals through continuous water columns from the roots to the foliage. Water under such negative pressures is extremely unstable, particularly given the hydrophobicity of the inner xylem walls and sap composition (lipids, proteins, mucopolysaccharides, etc.) that prevents the development of stable negative pressures larger than about -1 MPa. However, many plant physiologists still view the Cohesion Theory as the absolute and universal truth because clever wording from the proponents of this theory has concealed the recent breakdown of the Scholander pressure bomb (and other indirect methods) as qualified tools for measuring negative pressures in transpiring plants. Here we show that the arguments of the proponents of the Cohesion Theory are completely misleading. We further present an enormous bulk of evidence supporting the view that - depending on the species and ecophysiological context - many other forces, additional to low tensions, can be involved in water ascent and that water can be lifted by a series of watergates (like ships in staircase locks). Contents I. Introduction 576 II. Can water sustain negative pressures? 577 III. Negative xylem pressures of several megapascals: fact or mystery? 579 IV. The continuity of the xylem water columns: fact or hypothesis? 588 V. The 'Multi-Force' or 'Watergate' Theory 590 VI. Conclusions 604 Acknowledgements 605 References 605 Appendix 1 612 Appendix 2 613.

Diurnal and light-regulated expression of AtSTP1 in guard cells of Arabidopsis

Guard cell chloroplasts are unable to perform significant photosynthetic CO2 fixation via Rubisco. Therefore, guard cells depend on carbon supply from adjacent cells even during the light period. Due to their reversible turgor changes, this import cannot be mediated by plasmodesmata. Nevertheless, guard cells of several plants were shown to use extracellular sugars or to accumulate sucrose as an osmoticum that drives water influx to increase stomatal aperture. This paper describes the first localization of a guard cell-specific Arabidopsis sugar transporter involved in carbon acquisition of these symplastically isolated cells. Expression of the AtSTP1 H+-monosacharide symporter gene in guard cells was demonstrated by in situ hybridization and by immunolocalization with an AtSTP1-specific antiserum. Additional RNase protection analyses revealed a strong increase of AtSTP1 expression in the dark and a transient, diurnally regulated increase during the photoperiod around midday. This transient increase in AtSTP1 expression correlates in time with the described guard cell-specific accumulation of sucrose. Our data suggest a function of AtSTP1 in monosaccharide import into guard cells during the night and a possible role in osmoregulation during the day.