New Insights into the Regulation of Stomatal Opening by Blue Light and Plasma Membrane H+-ATPase

Integrative regulatory mechanisms of stomatal movements under changing climate

Global climate change-caused drought stress, high temperatures and other extreme weather profoundly impact plant growth and development, restricting sustainable crop production. To cope with various environmental stimuli, plants can optimize the opening and closing of stomata to balance CO2 uptake for photosynthesis and water loss from leaves. Guard cells perceive and integrate various signals to adjust stomatal pores through turgor pressure regulation. Molecular mechanisms and signaling networks underlying the stomatal movements in response to environmental stresses have been extensively studied and elucidated. This review focuses on the molecular mechanisms of stomatal movements mediated by abscisic acid, light, CO2 , reactive oxygen species, pathogens, temperature, and other phytohormones. We discussed the significance of elucidating the integrative mechanisms that regulate stomatal movements in helping design smart crops with enhanced water use efficiency and resilience in a climate-changing world.

Spectral composition of LED light differentially affects biomass, photosynthesis, nutrient profile, and foliar nitrate accumulation of lettuce grown under various replacement methods of nutrient solution

To enhance crop yield and quality, plant cultivation in controlled-growing systems is an alternative to traditional open-field farming. The use of light-emitting diode (LED) as an adjustable light source represents a promising approach to improve plant growth, metabolism, and function. The objective of this study was to assess the impact of different light spectra (red, red/blue (3:1), blue, and white) with an emission peak of around 656, 656, 450, and 449 nm, respectively, under various replacement methods of nutrient solution (complete replacement (CR), EC-based replacement (ECBR), and replacing based on plant needs (RBPN)), on biomass, physiological traits, and macro- and micronutrient contents of two best-known lettuce varieties, Lollo Rossa (LR) and Lollo Bionda (LB), in the nutrient film technique (NFT) hydroponic system. The results indicated that mix of red and blue LED spectra under RBPN method is the most effective treatment to enhance fresh and dry weights of lettuce plants. In addition, red LED spectrum under RBPN, and red and blue light under ECBR nutrient solution significantly increased leaf stomatal conductance, net photosynthesis and transpiration rate, and intercellular CO2 concentration of LR variety. Phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mn) content in LR variety, and iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn) content in both varieties increased upon exposure to blue and red LED light spectrum with RBPN method. Our results suggest that exposure to combination of red and blue light along with feeding plants using RBPN and ECBR methods can increase absorption of macro- and micronutrient elements and improve photosynthetic properties, and eventually increase lettuce yield with lower nitrate accumulation.

Growth, photosynthetic function, and stomatal characteristics of Persian walnut explants in vitro under different light spectra

Light plays a crucial role in photosynthesis, which is an essential process for plantlets produced during in vitro tissue culture practices and ex vitro acclimatization. LED lights are an appropriate technology for in vitro lighting but their effect on propagation and photosynthesis under in vitro condition is not well understood. This study aimed to investigate the impact of different light spectra on growth, photosynthetic functionality, and stomatal characteristics of micropropagated shoots of Persian walnut (cv. Chandler). Tissue-cultured walnut nodal shoots were grown under different light qualities including white, blue, red, far-red, green, combination of red and blue (70:30), combination of red and far-red (70:30), and fluorescent light as the control. Results showed that the best growth and vegetative characteristics of in vitro explants of Persian walnut were achieved under combination of red and blue light. The biggest size of stomata was detected under white and blue lights. Red light stimulated stomatal closure, while stomatal opening was induced under blue and white lights. Although the red and far-red light spectra resulted in the formation of elongated explants with more lateral shoots and anthocyanin content, they significantly reduced the photosynthetic functionality. Highest soluble carbohydrate content and maximum quantum yield of photosystem II were detected in explants grown under blue and white light spectra. In conclusion, growing walnut explants under combination of red and blue lights leads to better growth, photosynthesis functionality, and the emergence of functional stomata in in vitro explants of Persian walnuts.

The molecular mechanism of plasma membrane H+-ATPases in plant responses to abiotic stress

Plasma membrane H⁺-ATPases (PM H⁺-ATPases) are critical proton pumps that export protons from the cytoplasm to the apoplast. The resulting proton gradient and difference in electrical potential energize various secondary active transport events. PM H⁺-ATPases play essential roles in plant growth, development, and stress responses. In this review, we focus on recent studies of the mechanism of PM H⁺-ATPases in response to abiotic stresses in plants, such as salt and high pH, temperature, drought, light, macronutrient deficiency, acidic soil and aluminum stress, as well as heavy metal toxicity. Moreover, we discuss remaining outstanding questions about how PM H⁺-ATPases contribute to abiotic stress responses.

First Multi-Organ Full-Length Transcriptome of Tree Fern Alsophila spinulosa Highlights the Stress-Resistant and Light-Adapted Genes

Alsophila spinulosa, a relict tree fern, is a valuable plant for investigating environmental adaptations. Its genetic resources, however, are scarce. We used the PacBio and Illumina platforms to sequence the polyadenylated RNA of A. spinulosa root, rachis, and pinna, yielding 125,758, 89,107, and 89,332 unigenes, respectively. Combining the unigenes from three organs yielded a non-redundant reference transcriptome with 278,357 unigenes and N50 of 4141 bp, which were further reconstructed into 38,470 UniTransModels. According to functional annotation, pentatricopeptide repeat genes and retrotransposon-encoded polyprotein genes are the most abundant unigenes. Clean reads mapping to the full-length transcriptome is used to assess the expression of unigenes. The stress-induced ASR genes are highly expressed in all three organs, which is validated by qRT-PCR. The organ-specific upregulated genes are enriched for pathways involved in stress response, secondary metabolites, and photosynthesis. Genes for five types of photoreceptors, CRY signaling pathway, ABA biosynthesis and transduction pathway, and stomatal movement-related ion channel/transporter are profiled using the high-quality unigenes. The gene expression pattern coincides with the previously identified stomatal characteristics of fern. This study is the first multi-organ full-length transcriptome report of a tree fern species, the abundant genetic resources and comprehensive analysis of A. spinulosa, which provides the groundwork for future tree fern research.

High external Na+, but not K+, stimulates the growth of Ulva lactuca (L.) via induction of the plasma membrane ATPases and achievement of K+/Na+ homeostasis

This study aims at investigating the specific ion effects of Na⁺ and K⁺ on Ulva lactuca (L.) growth. U. lactuca was grown in balanced nutrient solutions with 10, 100, 300 and 600 mM NaCl or KCl. The growth was significantly higher at 300 and 600 mM NaCl compared to KCl, with the highest growth rate at 300 mM NaCl. NaCl-treated alga showed increases in the photosynthetic pigments and Rubisco protein content. However, KCl treatments adversely affected these photosynthetic attributes. U. lactuca needs adjusted, but not high K⁺/Na⁺ ratio for a proper growth, since the high K⁺/Na⁺ ratio in KCl-treated alga was associated with growth retardation. The cell wall was more extensible at high concentrations of NaCl compared to KCl. Therefore, the deleterious effect of K⁺ could be mainly on the cell wall and hence inhibiting the growth and perhaps the vitality of the whole cell. The transcript of plasma membrane (PM) H⁺-ATPase was detected only at 300 and 600 mM NaCl, implying that this gene was specifically induced by high concentrations of Na⁺ but not K⁺. The transcript of PM-Na⁺/K⁺-ATPase-like exhibited no Na⁺ specificity and its induction alone could not improve the growth of KCl-treated U. lactuca. The simultaneous induction of the two PM-ATPases could positively affect the algal growth at high NaCl concentrations by maintaining the proper cellular K⁺/Na⁺ ratio. Also, both PM-ATPases might contribute to energizing the plasma membrane and thereby promoting the cellular growth of U. lactuca at high Na⁺, but not K⁺, concentrations.

Appropriate NH4 +/NO3 - Ratio Triggers Plant Growth and Nutrient Uptake of Flowering Chinese Cabbage by Optimizing the pH Value of Nutrient Solution

Compared with sole nitrogen (N), the nutrition mixture of ammonium (NH₄ ⁺) and nitrate (NO₃ ^-) is known to better improve crop yield and quality. However, the mechanism underlying this improvement remains unclear. In the present study, we analyzed the changes in nutrient solution composition, content of different N forms in plant tissues and exudates, and expression of plasma membrane (PM) H⁺-ATPase genes (HAs) under different NH₄ ⁺/NO₃ ^- ratios (0/100, 10/90, 25/75, 50/50 as control, T1, T2, and T3) in flowering Chinese cabbage. We observed that compared with the control, T1 and T2 increased the economical yield of flowering Chinese cabbage by 1.26- and 1.54-fold, respectively, whereas T3 significantly reduced plant yield. Compared with the control, T1-T3 significantly reduced the NO₃ ^- content and increased the NH₄ ⁺, amino acid, and soluble protein contents of flowering Chinese cabbage to varying extents. T2 significantly increased the N use efficiency (NUE), whereas T3 significantly decreased it to only being 70.25% of that of the control. Owing to the difference in N absorption and utilization among seedlings, the pH value of the nutrient solution differed under different NH₄ ⁺/NO₃ ^- ratios. At harvest, the pH value of T2 was 5.8; in the control and T1, it was approximately 8.0, and in T3 it was only 3.6. We speculated that appropriate NH₄ ⁺/NO₃ ^- ratios may improve N absorption and assimilation and thus promote the growth of flowering Chinese cabbage, owing to the suitable pH value. On the contrary, addition of excessive NH₄ ⁺ may induce rhizosphere acidification and ammonia toxicity, causing plant growth inhibition. We further analyzed the transcription of PM H⁺-ATPase genes (HAs). HA1 and HA7 transcription in roots was significantly down-regulated by the addition of the mixture of NH₄ ⁺ and NO₃ ^-, whereas the transcription of HA2, HA9 in roots and HA7, HA8, and HA10 in leaves was sharply up-regulated by the addition of the mixture; the transcription of HA3 was mainly enhanced by the highest ratio of NH₄ ⁺/NO₃ ^-. Our results provide valuable information about the effects of treatments with different NH₄ ⁺/NO₃ ^- ratios on plant growth and N uptake and utilization.

Light-Mediated Signaling and Metabolic Changes Coordinate Stomatal Opening and Closure

Stomata are valves on the leaf surface controlling carbon dioxide (CO2) influx for photosynthesis and water loss by transpiration. Thus, plants have to evolve elaborate mechanisms controlling stomatal aperture to allow efficient photosynthesis while avoid excessive water loss. Light is not only the energy source for photosynthesis but also an important signal regulating stomatal movement during dark-to-light transition. Our knowledge concerning blue and red light signaling and light-induced metabolite changes that contribute to stomatal opening are accumulating. This review summarizes recent advances on the signaling components that lie between the perception of blue/red light and activation of the PM H+-ATPases, and on the negative regulation of stomatal opening by red light-activated phyB signaling and ultraviolet (UV-B and UV-A) irradiation. Besides, light-regulated guard cell (GC)-specific metabolic levels, mesophyll-derived sucrose, and CO2 concentration within GCs also play dual roles in stomatal opening. Thus, light-induced stomatal opening is tightly accompanied by brake mechanisms, allowing plants to coordinate carbon gain and water loss. Knowledge on the mechanisms regulating the trade-off between stomatal opening and closure may have potential applications toward generating superior crops with improved water use efficiency (CO2 gain vs. water loss).

Elevation of secondary metabolites production through light-emitting diodes (LEDs) illumination in protocorm-like bodies (PLBs) of Dendrobium hybrid orchid rich in phytochemicals with therapeutic effects

Gas-chromatography-mass-spectrometry revealed the presence of various bioactive compounds with anticancer properties in protocorm-like-body (PLB) cultures of a Dendrobium hybrid orchid (Dendrobium Enopi x Dendrobium Pink Lady). Pre-illumination of red fluorescent light lessened the stimulating effects of light-emitting diodes (LEDs) on secondary metabolites production among in vitro PLB cultures, possibly due to habituation. The highest flavonoid content of 16.79 μmol/ g of fresh weight (FW) was achieved under blue-red (1:1) LED for PLBs pre-treated with white LED for more than 3 subculture cycles. Phenolics content significantly reduced as PLBs pre-cultured under red fluorescent light for 2 subculture cycles were exposed to LED illuminations, where far red LED resulted in the lowest total phenolic content (18.85 μmol/ g FW). High intensity green LED (16.9 μmol/s) enhanced the accumulation of phenolics while amino acids such as L-leucine, glycine and proline exhibited no significant stimulating effect for secondary metabolites production.

Postharvest Spectral Light Composition Affects Chilling Injury in Anthurium Cut Flowers

The effect of the lighting environment during postharvest storage of ornamentals has largely been neglected in previous research. Anthurium is a cold-sensitive species originating from tropical climates and is widely cultivated all around the world for its colorful spathes. To investigate the effects of light spectrum on the performance of Anthurium cut flowers under cold storage, two cultivars [Calore (red spathe) and Angel (withe spathe)] were placed at low temperature (4°C), either in darkness (D) or under different light spectra [red (R), blue (B), 70:30% red:blue (RB), and white (W)] at an intensity of 125 µmol.m-2.s-1. In both cultivars, the longest and shortest vase lives were observed in spathes exposed to the R and B spectra, respectively. In both cultivars, electrolyte leakage (EL) of spathe was highest under the B and W spectra and lowest under the R spectrum. The highest rate of flower water loss from the spathes was observed under the B-containing light spectra, whereas the lowest rate of water loss was observed in D and under the R spectrum. Negative correlations were observed between EL and vase life and between anthocyanin concentration and EL for both Anthurium cultivars. A positive correlation was found between anthocyanin concentration and vase life. For both Anthurium cultivars, spectral light composition with higher percentage of B resulted in higher EL and as a result shorter vase life in cut flowers under cold storage condition. The negative effect of the B light spectrum on vase life of Anthurium can be explained through its effect on water loss and on oxidative stress and membrane integrity. The quality of Anthurium cut flowers should benefit from environments with restricted B light spectrum during postharvest handling.

Evolution of Guard-Cell Theories: The Story of Sugars

Stomata are dynamic pores in the impermeable cuticle that coats the aerial parts of vascular plants, allowing the entry of CO₂ for photosynthesis and controlling water loss. They are composed of two guard cells that can swell or shrink due to an increase or decrease in their osmotic pressure, respectively. Swelling opens the stomata and shrinking closes the stomata. For more than a century, scientists have been working to uncover the nature of the osmolytes that modulate osmotic pressure in guard cells. Recent discoveries have undermined long-standing theories in this area, reversing the understood roles of sugars and demonstrating the evolution of scientific theories. Here, we describe the evolution of guard-cell osmoregulation theories with an emphasis on the role of sugars.

Cytosolic malate and oxaloacetate activate S-type anion channels in Arabidopsis guard cells

Intracellular malate-starch interconversion plays an important role in stomatal movements. We investigated whether malate or oxaloacetate from the cytosolic membrane side regulate anion channels in the plasma membrane of Arabidopsis thaliana guard cells. Physiological concentrations of cytosolic malate have been reported in the range of 0.4-3 mM in leaf cells. Guard cell patch clamp and two-electrode oocyte voltage-clamp experiments were pursued. We show that a concentration of 1 mM cytosolic malate greatly activates S-type anion channels in Arabidopsis thaliana guard cells. Interestingly, 1 mM cytosolic oxaloacetate also activates S-type anion channels. Malate activation was abrogated at 10 mM malate and in SLAC1 anion channel mutant alleles. Interestingly, malate activation of S-type anion currents was disrupted at below resting cytosolic-free calcium concentrations ([Ca2+ ]cyt ), suggesting a key role for basal [Ca2+ ]cyt signaling. Cytosolic malate was not able to directly activate or enhance SLAC1-mediated anion currents in Xenopus oocytes, whereas in positive controls, cytosolic NaHCO3 enhanced SLAC1 activity, suggesting that malate may not directly modulate SLAC1. Cytosolic malate activation of S-type anion currents was impaired in ost1 and in cpk5/6/11/23 quadruple mutant guard cells. Together these findings show that these cytosolic organic anions function in guard cell 'plasma membrane' ion channel regulation.

Metabolism within the specialized guard cells of plants

Contents 1018 I. 1018 II. 1019 III. 1022 IV. 1025 V. 1026 VI. 1029 1030 References 1030 SUMMARY: Stomata are leaf epidermal structures consisting of two guard cells surrounding a pore. Changes in the aperture of this pore regulate plant water-use efficiency, defined as gain of C by photosynthesis per leaf water transpired. Stomatal aperture is actively regulated by reversible changes in guard cell osmolyte content. Despite the fact that guard cells can photosynthesize on their own, the accumulation of mesophyll-derived metabolites can seemingly act as signals which contribute to the regulation of stomatal movement. It has been shown that malate can act as a signalling molecule and a counter-ion of potassium, a well-established osmolyte that accumulates in the vacuole of guard cells during stomatal opening. By contrast, their efflux from guard cells is an important mechanism during stomatal closure. It has been hypothesized that the breakdown of starch, sucrose and lipids is an important mechanism during stomatal opening, which may be related to ATP production through glycolysis and mitochondrial metabolism, and/or accumulation of osmolytes such as sugars and malate. However, experimental evidence supporting this theory is lacking. Here we highlight the particularities of guard cell metabolism and discuss this in the context of the guard cells themselves and their interaction with the mesophyll cells.

Contribution of the S-type Anion Channel SLAC1 to Stomatal Control and Its Dependence on Developmental Stage in Rice

Rice production depends on water availability and carbon fixation by photosynthesis. Therefore, optimal control of stomata, which regulate leaf transpiration and CO2 absorption, is important for high productivity. SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1) is an S-type anion channel protein that controls stomatal closure in response to elevated CO2. Rice slac1 mutants showed significantly increased stomatal conductance (gs) and enhanced CO2 assimilation. To discern the contribution of stomatal regulation to rice growth, we compared gs in the wild type (WT) and two mutants, slac1 and the dominant-positive mutant SLAC1-F461A, which expresses a point mutation causing an amino acid substitution (F461A) in SLAC1, at different growth stages. Because the side group of F461 is estimated to function as the channel gate, stomata in the SLAC1-F461A mutant are expected to close constitutively. All three lines had maximum gs during the tillering stage, when the gs values were 50% higher in slac1 and 70% lower in SLAC1-F461A, compared with the WT. At the tillering stage, the gs values were highest in the first leaves at the top of the stem and lower in the second and third leaves in all three lines. Both slac1 and SLAC1-F461A retained the ability to change gs in response to the day-night cycle, and showed differences in tillering rate and plant height compared with the WT, and lower grain yield. These observations show that SLAC1 plays a crucial role in regulating stomata in rice at the tillering stage.

Biology of SLAC1-type anion channels - from nutrient uptake to stomatal closure

Contents 46 I. 46 II. 47 III. 50 IV. 53 V. 56 VI. 57 58 58 References 58 SUMMARY: Stomatal guard cells control leaf CO₂ intake and concomitant water loss to the atmosphere. When photosynthetic CO₂ assimilation is limited and the ratio of CO₂ intake to transpiration becomes suboptimal, guard cells, sensing the rise in CO₂ concentration in the substomatal cavity, deflate and the stomata close. Screens for mutants that do not close in response to experimentally imposed high CO₂ atmospheres identified the guard cell-expressed Slowly activating anion channel, SLAC1, as the key player in the regulation of stomatal closure. SLAC1 evolved, though, before the emergence of guard cells. In Arabidopsis, SLAC1 is the founder member of a family of anion channels, which comprises four homologues. SLAC1 and SLAH3 mediate chloride and nitrate transport in guard cells, while SLAH1, SLAH2 and SLAH3 are engaged in root nitrate and chloride acquisition, and anion translocation to the shoot. The signal transduction pathways involved in CO₂ , water stress and nutrient-sensing activate SLAC/SLAH via distinct protein kinase/phosphatase pairs. In this review, we discuss the role that SLAC/SLAH channels play in guard cell closure, on the one hand, and in the root-shoot continuum on the other, along with the molecular basis of the channels' anion selectivity and gating.

Guard cell sensory systems: recent insights on stomatal responses to light, abscisic acid, and CO2

By controlling the opening and closure of the stomatal pores through which gas exchange occurs, guard cells regulate two of the most important plant physiological processes: photosynthesis and transpiration. Accordingly, guard cells have evolved exquisite sensory systems. Here we summarize recent literature on guard cell sensing of light, drought (via the phytohormone abscisic acid (ABA)), and CO₂. New advances in our understanding of how guard cells satisfy the energetic and osmotic requirements of stomatal opening and utilize phosphorylation to regulate the anion channels and aquaporins involved in ABA-stimulated stomatal closure are highlighted. Omics and modeling approaches are providing new information that will ultimately allow an integrated understanding of guard cell physiology.

Physiological Roles of Plant Post-Golgi Transport Pathways in Membrane Trafficking

Membrane trafficking is the fundamental system through which proteins are sorted to their correct destinations in eukaryotic cells. Key regulators of this system include RAB GTPases and soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs). Interestingly, the numbers of RAB GTPases and SNAREs involved in post-Golgi transport pathways in plant cells are larger than those in animal and yeast cells, suggesting that plants have evolved unique and complex post-Golgi transport pathways. The trans-Golgi network (TGN) is an important organelle that acts as a sorting station in the post-Golgi transport pathways of plant cells. The TGN also functions as the early endosome, which is the first compartment to receive endocytosed proteins. Several endocytosed proteins on the plasma membrane (PM) are initially targeted to the TGN/EE, then recycled back to the PM or transported to the vacuole for degradation. The recycling and degradation of the PM localized proteins is essential for the development and environmental responses in plant. The present review describes the post-Golgi transport pathways that show unique physiological functions in plants.

Plasma Membrane H(+)-ATPase Regulation in the Center of Plant Physiology

The plasma membrane (PM) H(+)-ATPase is an important ion pump in the plant cell membrane. By extruding protons from the cell and generating a membrane potential, this pump energizes the PM, which is a prerequisite for growth. Modification of the autoinhibitory terminal domains activates PM H(+)-ATPase activity, and on this basis it has been hypothesized that these regulatory termini are targets for physiological factors that activate or inhibit proton pumping. In this review, we focus on the posttranslational regulation of the PM H(+)-ATPase and place regulation of the pump in an evolutionary and physiological context. The emerging picture is that multiple signals regulating plant growth interfere with the posttranslational regulation of the PM H(+)-ATPase.

Arabidopsis phot1 and phot2 phosphorylate BLUS1 kinase with different efficiencies in stomatal opening

In Arabidopsis thaliana, phototropins (phot1 and phot2), light-activated receptor kinases, redundantly regulate various photoresponses such as phototropism, chloroplast photorelocation movement, stomatal opening, and leaf flattening. However, it is still unclear how phot1 and phot2 signals are integrated into a common target and regulate physiological responses. In the present study, we provide evidence that phot1 and phot2 phosphorylate BLUE LIGHT SIGNALING1 (BLUS1) kinase as a common substrate in stomatal opening. Biochemical analysis revealed that the recombinant phot2 protein directly phosphorylated BLUS1 in vitro in a blue light-dependent manner, as reported for phot1. BLUS1 phosphorylation was observed in both phot1 and phot2 mutants, and phot2 mutant exhibited higher phosphorylation of BLUS1 than did phot1 mutant. Transgenic plants expressing phot1-GFP (P1G) and phot2-GFP (P2G) at a similar level under the PHOT2 promoter demonstrated that P1G initiated higher phosphorylation of BLUS1 than P2G, suggesting that phot1 phosphorylates BLUS1 more efficiently. Similarly, P1G mediated a higher activation of the plasma membrane H(+)-ATPase and stomatal opening than P2G, indicating that the phosphorylation status of BLUS1 is a key determinant of physiological response. Together, these findings provide insights into the signal integration and different properties of phot1 and phot2 signaling.

Reconstitution of an Initial Step of Phototropin Signaling in Stomatal Guard Cells

Phototropins are light-activated receptor kinases that mediate a wide range of blue light responses responsible for the optimization of photosynthesis. Despite the physiological importance of phototropins, it is still unclear how they transduce light signals into physiological responses. Here, we succeeded in reproducing a primary step of phototropin signaling in vitro using a physiological substrate of phototropin, the BLUS1 (BLUE LIGHT SIGNALING1) kinase of guard cells. When PHOT1 and BLUS1 were expressed in Escherichia coli and the resulting recombinant proteins were incubated with ATP, white and blue light induced phosphorylation of BLUS1 but red light and darkness did not. Site-directed mutagenesis of PHOT1 and BLUS1 revealed that the phosphorylation was catalyzed by phot1 kinase. Similar to stomatal blue light responses, the BLUS1 phosphorylation depended on the fluence rate of blue light and was inhibited by protein kinase inhibitors, K-252a and staurosporine. In contrast to the result in vivo, BLUS1 was not dephosphorylated in vitro, suggesting the involvement of a protein phosphatase in the response in vivo. phot1 with a C-terminal kinase domain but devoid of the N-terminal domain, constitutively phosphorylated BLUS1 without blue light, indicating that the N-terminal domain has an autoinhibitory action and prevents substrate phosphorylation. The results provide the first reconstitution of a primary step of phototropin signaling and a clue for understanding the molecular nature of this process.

A flowering integrator, SOC1, affects stomatal opening in Arabidopsis thaliana

Stomatal movements are regulated by multiple environmental signals. Recent investigations indicate that photoperiodic flowering components, such as CRY, GI, CO, FT and TSF, are expressed in guard cells and positively affect stomatal opening in Arabidopsis thaliana. Here we show that SOC1, which encodes a MADS box transcription factor and integrates multiple flowering signals, also exerts a positive effect on stomatal opening. FLC encodes a potent repressor of FT and SOC1, and FRI acts as an activator of FLC. Thus, we examined stomatal phenotypes in FRI-Col, which contains an active FRI allele of accession Sf-2 by introgression. We found higher expression of FLC and lower expression of FT, SOC1 and TSF in guard cells from FRI-Col than in those from Col. Light-induced stomatal opening was significantly suppressed in FRI-Col. Interestingly, vernalization of FRI-Col partially restored light-induced stomatal opening, concomitant with a decrease of FLC and increase of FT, SOC1 and TSF. Furthermore, we observed the constitutive open-stomata phenotype in transgenic plants overexpressing SOC1-GFP (green fluorescent protein) in guard cells (SOC1-GFP overexpressor), and found that light-induced stomatal opening was significantly suppressed in a soc1 knockout mutant. RNA sequencing using epidermis from the SOC1-GFP overexpressor revealed that the expression levels of several genes involved in stomatal opening, such as BLUS1 and the plasma membrane H(+)-ATPases, were higher than those in background plants. From these results, we conclude that SOC1 is involved in the regulation of stomatal opening via transcriptional regulation in guard cells.

Iron nanoparticle-induced activation of plasma membrane H(+)-ATPase promotes stomatal opening in Arabidopsis thaliana

Engineered nanomaterials (ENMs) enable the control and exploration of intermolecular interactions inside microscopic systems, but the potential environmental impacts of their inevitable release remain largely unknown. Plants exposed to ENMs display effects, such as increase in biomass and chlorophyll, distinct from those induced by exposure to their bulk counterparts, but few studies have addressed the mechanisms underlying such physiological results. The current investigation found that exposure of Arabidopsis thaliana to nano zerovalent iron (nZVI) triggered high plasma membrane H(+)-ATPase activity. The increase in activity caused a decrease in apoplastic pH, an increase in leaf area, and also wider stomatal aperture. Analysis of gene expression indicated that the levels of the H(+)-ATPase isoform responsible for stomatal opening, AHA2, were 5-fold higher in plants exposed to nZVI than in unexposed control plants. This is the first study to show that nZVI enhances stomatal opening by inducing the activation of plasma membrane H(+)-ATPase, leading to the possibility of increased CO2 uptake.

Abscisic acid suppresses hypocotyl elongation by dephosphorylating plasma membrane H(+)-ATPase in Arabidopsis thaliana

Plasma membrane H(+)-ATPase is thought to mediate hypocotyl elongation, which is induced by the phytohormone auxin through the phosphorylation of the penultimate threonine of H(+)-ATPase. However, regulation of the H(+)-ATPase during hypocotyl elongation by other signals has not been elucidated. Hypocotyl elongation in etiolated seedlings of Arabidopsis thaliana was suppressed by the H(+)-ATPase inhibitors vanadate and erythrosine B, and was significantly reduced in aha2-5, which is a knockout mutant of the major H(+)-ATPase isoform in etiolated seedlings. Application of the phytohormone ABA to etiolated seedlings suppressed hypocotyl elongation within 30 min at the half-inhibitory concentration (4.2 µM), and induced dephosphorylation of the penultimate threonine of H(+)-ATPase without affecting the amount of H(+)-ATPase. Interestingly, an ABA-insensitive mutant, abi1-1, did not show ABA inhibition of hypocotyl elongation or ABA-induced dephosphorylation of H(+)-ATPase. This indicates that ABI1, which is an early ABA signaling component through the ABA receptor PYR/PYL/RCARs (pyrabactin resistance/pyrabactin resistance 1-like/regulatory component of ABA receptor), is involved in these responses. In addition, we found that the fungal toxin fusiccocin (FC), an H(+)-ATPase activator, induced hypocotyl elongation and phosphorylation of the penultimate threonine of H(+)-ATPase, and that FC-induced hypocotyl elongation and phosphorylation of H(+)-ATPase were significantly suppressed by ABA. Taken together, these results indicate that ABA has an antagonistic effect on hypocotyl elongation through, at least in part, dephosphorylation of H(+)-ATPase in etiolated seedlings.

Dynamics and environmental responses of PATROL1 in Arabidopsis subsidiary cells

The Arabidopsis stomatal complex is composed of a pair of guard cells and surrounding anisocytic subsidiary cells. Subsidiary cells are thought to function as a supplier and receiver of bulk water and ions, and to assist turgor-driven stomatal movement, but the molecular mechanisms are largely unknown. In this work, we studied the dynamic behavior and environmental responses of PATROL1, which has been identified as a translocation factor of the plasma membrane proton pump ATPase (PM H(+)-ATPase) AHA1 in guard cells and subsidiary cells in Arabidopsis thaliana. Variable-angle epifluorescence microscopic observation revealed that green fluorescent protein (GFP)-PATROL1 localized on dot-like compartments that resided on plasma membranes for several seconds. The GFP-PATROL1-labeled dots were sensitive to phosphatidylinositol 4-kinase inhibitors but not to a phosphatidylinositol 3-kinase inhibitor. GFP-PATROL1 and red fluorescent protein (RFP)-AHA1 co-localized in hyperosmotic conditions, and a mutation of PATROL1 resulted in an increase in GFP-AHA1 internalization, suggesting a role in the translocation of PM H(+)-ATPase in subsidiary cells. Interestingly, subsidiary cells showed changes in localization of GFP-PATROL1 in response to environmental stimuli that were opposite to those in guard cells. Our observations suggested that PATROL1 may contribute to stomatal movement by translocations of PM H(+)-ATPase in subsidiary cells.

New approaches to the biology of stomatal guard cells

CO2 acts as an environmental signal that regulates stomatal movements. High CO2 concentrations reduce stomatal aperture, whereas low concentrations trigger stomatal opening. In contrast to our advanced understanding of light and drought stress responses in guard cells, the molecular mechanisms underlying stomatal CO2 sensing and signaling are largely unknown. Leaf temperature provides a convenient indicator of transpiration, and can be used to detect mutants with altered stomatal control. To identify genes that function in CO2 responses in guard cells, CO2-insensitive mutants were isolated through high-throughput leaf thermal imaging. The isolated mutants are categorized into three groups according to their phenotypes: (i) impaired in stomatal opening under low CO2 concentrations; (ii) impaired in stomatal closing under high CO2 concentrations; and (iii) impaired in stomatal development. Characterization of these mutants has begun to yield insights into the mechanisms of stomatal CO2 responses. In this review, we summarize the current status of the field and discuss future prospects.

A Munc13-like protein in Arabidopsis mediates H+-ATPase translocation that is essential for stomatal responses

Plants control CO2 uptake and water loss by modulating the aperture of stomata located in the epidermis. Stomatal opening is initiated by the activation of H(+)-ATPases in the guard-cell plasma membrane. In contrast to regulation of H(+)-ATPase activity, little is known about the translocation of the guard cell H(+)-ATPase to the plasma membrane. Here we describe the isolation of an Arabidopsis gene, PATROL1, that controls the translocation of a major H(+)-ATPase, AHA1, to the plasma membrane. PATROL1 encodes a protein with a MUN domain, known to mediate synaptic priming in neuronal exocytosis in animals. Environmental stimuli change the localization of plasma membrane-associated PATROL1 to an intracellular compartment. Plasma membrane localization of AHA1 and stomatal opening require the association of PATROL1 with AHA1. Increased stomatal opening responses in plants overexpressing PATROL1 enhance the CO2 assimilation rate, promoting plant growth.

Expression of a translationally fused TAP-tagged plasma membrane proton pump in Arabidopsis thaliana

The Arabidopsis thaliana plasma membrane proton ATPase genes, AHA1 and AHA2, are the two most highly expressed isoforms of an 11 gene family and are collectively essential for embryo development. We report the translational fusion of a tandem affinity-purification tag to the 5′ end of the AHA1 open reading frame in a genomic clone. Stable expression of TAP-tagged AHA1 in Arabidopsis rescues the embryonic lethal phenotype of endogenous double aha1/aha2 knockdowns. Western blots of SDS-PAGE and Blue Native gels show enrichment of AHA1 in plasma membrane fractions and indicate a hexameric quaternary structure. TAP-tagged AHA1 rescue lines exhibited reduced vertical root growth. Analysis of the plasma membrane and soluble proteomes identified several plasma membrane-localized proteins with alterred abundance in TAP-tagged AHA1 rescue lines compared to wild type. Using affinity-purification mass spectrometry, we uniquely identified two additional AHA isoforms, AHA9 and AHA11, which copurified with TAP-tagged AHA1. In conclusion, we have generated transgenic Arabidopsis lines in which a TAP-tagged AHA1 transgene has complemented all essential endogenous AHA1 and AHA2 functions and have shown that these plants can be used to purify AHA1 protein and to identify in planta interacting proteins by mass spectrometry.

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.

Overexpression of plasma membrane H+-ATPase in guard cells promotes light-induced stomatal opening and enhances plant growth

Stomatal pores surrounded by a pair of guard cells in the plant epidermis control gas exchange between plants and the atmosphere in response to light, CO2, and the plant hormone abscisic acid. Light-induced stomatal opening is mediated by at least three key components: the blue light receptor phototropin (phot1 and phot2), plasma membrane H(+)-ATPase, and plasma membrane inward-rectifying K(+) channels. Very few attempts have been made to enhance stomatal opening with the goal of increasing photosynthesis and plant growth, even though stomatal resistance is thought to be the major limiting factor for CO2 uptake by plants. Here, we show that transgenic Arabidopsis plants overexpressing H(+)-ATPase using the strong guard cell promoter GC1 showed enhanced light-induced stomatal opening, photosynthesis, and plant growth. The transgenic plants produced larger and increased numbers of rosette leaves, with ∼42-63% greater fresh and dry weights than the wild type in the first 25 d of growth. The dry weights of total flowering stems of 45-d-old transgenic plants, including seeds, siliques, and flowers, were ∼36-41% greater than those of the wild type. In addition, stomata in the transgenic plants closed normally in response to darkness and abscisic acid. In contrast, the overexpression of phototropin or inward-rectifying K(+) channels in guard cells had no effect on these phenotypes. These results demonstrate that stomatal aperture is a limiting factor in photosynthesis and plant growth, and that manipulation of stomatal opening by overexpressing H(+)-ATPase in guard cells is useful for the promotion of plant growth.

TWIN SISTER OF FT, GIGANTEA, and CONSTANS have a positive but indirect effect on blue light-induced stomatal opening in Arabidopsis

FLOWERING LOCUS T (FT) is the major regulatory component controlling photoperiodic floral transition. It is expressed in guard cells and affects blue light-induced stomatal opening induced by the blue-light receptor phototropins phot1 and phot2. Roles for other flowering regulators in stomatal opening have yet to be determined. We show in Arabidopsis (Arabidopsis thaliana) that TWIN SISTER OF FT (TSF), CONSTANS (CO), and GIGANTEA (GI) provide a positive effect on stomatal opening. TSF, which is the closest homolog of FT, was transcribed in guard cells, and light-induced stomatal opening was repressed in tsf-1, a T-DNA insertion mutant of TSF. Overexpression of TSF in a phot1 phot2 mutant background gave a constitutive open-stomata phenotype. Then, we examined whether CO and GI, which are upstream regulators of FT and TSF in photoperiodic flowering, are involved in stomatal opening. Similar to TSF, light-induced stomatal opening was suppressed in the GI and CO mutants gi-1 and co-1. A constitutive open-stomata phenotype was observed in GI and CO overexpressors with accompanying changes in the transcription of both FT and TSF. In photoperiodic flowering, photoperiod is sensed by photoreceptors such as the cryptochromes cry1 and cry2. We examined stomatal phenotypes in a cry1 cry2 mutant and in CRY2 overexpressors. Light-induced stomatal opening was suppressed in cry1 cry2, and the transcription of FT and TSF was down-regulated. In contrast, the stomata in CRY2 overexpressors opened even in the dark, and FT and TSF transcription was up-regulated. We conclude that the photoperiodic flowering components TSF, GI, and CO positively affect stomatal opening.

Sweet immunity in the plant circadian regulatory network

All organisms have an internal timing mechanism, termed the circadian clock, to anticipate the light/dark cycle. The clock, with an oscillating rhythm that approximates 24h, is a rather robust system persisting to a great extent in continuous light and dark. It is widely accepted that plant growth and development are regulated by the clock, hormones, and sugar signals. On the one hand, sugar signalling can affect circadian rhythms by altering the expression pattern of clock-regulated genes. More in particular, the clock seems to be particularly sensitive to sucrose-mediated signalling which is also associated with immunity and abiotic stress responses. Also, hormonal interaction with the clock can contribute to appropriate plant immune responses. Recent data show a prominent role for the clock in growth and stress responses. On the other hand, the clock seems to be essential in controlling the gene expression and activity of an array of carbohydrate-metabolizing enzymes, suggesting a complex reciprocal relationship between the clock and metabolic signalling processes. Therefore, the clock fulfils a crucial role at the heart of cellular networks. The players involved in the complex plant circadian network and their possible contribution to the novel 'sweet immunity' concept are discussed.

Overexpression of the Mg-chelatase H subunit in guard cells confers drought tolerance via promotion of stomatal closure in Arabidopsis thaliana

The Mg-chelatase H subunit (CHLH) has been shown to mediate chlorophyll biosynthesis, as well as plastid-to-nucleus and abscisic acid (ABA)-mediated signaling. A recent study using a novel CHLH mutant, rtl1, indicated that CHLH specifically affects ABA-induced stomatal closure, but also that CHLH did not serve as an ABA receptor in Arabidopsis thaliana. However, the molecular mechanism by which CHLH engages in ABA-mediated signaling in guard cells remains largely unknown. In the present study, we examined CHLH function in guard cells and explored whether CHLH expression might influence stomatal aperture. Incubation of rtl1 guard cell protoplasts with ABA induced expression of the ABA-responsive genes RAB18 and RD29B, as also observed in wild-type (WT) cells, indicating that CHLH did not affect the expression of ABA-responsive genes. Earlier, ABA was reported to inhibit blue light (BL)-mediated stomatal opening, at least in part through dephosphorylating/inhibiting guard cell H(+)-ATPase (which drives opening). Therefore, we immunohistochemically examined the phosphorylation status of guard cell H(+)-ATPase. Notably, ABA inhibition of BL-induced phosphorylation of H(+)-ATPase was impaired in rtl1 cells, suggesting that CHLH influences not only ABA-induced stomatal closure but also inhibition of BL-mediated stomatal opening by ABA. Next, we generated CHLH-GFP-overexpressing plants using CER6 promoter, which induces gene expression in the epidermis including guard cells. CHLH-transgenic plants exhibited a closed stomata phenotype even when brightly illuminated. Moreover, plant growth experiments conducted under water-deficient conditions showed that CHLH transgenic plants were more tolerant of drought than WT plants. In summary, we show that CHLH is involved in the regulation of stomatal aperture in response to ABA, but not in ABA-induced gene expression, and that manipulation of stomatal aperture via overexpression of CHLH in guard cells improves plant drought tolerance.

Evolutionary appearance of the plasma membrane H (+) -ATPase containing a penultimate threonine in the bryophyte

The plasma membrane H (+) -ATPase provides the driving force for solute transport via an electrochemical gradient of H (+) across the plasma membrane, and regulates pH homeostasis and membrane potential in plant cells. However, the plasma membrane H (+) -ATPase in non-vascular plant bryophyte is largely unknown. Here, we show that the moss Physcomitrella patens, which is known as a model bryophyte, expresses both the penultimate Thr-containing H (+) -ATPase (pT H (+) -ATPase) and non-pT H (+) -ATPase as in the green algae, and that pT H (+) -ATPase is regulated by phosphorylation of its penultimate Thr. A search in the P. patens genome database revealed seven H (+) -ATPase genes, designated PpHA (Physcomitrella patens H (+) -ATPase). Six isoforms are the pT H (+) -ATPase; a remaining isoform is non-pT H (+) -ATPase. An apparent 95-kD protein was recognized by anti-H (+) -ATPase antibodies against an isoform of Arabidopsis thaliana and was phosphorylated on the penultimate Thr in response to a fungal toxin fusicoccin and light in protonemata, indicating that the 95-kD protein contains pT H (+) -ATPase. Furthermore, we could not detect the pT H (+) -ATPase in the charophyte alga Chara braunii, which is the closest relative of the land plants, by immunological methods. These results strongly suggest the pT H (+) -ATPase most likely appeared for the first time in bryophyte.

Auxin activates the plasma membrane H+-ATPase by phosphorylation during hypocotyl elongation in Arabidopsis

The phytohormone auxin is a major regulator of diverse aspects of plant growth and development. The ubiquitin-ligase complex SCF(TIR1/AFB) (for Skp1-Cul1-F-box protein), which includes the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX (TIR1/AFB) auxin receptor family, has recently been demonstrated to be critical for auxin-mediated transcriptional regulation. Early-phase auxin-induced hypocotyl elongation, on the other hand, has long been explained by the acid-growth theory, for which proton extrusion by the plasma membrane H(+)-ATPase is a functional prerequisite. However, the mechanism by which auxin mediates H(+)-ATPase activation has yet to be elucidated. Here, we present direct evidence for H(+)-ATPase activation in etiolated hypocotyls of Arabidopsis (Arabidopsis thaliana) by auxin through phosphorylation of the penultimate threonine during early-phase hypocotyl elongation. Application of the natural auxin indole-3-acetic acid (IAA) to endogenous auxin-depleted hypocotyl sections induced phosphorylation of the penultimate threonine of the H(+)-ATPase and increased H(+)-ATPase activity without altering the amount of the enzyme. Changes in both the phosphorylation level of H(+)-ATPase and IAA-induced elongation were similarly concentration dependent. Furthermore, IAA-induced H(+)-ATPase phosphorylation occurred in a tir1-1 afb2-3 double mutant, which is severely defective in auxin-mediated transcriptional regulation. In addition, α-(phenylethyl-2-one)-IAA, the auxin antagonist specific for the nuclear auxin receptor TIR1/AFBs, had no effect on IAA-induced H(+)-ATPase phosphorylation. These results suggest that the TIR1/AFB auxin receptor family is not involved in auxin-induced H(+)-ATPase phosphorylation. Our results define the activation mechanism of H(+)-ATPase by auxin during early-phase hypocotyl elongation; this is the long-sought-after mechanism that is central to the acid-growth theory.

Characterization of the plasma membrane H+-ATPase in the liverwort Marchantia polymorpha

The plasma membrane H(+)-ATPase generates an electrochemical gradient of H(+) across the plasma membrane that provides the driving force for solute transport and regulates pH homeostasis and membrane potential in plant cells. Recent studies have demonstrated that phosphorylation of the penultimate threonine in H(+)-ATPase and subsequent binding of a 14-3-3 protein is the major common activation mechanism for H(+)-ATPase in vascular plants. However, there is very little information on the plasma membrane H(+)-ATPase in nonvascular plant bryophytes. Here, we show that the liverwort Marchantia polymorpha, which is the most basal lineage of extant land plants, expresses both the penultimate threonine-containing H(+)-ATPase (pT H(+)-ATPase) and non-penultimate threonine-containing H(+)-ATPase (non-pT H(+)-ATPase) as in the green algae and that pT H(+)-ATPase is regulated by phosphorylation of its penultimate threonine. A search in the expressed sequence tag database of M. polymorpha revealed eight H(+)-ATPase genes, designated MpHA (for M. polymorpha H(+)-ATPase). Four isoforms are the pT H(+)-ATPase; the remaining isoforms are non-pT H(+)-ATPase. An apparent 95-kD protein was recognized by anti-H(+)-ATPase antibodies against an Arabidopsis (Arabidopsis thaliana) isoform and was phosphorylated on the penultimate threonine in response to the fungal toxin fusicoccin in thalli, indicating that the 95-kD protein contains pT H(+)-ATPase. Furthermore, we found that the pT H(+)-ATPase in thalli is phosphorylated in response to light, sucrose, and osmotic shock and that light-induced phosphorylation depends on photosynthesis. Our results define physiological signals for the regulation of pT H(+)-ATPase in the liverwort M. polymorpha, which is one of the earliest plants to acquire pT H(+)-ATPase.

Light-regulated stomatal aperture in Arabidopsis

The stomatal pores of plant leaves, situated in the epidermis and surrounded by a pair of guard cells, allow CO2 uptake for photosynthesis and water loss through transpiration. Blue light is one of the dominant environmental signals that control stomatal movements in leaves of plants in a natural environment. This blue light response is mediated by blue/UV A light-absorbing phototropins (phots) and cryptochromes (crys). Red/far-red light-absorbing phytochromes (phys) also play a role in the control of stomatal aperture. The signaling components that link the perception of light signals to the stomatal opening response are largely unknown. This review discusses a few newly discovered nuclear genes, their function with respect to the phot-, cry-, and phy-mediated signal transduction cascades, and possible involvement of circadian clock.

Crosstalk between blue-light- and ABA-signaling pathways in stomatal guard cells

We recently established an immunohistochemical method for the detection of blue light (BL)-induced and phototropin-mediated phosphorylation of plasma-membrane H+-ATPase in stomatal guard cells of Arabidopsis thaliana. This technique makes it possible to detect the phosphorylation/activation status of guard-cell H+-ATPase in the epidermis of a single rosette leaf, without the need to prepare guard-cell protoplasts (GCPs) from a large number of plants. Moreover, it can detect guard-cell responses under more natural and stress-free conditions compared to using GCPs. Taking advantage of these properties, we examined the effect of abscisic acid (ABA) on BL-induced phosphorylation of guard-cell H+-ATPase by using ABA-insensitive mutants. This revealed inhibition of BL-induced phosphorylation of guard-cell H+-ATPase via the early ABA-signaling components PYR/PYL/RCAR-PP2Cs-SnRK2s, which are known to be early ABA-signaling components for a wide range of ABA responses in plants.