Light-induced modification of plant plasma membrane ion transport

Response of photosynthesis and electrical reactions of wheat plants upon the action of magnetic fields in the Schumann resonance frequency band

Alternating magnetic fields (MF) with Schumann resonance frequencies accompanied the development of living organisms throughout evolution, but today it remains unclear whether they can have a special biological effect in comparison with surrounding non-resonant frequencies. This work shows some stimulating effect of extremely low-frequency MFs on morphometric parameters and the activity of physiological processes in wheat (Triticum aestivum L.). It is shown that the MF effect is more pronounced for transient processes - photosynthesis reactions and changes in electrical potential caused by turning on light. For light-induced electrical reactions, the dependence of the severity of the effect on the frequency of the applied MF was demonstrated. It is shown that the most pronounced effect occurs in the 14.3 Hz field, which corresponds to the second harmonic of the Schumann resonance. The predominant sensitivity of signal-regulatory systems gives reason to assume the influence of MFs with Schumann resonance frequencies on the interaction of plants with environmental factors under conditions of a changed electromagnetic environment. Such conditions can occur, for example, with an increase in lightning activity caused by climate change, which serves as the basis for the generation of Schumann resonances, and with the development of artificial ecosystems outside the Earth's atmosphere.

Exploring the role of the LkABCG36 transporter in lignin accumulation

Lignin is a complex biopolymer formed through the condensation of three monomeric precursors known as monolignols. However, the mechanism underlying lignin precursor transport remains elusive, with uncertainty over whether it occurs through passive diffusion or an active energized process. ATP-binding cassette 36 (ABCG36) plays important roles in abiotic stress resistance. In this study, we investigated the transport functions of LkABCG36 (Larix kaempferi) for lignin precursors and the potential effects of LkABCG36 overexpression in plants. LkABCG36 enhanced the ability of tobacco (Nicotiana tabacum) bright yellow-2 (BY-2) cells to resist monolignol alcohol stress. Furthermore, LkABCG36 overexpression promoted lignin deposition in tobacco plant stem tissue. To understand the underlying mechanism, we measured the BY-2 cell ability to export lignin monomers and the uptake of monolignol precursors in inside-out (inverted) plasma membrane vesicles. We found that the transport of coniferyl and sinapyl alcohols is an ATP-dependent process. Our data suggest that LkABCG36 contributes to lignin accumulation in tobacco stem tissues through a mechanism involving the active transport of lignin precursors to the cell wall. These findings shed light on the lignin biosynthesis process, with important implications for enhancing lignin deposition in plants, potentially leading to improved stress tolerance and biomass production.

Chloroplast envelope K+/H+ antiporters are involved in cytosol pH regulation

Variations in light intensity induce cytosol pH changes in photosynthetic tissues, providing a possible signal to adjust a variety of biochemical, physiological and developmental processes to the energy status of the cells. It was shown that these pH changes are partially due to the transport of protons in or out of the thylakoid lumen. However, the ion transporters in the chloroplast that transmit these pH changes to the cytosol are not known. KEA1 and KEA2 are K+/H+ antiporters in the chloroplast inner envelope that adjust stromal pH in light-to-dark transitions. We previously determined that stromal pH is higher in kea1kea2 mutant cells. In this study, we now show that KEA1 and KEA2 are required to attenuate cytosol pH variations upon sudden light intensity changes in leaf mesophyll cells, showing they are important components of the light-modulated pH signalling module. The kea1kea2 mutant mesophyll cells also have a considerably less negative membrane potential. Membrane potential is dependent on the activity of the plasma membrane proton ATPase and is regulated by secondary ion transporters, mainly potassium channels in the plasma membrane. We did not find significant differences in the activity of the plasma membrane proton pump but found a strongly increased membrane permeability to protons, especially potassium, of the double mutant plasma membranes. Our results indicate that chloroplast envelope K+/H+ antiporters not only affect chloroplast pH but also have a strong impact on cellular ion homeostasis and energization of the plasma membrane.

Adaptive traits of cysts of the snow alga Sanguina nivaloides unveiled by 3D subcellular imaging

Sanguina nivaloides is the main alga forming red snowfields in high mountains and Polar Regions. It is non-cultivable. Analysis of environmental samples by X-ray tomography, focused-ion-beam scanning-electron-microscopy, physicochemical and physiological characterization reveal adaptive traits accounting for algal capacity to reside in snow. Cysts populate liquid water at the periphery of ice, are photosynthetically active, can survive for months, and are sensitive to freezing. They harbor a wrinkled plasma membrane expanding the interface with environment. Ionomic analysis supports a cell efflux of K+, and assimilation of phosphorus. Glycerolipidomic analysis confirms a phosphate limitation. The chloroplast contains thylakoids oriented in all directions, fixes carbon in a central pyrenoid and produces starch in peripheral protuberances. Analysis of cells kept in the dark shows that starch is a short-term carbon storage. The biogenesis of cytosolic droplets shows that they are loaded with triacylglycerol and carotenoids for long-term carbon storage and protection against oxidative stress.

Plasma membrane-chloroplast interactions activated by the hyperpolarizing response in characean cells

Signaling pathways in plant cells often comprise electrical phenomena developing at the plasma membrane. The action potentials in excitable plants like characean algae have a marked influence on photosynthetic electron transport and CO2 assimilation. The internodal cells of Characeae can also generate active electrical signals of a different type. The so called hyperpolarizing response develops under the passage of electric current whose strength is comparable to physiological currents circulating between nonuniform cell regions. The plasma membrane hyperpolarization is involved in multiple physiological events in aquatic and terrestrial plants. The hyperpolarizing response may represent an unexplored tool for studying the plasma membrane-chloroplast interactions in vivo. This study shows that the hyperpolarizing response of Chara australis internodes whose plasmalemma was preliminary converted into the K+-conductive state induces transient changes in maximal (Fm') and actual (F') fluorescence yields of chloroplasts in vivo. These fluorescence transients were light dependent, suggesting their relation to photosynthetic electron and H+ transport. The cell hyperpolarization promoted H+ influx that was inactivated after a single electric stimulus. The results indicate that the plasma membrane hyperpolarization drives transmembrane ion fluxes and modifies the ionic composition of cytoplasm, which indirectly (via envelope transporters) affects the pH of chloroplast stroma and chlorophyll fluorescence. Remarkably, the functioning of envelope ion transporters can be revealed in short-term experiments in vivo, without growing plants on solutions with various mineral compositions.

Effect of extremely low-frequency magnetic fields on light-induced electric reactions in wheat

Magnetic field oscillations resulting from atmospheric events could have an effect on growth and development of plants and on the responsive reactions of plants to other environmental factors. In the current work, extremely low-frequency magnetic field (14.3 Hz) was shown to modulate light-induced electric reactions of wheat (Triticum aestivum L.). Blue light-induced electric reaction in wheat leaf comprises depolarization and two waves of hyperpolarization resulting in an increase of the potential to a higher level compared to the dark one. Fluorescent and inhibitory analysis demonstrate a key role of calcium ions and calcium-dependent H⁺-ATPase of the plasma membrane in the development of the reaction. Activation of H⁺-ATPase by the increased calcium influx is suggested as a mechanism of the influence of magnetic field on light-induced electric reaction.

A tonoplast-localized magnesium transporter is crucial for stomatal opening in Arabidopsis under high Mg2+ conditions

Plant stomata play an important role in CO₂ uptake for photosynthesis and transpiration, but the mechanisms underlying stomatal opening and closing under changing environmental conditions are still not completely understood. Through large-scale genetic screening, we isolated an Arabidopsis mutant (closed stomata2 (cst2)) that is defective in stomatal opening. We cloned the causal gene (MGR1/CST2) and functionally characterized this gene. The mutant phenotype was caused by a mutation in a gene encoding an unknown protein with similarities to the human magnesium (Mg²⁺ ) efflux transporter ACDP/CNNM. MGR1/CST2 was localized to the tonoplast and showed transport activity for Mg²⁺ . This protein was constitutively and highly expressed in guard cells. Knockout of this gene resulted in stomatal closing, decreased photosynthesis and growth retardation, especially under high Mg²⁺ conditions, while overexpression of this gene increased stomatal opening and tolerance to high Mg²⁺ concentrations. Furthermore, guard cell-specific expression of MGR1/CST2 in the mutant partially restored its stomatal opening. Our results indicate that MGR1/CST2 expression in the leaf guard cells plays an important role in maintaining cytosolic Mg²⁺ concentrations through sequestering Mg²⁺ into vacuoles, which is required for stomatal opening, especially under high Mg²⁺ conditions.

Effect of Photoconversion Coatings for Greenhouses on Electrical Signal-Induced Resistance to Heat Stress of Tomato Plants

The use of photoconversion coatings is a promising approach to improving the quality of light when growing plants in greenhouses in low light conditions. In this work, we studied the effect of fluoropolymer coatings, which produce photoconversion of UV-A radiation and violet light into blue and red light, on the growth and resistance to heat stress of tomato plants (Solanum lycopersicum L.). The stimulating effect of the spectrum obtained as a result of photoconversion on plant growth and the activity of the photosynthesis process are shown. At the same time, the ability to withstand heat stress is reduced in plants grown under a photoconversion coating. Stress electrical signals, which normally increase resistance, in such plants have a much weaker protective effect on the photosynthetic apparatus. The observed effects are apparently explained by a decrease in the concentration of H₂O₂ in plants grown using photoconversion technologies, which leads to a shift in the development program towards increased productivity to the detriment of the protective function. Thus, when using photoconversion technologies in agricultural practice, it is necessary to pay increased attention to maintaining stable conditions during plant cultivation.

Plant electrical signals: A multidisciplinary challenge

Plant electrical signals, an early event in the plant-stimulus interaction, rapidly transmit information generated by the stimulus to other organs, and even the whole plant, to promote the corresponding response and trigger a regulatory cascade. In recent years, many promising state-of-the-art technologies applicable to study plant electrophysiology have emerged. Research focused on expression of genes associated with electrical signals has also proliferated. We propose that it is appropriate for plant electrical signals to be considered in the form of a "plant electrophysiological phenotype". This review synthesizes research on plant electrical signals from a novel, interdisciplinary perspective, which is needed to improve the efficient aggregation and use of plant electrical signal data and to expedite interpretation of plant electrical signals.

CIPK23 regulates blue light-dependent stomatal opening in Arabidopsis thaliana

Phototropins (phot1 and phot2) are plant blue light receptor kinases that function to mediate phototropism, chloroplast movement, leaf flattening, and stomatal opening in Arabidopsis. Considerable progress has been made in understanding the mechanisms associated with phototropin receptor activation by light. However, the identities of phototropin signaling components are less well understood by comparison. In this study, we specifically searched for protein kinases that interact with phototropins by using an in vitro screening method (AlphaScreen) to profile interactions against an Arabidopsis protein kinase library. We found that CBL-interacting protein kinase 23 (CIPK23) interacts with both phot1 and phot2. Although these interactions were verified by in vitro pull-down and in vivo bimolecular fluorescence complementation assays, CIPK23 was not phosphorylated by phot1, as least in vitro. Mutants lacking CIPK23 were found to exhibit impaired stomatal opening in response to blue light but no deficits in other phototropin-mediated responses. We further found that blue light activation of inward-rectifying K⁺ (K⁺ _in ) channels was impaired in the guard cells of cipk23 mutants, whereas activation of the plasma membrane H⁺ -ATPase was not. The blue light activation of K⁺ _in channels was also impaired in the mutant of BLUS1, which is one of the phototropin substrates in guard cells. We therefore conclude that CIPK23 promotes stomatal opening through activation of K⁺ _in channels most likely in concert with BLUS1, but through a mechanism other than activation of the H⁺ -ATPase. The role of CIPK23 as a newly identified component of phototropin signaling in stomatal guard cells is discussed.

Does Molecular and Structural Evolution Shape the Speedy Grass Stomata?

It has been increasingly important for breeding programs to be aimed at crops that are capable of coping with a changing climate, especially with regards to higher frequency and intensity of drought events. Grass stomatal complex has been proposed as an important factor that may enable grasses to adapt to water stress and variable climate conditions. There are many studies focusing on the stomatal morphology and development in the eudicot model plant Arabidopsis and monocot model plant Brachypodium. However, the comprehensive understanding of the distinction of stomatal structure and development between monocots and eudicots, especially between grasses and eudicots, are still less known at evolutionary and comparative genetic levels. Therefore, we employed the newly released version of the One Thousand Plant Transcriptome (OneKP) database and existing databases of green plant genome assemblies to explore the evolution of gene families that contributed to the formation of the unique structure and development of grass stomata. This review emphasizes the differential stomatal morphology, developmental mechanisms, and guard cell signaling in monocots and eudicots. We provide a summary of useful molecular evidences for the high water use efficiency of grass stomata that may offer new horizons for future success in breeding climate resilient crops.

Illumination/Darkness-Induced Changes in Leaf Surface Potential Linked With Kinetics of Ion Fluxes

A highly reproducible plant electrical signal-light-induced bioelectrogenesis (LIB) was obtained by means of periodic illumination/darkness stimulation of broad bean (Vicia faba L.) leaves. By stimulating the same position of the same leaf with different concentrations of NaCl, we observed that the amplitude and waveform of the LIB was correlated with the intensity of stimulation. This method allowed us to link dynamic ion fluxes induced by periodic illumination/darkness to salt stress. The self-referencing ion electrode technique was used to explore the ionic mechanisms of the LIB. Fluxes of H+, Ca2+, K+, and Cl- showed periodic changes under periodic illumination/darkness before and after 50 mM NaCl stimulation. Gray relational analysis was used to analyze correlations between each of these ions and LIB. The results showed that different ions are involved in surface potential changes at different stages under periodic illumination/darkness. The gray relational grade reflected the contribution of each ion to the change in surface potential at a certain time period. The ion fluxes data obtained under periodic illumination/darkness stimulation will contribute to the future development of a dynamic model for interpretation of electrophysiological events in plant cells.

PH-dependent cell-cell interactions in the green alga Chara

Characean internodal cells develop alternating patterns of acid and alkaline zones along their surface in order to facilitate uptake of carbon required for photosynthesis. In this study, we used a pH-indicating membrane dye, 4-heptadecylumbiliferone, to study the kinetics of alkaline band formation and decomposition. The differences in growth/decay kinetics suggested that growth occurred as an active, autocatalytic process, whereas decomposition was due to diffusion. We further investigated mutual interactions between internodal cells and found that their alignment parallel to each other induced matching of the pH banding patterns, which was mirrored by chloroplast activity. In non-aligned cells, the lowered photosynthetic activity was noted upon a rise of the external pH, suggesting that the matching of pH bands was due to a local elevation of membrane conductance by the high pH of the alkaline zones of neighboured cells. Finally, we show that the altered pH banding pattern caused the reorganization of the cortical cytoplasm. Complex plasma membrane elaborations (charasomes) were degraded via endocytosis, and mitochondria were moved away from the cortex when a previously acid region became alkaline and vice versa. Our data show that characean internodal cells react flexibly to environmental cues, including those originating from neighboured cells.

Molecular Evolution and Interaction of Membrane Transport and Photoreception in Plants

Light is a vital regulator that controls physiological and cellular responses to regulate plant growth, development, yield, and quality. Light is the driving force for electron and ion transport in the thylakoid membrane and other membranes of plant cells. In different plant species and cell types, light activates photoreceptors, thereby modulating plasma membrane transport. Plants maximize their growth and photosynthesis by facilitating the coordinated regulation of ion channels, pumps, and co-transporters across membranes to fine-tune nutrient uptake. The signal-transducing functions associated with membrane transporters, pumps, and channels impart a complex array of mechanisms to regulate plant responses to light. The identification of light responsive membrane transport components and understanding of their potential interaction with photoreceptors will elucidate how light-activated signaling pathways optimize plant growth, production, and nutrition to the prevailing environmental changes. This review summarizes the mechanisms underlying the physiological and molecular regulations of light-induced membrane transport and their potential interaction with photoreceptors in a plant evolutionary and nutrition context. It will shed new light on plant ecological conservation as well as agricultural production and crop quality, bringing potential nutrition and health benefits to humans and animals.

Brassinosteroid Involvement in Arabidopsis thaliana Stomatal Opening

Stomata within the plant epidermis regulate CO2 uptake for photosynthesis and water loss through transpiration. Stomatal opening in Arabidopsis thaliana is determined by various factors, including blue light as a signal and multiple phytohormones. Plasma membrane transporters, including H+-ATPase, K+ channels and anion channels in guard cells, mediate these processes, and the activities and expression levels of these components determine stomatal aperture. However, the regulatory mechanisms involved in these processes are not fully understood. In this study, we used infrared thermography to isolate a mutant defective in stomatal opening in response to light. The causative mutation was identified as an allele of the brassinosteroid (BR) biosynthetic mutant dwarf5. Guard cells from this mutant exhibited normal H+-ATPase activity in response to blue light, but showed reduced K+ accumulation and inward-rectifying K+ (K+in) channel activity as a consequence of decreased expression of major K+in channel genes. Consistent with these results, another BR biosynthetic mutant, det2-1, and a BR receptor mutant, bri1-6, exhibited reduced blue light-dependent stomatal opening. Furthermore, application of BR to the hydroponic culture medium completely restored stomatal opening in dwarf5 and det2-1 but not in bri1-6. However, application of BR to the epidermis of dwarf5 did not restore stomatal response. From these results, we conclude that endogenous BR acts in a long-term manner and is required in guard cells with the ability to open stomata in response to light, probably through regulation of K+in channel activity.

Photorespiration Is Crucial for Dynamic Response of Photosynthetic Metabolism and Stomatal Movement to Altered CO2 Availability

The photorespiratory pathway or photorespiration is an essential process in oxygenic photosynthetic organisms, which can reduce the efficiency of photosynthetic carbon assimilation and is hence frequently considered as a wasteful process. By comparing the response of the wild-type plants and mutants impaired in photorespiration to a shift in ambient CO₂ concentrations, we demonstrate that photorespiration also plays a beneficial role during short-term acclimation to reduced CO₂ availability. The wild-type plants responded with few differentially expressed genes, mostly involved in drought stress, which is likely a consequence of enhanced opening of stomata and concomitant water loss upon a shift toward low CO₂. In contrast, mutants with impaired activity of photorespiratory enzymes were highly stressed and not able to adjust stomatal conductance to reduced external CO₂ availability. The transcriptional response of mutant plants was congruent, indicating a general reprogramming to deal with the consequences of reduced CO₂ availability, signaled by enhanced oxygenation of ribulose-1,5-bisphosphate and amplified by the artificially impaired photorespiratory metabolism. Central in this reprogramming was the pronounced reallocation of resources from growth processes to stress responses. Taken together, our results indicate that unrestricted photorespiratory metabolism is a prerequisite for rapid physiological acclimation to a reduction in CO₂ availability.

Guard cell chloroplasts are essential for blue light-dependent stomatal opening in Arabidopsis

Blue light (BL) induces stomatal opening through the activation of H+-ATPases with subsequent ion accumulation in guard cells. In most plant species, red light (RL) enhances BL-dependent stomatal opening. This RL effect is attributable to the chloroplasts of guard cell, the only cells in the epidermis possessing this organelle. To clarify the role of chloroplasts in stomatal regulation, we investigated the effects of RL on BL-dependent stomatal opening in isolated epidermis, guard cell protoplasts, and intact leaves of Arabidopsis thaliana. In isolated epidermal tissues and intact leaves, weak BL superimposed on RL enhanced stomatal opening while BL alone was less effective. In guard cell protoplasts, RL enhanced BL-dependent H+-pumping and DCMU, a photosynthetic electron transport inhibitor, eliminated this effect. RL enhanced phosphorylation levels of the H+-ATPase in response to BL, but this RL effect was not suppressed by DCMU. Furthermore, DCMU inhibited both RL-induced and BL-dependent stomatal opening in intact leaves. The photosynthetic rate in leaves correlated positively with BL-dependent stomatal opening in the presence of DCMU. We conclude that guard cell chloroplasts provide ATP and/or reducing equivalents that fuel BL-dependent stomatal opening, and that they indirectly monitor photosynthetic CO2 fixation in mesophyll chloroplasts by absorbing PAR in the epidermis.

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.

Light sensitivity of shoot hydraulic conductance in five temperate deciduous tree species

The light sensitivity of the shoot hydraulic conductance in five temperate deciduous tree species was measured using two methods to clarify the role of light sensitivity and the suitability of the methods used to study it. The light sensitivity measured using a method that included an interruption of ≤10min in shoot light acclimation did not differ from that measured using a method with continuous illumination. The 'noncontinuous light' methods are suitable for measuring hydraulic conductance and its light response. Light sensitivity correlated with other leaf water traits as follows: positively with the ion-mediated increase in xylem hydraulic conductance; a relative decrease in the hydraulic conductance of the laminae in response to HgCl2; a relative change in stomatal conductance in response to changes in PAR intensity or atmospheric CO2 concentration, or to a decrease in air humidity or leaf water potential; and with instantaneous water use efficiency. The traits correlated negatively with shoot hydraulic conductance, stomatal conductance and relative increases in stomatal conductance in response to increases in leaf water potential. We suggest that high light sensitivity should be considered as one of the characteristics of conservative water use in trees. Low blue light increased shoot hydraulic conductance to a similar extent to moderate white light and twice as much as moderate red light. Blue light perception is important in the light sensitivity mechanism.

Development of real-time radioisotope imaging systems for plant nutrient uptake studies

Ionic nutrition is essential for plant development. Many techniques have been developed to image and (or) measure ionic movement in plants. Nevertheless, most of them are destructive and limit the analysis. Here, we present the development of radioisotope imaging techniques that overcome such restrictions and allow for real-time imaging of ionic movement. The first system, called macroimaging, was developed to visualize and measure ion uptake and translocation between organs at a whole-plant scale. Such a device is fully compatible with illumination of the sample. We also modified fluorescent microscopes to set up various solutions for ion uptake analysis at the microscopic level. Both systems allow numerical analysis of images and possess a wide dynamic range of detection because they are based on radioactivity.

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.

Anion channels: master switches of stress responses

During stress, plant cells activate anion channels and trigger the release of anions across the plasma membrane. Recently, two new gene families have been identified that encode major groups of anion channels. The SLAC/SLAH channels are characterized by slow voltage-dependent activation (S-type), whereas ALMT genes encode rapid-activating channels (R-type). Both S- and R-type channels are stimulated in guard cells by the stress hormone ABA, which leads to stomatal closure. Besides their role in ABA-dependent stomatal movement, anion channels are also activated by biotic stress factors such as microbe-associated molecular patterns (MAMPs). Given that anion channels occur throughout the plant kingdom, they are likely to serve a general function as master switches of stress responses.

Responses of stomatal conductance to simultaneous changes in two environmental factors

To clarify interactions between stomatal responses to two simultaneous environmental changes, the rates of change in stomatal conductance were measured after simultaneously changing two environmental factors from the set of air humidity, leaf water potential (hydraulic environmental factors), air CO(2) concentration and light intensity (photosynthetic environmental factors). The stomatal responses to changes in leaf water potential were not significantly modified by any other simultaneous environmental change. A decrease in air humidity was followed by a decrease in stomatal conductance, and an increase in air humidity was followed by an increase in the conductance, irrespective of the character of the simultaneous change in the photosynthetic environmental factor. If the simultaneous change had an opposite effect on stomatal conductance, the rate of change in stomatal conductance was higher than the theoretical summed rate-the sum of the rate following one environmental change and the rate following another environmental change, measured separately. That is, the stomatal response to air humidity dominated over the responses to photosynthetic environmental factors. Yet, if the simultaneous change in photosynthetic factors had a codirectional effect on stomatal conductance, the rate of stomatal conductance change was lower than the theoretical summed rate. After a simultaneous change of two photosynthetic environmental factors, the rate of stomatal conductance change was very similar to the theoretical rate, if both the environmental changes had a codirectional effect on stomatal conductance. If the changes in the photosynthetic factors had opposite effects on stomatal conductance, the conductance increased, irrespective of the character of the increasing environmental factor. In drought-stressed trees, the rates of change in stomatal conductance tended to differ from the theoretical summed rates more than in well-watered trees. Stomatal closure following an increase in CO(2) concentration was the stomatal response that was most strongly suppressed by the response to another simultaneous environmental change. Six species of temperate deciduous trees were shown to be similar in their relations between the stomatal responses to two simultaneous environmental changes. The mechanism and ecological significance of the interactions between the two signal response pathways of stomata are discussed.