CO2 provides an intermediate link in the red light response of guard cells

Calvin cycle and guard cell metabolism impact stomatal function

Stomatal conductance (gs) determines CO2 uptake for photosynthesis (A) and water loss through transpiration, which is essential for evaporative cooling and maintenance of optimal leaf temperature as well as nutrient uptake. Stomata adjust their aperture to maintain an appropriate balance between CO2 uptake and water loss and are therefore critical to overall plant water status and productivity. Although there is considerable knowledge regarding guard cell (GC) osmoregulation (which drives differences in GC volume and therefore stomatal opening and closing), as well as the various signal transduction pathways that enable GCs to sense and respond to different environmental stimuli, little is known about the signals that coordinate mesophyll demands for CO2. Furthermore, chloroplasts are a key feature in GCs of many species, however, their role in stomatal function is unclear and a subject of debate. In this review we explore the current evidence regarding the role of these organelles in stomatal behaviour, including GC electron transport and Calvin-Benson-Bassham (CBB) cycle activity as well as their possible involvement correlating gs and A along with other potential mesophyll signals. We also examine the roles of other GC metabolic processes in stomatal function.

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.

A comparison of stomatal conductance responses to blue and red light between C3 and C4 photosynthetic species in three phylogenetically-controlled experiments

Introduction

C4 photosynthesis is an adaptation that has independently evolved at least 66 times in angiosperms. C4 plants, unlike their C3 ancestral, have a carbon concentrating mechanism which suppresses photorespiration, often resulting in faster photosynthetic rates, higher yields, and enhanced water use efficiency. Moreover, the presence of C4 photosynthesis greatly alters the relation between CO2 assimilation and stomatal conductance. Previous papers have suggested that the adjustment involves a decrease in stomatal density. Here, we tested if C4 species also have differing stomatal responses to environmental cues, to accommodate the modified CO2 assimilation patterns compared to C3 species.

Methods

To test this hypothesis, stomatal responses to blue and red-light were analysed in three phylogenetically linked pairs of C3 and C4 species from the Cleomaceae (Gynandropsis and Tarenaya), Flaveria, and Alloteropsis, that use either C3 or C4 photosynthesis.

Results

The results showed strongly decreased stomatal sensitivity to blue light in C4 dicots, compared to their C3 counterparts, which exhibited significant blue light responses. In contrast, in C3 and C4 subspecies of the monocot A. semialata, the blue light response was observed regardless of photosynthetic type. Further, the quantitative red-light response varied across species, but the presence or absence of a significant stomatal red-light response was not directly associated with differences in photosynthetic pathway. Interestingly, stomatal density and morphology patterns observed across the three comparisons were also not consistent with patterns commonly asserted for C3 and C4 species.

Discussion

The strongly diminished blue-light sensitivity of stomatal responses in C4 species across two of the comparisons suggests a common C4 feature that may have functional implications. Altogether, the strong prevalence of species-specific effects clearly emphasizes the importance of phylogenetic controls in comparisons between C3 and C4 photosynthetic pathways.

Changes in the light environment: Short-term responses of photosynthesis and metabolism in spinach

Modification in the light environment can induce several changes even within a short time. In this article, light intensity and spectrum-dependent changes in photosynthetic and metabolic processes were investigated in spinach leaves. Short-term exposure of the youngest fully developed leaves provided an elevated CO2 assimilation capacity under red light compared with blue or white light, although the electron transport rate was lower. The stomatal opening was mainly stimulated by blue light. These spectrum-induced changes also depended on light intensity. When white light was used to activate the photosynthesis, the white light showed a similar light response to blue light regarding the electron transport processes and red light in terms of stomatal opening. In contrast, concerning CO2 assimilation characteristics, the white light resembled blue light at low and red light at high light intensities. These results indicate that the photosynthetic processes strongly interact with the light intensity and spectral composition. Furthermore, changes in spectral composition modified the primary metabolic processes as well. Red light induced the sugar accumulation, while more organic acids that belong to the respiration pathway were produced under blue and white lights. These changes occurred even within a short (30 min) time frame. These results also draw attention to the importance of the light environment used during the measurements of the photosynthetic activity of plants and/or sample collections.

Polyamines inhibit abscisic acid-induced stomatal closure by scavenging hydrogen peroxide

Stomatal closure is regulated by plant hormones and some small molecules to reduce water loss under stress conditions. Both abscisic acid (ABA) and polyamines alone induce stomatal closure; however, whether the physiological functions of ABA and polyamines are synergistic or antagonistic with respect to inducing stomatal closure is still unknown. Here, stomatal movement in response to ABA and/or polyamines was tested in Vicia faba and Arabidopsis thaliana, and the change in the signaling components under stomatal closure was analyzed. We found that both polyamines and ABA could induce stomatal closure through similar signaling components, including the synthesis of hydrogen peroxide (H₂ O₂ ) and nitric oxide (NO) and the accumulation of Ca²⁺ . However, polyamines partially inhibited ABA-induced stomatal closure both in epidermal peels and in planta by activating antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), to eliminate the ABA-induced increase in H₂ O₂ . These results strongly indicate that polyamines inhibit abscisic acid-induced stomatal closure, suggesting that polyamines could be used as potential plant growth regulators to increase photosynthesis under mild drought stress.

Elevated CO2 induces rapid dephosphorylation of plasma membrane H+ -ATPase in guard cells

Light induces stomatal opening, which is driven by plasma membrane (PM) H⁺ -ATPase in guard cells. The activation of guard-cell PM H⁺ -ATPase is mediated by phosphorylation of the penultimate C-terminal residue, threonine. The phosphorylation is induced by photosynthesis as well as blue light photoreceptor phototropin. Here, we investigated the effects of cessation of photosynthesis on the phosphorylation level of guard-cell PM H⁺ -ATPase in Arabidopsis thaliana. Immunodetection of guard-cell PM H⁺ -ATPase, time-resolved leaf gas-exchange analyses and stomatal aperture measurements were carried out. We found that light-dark transition of leaves induced dephosphorylation of the penultimate residue at 1 min post-transition. Gas-exchange analyses confirmed that the dephosphorylation is accompanied by an increase in the intercellular CO₂ concentration, caused by the cessation of photosynthetic CO₂ fixation. We discovered that CO₂ induces guard-cell PM H⁺ -ATPase dephosphorylation as well as stomatal closure. Interestingly, reverse-genetic analyses using guard-cell CO₂ signal transduction mutants suggested that the dephosphorylation is mediated by a mechanism distinct from the established CO₂ signalling pathway. Moreover, type 2C protein phosphatases D6 and D9 were required for the dephosphorylation and promoted stomatal closure upon the light-dark transition. Our results indicate that CO₂ -mediated dephosphorylation of guard-cell PM H⁺ -ATPase underlies stomatal closure.

The green light gap: a window of opportunity for optogenetic control of stomatal movement

Green plants are equipped with photoreceptors that are capable of sensing radiation in the ultraviolet-to-blue and the red-to-far-red parts of the light spectrum. However, plant cells are not particularly sensitive to green light (GL), and light which lies within this part of the spectrum does not efficiently trigger the opening of stomatal pores. Here, we discuss the current knowledge of stomatal responses to light, which are either provoked via photosynthetically active radiation or by specific blue light (BL) signaling pathways. The limited impact of GL on stomatal movements provides a unique option to use this light quality to control optogenetic tools. Recently, several of these tools have been optimized for use in plant biological research, either to control gene expression, or to provoke ion fluxes. Initial studies with the BL-activated potassium channel BLINK1 showed that this tool can speed up stomatal movements. Moreover, the GL-sensitive anion channel GtACR1 can induce stomatal closure, even at conditions that provoke stomatal opening in wild-type plants. Given that crop plants in controlled-environment agriculture and horticulture are often cultivated with artificial light sources (i.e. a combination of blue and red light from light-emitting diodes), GL signals can be used as a remote-control signal that controls stomatal transpiration and water consumption.

Phytochrome interacting factor regulates stomatal aperture by coordinating red light and abscisic acid

Stomata are crucial valves coordinating the fixation of carbon dioxide by photosynthesis and water loss through leaf transpiration. Phytochrome interacting factors (PIFs) are negative regulators of red light responses that belong to the basic helix-loop-helix family of transcription factors. Here, we show that the rice (Oryza sativa) PIF family gene OsPIL15 acts as a negative regulator of stomatal aperture to control transpiration in rice. OsPIL15 reduces stomatal aperture by activating rice ABSCISIC ACID INSENSITIVE 5 (OsABI5), which encodes a critical positive regulator of ABSCISIC ACID (ABA) signaling in rice. Moreover, OsPIL15 interacts with the NIGT1/HRS1/HHO family transcription factor rice HRS1 HOMOLOG 3 (OsHHO3) to possibly enhance the regulation of stomatal aperture. Notably, we discovered that the maize (Zea mays) PIF family genes ZmPIF1 and ZmPIF3, which are homologous to OsPIL15, are also involved in the regulation of stomatal aperture in maize, indicating that PIF-mediated regulation of stomatal aperture may be conserved in the plant lineage. Our findings explain the molecular mechanism by which PIFs play a role in red-light-mediated stomatal opening, and demonstrate that PIFs regulate stomatal aperture by coordinating the red light and ABA signaling pathways.

Promotion and Upregulation of a Plasma Membrane Proton-ATPase Strategy: Principles and Applications

Plasma membrane proton-ATPase (PM H+-ATPase) is a primary H+ transporter that consumes ATP in vivo and is a limiting factor in the blue light-induced stomatal opening signaling pathway. It was recently reported that manipulation of PM H+-ATPase in stomatal guard cells and other tissues greatly improved leaf photosynthesis and plant growth. In this report, we review and discuss the function of PM H+-ATPase in the context of the promotion and upregulation H+-ATPase strategy, including associated principles pertaining to enhanced stomatal opening, environmental plasticity, and potential applications in crops and nanotechnology. We highlight the great potential of the promotion and upregulation H+-ATPase strategy, and explain why it may be applied in many crops in the future.

Deep dive into CO2-dependent molecular mechanisms driving stomatal responses in plants

Recent advances are revealing mechanisms mediating CO₂-regulated stomatal movements in Arabidopsis, stomatal architecture and stomatal movements in grasses, and the long-term impact of CO₂ on growth.

Jasmonic acid and salicylic acid play minor roles in stomatal regulation by CO2 , abscisic acid, darkness, vapor pressure deficit and ozone

Jasmonic acid (JA) and salicylic acid (SA) regulate stomatal closure, preventing pathogen invasion into plants. However, to what extent abscisic acid (ABA), SA and JA interact, and what the roles of SA and JA are in stomatal responses to environmental cues, remains unclear. Here, by using intact plant gas-exchange measurements in JA and SA single and double mutants, we show that stomatal responsiveness to CO₂ , light intensity, ABA, high vapor pressure deficit and ozone either did not or, for some stimuli only, very slightly depended upon JA and SA biosynthesis and signaling mutants, including dde2, sid2, coi1, jai1, myc2 and npr1 alleles. Although the stomata in the mutants studied clearly responded to ABA, CO₂ , light and ozone, ABA-triggered stomatal closure in npr1-1 was slightly accelerated compared with the wild type. Stomatal reopening after ozone pulses was quicker in the coi1-16 mutant than in the wild type. In intact Arabidopsis plants, spraying with methyl-JA led to only a modest reduction in stomatal conductance 80 min after treatment, whereas ABA and CO₂ induced pronounced stomatal closure within minutes. We could not document a reduction of stomatal conductance after spraying with SA. Coronatine-induced stomatal opening was initiated slowly after 1.5-2.0 h, and reached a maximum by 3 h after spraying intact plants. Our results suggest that ABA, CO₂ and light are major regulators of rapid guard cell signaling, whereas JA and SA could play only minor roles in the whole-plant stomatal response to environmental cues in Arabidopsis and Solanum lycopersicum (tomato).

A BLUS1 kinase signal and a decrease in intercellular CO2 concentration are necessary for stomatal opening in response to blue light

Light-induced stomatal opening stimulates CO2 uptake and transpiration in plants. Weak blue light under strong red light effectively induces stomatal opening. Blue light-dependent stomatal opening initiates light perception by phototropins, and the signal is transmitted to a plasma membrane H+-ATPase in guard cells via BLUE LIGHT SIGNALING 1 (BLUS1) kinase. However, it is unclear how BLUS1 transmits the signal to H+-ATPase. Here, we characterized BLUS1 signaling in Arabidopsis thaliana, and showed that the BLUS1 C-terminus acts as an auto-inhibitory domain and that phototropin-mediated Ser-348 phosphorylation within the domain removes auto-inhibition. C-Terminal truncation and phospho-mimic Ser-348 mutation caused H+-ATPase activation in the dark, but did not elicit stomatal opening. Unexpectedly, the plants exhibited stomatal opening under strong red light and stomatal closure under weak blue light. A decrease in intercellular CO2 concentration via red light-driven photosynthesis together with H+-ATPase activation caused stomatal opening. Furthermore, phototropins caused H+-ATPase dephosphorylation in guard cells expressing constitutive signaling variants of BLUS1 in response to blue light, possibly for fine-tuning stomatal opening. Overall, our findings provide mechanistic insights into the blue light regulation of stomatal opening.

Optogenetic control of the guard cell membrane potential and stomatal movement by the light-gated anion channel GtACR1

Guard cells control the aperture of plant stomata, which are crucial for global fluxes of CO₂ and water. In turn, guard cell anion channels are seen as key players for stomatal closure, but is activation of these channels sufficient to limit plant water loss? To answer this open question, we used an optogenetic approach based on the light-gated anion channelrhodopsin 1 (GtACR1). In tobacco guard cells that express GtACR1, blue- and green-light pulses elicit Cl^- and NO₃ ^- currents of -1 to -2 nA. The anion currents depolarize the plasma membrane by 60 to 80 mV, which causes opening of voltage-gated K⁺ channels and the extrusion of K⁺ As a result, continuous stimulation with green light leads to loss of guard cell turgor and closure of stomata at conditions that provoke stomatal opening in wild type. GtACR1 optogenetics thus provides unequivocal evidence that opening of anion channels is sufficient to close stomata.

Engineering of Crassulacean Acid Metabolism

Crassulacean acid metabolism (CAM) has evolved from a C₃ ground state to increase water use efficiency of photosynthesis. During CAM evolution, selective pressures altered the abundance and expression patterns of C₃ genes and their regulators to enable the trait. The circadian pattern of CO₂ fixation and the stomatal opening pattern observed in CAM can be explained largely with a regulatory architecture already present in C₃ plants. The metabolic CAM cycle relies on enzymes and transporters that exist in C₃ plants and requires tight regulatory control to avoid futile cycles between carboxylation and decarboxylation. Ecological observations and modeling point to mesophyll conductance as a major factor during CAM evolution. The present state of knowledge enables suggestions for genes for a minimal CAM cycle for proof-of-concept engineering, assuming altered regulation of starch synthesis and degradation are not critical elements of CAM photosynthesis and sufficient malic acid export from the vacuole is possible.

Mesophyll-derived sugars are positive regulators of light-driven stomatal opening

Guard cell membrane ion transport and metabolism are deeply interconnected, and their coordinated regulation is integral to stomatal opening. Whereas ion transport is exceptionally well understood, how guard cell metabolism influences stomatal movements is less well known. Organic metabolites, such as malate and sugars, fulfill several functions in guard cells during stomatal opening as allosteric activators, counter-ions, energy source and osmolytes. However, their origin and exact fate remain debated. Recent work revealed that the guard cell carbon pool regulating stomatal function and plant growth is mostly of mesophyll origin, highlighting a tight correlation between mesophyll and guard cell metabolism. This review discusses latest research into guard cell carbon metabolism and its impact on stomatal function and whole plant physiology.

Integrating stomatal physiology and morphology: evolution of stomatal control and development of future crops

Stomata are central players in the hydrological and carbon cycles, regulating the uptake of carbon dioxide (CO₂) for photosynthesis and transpirative loss of water (H₂O) between plants and the atmosphere. The necessity to balance water-loss and CO₂-uptake has played a key role in the evolution of plants, and is increasingly important in a hotter and drier world. The conductance of CO₂ and water vapour across the leaf surface is determined by epidermal and stomatal morphology (the number, size, and spacing of stomatal pores) and stomatal physiology (the regulation of stomatal pore aperture in response to environmental conditions). The proportion of the epidermis allocated to stomata and the evolution of amphistomaty are linked to the physiological function of stomata. Moreover, the relationship between stomatal density and [CO₂] is mediated by physiological stomatal behaviour; species with less responsive stomata to light and [CO₂] are most likely to adjust stomatal initiation. These differences in the sensitivity of the stomatal density—[CO₂] relationship between species influence the efficacy of the ‘stomatal method’ that is widely used to infer the palaeo-atmospheric [CO₂] in which fossil leaves developed. Many studies have investigated stomatal physiology or morphology in isolation, which may result in the loss of the ‘overall picture’ as these traits operate in a coordinated manner to produce distinct mechanisms for stomatal control. Consideration of the interaction between stomatal morphology and physiology is critical to our understanding of plant evolutionary history, plant responses to on-going climate change and the production of more efficient and climate-resilient food and bio-fuel crops.

Stomatal Responses to Light, CO2, and Mesophyll Tissue in Vicia faba and Kalanchoë fedtschenkoi

The responses of stomatal aperture to light intensity and CO₂ concentration were studied in both Vicia faba (C₃) and Kalanchoë fedtschenkoi (Crassulacean acid metabolism; CAM), in material sampled from both light and dark periods. Direct comparison was made between intact leaf segments, epidermises grafted onto exposed mesophyll, and isolated epidermal peels, including transplantations between species and between diel periods. We reported the stomatal opening in response to darkness in isolated CAM peels from the light period, but not from the dark. Furthermore, we showed that C₃ mesophyll has stimulated CAM stomata in transplanted peels to behave as C₃ in response to light and CO₂. By using peels and mesophyll from plants sampled in the dark and the light period, we provided clear evidence that CAM stomata behaved differently from C₃. This might be linked to stored metabolites/ions and signalling pathway components within the guard cells, and/or a mesophyll-derived signal. Overall, our results provided evidence for both the involvement of guard cell metabolism and mesophyll signals in stomatal responses in both C₃ and CAM species.

Early Changes in Nitrate Uptake and Assimilation Under Drought in Relation to Transpiration

Soil drying combined with nitrogen (N) deficiency poses a grave threat to agricultural crop production. The rate at which nitrate (NO3 -) is taken up depends partly on the uptake and transpiration of water. Rapid changes in nitrate assimilation, in contrast to other N forms, may serve as a component of the plant stress response to drought because nitrate assimilation may lead to changes in xylem pH. The modulation of xylem sap pH may be relevant for stomata regulation via the delivery of abscisic acid (ABA) to guard cells. In several factorial experiments, we investigated the interactions between nitrate and water availability on nitrate fate in the plant, as well as their possible implications for the early drought-stress response. We monitored the short-term response (2-6 days) of nitrate in biomass, transport to shoot and reduction in Pisum sativum, Hordeum vulgare, Vicia faba, and Nicotiana tabacum and correlated this with sap pH and transpiration rates (TRs). Cultivation on inorganic substrate ensured control over nutrient and water supply and prevented nodulation in legume species. NO3 - content in biomass decreased in most of the species under drought indicating significant decline in NO3 - uptake. Hordeum vulgare had the highest NO3 - concentrations in all organs even under drought and low NO3 - treatment. This species can likely respond much better to the combined adverse effects of low NO3 - and water scarcity. Nitrate reductase activity (NRA) was reduced in both roots and leaves of water deficient (WD) plants in all species except H. vulgare, presumably due to its high NO3 - contents. Further, transient reduction in NO3 - availability had no effect on sap pH. Therefore, it seems unlikely that NRA shifts from shoot root leading to the supposed alkalization of sap. We also did not observe any interactive effects of NO3 - and water deficiency on transpiration. Hence, as long as leaf NO3 - content remains stable, NO3 - availability in soil is not linked to short-term modulation of transpiration.

Stomatal conductance bears no correlation with transpiration rate in wheat during their diurnal variation under high air humidity

A good understanding of the response of photosynthesis rate (P N) and transpiration rate (Tr) to stomatal alteration during the diurnal variations is important to cumulative photosynthetic production and water loss of crops. Six wheat genotypes were studied for 2 years with pot cultivation in rain-shelter. Among different genotypes, stomatal conductance (g s) was significantly correlated with both P N and Tr. But for each genotype, though g s was significantly correlated with P N regardless of relative air humidity (RH) status and it was also significantly correlated with Tr under lower RH (LRH, 15.4%) and moderate RH (MRH, 28.3%), it was not correlated with Tr under higher RH (HRH, 36.7%) during the diurnal changes. The conditional correlation between g s and Tr of wheat evoked new thinking on the relationships among g s, P N and Tr. Path analysis was further carried out to clarify the correlations of g s with the four atmospheric factors, that of Tr with g s and the four factors and the direct and indirect effects of the factors, during their diurnal dynamic variation. The effects of these factors on g s or Tr were related to RH. All the four factors had a much higher correlation with g s under HRH than that under LRH and MRH. Air temperature (T) had a rather higher direct effect than RH and photosynthetically active radiation (PAR). Also, the other factors had a much higher indirect effect on g s through vapor pressure deficit (VPD) and T. Transpiration rate was highly correlated with g s under LRH and MRH, with g s having a higher direct effect on it. In comparison, Tr was not correlated with g s under HRH but highly correlated with the atmospheric factors, with T, RH, and PAR having a higher indirect effect through VPD.

Role of blue and red light in stomatal dynamic behaviour

Plants experience changes in light intensity and quality due to variations in solar angle and shading from clouds and overlapping leaves. Stomatal opening to increasing irradiance is often an order of magnitude slower than photosynthetic responses, which can result in CO2 diffusional limitations on leaf photosynthesis, as well as unnecessary water loss when stomata continue to open after photosynthesis has reached saturation. Stomatal opening to light is driven by two distinct pathways; the 'red' or photosynthetic response that occurs at high fluence rates and saturates with photosynthesis, and is thought to be the main mechanism that coordinates stomatal behaviour with photosynthesis; and the guard cell-specific 'blue' light response that saturates at low fluence rates, and is often considered independent of photosynthesis, and important for early morning stomatal opening. Here we review the literature on these complicated signal transduction pathways and osmoregulatory processes in guard cells that are influenced by the light environment. We discuss the possibility of tuning the sensitivity and magnitude of stomatal response to blue light which potentially represents a novel target to develop ideotypes with the 'ideal' balance between carbon gain, evaporative cooling, and maintenance of hydraulic status that is crucial for maximizing crop performance and productivity.

Metabolomics of red-light-induced stomatal opening in Arabidopsis thaliana: Coupling with abscisic acid and jasmonic acid metabolism

Environmental stimuli-triggered stomatal movement is a key physiological process that regulates CO₂ uptake and water loss in plants. Stomata are defined by pairs of guard cells that perceive and transduce external signals, leading to cellular volume changes and consequent stomatal aperture change. Within the visible light spectrum, red light induces stomatal opening in intact leaves. However, there has been debate regarding the extent to which red-light-induced stomatal opening arises from direct guard cell sensing of red light versus indirect responses as a result of red light influences on mesophyll photosynthesis. Here we identify conditions that result in red-light-stimulated stomatal opening in isolated epidermal peels and enlargement of protoplasts, firmly establishing a direct guard cell response to red light. We then employ metabolomics workflows utilizing gas chromatography mass spectrometry and liquid chromatography mass spectrometry for metabolite profiling and identification of Arabidopsis guard cell metabolic signatures in response to red light in the absence of the mesophyll. We quantified 223 metabolites in Arabidopsis guard cells, with 104 found to be red light responsive. These red-light-modulated metabolites participate in the tricarboxylic acid cycle, carbon balance, phytohormone biosynthesis and redox homeostasis. We next analyzed selected Arabidopsis mutants, and discovered that stomatal opening response to red light is correlated with a decrease in guard cell abscisic acid content and an increase in jasmonic acid content. The red-light-modulated guard cell metabolome reported here provides fundamental information concerning autonomous red light signaling pathways in guard cells.

From reproduction to production, stomata are the master regulators

The best predictor of leaf level photosynthetic rate is the porosity of the leaf surface, as determined by the number and aperture of stomata on the leaf. This remarkable correlation between stomatal porosity (or diffusive conductance to water vapour g_s ) and CO₂ assimilation rate (A) applies to all major lineages of vascular plants (Figure 1) and is sufficiently predictable that it provides the basis for the model most widely used to predict water and CO₂ fluxes from leaves and canopies. Yet the Ball-Berry formulation is only a phenomenological approximation that captures the emergent character of stomatal behaviour. Progressing to a more mechanistic prediction of plant gas exchange is challenging because of the diversity of biological components regulating stomatal action. These processes are the product of more than 400 million years of co-evolution between stomatal, vascular and photosynthetic tissues. Both molecular and structural components link the abiotic world of the whole plant with the turgor pressure of the epidermis and guard cells, which ultimately determine stomatal pore size and porosity to water and CO₂ exchange (New Phytol., 168, 2005, 275). In this review we seek to simplify stomatal behaviour by using an evolutionary perspective to understand the principal selective pressures involved in stomatal evolution, thus identifying the primary regulators of stomatal aperture. We start by considering the adaptive process that has locked together the regulation of water and carbon fluxes in vascular plants, finally examining specific evidence for evolution in the proteins responsible for regulating guard cell turgor.

Contributions of cryptochromes and phototropins to stomatal opening through the day

The UV-A/blue photoreceptors phototropins and cryptochromes are both known to contribute to stomatal opening (Δgs) in blue light. However, their relative contributions to the maintenance of gs in blue light through the whole photoperiod remain unknown. To elucidate this question, Arabidopsis phot1 phot2 and cry1 cry2 mutants (MTs) and their respective wild types (WTs) were irradiated with 200 μmolm-2s-1 of blue-, green- or red-light (BL, GL or RL) throughout a 11-h photoperiod. Stomatal conductance (gs) was higher under BL than under RL or GL. Under RL, gs was not affected by either of the photoreceptor mutations, but under GL gs was slightly lower in cry1 cry2 than its WT. Under BL, the presence of phototropins was essential for rapid stomatal opening at the beginning of the photoperiod, and maximal stomatal opening beyond 3 h of irradiation required both phototropins and cryptochromes. Time courses of whole-plant net carbon assimilation rate (Anet) and the effective quantum yield of PSII photochemistry (ΦPSII) were consistent with an Anet-independent contribution of BL on gs both in phot1 phot2 and cry1 cry2 mutants. The changing roles of phototropins and cryptochromes through the day may allow more flexible coordination between gs and Anet.

Insights into the Molecular Mechanisms of CO2-Mediated Regulation of Stomatal Movements

Plants must continually balance the influx of CO₂ for photosynthesis against the loss of water vapor through stomatal pores in their leaves. This balance can be achieved by controlling the aperture of the stomatal pores in response to several environmental stimuli. Elevation in atmospheric [CO₂] induces stomatal closure and further impacts leaf temperatures, plant growth and water-use efficiency, and global crop productivity. Here, we review recent advances in understanding CO₂-perception mechanisms and CO₂-mediated signal transduction in the regulation of stomatal movements, and we explore how these mechanisms are integrated with other signaling pathways in guard cells.

Confirmation of mesophyll signals controlling stomatal responses by a newly devised transplanting method

There are opposing views on whether the responses of stomata to environmental stimuli are all autonomous reactions of stomatal guard cells or whether mesophyll is involved in these responses. Transplanting isolated epidermis onto mesophyll is a potent methodology for examining the roles of mesophyll-derived signals in stomatal responses. Here we report on development of a new transplanting method. Leaf segments of Commelina communis L. were pretreated in the light or dark at 10, 39 or 70Pa ambient CO2 for 1h. Then the abaxial epidermises were removed and the epidermal strips prepared from the other leaves kept in the dark at 39Pa CO2, were transplanted onto the mesophyll. After illumination of the transplants for 1h at 39Pa CO2, stomatal apertures were measured. We also examined the molecular sizes of the mesophyll signals by inserting the dialysis membrane permeable to molecules smaller than 100-500Da or 500-1000Da between the epidermis and mesophyll. Mesophyll pretreatments in the light at low CO2 partial pressures accelerated stomatal opening in the transplanted epidermal strips, whereas pretreatments at 70Pa CO2 suppressed stomatal opening. Insertion of these dialysis membranes did not suppress stomatal opening significantly at 10Pa CO2 in the light, whereas insertion of the 100-500Da membrane decelerated stomatal closure at high CO2. It is probable that the mesophyll signals inducing stomatal opening at low CO2 in the light would permeate both membranes, and that those inducing stomatal closure at high CO2 would not permeate the 100-500Da membrane. Possible signal compounds are discussed.

Acquiring Control: The Evolution of Stomatal Signalling Pathways

In vascular plants, stomata balance two opposing functions: they open to facilitate CO₂ uptake and close to prevent excessive water loss. Here, we discuss the evolution of three major signalling pathways that are known to control stomatal movements in angiosperms in response to light, CO₂, and abscisic acid (ABA). We examine the evolutionary origins of key signalling genes involved in these pathways, and compare their expression patterns between an angiosperm and moss. We propose that variation in stomatal sensitivity to stimuli between plant groups are rooted in differences in: (i) gene presence/absence, (ii) specificity of gene spatial expression pattern, and (iii) protein characteristics and functional interactions.

Reactive Oxygen Species, Photosynthesis, and Environment in the Regulation of Stomata

SIGNIFICANCE

Stomata sense the intercellular carbon dioxide (CO₂) concentration (C_i) and water availability under changing environmental conditions and adjust their apertures to maintain optimal cellular conditions for photosynthesis. Stomatal movements are regulated by a complex network of signaling cascades where reactive oxygen species (ROS) play a key role as signaling molecules. Recent Advances: Recent research has uncovered several new signaling components involved in CO₂- and abscisic acid-triggered guard cell signaling pathways. In addition, we are beginning to understand the complex interactions between different signaling pathways.

CRITICAL ISSUES

Plants close their stomata in reaction to stress conditions, such as drought, and the subsequent decrease in C_i leads to ROS production through photorespiration and over-reduction of the chloroplast electron transport chain. This reduces plant growth and thus drought may cause severe yield losses for agriculture especially in arid areas.

FUTURE DIRECTIONS

The focus of future research should be drawn toward understanding the interplay between various signaling pathways and how ROS, redox, and hormonal balance changes in space and time. Translating this knowledge from model species to crop plants will help in the development of new drought-resistant crop species with high yields.

Dawn regulates guard cell proteins in Arabidopsis thaliana that function in ATP production from fatty acid beta-oxidation

Based on the nature of the proteins that are altered in abundance, we conclude that guard cells switch their energy source from fatty acid metabolism to chloroplast activity, at the onset of dawn. During stomatal opening at dawn, evidence was recently presented for a breakdown and liquidation of stored triacylglycerols in guard cells to supply ATP for use in stomatal opening. However, proteome changes that happen in the guard cells during dawn were until now poorly understood. Bad accessibility to pure and intact guard cell samples can be considered as the primary reason behind this lack of knowledge. To overcome these technical constraints, epidermal guard cell samples with ruptured pavement cells were isolated at 1 h pre-dawn, 15 min post-dawn and 1 h post-dawn from Arabidopsis thaliana. Proteomic changes were analysed by ultra-performance-liquid-chromatography-mass-spectrometry. With 994 confidently identified proteins, we present the first analysis of the A. thaliana guard cell proteome that is not influenced by side effects of guard cell protoplasting. Data are available via ProteomeXchange with identifier PXD009918. By elucidating the identities of enzymes that change in abundance by the transition from dark to light, we corroborate the hypothesis that respiratory ATP production for stomatal opening results from fatty acid beta-oxidation. Moreover, we identified many proteins that were never reported in the context of guard cell biology. Among them are proteins that might play a role in signalling or circadian rhythm.

Effects of the mesophyll on stomatal responses in amphistomatous leaves

The role of the mesophyll in stomatal functioning in thin amphistomatous leaves was investigated by altering gas exchange for one surface and observing the effects on stomatal conductance for the other surface. Three methods of perturbing gas exchange on the adaxial surface were used. First, gas exchange for the adaxial surface was completely blocked with plastic wrap or vacuum grease. Second, leaves were inverted to induce closure of the adaxial stomata. And third, ambient humidity for the adaxial surface was reduced to induce stomatal closure on that surface. Experiments were performed at low light intensity and three different CO₂ concentrations to test the idea that stomatal responses in thin amphistomatous leaves are partially controlled by a signal from the mesophyll that varies with light and CO₂ . In general, stomata on the abaxial surface opened when gas exchange on the adaxial surface was reduced, with the largest increases in conductance occurring at high CO₂ concentration. The data are discussed with respect to role of a purported signal from the mesophyll and the partitioning of that signal between the two surfaces of the leaf.

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.

Guard cells in fern stomata are connected by plasmodesmata, but control cytosolic Ca2+ levels autonomously

Recent studies have revealed that some responses of fern stomata to environmental signals differ from those of their relatives in seed plants. However, it is unknown whether the biophysical properties of guard cells differ fundamentally between species of both clades. Intracellular micro-electrodes and the fluorescent Ca²⁺ reporter FURA2 were used to study voltage-dependent cation channels and Ca²⁺ signals in guard cells of the ferns Polypodium vulgare and Asplenium scolopendrium. Voltage clamp experiments with fern guard cells revealed similar properties of voltage-dependent K⁺ channels as found in seed plants. However, fluorescent dyes moved within the fern stomata, from one guard cell to the other, which does not occur in most seed plants. Despite the presence of plasmodesmata, which interconnect fern guard cells, Ca²⁺ signals could be elicited in each of the cells individually. Based on the common properties of voltage-dependent channels in ferns and seed plants, it is likely that these key transport proteins are conserved in vascular plants. However, the symplastic connections between fern guard cells in mature stomata indicate that the biophysical mechanisms that control stomatal movements differ between ferns and seed plants.

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.

Blue light and CO2 signals converge to regulate light-induced stomatal opening

Stomata regulate gas exchange between plants and atmosphere by integrating opening and closing signals. Stomata open in response to low CO₂ concentrations to maximize photosynthesis in the light; however, the mechanisms that coordinate photosynthesis and stomatal conductance have yet to be identified. Here we identify and characterize CBC1/2 (CONVERGENCE OF BLUE LIGHT (BL) AND CO₂ 1/2), two kinases that link BL, a major component of photosynthetically active radiation (PAR), and the signals from low concentrations of CO₂ in guard cells. CBC1/CBC2 redundantly stimulate stomatal opening by inhibition of S-type anion channels in response to both BL and low concentrations of CO₂. CBC1/CBC2 function in the signaling pathways of phototropins and HT1 (HIGH LEAF TEMPERATURE 1). CBC1/CBC2 interact with and are phosphorylated by HT1. We propose that CBCs regulate stomatal aperture by integrating signals from BL and CO₂ and act as the convergence site for signals from BL and low CO_2.

Stomata open in response to low CO₂ conditions in the light to maximise photosynthesis. Here, Hiyama et al. identify two kinases that promote stomatal opening by inhibiting S-type anion channels downstream of phototropin and HT1 thereby acting as a convergence point for blue light and CO₂ signaling.

The desert plant Phoenix dactylifera closes stomata via nitrate-regulated SLAC1 anion channel

Date palm Phoenix dactylifera is a desert crop well adapted to survive and produce fruits under extreme drought and heat. How are palms under such harsh environmental conditions able to limit transpirational water loss? Here, we analysed the cuticular waxes, stomata structure and function, and molecular biology of guard cells from P. dactylifera. To understand the stomatal response to the water stress phytohormone of the desert plant, we cloned the major elements necessary for guard cell fast abscisic acid (ABA) signalling and reconstituted this ABA signalosome in Xenopus oocytes. The PhoenixSLAC1-type anion channel is regulated by ABA kinase PdOST1. Energy-dispersive X-ray analysis (EDXA) demonstrated that date palm guard cells release chloride during stomatal closure. However, in Cl^- medium, PdOST1 did not activate the desert plant anion channel PdSLAC1 per se. Only when nitrate was present at the extracellular face of the anion channel did the OST1-gated PdSLAC1 open, thus enabling chloride release. In the presence of nitrate, ABA enhanced and accelerated stomatal closure. Our findings indicate that, in date palm, the guard cell osmotic motor driving stomatal closure uses nitrate as the signal to open the major anion channel SLAC1. This initiates guard cell depolarization and the release of anions together with potassium.

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.

The acclimation of Tilia cordata stomatal opening in response to light, and stomatal anatomy to vegetational shade and its components

Stomatal anatomical traits and rapid responses to several components of visible light were measured in Tilia cordata Mill. seedlings grown in an open, fully sunlit field (C-set), or under different kinds of shade. The main questions were: (i) stomatal responses to which visible light spectrum regions are modified by growth-environment shade and (ii) which separate component of vegetational shade is most effective in eliciting the acclimation effects of the full vegetational shade. We found that stomatal opening in response to red or green light did not differ between the plants grown in the different environments. Stomatal response to blue light was increased (in comparison with that of C-set) in the leaves grown in full vegetational shade (IABW-set), in attenuated UVAB irradiance (AB-set) or in decreased light intensity (neutral shade) plus attenuated UVAB irradiance (IAB-set). In all sets, the addition of green light-two or four times stronger-into induction light barely changed the rate of the blue-light-stimulated stomatal opening. In the AB-set, stomatal response to blue light equalled the strong IABW-set response. In attenuated UVB-grown leaves, stomatal response fell midway between IABW- and C-set results. Blue light response by neutral shade-grown leaves did not differ from that of the C-set, and the response by the IAB-set did not differ from that of the AB-set. Stomatal size was not modified by growth environments. Stomatal density and index were remarkably decreased only in the IABW- and IAB-sets. It was concluded that differences in white light responses between T. cordata leaves grown in different light environments are caused only by their different blue light response. Differences in stomatal sensitivity are not dependent on altered stomatal anatomy. Attenuated UVAB irradiance is the most efficient component of vegetational shade in stimulating acclimation of stomata, whereas decreased light intensity plays a minor role.

Impaired Stomatal Control Is Associated with Reduced Photosynthetic Physiology in Crop Species Grown at Elevated [CO2]

Physiological control of stomatal conductance (Gs) permits plants to balance CO2-uptake for photosynthesis (PN) against water-loss, so optimizing water use efficiency (WUE). An increase in the atmospheric concentration of carbon dioxide ([CO2]) will result in a stimulation of PN and reduction of Gs in many plants, enhancing carbon gain while reducing water-loss. It has also been hypothesized that the increase in WUE associated with lower Gs at elevated [CO2] would reduce the negative impacts of drought on many crops. Despite the large number of CO2-enrichment studies to date, there is relatively little information regarding the effect of elevated [CO2] on stomatal control. Five crop species with active physiological stomatal behavior were grown at ambient (400 ppm) and elevated (2000 ppm) [CO2]. We investigated the relationship between stomatal function, stomatal size, and photosynthetic capacity in the five species, and then assessed the mechanistic effect of elevated [CO2] on photosynthetic physiology, stomatal sensitivity to [CO2] and the effectiveness of stomatal closure to darkness. We observed positive relationships between the speed of stomatal response and the maximum rates of PN and Gs sustained by the plants; indicative of close co-ordination of stomatal behavior and PN. In contrast to previous studies we did not observe a negative relationship between speed of stomatal response and stomatal size. The sensitivity of stomata to [CO2] declined with the ribulose-1,5-bisphosphate limited rate of PN at elevated [CO2]. The effectiveness of stomatal closure was also impaired at high [CO2]. Growth at elevated [CO2] did not affect the performance of photosystem II indicating that high [CO2] had not induced damage to the photosynthetic physiology, and suggesting that photosynthetic control of Gs is either directly impaired at high [CO2], sensing/signaling of environmental change is disrupted or elevated [CO2] causes some physical effect that constrains stomatal opening/closing. This study indicates that while elevated [CO2] may improve the WUE of crops under normal growth conditions, impaired stomatal control may increase the vulnerability of plants to water deficit and high temperatures.

Elevated-CO2 Response of Stomata and Its Dependence on Environmental Factors

Stomata control the flow of gases between plants and the atmosphere. This review is centered on stomatal responses to elevated CO2 concentration and considers other key environmental factors and underlying mechanisms at multiple levels. First, an outline of general responses in stomatal conductance under elevated CO2 is presented. Second, stomatal density response, its development, and the trade-off with leaf growth under elevated CO2 conditions are depicted. Third, the molecular mechanism regulating guard cell movement at elevated CO2 is suggested. Finally, the interactive effects of elevated CO2 with other factors critical to stomatal behavior are reviewed. It may be useful to better understand how stomata respond to elevated CO2 levels while considering other key environmental factors and mechanisms, including molecular mechanism, biochemical processes, and ecophysiological regulation. This understanding may provide profound new insights into how plants cope with climate change.

Dominant and recessive mutations in the Raf-like kinase HT1 gene completely disrupt stomatal responses to CO2 in Arabidopsis

HT1 (HIGH LEAF TEMPERATURE 1) is the first component associated with changes in stomatal aperture in response to CO2 to be isolated by forward genetic screening. The HT1 gene encodes a protein kinase expressed mainly in guard cells. The loss-of-function ht1-1 and ht1-2 mutants in Arabidopsis thaliana have CO2-hypersensitive stomatal closure with concomitant reductions in their kinase activities in vitro In addition to these mutants, in this study we isolate or obtaine five new ht1 alleles (ht1-3, ht1-4, ht1-5, ht1-6, and ht1-7). Among the mutants, only ht1-3 has a dominant mutant phenotype and has widely opened stomata due to CO2 insensitivity. The ht1-3 mutant has a missense mutation affecting a non-conserved residue (R102K), whereas the other six recessive mutants have mutations in highly conserved residues in the catalytic domains required for kinase activity. We found that the dominant mutation does not affect the expression of HT1 or the ability to phosphorylate casein, a universal kinase substrate, but it does affect autophosphorylation activity in vitro A 3D structural model of HT1 also shows that the R102 residue protrudes from the surface of the kinase, implying a role for the formation of oligomers and/or interaction with its targets. We demonstrate that both the loss-of-function and gain-of-function ht1 mutants have completely disrupted CO2 responses, although they have normal responses to ABA. Furthermore, light-induced stomatal opening is smaller in ht1-3 and much smaller in ht1-2 Taken together, these results indicate that HT1 is a critical regulator for CO2 signaling and is partially involved in the light-induced stomatal opening pathway.

The reduced state of the plastoquinone pool is required for chloroplast-mediated stomatal closure in response to calcium stimulation

Besides their participation in photosynthesis, leaf chloroplasts function in plant responses to stimuli, yet how they direct stimulus-induced stomatal movement remains elusive. Here, we showed that over-reduction of the plastoquinone (PQ) pool by dibromothymoquinone (DBMIB) was closely associated with stomatal closure in plants which required chloroplastic H2O2 generation in the mesophyll. External application of H2 O2 reduced the PQ pool, whereas the cell-permeable reactive oxygen species (ROS) scavenger N-acetylcysteine (NAC) reversed the DBMIB-induced over-reduction of the PQ pool and stomatal closure. Mesophyll chloroplasts are key players of extracellular Ca(2+) (Ca(2+)o)-induced stomatal closure, but when treated with either 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) or NAC they failed to facilitate Ca(2+)o-induced stomatal closure due to the inhibition of chloroplastic H2 O2 synthesis in mesophyll. Similarly, the Arabidopsis electron transfer chain-related mutants npq4-1, stn7 and cas-1 exhibited diverse responses to Ca(2+)o or DBMIB. Transcriptome analysis also demonstrated that the PQ pool signaling pathway shared common responsive genes with the H2 O2 signaling pathway. These results implicated a mechanism for chloroplast-mediated stomatal closure involving the generation of mesophyll chloroplastic H2O2 based on the reduced state of the PQ pool, which is calcium-sensing receptor (CAS) and LHCII phosphorylation dependent.

CO2 Sensing and CO2 Regulation of Stomatal Conductance: Advances and Open Questions

Guard cells form epidermal stomatal gas-exchange valves in plants and regulate the aperture of stomatal pores in response to changes in the carbon dioxide (CO2) concentration ([CO2]) in leaves. Moreover, the development of stomata is repressed by elevated CO2 in diverse plant species. Evidence suggests that plants can sense [CO2] changes via guard cells and via mesophyll tissues in mediating stomatal movements. We review new discoveries and open questions on mechanisms mediating CO2-regulated stomatal movements and CO2 modulation of stomatal development, which together function in the CO2 regulation of stomatal conductance and gas exchange in plants. Research in this area is timely in light of the necessity of selecting and developing crop cultivars that perform better in a shifting climate.

Elevated-CO2 Response of Stomata and Its Dependence on Environmental Factors

Stomata control the flow of gases between plants and the atmosphere. This review is centered on stomatal responses to elevated CO2 concentration and considers other key environmental factors and underlying mechanisms at multiple levels. First, an outline of general responses in stomatal conductance under elevated CO2 is presented. Second, stomatal density response, its development, and the trade-off with leaf growth under elevated CO2 conditions are depicted. Third, the molecular mechanism regulating guard cell movement at elevated CO2 is suggested. Finally, the interactive effects of elevated CO2 with other factors critical to stomatal behavior are reviewed. It may be useful to better understand how stomata respond to elevated CO2 levels while considering other key environmental factors and mechanisms, including molecular mechanism, biochemical processes, and ecophysiological regulation. This understanding may provide profound new insights into how plants cope with climate change.

The HT1 protein kinase is essential for red light-induced stomatal opening and genetically interacts with OST1 in red light and CO2 -induced stomatal movement responses

The question of whether red light-induced stomatal opening is mediated by a photosynthesis-derived reduction in intercellular [CO2 ] (Ci ) remains controversial and genetic analyses are needed. The Arabidopsis thaliana protein kinase HIGH TEMPERATURE 1 (HT1) is a negative regulator of [CO2 ]-induced stomatal closing and ht1-2 mutant plants do not show stomatal opening to low [CO2 ]. The protein kinase mutant ost1-3 exhibits slowed stomatal responses to CO2 . The functions of HT1 and OPEN STOMATA 1 (OST1) to changes in red, blue light or [CO2 ] were analyzed. For comparison we assayed recessive ca1ca4 carbonic anhydrase double mutant plants, based on their slowed stomatal response to CO2 . Here, we report a strong impairment in ht1 in red light-induced stomatal opening whereas blue light was able to induce stomatal opening. The effects on photosynthetic performance in ht1 were restored when stomatal limitation of CO2 uptake, by control of [Ci ], was eliminated. HT1 was found to interact genetically with OST1 both during red light- and low [CO2 ]-induced stomatal opening. Analyses of ca1ca4 plants suggest that more than a low [Ci ]-dependent pathway may function in red light-induced stomatal opening. These results demonstrate that HT1 is essential for red light-induced stomatal opening and interacts genetically with OST1 during stomatal responses to red light and altered [CO2 ].

Guard cell photosynthesis is critical for stomatal turgor production, yet does not directly mediate CO2 - and ABA-induced stomatal closing

Stomata mediate gas exchange between the inter-cellular spaces of leaves and the atmosphere. CO2 levels in leaves (Ci) are determined by respiration, photosynthesis, stomatal conductance and atmospheric [CO2 ]. [CO2 ] in leaves mediates stomatal movements. The role of guard cell photosynthesis in stomatal conductance responses is a matter of debate, and genetic approaches are needed. We have generated transgenic Arabidopsis plants that are chlorophyll-deficient in guard cells only, expressing a constitutively active chlorophyllase in a guard cell specific enhancer trap line. Our data show that more than 90% of guard cells were chlorophyll-deficient. Interestingly, approximately 45% of stomata had an unusual, previously not-described, morphology of thin-shaped chlorophyll-less stomata. Nevertheless, stomatal size, stomatal index, plant morphology, and whole-leaf photosynthetic parameters (PSII, qP, qN, FV '/FM' ) were comparable with wild-type plants. Time-resolved intact leaf gas-exchange analyses showed a reduction in stomatal conductance and CO2 -assimilation rates of the transgenic plants. Normalization of CO2 responses showed that stomata of transgenic plants respond to [CO2 ] shifts. Detailed stomatal aperture measurements of normal kidney-shaped stomata, which lack chlorophyll, showed stomatal closing responses to [CO2 ] elevation and abscisic acid (ABA), while thin-shaped stomata were continuously closed. Our present findings show that stomatal movement responses to [CO2 ] and ABA are functional in guard cells that lack chlorophyll. These data suggest that guard cell CO2 and ABA signal transduction are not directly modulated by guard cell photosynthesis/electron transport. Moreover, the finding that chlorophyll-less stomata cause a 'deflated' thin-shaped phenotype, suggests that photosynthesis in guard cells is critical for energization and guard cell turgor production.

Natural variation in stomatal responses to environmental changes among Arabidopsis thaliana ecotypes

Stomata are small pores surrounded by guard cells that regulate gas exchange between plants and the atmosphere. Guard cells integrate multiple environmental signals and control the aperture width to ensure appropriate stomatal function for plant survival. Leaf temperature can be used as an indirect indicator of stomatal conductance to environmental signals. In this study, leaf thermal imaging of 374 Arabidopsis ecotypes was performed to assess their stomatal responses to changes in environmental CO2 concentrations. We identified three ecotypes, Köln (Kl-4), Gabelstein (Ga-0), and Chisdra (Chi-1), that have particularly low responsiveness to changes in CO2 concentrations. We next investigated stomatal responses to other environmental signals in these selected ecotypes, with Col-0 as the reference. The stomatal responses to light were also reduced in the three selected ecotypes when compared with Col-0. In contrast, their stomatal responses to changes in humidity were similar to those of Col-0. Of note, the responses to abscisic acid, a plant hormone involved in the adaptation of plants to reduced water availability, were not entirely consistent with the responses to humidity. This study demonstrates that the stomatal responses to CO2 and light share closely associated signaling mechanisms that are not generally correlated with humidity signaling pathways in these ecotypes. The results might reflect differences between ecotypes in intrinsic response mechanisms to environmental signals.

Are fern stomatal responses to different stimuli coordinated? Testing responses to light, vapor pressure deficit, and CO2 for diverse species grown under contrasting irradiances

The stomatal behavior of ferns provides an excellent system for disentangling responses to different environmental signals, which balance carbon gain against water loss. Here, we measured responses of stomatal conductance (gs ) to irradiance, CO2 , and vapor pressure deficit (VPD) for 13 phylogenetically diverse species native to open and shaded habitats, grown under high- and low-irradiance treatments. We tested two main hypotheses: that plants adapted and grown in high-irradiance environments would have greater responsiveness to all stimuli given higher flux rates; and that species' responsiveness to different factors would be correlated because of the relative simplicity of fern stomatal control. We found that species with higher light-saturated gs had larger responses, and that plants grown under high irradiance were more responsive to all stimuli. Open habitat species showed greater responsiveness to irradiance and CO2 , but lower responsiveness to VPD; a case of plasticity and adaptation tending in different directions. Responses of gs to irradiance and VPD were positively correlated across species, but CO2 responses were independent and highly variable. The novel finding of correlations among stomatal responses to different stimuli suggests coordination of hydraulic and photosynthetic signaling networks modulating fern stomatal responses, which show distinct optimization at growth and evolutionary time-scales.

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.