Mesophyll photosynthesis and guard cell metabolism impacts on stomatal behaviour

Metabolite-mediated adaptation of crops to drought and the acquisition of tolerance

Drought is one of the major and growing threats to agriculture productivity and food security. Metabolites are involved in the regulation of plant responses to various environmental stresses, including drought stress. The complex drought tolerance can be ascribed to several simple metabolic traits. These traits could then be used for detecting the genetic architecture of drought tolerance. Plant metabolomes show dynamic differences when drought occurs during different developmental stages or upon different levels of drought stress. Here, we reviewed the major and most recent findings regarding the metabolite-mediated plant drought response. Recent progress in the development of drought-tolerant agents is also discussed. We provide an updated schematic overview of metabolome-driven solutions for increasing crop drought tolerance and thereby addressing an impending agricultural challenge.

Greater mesophyll conductance and leaf photosynthesis in the field through modified cell wall porosity and thickness via AtCGR3 expression in tobacco

Mesophyll conductance (gm) describes the ease with which CO2 passes from the sub-stomatal cavities of the leaf to the primary carboxylase of photosynthesis, Rubisco. Increasing gm is suggested as a means to engineer increases in photosynthesis by increasing [CO2] at Rubisco, inhibiting oxygenation and accelerating carboxylation. Here, tobacco was transgenically up-regulated with Arabidopsis Cotton Golgi-related 3 (CGR3), a gene controlling methylesterification of pectin, as a strategy to increase CO2 diffusion across the cell wall and thereby increase gm. Across three independent events in tobacco strongly expressing AtCGR3, mesophyll cell wall thickness was decreased by 7%-13%, wall porosity increased by 75% and gm measured by carbon isotope discrimination increased by 28%. Importantly, field-grown plants showed an average 8% increase in leaf photosynthetic CO2 uptake. Up-regulating CGR3 provides a new strategy for increasing gm in dicotyledonous crops, leading to higher CO2 assimilation and a potential means to sustainable crop yield improvement.

The poorly-explored stomatal response to temperature at constant evaporative demand

Changes in leaf temperature are known to drive stomatal responses, because the leaf-to-air water vapour gradient (Δw) increases with temperature if ambient vapour pressure is held constant, and stomata respond to changes in Δw. However, the direct response of stomata to temperature (DRST; the response when Δw is held constant by adjusting ambient humidity) has been examined far less extensively. Though the meagre available data suggest the response is usually positive, results differ widely and defy broad generalisation. As a result, little is known about the DRST. This review discusses the current state of knowledge about the DRST, including numerous hypothesised biophysical mechanisms, potential implications of the response for plant adaptation, and possible impacts of the DRST on plant-atmosphere carbon and water exchange in a changing climate.

Potassium deficiency reduces grapevine transpiration through decreased leaf area and stomatal conductance

Plants require potassium (K) to support growth and regulate hydraulics. Yet, K's effects on transpiration are still speculated. We hypothesized that K deficiency would limit grapevine water uptake by limiting canopy size and stomatal conductance (gs). Hence, we constructed large (2 m3) lysimeters and recorded vine transpiration for three years (2020-2022) under three fertilization application rates (8, 20, or 58 mg K L-1 in irrigation). Maximal K availability supported transpiration up to 75 L day-1, whereas K-deficient vines transpired only 60 L day-1 in midsummer. Limited vine growth and canopy size mainly accounted for reduced transpiration under low K conditions. Hence, considering K demand in addition to supply, we compared K deficiency effects on vines bearing 20 or 50 fruit clusters and found that reduced gs further limited transpiration when yields were high. Although fruits were strong K sinks, high yields did not alter K uptake because lower vegetative growth countered the additional K demands. Potassium deficiency leads to lower transpiration and productivity. Yet, internal mineral allocation compensates for fruit K uptake and masks biochemical indices or physiological proxies for K deficiency. Thus, decision support tools should integrate mineral availability, seasonal growth, and yield projections to determine grapevine water demands.

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.

The starch-deficient plastidic PHOSPHOGLUCOMUTASE mutant of the constitutive crassulacean acid metabolism (CAM) species Kalanchoë fedtschenkoi impacts diel regulation and timing of stomatal CO2 responsiveness

BACKGROUND AND AIMS

Crassulacean acid metabolism (CAM) is a specialized type of photosynthesis characterized by a diel pattern of stomatal opening at night and closure during the day, which increases water-use efficiency. Starch degradation is a key regulator of CAM, providing phosphoenolpyruvate as a substrate in the mesophyll for nocturnal assimilation of CO2. Growing recognition of a key role for starch degradation in C3 photosynthesis guard cells for mediating daytime stomatal opening presents the possibility that starch degradation might also impact CAM by regulating the provision of energy and osmolytes to increase guard cell turgor and drive stomatal opening at night. In this study, we tested the hypothesis that the timing of diel starch turnover in CAM guard cells has been reprogrammed during evolution to enable nocturnal stomatal opening and daytime closure.

METHODS

Biochemical and genetic characterization of wild-type and starch-deficient RNAi lines of Kalanchoë fedtschenkoi with reduced activity of plastidic phosphoglucomutase (PGM) constituted a preliminary approach for the understanding of starch metabolism and its implications for stomatal regulation in CAM plants.

KEY RESULTS

Starch deficiency reduced nocturnal net CO2 uptake but had negligible impact on nocturnal stomatal opening. In contrast, daytime stomatal closure was reduced in magnitude and duration in the starch-deficient rPGM RNAi lines, and their stomata were unable to remain closed in response to elevated concentrations of atmospheric CO2 administered during the day. Curtailed daytime stomatal closure was linked to higher soluble sugar contents in the epidermis and mesophyll.

CONCLUSIONS

Nocturnal stomatal opening is not reliant upon starch degradation, but starch biosynthesis is an important sink for carbohydrates, ensuring daytime stomatal closure in this CAM species.

Engineering stomata for enhanced carbon capture and water-use efficiency

Stomatal pores facilitate gaseous exchange between the inner air spaces of the leaf and the atmosphere. As gatekeepers that balance CO2 entry for photosynthesis against transpirational water loss, they are a focal point for efforts to improve crop performance, especially in the efficiency of water use, within the changing global environment. Until recently, engineering strategies had focused on stomatal conductance in the steady state. These strategies are limited by the physical constraints of CO2 and water exchange such that gains in water-use efficiency (WUE) commonly come at a cost in carbon assimilation. Attention to stomatal speed and responsiveness circumvents these constraints and offers alternatives to enhancing WUE that also promise increases in carbon assimilation in the field.

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.

Silicon as an attenuator of the toxic effects of aluminum in Schinus terebinthifolius plants

Aluminum (Al) is highly toxic to plants, since it causes stress and inhibits plant growth. Silicon (Si) is known to mitigate the stress caused by Al in several plant species. Thus, the current study aims to investigate the soothing effects of Si on morphophysiological and photosynthetic variables, and the attributes associated with oxidative stress in Schinus terebinthifolius plants exposed to Al. Treatments have followed a completely randomized design, with three repetitions based on the following Al/Si combinations (in mM): Treatment 1: 0 Al + 0 Si; Treatment 2: 0 Al + 2.5 Si; Treatment 3: 1.85 Al + 0 Si; Treatment 4: 1.85 Al + 2.5 Si; Treatment 5: 3.71 Al + 0 Si; Treatment 6: 3.71 Al + 2.5 Si. Each sampling unit consisted of a tray with 15 plants, totaling forty-five per treatment. Shoot and root morphological variables, photosynthetic variables, photosynthetic pigments, hydrogen peroxide concentration, lipid peroxidation (MDA), guaiacol peroxidase (POD) and superoxide dismutase (SOD) enzymes, and non-enzymatic antioxidant such as Ascorbic acid (AsA) and non-protein thiol (NPSH) concentration were assessed. Root growth inhibition followed by changes in root morphological variables have negatively affected root and shoot biomass production in plants only subjected to Al. However, adding 2.5 mM Si to the treatment has mitigated the toxic effects caused by 1.85 mM of aluminum on S. terebinthifolius plants.

Plastids: diving into their diversity, their functions, and their role in plant development

Plastids are a group of essential, heterogenous semi-autonomous organelles characteristic of plants that perform photosynthesis and a diversity of metabolic pathways that impact growth and development. Plastids are remarkably dynamic and can interconvert in response to specific developmental and environmental cues, functioning as a central metabolic hub in plant cells. By far the best studied plastid is the chloroplast, but in recent years the combination of modern techniques and genetic analyses has expanded our current understanding of plastid morphological and functional diversity in both model and non-model plants. These studies have provided evidence of an unexpected diversity of plastid subtypes with specific characteristics. In this review, we describe recent findings that provide insights into the characteristics of these specialized plastids and their functions. We concentrate on the emerging evidence that supports the model that signals derived from particular plastid types play pivotal roles in plant development, environmental, and defense responses. Furthermore, we provide examples of how new technologies are illuminating the functions of these specialized plastids and the overall complexity of their differentiation processes. Finally, we discuss future research directions such as the use of ectopic plastid differentiation as a valuable tool to characterize factors involved in plastid differentiation. Collectively, we highlight important advances in the field that can also impact future agricultural and biotechnological improvement in plants.

Changes in endogenous abscisic acid and stomata of the resurrection fern, Pleopeltis polypodioides, in response to de- and rehydration

PREMISE

While angiosperms respond uniformly to abscisic acid (ABA) by stomatal closure, the response of ferns to ABA is ambiguous. We evaluated the effect of endogenous ABA, hydrogen peroxide (H₂ O₂ ), nitric oxide (NO), and Ca²⁺ , low and high light intensities, and blue light (BL) on stomatal opening of Pleopeltis polypodioides.

METHODS

Endogenous ABA was quantified using gas chromatography-mass spectrometry; microscopy results and stomatal responses to light and chemical treatments were analyzed with Image J.

RESULTS

The ABA content increases during initial dehydration, peaks at 15 h and then decreases to one fourth of the ABA content of hydrated fronds. Following rehydration, ABA content increases within 24 h to the level of hydrated tissue. The stomatal aperture opens under BL and remains open even in the presence of ABA. Closure was strongly affected by BL, NO, and Ca²⁺ , regardless of ABA, H₂ O₂ effect was weak.

CONCLUSIONS

The decrease in the ABA content during extended dehydration and insensitivity of the stomata to ABA suggests that the drought tolerance mechanism of Pleopeltis polypodioides is independent of ABA.

Guard cell activity of PIF4 and HY5 control transpiration

Whole-plant transpiration, controlled by plant hydraulics and stomatal movement, is regulated by endogenous and environmental signals, with the light playing a dominant role. Stomatal pore size continuously adjusts to changes in light intensity and quality to ensure optimal CO₂ intake for photosynthesis on the one hand, together with minimal water loss on the other. The link between light and transpiration is well established, but the genetic knowledge of how guard cells perceive those signals to affect stomatal conductance is still somewhat limited. In the current study, we evaluated the role of two central light-responsive transcription factors; a bZIP-family transcription factor ELONGATED HYPOCOTYL5 (HY5) and the basic helix-loop-helix (BHLH) transcription factor PHYTOCHROME INTERACTING FACTOR4 (PIF4), in the regulation of steady-state transpiration. We show that overexpression of PIF4 exclusively in guard cells (GCPIF4) decreases transpiration, and can restrain the high transpiration of the pif4 mutant. Expression of HY5 specifically in guard cells (GCHY5) had the opposite effect of enhancing transpiration rates of WT- Arabidopsis and tobacco plants and of the hy5 mutant in Arabidopsis. In addition, we show that GCHY5 can reverse the low transpiration caused by guard cell overexpression of the sugar sensor HEXOKINASE1 (HXK1, GCHXK), an established low transpiring genotype. Finally, we suggest that the GCHY5 reversion of low transpiration by GCHXK requires the auto-activation of the endogenous HY5 in other tissues. These findings support the existence of an ongoing diurnal regulation of transpiration by the light-responsive transcription factors HY5 and PIF4 in the stomata, which ultimately determine the whole-plant water use efficiency.

PtrVCS2 Regulates Drought Resistance by Changing Vessel Morphology and Stomatal Closure in Populus trichocarpa

Drought has severe effects on plant growth, forest productivity, and survival throughout the world. Understanding the molecular regulation of drought resistance in forest trees can enable effective strategic engineering of novel drought-resistant genotypes of tree species. In this study, we identified a gene, PtrVCS2, encoding a zinc finger (ZF) protein of the ZF-homeodomain transcription factor in Populus trichocarpa (Black Cottonwood) Torr. & A. Gray. ex Hook. Overexpression of PtrVCS2 (OE-PtrVCS2) in P. trichocarpa resulted in reduced growth, a higher proportion of smaller stem vessels, and strong drought-resistance phenotypes. Stomatal movement experiments revealed that the OE-PtrVCS2 transgenics showed lower stomata apertures than wild-type plants under drought conditions. RNA-seq analysis of the OE-PtrVCS2 transgenics showed that PtrVCS2 regulates the expression of multiple genes involved in regulation of stomatal opening and closing, particularly the PtrSULTR3;1-1 gene, and several genes related to cell wall biosynthesis, such as PtrFLA11-12 and PtrPR3-3. Moreover, we found that the water use efficiency of the OE-PtrVCS2 transgenic plants was consistently higher than that of wild type plants when subjected to chronic drought stress. Taken together, our results suggest that PtrVCS2 plays a positive role in improving drought adaptability and resistance in P. trichocarpa.

Evaluation of morpho-physiological responses and genotoxicity in Eruca sativa (Mill.) grown in hydroponics from seeds exposed to X-rays

Due to its potential applications in cultivated plants, ionizing radiation (IR) and its effect on organisms is increasingly studied. Here we measured the effects of ionizing radiation on Eruca sativa by analyzing plants from irradiated seeds (1 and 10 Gy) grown in hydroponics. We measured several morpho-physiological traits and genotoxicity. Radiation stress induced a noticeable variability of the morpho-physiological traits highlighting decreased plant vigor. Shoot length and leaf number were significantly higher in 1 Gy-treated samples, whereas root length was significantly higher in 10 Gy treated plants. Stomata number significantly increased with IR dose, whereas both pigment and Rubisco content decreased under radiation stress. Phenol content significantly increased in 1 Gy treated samples, otherwise from total antioxidants, which were not different from control. Most results could find a feasible explanation in a hormesis-like pattern and in a decreased plant vigor under radiation stress. IR induced genotoxic damage, evaluated by ISSR markers, in 15 day old leaves; specifically, a severe decrease in the genome template stability was observed. However, a partial recovery occurred after 2 weeks, especially under the lowest dose (i.e., 1 Gy), suggesting that DNA damage detection and repair mechanisms are active. Pigment content and genotoxic damage may serve as proxies for evaluating plant responses to IR stress, since they show univocal dose-dependent trends. The use of more checkpoints for analyses and more doses over a wider range, as well as the focus on different metabolites, could help elucidate plant response in terms of morpho-physiological changes.

Determination of stomatic density, index, and area as exposition biomarkers of pollution in Deschampsia antárctica Desv. (Poaceae)

Until not so long ago, Antarctica was considered to be a polar region practically pristine. The Antarctic Peninsula has the highest concentration of scientific stations from different countries. Anthropogenic activity has caused alterations in the Antarctic ecosystems directly affecting terrestrial vegetation. This fact requires the finding of biomarkers in native plants to estimate the effects of human impact. Deschampsia antarctica Desv. (Poaceae) is the unique native grass described so far for Antarctica and was used for multiple investigations. In this study, plants were collected on Carlini scientific station, 25 de Mayo (King George) Island, Potter Peninsula, South Shetland Islands. Thus, the main objective planned consists of the evaluation of leaf stomata-related parameters as pollution biomarkers. The results of the stomatic index (SI), density (SD), and area (SA) were shown at sites with different levels of human impact (close and far away from the scientific station). It was found that the correlation between SD and SI, on the adaxial side of the leaves, resulted in a good biomarker for estimating the degree of anthropogenic impact in each studied area. Graphical abstract.

Stomata on the abaxial and adaxial leaf surfaces contribute differently to leaf gas exchange and photosynthesis in wheat

Although stomata are typically found in greater numbers on the abaxial surface, wheat flag leaves have greater densities on the adaxial surface. We determine the impact of this less common stomatal patterning on gaseous fluxes using a novel chamber that simultaneously measures both leaf surfaces. Using a combination of differential illuminations and CO₂ concentrations at each leaf surface, we found that mesophyll cells associated with the adaxial leaf surface have a higher photosynthetic capacity than those associated with the abaxial leaf surface, which is supported by an increased stomatal conductance (driven by differences in stomatal density). When vertical gas flux at the abaxial leaf surface was blocked, no compensation by adaxial stomata was observed, suggesting each surface operates independently. Similar stomatal kinetics suggested some co-ordination between the two surfaces, but factors other than light intensity played a role in these responses. Higher photosynthetic capacity on the adaxial surface facilitates greater carbon assimilation, along with higher adaxial stomatal conductance, which would also support greater evaporative leaf cooling to maintain optimal leaf temperatures for photosynthesis. Furthermore, abaxial gas exchange contributed c. 50% to leaf photosynthesis and therefore represents an important contributor to overall leaf gas exchange.

Combinational Variation Temperature and Soil Water Response of Stomata and Biomass Production in Maize, Millet, Sorghum and Rice

Environmental responses of stomatal conductance (gs) as basic information for a photosynthesis-transpiration-coupled model have been increasing under global warming. This study identified the impact of gs behavior under different soil water statuses and temperatures in rice, maize, millet, and sorghum. The experiments consisted of various soil moisture statuses from flooding to drying and combination of soil moisture status and temperature. There was a reduction in shoot biomass of maize and sorghum caused by decreasing of gs, photosynthesis (A), and transpiration (E) in early imposed waterlogging without dependent temperature, whereas millet and rice were dependent on temperature variation. The effect of gradual soil drying, gs, A, and E of maize, millet, and sorghum were caused by low temperature, except rice. The impact of the combination of various soil water statuses and temperatures on gs is important for the trade-off between A and E, and consequently shoot biomass. However, we discovered that an ability to sustain gs is essential for photo assimilation and maintaining leaf temperature through evapotranspiration for biomass production, a mechanism of crop avoidance in variable soil water status and temperature.

Cytosolic fumarase acts as a metabolic fail-safe for both high and low temperature acclimation of Arabidopsis thaliana

Plants acclimate their photosynthetic capacity (Pmax) in response to changing environmental conditions. In Arabidopsis thaliana, photosynthetic acclimation to cold requires the accumulation of the organic acid fumarate, catalysed by a cytosolically localized fumarase, FUM2. However, the role of this accumulation is currently unknown. Here, we use an integrated experimental and modelling approach to examine the role of FUM2 and fumarate across the physiological temperature range. We have studied three genotypes: Col-0; a fum2 mutant in a Col-0 background; and C24, an accession with reduced FUM2 expression. While low temperature causes an increase in Pmax in the Col-0 plants, this parameter decreases following exposure of plants to 30 °C for 7 d. Plants in which fumarate accumulation is partially (C24) or completely (fum2) abolished show a reduced acclimation of Pmax across the physiological temperature range (i.e. Pmax changes less in response to changing temperature). To understand the role of fumarate accumulation, we have adapted a reliability engineering technique, Failure Mode and Effect Analysis (FMEA), to formalize a rigorous approach for ranking metabolites according to the potential risk that they pose to the metabolic system. FMEA identifies fumarate as a low-risk metabolite, while its precursor, malate, is shown to be high risk and liable to cause system instability. We propose that the role of FUM2 is to provide a fail-safe in order to control malate concentration, maintaining system stability in a changing environment. We suggest that FMEA is a technique that is not only useful in understanding plant metabolism but can also be used to study reliability in other systems and synthetic pathways.

Wearable Crop Sensor Based on Nano-Graphene Oxide for Noninvasive Real-Time Monitoring of Plant Water

Real-time noninvasive monitoring of crop water information is an important basis for water-saving irrigation and precise management. Nano-electronic technology has the potential to enable smart plant sensors to communicate with electronic devices and promote the automatic and accurate distribution of water, fertilizer, and medicine to improve crop productivity. In this work, we present a new flexible graphene oxide (GO)-based noninvasive crop water sensor with high sensitivity, fast responsibility and good bio-interface compatibility. The humidity monitoring sensitivity of the sensor reached 7945 Ω/% RH, and the response time was 20.3 s. We first present the correlation monitoring of crop physiological characteristics by using flexible wearable sensors and photosynthesis systems, and have studied the response and synergistic effect of net photosynthetic rate and transpiration rate of maize plants under different light environments. Results show that in situ real-time sensing of plant transpiration was realized, and the internal water transportation within plants could be monitored dynamically. The synergistic effect of net photosynthetic rate and transpiration of maize plants can be jointly tested. This study provides a new technical method to carry out quantitative monitoring of crop water in the entire life cycle and build smart irrigation systems. Moreover, it holds great potential in studying individual plant biology and could provide basic support to carry out precise monitoring of crop physiological information.

The Impact of Treated Wastewater Irrigation on the Metabolism of Barley Grown in Arid and Semi-Arid Regions

The use of treated wastewater (TWW) for irrigation has gained global attention since it reduces pressure on groundwater (GW) and surface water. This study aimed to evaluate the effect of TWW on agronomic, photosynthetic, stomatal, and nutritional characteristics of barley plants. The experiment with barley was established on two bands: one band was irrigated with GW and the other with TWW. The evaluation was performed 25, 40, 60, 90, and 115 days after sowing (DAS). Results showed that irrigation with TWW increased (p < 0.01) grain yield by 54.3% and forage yield by 39.4% compared to GW irrigation. In addition, it increased plant height (PH) (p = 0.013), chlorophyll concentration index (CCI) (p = 0.006), and leaf area index (LAI) (p = 0.002). TWW also produced a positive effect (p < 0.05) in all the photosynthetic efficiency parameters evaluated. Barley plants irrigated with TWW had lower stomatal density (SD) and area (SA) (p < 0.001) than plants irrigated with GW. Plants irrigated with TWW had a higher P concentration (p < 0.05) in stems and roots and K concentration in leaves than plants irrigated with GW. We concluded that the use of TWW induced important biochemical, physiological, and agronomic changes in barley plants. Hence, the use of TWW may be a sustainable alternative for barley production in arid and semi-arid regions. This study was part of a government project, which aimed to develop a new metropolitan irrigation district with TWW. This study may contribute to the sustainability of water resources and agricultural practices in northern Mexico.

Metabolism-mediated mechanisms underpin the differential stomatal speediness regulation among ferns and angiosperms

Recent results suggest that metabolism-mediated stomatal closure mechanisms are important to regulate differentially the stomatal speediness between ferns and angiosperms. However, evidence directly linking mesophyll metabolism and the slower stomatal conductance (g_s ) in ferns is missing. Here, we investigated the effect of exogenous application of abscisic acid (ABA), sucrose and mannitol on stomatal kinetics and carried out a metabolic fingerprinting analysis of ferns and angiosperms leaves harvested throughout a diel course. Fern stomata did not respond to ABA in the time period analysed. No differences in the relative decrease in g_s was observed between ferns and the angiosperm following provision of sucrose or mannitol. However, ferns have slower g_s responses to these compounds than angiosperms. Metabolomics analysis highlights that ferns have a higher accumulation of secondary rather than primary metabolites throughout the diel course, with the opposite being observed in angiosperms. Our results indicate that metabolism-mediated stomatal closure mechanisms underpin the differential stomatal speediness regulation among ferns and angiosperms, in which the slower stomatal closure in ferns is associated with the lack of ABA-responsiveness, to a reduced capacity to respond to mesophyll-derived sucrose and to a higher carbon allocation toward secondary metabolism, which likely modulates both photosynthesis-g_s and growth-stress tolerance trade-offs.

The emerging role of GABA as a transport regulator and physiological signal

While the proposal that γ-aminobutyric acid (GABA) acts a signal in plants is decades old, a signaling mode of action for plant GABA has been unveiled only relatively recently. Here, we review the recent research that demonstrates how GABA regulates anion transport through aluminum-activated malate transporters (ALMTs) and speculation that GABA also targets other proteins. The ALMT family of anion channels modulates multiple physiological processes in plants, with many members still to be characterized, opening up the possibility that GABA has broad regulatory roles in plants. We focus on the role of GABA in regulating pollen tube growth and stomatal pore aperture, and we speculate on its role in long-distance signaling and how it might be involved in cross talk with hormonal signals. We show that in barley (Hordeum vulgare), guard cell opening is regulated by GABA, as it is in Arabidopsis (Arabidopsis thaliana), to regulate water use efficiency, which impacts drought tolerance. We also discuss the links between glutamate and GABA in generating signals in plants, particularly related to pollen tube growth, wounding, and long-distance electrical signaling, and explore potential interactions of GABA signals with hormones, such as abscisic acid, jasmonic acid, and ethylene. We conclude by postulating that GABA encodes a signal that links plant primary metabolism to physiological status to fine tune plant responses to the environment.

Genetic divergence and local adaptation of Liriodendron driven by heterogeneous environments

Ecological adaptive differentiation alters both the species diversity and intraspecific genetic diversity in forests, thus affecting the stability of forest ecosystems. Therefore, knowledge of the genetic underpinnings of the ecological adaptive differentiation of forest species is critical for effective species conservation. In this study, single-nucleotide polymorphisms (SNPs) from population transcriptomes were used to investigate the spatial distribution of genetic variation in Liriodendron to assess whether environmental variables can explain genetic divergence. We examined the contributions of environmental variables to population divergence and explored the genetic underpinnings of local adaptation using a landscape genomic approach. Niche models and statistical analyses showed significant niche divergence between L. chinense and L. tulipifera, suggesting that ecological adaptation may play a crucial role in driving interspecific divergence. We detected a new fine-scale genetic structure in L. chinense, and divergence of the six groups occurred during the late Pliocene to early Pleistocene. Redundancy analysis (RDA) revealed significant associations between genetic variation and multiple environmental variables. Environmental association analyses identified 67 environmental association loci (EALs; nonsynonymous SNPs) that underwent interspecific or intraspecific differentiation, 28 of which were associated with adaptive genes. These 28 candidate adaptive loci provide substantial evidence for local adaptation in Liriodendron. Our findings reveal ecological adaptive divergence pattern between Liriodendron species and provide novel insight into the role of heterogeneous environments in shaping genetic structure and driving local adaptation among populations, informing future L. chinense conservation efforts.

Optical topometry and machine learning to rapidly phenotype stomatal patterning traits for maize QTL mapping

Stomata are adjustable pores on leaf surfaces that regulate the tradeoff of CO2 uptake with water vapor loss, thus having critical roles in controlling photosynthetic carbon gain and plant water use. The lack of easy, rapid methods for phenotyping epidermal cell traits have limited discoveries about the genetic basis of stomatal patterning. A high-throughput epidermal cell phenotyping pipeline is presented here and used for quantitative trait loci (QTL) mapping in field-grown maize (Zea mays). The locations and sizes of stomatal complexes and pavement cells on images acquired by an optical topometer from mature leaves were automatically determined. Computer estimated stomatal complex density (SCD; R2 = 0.97) and stomatal complex area (SCA; R2 = 0.71) were strongly correlated with human measurements. Leaf gas exchange traits were genetically correlated with the dimensions and proportions of stomatal complexes (rg = 0.39-0.71) but did not correlate with SCD. Heritability of epidermal traits was moderate to high (h2 = 0.42-0.82) across two field seasons. Thirty-six QTL were consistently identified for a given trait in both years. Twenty-four clusters of overlapping QTL for multiple traits were identified, with univariate versus multivariate single marker analysis providing evidence consistent with pleiotropy in multiple cases. Putative orthologs of genes known to regulate stomatal patterning in Arabidopsis (Arabidopsis thaliana) were located within some, but not all, of these regions. This study demonstrates how discovery of the genetic basis for stomatal patterning can be accelerated in maize, a C4 model species where these processes are poorly understood.

Carotenoids and Apocarotenoids in Planta: Their Role in Plant Development, Contribution to the Flavour and Aroma of Fruits and Flowers, and Their Nutraceutical Benefits

Carotenoids and apocarotenoids are diverse classes of compounds found in nature and are important natural pigments, nutraceuticals and flavour/aroma molecules. Improving the quality of crops is important for providing micronutrients to remote communities where dietary variation is often limited. Carotenoids have also been shown to have a significant impact on a number of human diseases, improving the survival rates of some cancers and slowing the progression of neurological illnesses. Furthermore, carotenoid-derived compounds can impact the flavour and aroma of crops and vegetables and are the origin of important developmental, as well as plant resistance compounds required for defence. In this review, we discuss the current research being undertaken to increase carotenoid content in plants and research the benefits to human health and the role of carotenoid derived volatiles on flavour and aroma of fruits and vegetables.

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).

Guard cell endomembrane Ca2+-ATPases underpin a 'carbon memory' of photosynthetic assimilation that impacts on water-use efficiency

Stomata of most plants close to preserve water when the demand for CO₂ by photosynthesis is reduced. Stomatal responses are slow compared with photosynthesis, and this kinetic difference erodes assimilation and water-use efficiency under fluctuating light. Despite a deep knowledge of guard cells that regulate the stoma, efforts to enhance stomatal kinetics are limited by our understanding of its control by foliar CO₂. Guided by mechanistic modelling that incorporates foliar CO₂ diffusion and mesophyll photosynthesis, here we uncover a central role for endomembrane Ca²⁺ stores in guard cell responsiveness to fluctuating light and CO₂. Modelling predicted and experiments demonstrated a delay in Ca²⁺ cycling that was enhanced by endomembrane Ca²⁺-ATPase mutants, altering stomatal conductance and reducing assimilation and water-use efficiency. Our findings illustrate the power of modelling to bridge the gap from the guard cell to whole-plant photosynthesis, and they demonstrate an unforeseen latency, or 'carbon memory', of guard cells that affects stomatal dynamics, photosynthesis and water-use efficiency.

EkFLS overexpression promotes flavonoid accumulation and abiotic stress tolerance in plant

Flavonoids with great medicinal value play an important role in plant individual growth and stress resistance. Flavonol synthetase (FLS) is one of the key enzymes to synthesize flavonoids. However, the role of the FLS gene in flavonoid accumulation and tolerance to abiotic stresses, as well as its mechanism has not yet been investigated systematically in plants. The aim of this research is to evaluate the effect of FLS overexpression on the accumulation of active ingredients and stress resistance in Euphorbia kansui Liou. The results showed that when the EkFLS gene was overexpressed in Arabidopsis thaliana, the accumulation of flavonoids was improved. In addition, when the wild-type and EkFLS overexpressed Arabidopsis plants were treated with ABA and MeJA, compared with WT Arabidopsis, EkFLS overexpressed Arabidopsis promoted stomatal aperture to influence photosynthesis of the plants, which in turn can promote stress resistance. Meanwhile, under MeJA, NaCl, and PEG treatment, EkFLS overexpressed in Arabidopsis induced higher accumulation of flavonoids, which significantly enhanced peroxidase (POD) and superoxide dismutase (SOD) activities that can scavenge reactive oxygen species in cells to protect the plant. These results indicated that EkFLS overexpression is strongly correlated to the increase of flavonoid synthesis and therefore the tolerance to abiotic stresses in plants, providing a theoretical basis for further improving the quality of medicinal plants and their resistance to abiotic stresses simultaneously.

Stomatal Traits and Barley (Hordeum vulgare L.) Forage Yield in Drought Conditions of Northeastern Mexico

Infrared technology is a practical, fast, non-destructive method that helps in forecasting plant development and can be used to select physiological traits, instead of other methodologies that require more time and breeding efforts. According to the statistical analyses and the relationship between the direct and indirect effects of the variables, this technology could serve as the basis for implementing a genotype selection methodology. Awnless barley was assessed in a randomized block design with three replicates in two crop seasons at Mexico's northeastern region. Two samplings were carried out during crop development: at 75 and 90 days after sowing. The infrared and stomatal sensors were used to identify the direct and indirect effects of stomata's traits on dry forage yield. The data were analyzed in a subdivided plot design, using mean comparison tests, correlation coefficients and path analyses, finding significant differences (p < 0.05) among localities and among samplings. Dry forage yield was significant and positively correlated with plant height (r = 0.578) and canopy temperature (r = 0.724), and negatively correlated with the leaf upper side stomatal width (r = -0.409) and the leaf lower side stomatal width (r = -0.641), chlorophyll content and vegetation index. Temperature, chlorophyll, density and leaf lower side stomatal index had the strongest direct effects on yield. Therefore, the infrared technology appears as a way to select high yielding awnless forage barley, to obtain the correlation, the positive direct effect of temperature and the negative effect of chlorophyll. Due to their direct effects, low density and low leaf underside stomatal indexes can also help in the indirect selection of higher yielding forage barley genotypes.

The effect of increasing temperature on crop photosynthesis: from enzymes to ecosystems

As global land surface temperature continues to rise and heatwave events increase in frequency, duration, and/or intensity, our key food and fuel cropping systems will likely face increased heat-related stress. A large volume of literature exists on exploring measured and modelled impacts of rising temperature on crop photosynthesis, from enzymatic responses within the leaf up to larger ecosystem-scale responses that reflect seasonal and interannual crop responses to heat. This review discusses (i) how crop photosynthesis changes with temperature at the enzymatic scale within the leaf; (ii) how stomata and plant transport systems are affected by temperature; (iii) what features make a plant susceptible or tolerant to elevated temperature and heat stress; and (iv) how these temperature and heat effects compound at the ecosystem scale to affect crop yields. Throughout the review, we identify current advancements and future research trajectories that are needed to make our cropping systems more resilient to rising temperature and heat stress, which are both projected to occur due to current global fossil fuel emissions.

Acclimatisation of guard cell metabolism to long-term salinity

Stomatal movements are enabled by changes in guard cell turgor facilitated via transient accumulation of inorganic and organic ions imported from the apoplast or biosynthesized within guard cells. Under salinity, excess salt ions accumulate within plant tissues resulting in osmotic and ionic stress. To elucidate whether (a) Na⁺ and Cl^- concentrations increase in guard cells in response to long-term NaCl exposure and how (b) guard cell metabolism acclimates to the anticipated stress, we profiled the ions and primary metabolites of leaves, the apoplast and isolated guard cells at darkness and during light, that is, closed and fully opened stomata. In contrast to leaves, the primary metabolism of guard cell preparations remained predominantly unaffected by increased salt ion concentrations. Orchestrated reductions of stomatal aperture and guard cell osmolyte synthesis were found, but unlike in leaves, no increases of stress responsive metabolites or compatible solutes occurred. Diverging regulation of guard cell metabolism might be a prerequisite to facilitate the constant adjustment of turgor that affects aperture. Moreover, the photoperiod-dependent sucrose accumulation in the apoplast and guard cells changed to a permanently replete condition under NaCl, indicating that stress-related photosynthate accumulation in leaves contributes to the permanent closing response of stomata under stress.

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.

Effect of Stomatal Morphology on Leaf Photosynthetic Induction Under Fluctuating Light in Rice

Plants are often confronted with light fluctuations from seconds to minutes due to altering sun angles, mutual shading, and clouds under natural conditions, which causes a massive carbon loss and water waste. The effect of stomatal morphology on the response of leaf gas exchange to fluctuating light remains disputable. In this study, we investigated the differences in leaf stomatal morphology and photosynthetic induction across twelve rice genotypes after a stepwise increase in light intensity. A negative correlation was observed between stomatal size and density across rice genotypes. Smaller and denser stomata contributed to a faster stomatal response under fluctuating light. Plants with faster stomatal opening also showed faster photosynthetic induction and higher biomass accumulation but lower intrinsic water use efficiency ( _i WUE) under fluctuating light. Moreover, stomatal morphology seemed to have less effect on the initial and final stomatal conductance, and there was a minimal correlation between steady-state and non-steady-state stomatal conductance among different rice genotypes. These results highlight the important role of stomatal morphology in regulating photosynthetic efficiency and plant growth under fluctuating light conditions. To simultaneously enhance leaf _i WUE when improving the photosynthetic efficiency under fluctuating light, it may be necessary to take biochemical processes into account in the future.

A multiphase flux balance model reveals flexibility of central carbon metabolism in guard cells of C3 plants

Experimental research into guard cell metabolism has revealed the roles of the accumulation of various metabolites in guard cell function, but a comprehensive understanding of their metabolism over the diel cycle is still incomplete due to the limitations of current experimental methods. In this study we constructed a four-phase flux balance model of guard cell metabolism to investigate the changes in guard cell metabolism over the diel cycle, including the day and night and stomatal opening and closing. Our model predicted metabolic flexibility in guard cells of C₃ plants, showing that multiple metabolic processes can contribute to the synthesis and metabolism of malate and sucrose as osmolytes during stomatal opening and closing. Our model showed the possibility of guard cells adapting to varying light availability and sucrose uptake from the apoplast during the day by operating in a mixotrophic mode with a switch between sucrose synthesis via the Calvin-Benson cycle and sucrose degradation via the oxidative pentose phosphate pathway. During stomatal opening, our model predicted an alternative flux mode of the Calvin-Benson cycle with all dephosphorylating steps diverted to diphosphate-fructose-6-phosphate 1-phosphotransferase to produce inorganic pyrophosphate, which is used to pump protons across the tonoplast for the accumulation of osmolytes. An analysis of the energetics of the use of different osmolytes in guard cells showed that malate and Cl^- are similarly efficient as the counterion of K⁺ during stomatal opening.

Natural Diversity in Stomatal Features of Cultivated and Wild Oryza Species

BACKGROUND

Stomata in rice control a number of physiological processes by regulating gas and water exchange between the atmosphere and plant tissues. The impact of the structural diversity of these micropores on its conductance level is an important area to explore before introducing stomatal traits into any breeding program in order to increase photosynthesis and crop yield. Therefore, an intensive measurement of structural components of stomatal complex (SC) of twenty three Oryza species spanning the primary, secondary and tertiary gene pools of rice has been conducted.

RESULTS

Extensive diversity was found in stomatal number and size in different Oryza species and Oryza complexes. Interestingly, the dynamics of stomatal traits in Oryza family varies differently within different Oryza genetic complexes. Example, the Sativa complex exhibits the greatest diversity in stomatal number, while the Officinalis complex is more diverse for its stomatal size. Combining the structural information with the Oryza phylogeny revealed that speciation has tended towards increasing stomatal density rather than stomatal size in rice family. Thus, the most recent species (i.e. the domesticated rice) eventually has developed smaller yet numerous stomata. Along with this, speciation has also resulted in a steady increase in stomatal conductance (anatomical, gmax) in different Oryza species. These two results unambiguously prove that increasing stomatal number (which results in stomatal size reduction) has increased the stomatal conductance in rice. Correlations of structural traits with the anatomical conductance, leaf carbon isotope discrimination (∆13C) and major leaf morphological and anatomical traits provide strong supports to untangle the ever mysterious dependencies of these traits in rice. The result displayed an expected negative correlation in the number and size of stomata; and positive correlations among the stomatal length, width and area with guard cell length, width on both abaxial and adaxial leaf surfaces. In addition, gmax is found to be positively correlated with stomatal number and guard cell length. The ∆13C values of rice species showed a positive correlation with stomatal number, which suggest an increased water loss with increased stomatal number. Interestingly, in contrast, the ∆13C consistently shows a negative relationship with stomatal and guard cell size, which suggests that the water loss is less when the stomata are larger. Therefore, we hypothesize that increasing stomatal size, instead of numbers, is a better approach for breeding programs in order to minimize the water loss through stomata in rice.

CONCLUSION

Current paper generates useful data on stomatal profile of wild rice that is hitherto unknown for the rice science community. It has been proved here that the speciation has resulted in an increased stomatal number accompanied by size reduction during Oryza's evolutionary course; this has resulted in an increased gmax but reduced water use efficiency. Although may not be the sole driver of water use efficiency in rice, our data suggests that stomata are a potential target for modifying the currently low water use efficiency in domesticated rice. It is proposed that Oryza barthii can be used in traditional breeding programs in enhancing the stomatal size of elite rice cultivars.

Developing and validating protocols for mechanical isolation of guard-cell enriched epidermal peels for omics studies

Stomata, which are microscopic valves on the leaf surface formed by two guard cells (GC), play a critical role in the regulation of leaf water and gas exchange and, hence, determine plant adaptive potential. However, little data is available on GC biochemistry, protein abundance and gene expression, mainly due to technical difficulties and challenges in isolating sufficient amounts of high-quality pure GC. In the present study we applied some modifications to the mechanical isolation of guard-cell to generalise this method for diverse growth conditions as well as plant species. Epidermal peel fragments enriched in guard cells were mechanically isolated from quinoa, spinach and sugar beet leaves grown at two conditions (normal and salt stress). Multiple analysis was performed to confirm the suitability and superiority of the modified technique to the original method. At the first step, the viability and purity of GC-enriched epidermal fragments were assessed under the microscope. Then, the RNA integrity, gene expression, and 1D SDS-PAGE tests were performed to validate the suitability of this technique for omics studies. The data revealed a wide range of proteins as well as a high integrity of RNA extracted from guard cell samples. The expression level of several GC-specific genes and mesophyll-dominant genes were investigated using a comparative analysis of transcriptome datasets of GC and whole-leaf samples. We found that Rubisco and photosynthesis-related proteins such as chlorophyll a/b binding protein were substantially higher in the whole leaf compared with the GCs. More importantly, GC-specific genes such as OST1, SLAC1, MYB60, FAMA and HT1 were highly expressed in the GCs, confirming that our guard cell preparation was highly enriched in GC gene transcripts. Real-time quantitative reverse transcription PCR further confirmed the efficacy of the GC isolation technique for exploring responses of GC to diverse types of stress at the molecular level.

Metabolomic, proteomic and physiological insights into the potential mode of action of thymol, a phytotoxic natural monoterpenoid phenol

Thymol is a natural phenolic monoterpene widely produced by different species belonging to the Labiateae family. Although the thymol phytotoxicity is well known, the knowledge of its potential toxic mechanism is still limited. In this regard, the model species Arabidopsis thaliana was treated for 16 days by sub-irrigation with 300 μM of thymol. The results confirmed the high phytotoxic potential of this phenolic compound, which caused a reduction in plant growth and development. Thymol induced a water status alteration accompanied by an increase in ABA content and stomatal closure. Furthermore, leaves appeared necrotic in the margins and their temperature rinsed. The increase in H₂O₂ content suggested an oxidative stress experienced by treated plants. Both metabolomic and proteomic analysis confirmed this hypothesis showing a strong increase in osmoprotectants content, such as galactinol and proline, and a significant up-accumulation of proteins involved in ROS detoxification. Furthermore, the down-accumulation of proteins and pigments involved in the photosynthetic machinery, the increase in light sensitivity and the lower PSII efficiency well indicated a reduction in photosynthetic activity. Overall, we can postulate that thymol-induced phytotoxicity could be related to a combined osmotic and oxidative stress that resulted in reduced plant development.

Efficient photosynthesis in dynamic light environments: a chloroplast's perspective

In nature, light availability for photosynthesis can undergo massive changes on a very short timescale. Photosynthesis in such dynamic light environments requires that plants can respond swiftly. Expanding our knowledge of the rapid responses that underlie dynamic photosynthesis is an important endeavor: it provides insights into nature's design of a highly dynamic energy conversion system and hereby can open up new strategies for improving photosynthesis in the field. The present review focuses on three processes that have previously been identified as promising engineering targets for enhancing crop yield by accelerating dynamic photosynthesis, all three of them involving or being linked to processes in the chloroplast, i.e. relaxation of non-photochemical quenching, Calvin–Benson–Bassham cycle enzyme activation/deactivation and dynamics of stomatal conductance. We dissect these three processes on the functional and molecular level to reveal gaps in our understanding and critically discuss current strategies to improve photosynthesis in the field.

Arabidopsis thaliana trehalose-6-phosphate phosphatase gene TPPI enhances drought tolerance by regulating stomatal apertures

Transpiration occurs through stomata. The alteration of stomatal apertures in response to drought stress is an important process associated with water use efficiency (WUE). Trehalose-6-phosphate phosphatase (TPP) family genes have been reported to participate in adjustment of stomatal aperture. However, there have been no reports of the trehalose metabolism pathway genes improving WUE, and the upstream signalling pathway modulating these genes is not clear. Here, we demonstrate that a member of the TPP gene family, AtTPPI, confers drought resistance and improves WUE by decreasing stomatal apertures and improving root architecture. The reduced expression of AtTPPI caused a drought-sensitive phenotype, while its overexpression significantly increased drought tolerance. Abscisic acid (ABA)-induced stomatal closure experiments confirmed that AtTPPI mutation increased the stomatal aperture compared with that of wild-type plants; in contrast, overexpression plants had smaller stomatal apertures than those of wild-type plants. Moreover, AtTPPI mutation also caused stunted primary root length and compromised auxin transport, while overexpression plants had longer primary root lengths. Yeast one-hybrid assays showed that ABA-responsive element-binding factor1 (ABF1), ABF2, and ABF4 directly regulated AtTPPI expression. In summary, the way in which AtTPPI responds to drought stress suggests that AtTPPI-mediated stomatal regulation is an important mechanism to cope with drought stress and improve WUE.

Guard Cell Metabolism and Stomatal Function

The control of gaseous exchange between the leaf and external atmosphere is governed by stomatal conductance (g_s); therefore, stomata play a critical role in photosynthesis and transpiration and overall plant productivity. Stomatal conductance is determined by both anatomical features and behavioral characteristics. Here we review some of the osmoregulatory pathways in guard cell metabolism, genes and signals that determine stomatal function and patterning, and the recent work that explores coordination between g_s and carbon assimilation (A) and the influence of spatial distribution of functional stomata on underlying mesophyll anatomy. We also evaluate the current literature on mesophyll-driven signals that may coordinate stomatal behavior with mesophyll carbon assimilation and explore stomatal kinetics as a possible target to improve A and water use efficiency. By understanding these processes, we can start to provide insight into manipulation of these regulatory pathways to improve stomatal behavior and identify novel unexploited targets for altering stomatal behavior and improving crop plant productivity.

Are Aquaporins Expressed in Stomatal Complexes Promising Targets to Enhance Stomatal Dynamics?

The opening and closure of stomata depend on the turgor pressure adjustment by the influx or efflux of ions and water in guard cells. In this process, aquaporins may play important roles by facilitating the transport of water and other small molecules. In this perspective, we consider the potential roles of aquaporins in the membrane diffusion of different molecules (H₂O, CO₂, and H₂O₂), processes dependent on abscisic acid and CO₂ signaling in guard cells. While the limited data already available emphasizes the roles of aquaporins in stomatal movement, we propose additional approaches to elucidate the specific roles of single or several aquaporin isoforms in the stomata and evaluate the perspectives aquaporins might offer to improve stomatal dynamics.

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.

The Phloem as a Mediator of Plant Growth Plasticity

Developmental plasticity, defined as the capacity to respond to changing environmental conditions, is an inherent feature of plant growth. Recent studies have brought the phloem tissue, the quintessential conduit for energy metabolites and inter-organ communication, into focus as an instructive developmental system. Those studies have clarified long-standing questions about essential aspects of phloem development and function, such as the pressure flow hypothesis, mechanisms of phloem unloading, and source-sink relationships. Interestingly, plants with impaired phloem development show characteristic changes in body architecture, thereby highlighting the capacity of the phloem to integrate environmental cues and to fine-tune plant development. Therefore, understanding the plasticity of phloem development provides scenarios of how environmental stimuli are translated into differential plant growth. In this Review, we summarize novel insights into how phloem identity is established and how phloem cells fulfil their core function as transport units. Moreover, we discuss possible interfaces between phloem physiology and development as sites for mediating the plastic growth mode of plants.

Is nitric oxide a critical key factor in ABA-induced stomatal closure?

The role of nitric oxide (NO) in abscisic acid (ABA)-induced stomatal closure is a matter of debate. We conducted experiments in Vicia faba leaves using NO gas and sodium nitroprusside (SNP), a NO-donor compound, and compared their effects to those of ABA. In epidermal strips, stomatal closure was induced by ABA but not by NO, casting doubt on the role of NO in ABA-mediated stomatal closure. Leaf discs and intact leaves showed a dual dose response to NO: stomatal aperture widened at low dosage and narrowed at high dosage. Overcoming stomatal resistance by means of high CO2 concentration ([CO2]) restored photosynthesis in ABA-treated leaf discs but not in those exposed to NO. NO inhibited photosynthesis immediately, causing an instantaneous increase in intercellular [CO2] (Ci), followed by stomatal closure. However, lowering Ci by using low ambient [CO2] showed that it was not the main factor in NO-induced stomatal closure. In intact leaves, the rate of stomatal closure in response to NO was about one order of magnitude less than after ABA application. Because of the different kinetics of photosynthesis and stomatal closure that were observed, we conclude that NO is not likely to be the key factor in ABA-induced rapid stomatal closure, but that it fine-tunes stomatal aperture via different pathways.

The xanthophyll cycle as an early pathogenic target to deregulate guard cells during Sclerotinia sclerotiorum infection

Stomata not only control the important balance between gaseous fluxes and water loss, but also act as a route of invading pathogen entry into the plant. Here, the stomatal opening was observed to be induced by a necrotrophic pathogen Sclerotinia sclerotiorum at the early stages of infection. In contrast to uninfected control, the stomatal pores were still opened in S. sclerotiorum-infected regions after dark adaption. Mutation of violaxanthin de-epoxidase, a key enzyme in the xanthophyll cycle, could partially restore the S. sclerotiorum-induced stomatal opening. Further studies showed that S. sclerotiorum invasion led to a decrease in electron transport rate, but a significant increase in non-photochemical quenching (NPQ). The decay kinetics of NPQ revealed that zeaxanthin epoxidase (ZEP, also known as ABA1) was continuous deactivation in S. sclerotiorum-infected region. HPLC-MS/MS analysis showed a slight increase of jasmonate acid (JA), but a great decrease of abscisic acid (ABA) content in S. sclerotiorum-inoculated tissue. Exogenous application of ABA but not JA could rescue the abnormal stomatal opening. Together, these results suggested that the S. sclerotiorum-induced decrease of ABA biosynthesis reduced stomatal closing via dysfunction of the xanthophyll cycle during early pathogenesis.

Guard-Cell Hexokinase Increases Water-Use Efficiency Under Normal and Drought Conditions

Water is a limiting resource for many land plants. Most of the water taken up by plants is lost to the atmosphere through the stomata, which are adjustable pores on the leaf surface that allow for gas exchange between the plant and the atmosphere. Modulating stomatal activity might be an effective way to reduce plants' water consumption and enhance their productivity under normal, as well as water-limiting conditions. Our recent discovery of stomatal regulation by sugars that is mediated by guard-cell hexokinase (HXK), a sugar-sensing enzyme, has raised the possibility that HXK might be used to increase plant water-use efficiency (WUE; i.e., carbon gain per unit of water). We show here that transgenic tomato and Arabidopsis plants with increased expression of HXK in their guard cells (GCHXK plants) exhibit reduced transpiration and higher WUE without any negative effects on growth under normal conditions, as well as drought avoidance and improved photosynthesis and growth under limited-water conditions. Our results demonstrate that exclusive expression of HXK in guard cells is an effective tool for improving WUE, and plant performance under drought.

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.

The sucrose-to-malate ratio correlates with the faster CO2 and light stomatal responses of angiosperms compared to ferns

Stomatal responses to environmental signals differ substantially between ferns and angiosperms. However, the mechanisms that lead to such different responses remain unclear. Here we investigated the extent to which leaf metabolism contributes to coordinate the differential stomatal behaviour among ferns and angiosperms. Stomata from all species were responsive to light and CO₂ transitions. However, fern stomatal responses were slower and minor in both absolute and relative terms. Angiosperms have higher stomatal density, but this is not correlated with speed of stomatal closure. The metabolic responses throughout the diel course and under different CO₂ conditions differ substantially among ferns and angiosperms. Higher sucrose content and an increased sucrose-to-malate ratio during high CO₂ -induced stomatal closure was observed in angiosperms compared to ferns. Furthermore, the speed of stomatal closure was positively and negatively correlated with sugars and organic acids, respectively, suggesting that the balance between sugars and organic acids aids in explaining the faster stomatal responses of angiosperms. Our results suggest that mesophyll-derived metabolic signals, especially those associated with sucrose and malate, may also be important to modulate the differential stomatal behaviour between ferns and angiosperms, providing important new information that helps in understanding the metabolism-mediated mechanisms regulating stomatal movements across land plant evolution.

Predicting light-induced stomatal movements based on the redox state of plastoquinone: theory and validation

Prediction of stomatal conductance is a key element to relate and scale up leaf-level gas exchange processes to canopy, ecosystem and land surface models. The empirical models that are typically employed for this purpose are simple and elegant formulations which relate stomatal conductance on a leaf area basis to the net rate of CO₂ assimilation, humidity and CO₂ concentration. Although light intensity is not directly modelled as a stomatal opening cue, it is well-known that stomata respond strongly to light. One response mode depends specifically on the blue-light part of the light spectrum, whereas the quantitative or 'red' light response is less spectrally defined and relies more on the quantity of incident light. Here, we present a modification of an empirical stomatal conductance model which explicitly accounts for the stomatal red-light response, based on a mesophyll-derived signal putatively initiated by the chloroplastic plastoquinone redox state. The modified model showed similar prediction accuracy compared to models using a relationship between stomatal conductance and net assimilation rate. However, fitted parameter values with the modified model varied much less across different measurement conditions, lessening the need for frequent re-parameterization to different conditions required of the current model. We also present a simple and easy to parameterize extension to the widely used Farquhar-Von Caemmerer-Berry photosynthesis model to facilitate coupling with the modified stomatal conductance model, which should enable use of the new stomatal conductance model to simulate ecosystem water vapour exchange in terrestrial biosphere models.