Guard cells: a dynamic signaling model

A critical review on bioaerosols-dispersal of crop pathogenic microorganisms and their impact on crop yield

Bioaerosols are potential sources of pathogenic microorganisms that can cause devastating outbreaks of global crop diseases. Various microorganisms, insects and viroids are known to cause severe crop diseases impeding global agro-economy. Such losses threaten global food security, as it is estimated that almost 821 million people are underfed due to global crisis in food production. It is estimated that global population would reach 10 billion by 2050. Hence, it is imperative to substantially increase global food production to about 60% more than the existing levels. To meet the increasing demand, it is essential to control crop diseases and increase yield. Better understanding of the dispersive nature of bioaerosols, seasonal variations, regional diversity and load would enable in formulating improved strategies to control disease severity, onset and spread. Further, insights on regional and global bioaerosol composition and dissemination would help in predicting and preventing endemic and epidemic outbreaks of crop diseases. Advanced knowledge of the factors influencing disease onset and progress, mechanism of pathogen attachment and penetration, dispersal of pathogens, life cycle and the mode of infection, aid the development and implementation of species-specific and region-specific preventive strategies to control crop diseases. Intriguingly, development of R gene-mediated resistant varieties has shown promising results in controlling crop diseases. Forthcoming studies on the development of an appropriately stacked R gene with a wide range of resistance to crop diseases would enable proper management and yield. The article reviews various aspects of pathogenic bioaerosols, pathogen invasion and infestation, crop diseases and yield.

The osmotic stress of Vallisneria natans (Lour.) Hara leaves originating from the disruption of calcium and potassium homeostasis caused by MC-LR

Aquatic plants are potentially impacted by microcystins (MCs) in lakes experiencing harmful algal blooms. However, how these plants respond, and possibly adapt to osmotic stress caused by MCs is unclear. Vallisneria natans is a pioneer taxon with a global distribution in eutrophic lakes. In this study, we investigated the effect of MC-LR on morphological structure, water retention, osmoregulatory ability, and homeostasis of calcium (Ca2+) and potassium (K+) ions in V. natans leaves. Results showed that the morphological changes caused by MC-LR included increased volumes of epidermal and mesophyll cells, changes in their lignification level, and the degradation of chloroplast structure and dissolution of starch granules. The increased moisture content and water potential with MC-LR concentration were consistent with the occurrence of osmotic stress, and the decreased osmotic potential implied the activation of osmoregulation. Soluble sugar and free amino acid concentrations increased at MC-LR treatments ≥10 μg/L, while inorganic ion K+ content increased in all MC-LR treatments. Although instantaneous K+inflow and Ca2+outflow occurred at 10 μg/L and 100 μg/L MC-LR, respectively, ≥1 μg/L MC-LR resulted in continuous K+ inflow and Ca2+ outflow within 24 h. Moreover, plasma membrane hyperpolarization was caused by MC-LR, especially at 1 and 10 μg/L. We suggest that Ca2+ efflux served as a signal molecule from the cytoplasmic matrix via Ca2+-ATPase, and the uptake of K+ was activated passively through transporters in response to MC-LR-induced plasma membrane hyperpolarization. Therefore, the uptake of K+ was a part of the response but not an adaptation to MC-LR stress, and is considered the cause for the uptake of water in leaves. Ca2+ and K+ homeostasis of V. natans leaves was disrupted by MC-LR concentrations as low as 1 μg/L, suggesting that aquatic plants in most eutrophic lakes may experience negative impacts such as Ca2+ loss, impacts to cell water balance, and alteration in cellular morphology, due to osmotic stress caused by MC-LR.

Calcium-dependent protein kinase GhCDPK4 plays a role in drought and abscisic acid stress responses

Drought is an important factor limiting the yield and quality of cotton. In the present study, the gene encoding the cotton calcium-dependent protein kinase GhCDPK4 was identified and characterized in the transcriptome of cotton under PEG-induced drought stress. In RT-qPCR experiments, GhCDPK4 expression was found to be up-regulated under drought and abscisic acid (ABA) stress. Under drought conditions, heterologous overexpression of GhCDPK4 in tobacco showed a better phenotypic status, higher antioxidant enzyme activity, and lower relative electrolyte leakage (REL) and malondialdehyde (MDA) content. Meanwhile, ghcdpk4-silenced cotton plants, which were extremely sensitive to drought, exhibited higher levels of O^2-，H₂O₂, and MDA contents compared to the control. Meanwhile, silenced lines showed impaired stomatal closure under drought stress, resulting in increased water loss from transpiration in silenced lines. GhCDPK4 expression was induced by ABA, and there are five ABA-responsive elements in its promoter. and C2-DOMAIN ABA-RELATED 4(CAR4, Gh_D09G1653) were found to interact and be co-expressed in the GhCDPK4 interaction network. Therefore, GhCDPK4 may reduce the extent of water loss and oxidative damage in cotton under drought by positively regulating ABA-controlled stomatal closure and reactive oxygen species (ROS) scavenging systems. This study demonstrates the great potential of GhCDPK4 in improving drought resistance in crops.

Cell-type-specific metabolism in plants

Every plant organ contains tens of different cell types, each with a specialized function. These functions are intrinsically associated with specific metabolic flux distributions that permit the synthesis of the ATP, reducing equivalents and biosynthetic precursors demanded by the cell. Investigating such cell-type-specific metabolism is complicated by the mosaic of different cells within each tissue combined with the relative scarcity of certain types. However, techniques for the isolation of specific cells, their analysis in situ by microscopy, or modeling of their function in silico have permitted insight into cell-type-specific metabolism. In this review we present some of the methods used in the analysis of cell-type-specific metabolism before describing what we know about metabolism in several cell types that have been studied in depth; (i) leaf source and sink cells; (ii) glandular trichomes that are capable of rapid synthesis of specialized metabolites; (iii) guard cells that must accumulate large quantities of the osmolytes needed for stomatal opening; (iv) cells of seeds involved in storage of reserves; and (v) the mesophyll and bundle sheath cells of C4 plants that participate in a CO₂ concentrating cycle. Metabolism is discussed in terms of its principal features, connection to cell function and what factors affect the flux distribution. Demand for precursors and energy, availability of substrates and suppression of deleterious processes are identified as key factors in shaping cell-type-specific metabolism.

The disruption of calcium and hydrogen ion homeostasis of submerged macrophyte Vallisneria natans (Lour.) Hara caused by microcystin-LR

Aquatic plants play an important role in maintaining lake water status and ecosystem stability, but the effect of the cyanotoxin microcystin (MC) on ion homeostasis in aquatic plants and the resulting adverse consequences remains unclear. This study used non-invasive micro-test technology to detect the effect of MC-LR on homeostasis of calcium (Ca²⁺) and hydrogen ions (H⁺) in Vallisneria natans (Lour.) Hara, and examined the relationship between ion homeostasis and physiological indicators. Results showed that 1) MC-LR was enriched in V. natans tissues, with greater absorption in roots than in leaves, and 2) MC-LR induced a sustained and dose-dependent Ca²⁺ efflux from leaves and recoverable Ca²⁺ efflux from roots. Although H⁺-ATPase of leaves and roots was activated by MC-LR, the effluent of H⁺ from roots and influent of H⁺ into leaves was enhanced. By affecting the homeostasis of Ca²⁺ and H⁺, MC-LR directly or indirectly affected accumulation of nutrients essential for maintaining normal growth: accumulation of nitrogen, magnesium, phosphorus, calcium, iron, and zinc decreased in leaves; calcium, magnesium, and zinc decreased in roots; and potassium showed an increase in both leaves and roots. Microscopy revealed MC-LR results in leaf swelling and reduced accumulation of protein and starch, presumably due to changes in nutrient processes. In addition, efflux of Ca²⁺ and reduced accumulation of transition metals resulted in decreased ROS levels in leaves and roots. The disruption of ionic homeostasis in aquatic plants can be caused by as small a concentration as 1 μg/L MC-LR, indicating potential ecological impacts caused by microcystin need greater attention.

Role of guard cell- or mesophyll cell-localized phytochromes in stomatal responses to blue, red, and far-red light

Guard cell- or mesophyll cell-localized phytochromes do not have a predominant direct light sensory role in red- or blue-light-mediated stomatal opening or far-red-light-mediated stomatal closure of Arabidopsis. The role of phytochromes in blue- and red-light-mediated stomatal opening, and far-red-light- mediated decrease in opening, is still under debate. It is not clear whether reduced stomatal opening in a phytochrome B (phyB) mutant line, is due to phytochrome acting as a direct photosensor or an indirect growth effect. The exact tissue localization of the phytochrome photoreceptor important for stomatal opening is also not known. We studied differences in stomatal opening in an Arabidopsis phyB mutant, and lines showing mesophyll cell-specific or guard cell-specific inactivation of phytochromes. Stomatal conductance (g_s) of intact leaves was measured under red, blue, and blue + far-red light. Lines exhibiting guard cell-specific inactivation of phytochrome did not show a change in g_s under blue or red light compared to Col-0. phyB consistently exhibited a reduction in g_s under both blue and red light. Addition of far-red light did not have a significant impact on the blue- or red-light-mediated stomatal response. Treatment of leaves with DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea), a photosynthetic electron transport (PET) inhibitor, eliminated the response to red light in all lines, indicating that stomatal opening under red light is controlled by PET, and not directly by phytochrome. Similar to previous studies, leaves of the phyB mutant line had fewer stomata. Overall, phytochrome does not appear have a predominant direct sensory role in stomatal opening under red or blue light. However, phytochromes likely have an indirect effect on the degree of stomatal opening under light through effects on leaf growth and stomatal development.

SORTING NEXIN2 proteins mediate stomatal movement and the response to drought stress by modulating trafficking and protein levels of the ABA exporter ABCG25

The phytohormone abscisic acid (ABA) regulates ion channel activity and stomatal movement in response to drought stress. Cellular ABA levels change depending on cellular and environmental conditions via modulation of its biosynthesis, catabolism and transport. Although factors involved in ABA biosynthesis and degradation have been studied extensively, how ABA transporters are modulated to fine-tune ABA levels, especially under drought stress, remains elusive. Here, we show that Arabidopsis thaliana SORTING NEXIN 2 (SNX2) proteins play a critical role in endosomal trafficking of the ABA exporter ATP BINDING CASETTE G25 (ABCG25) via direct interaction at endosomes, leading to its degradation in the vacuole. In agreement, snx2a and snx2b mutant plants showed enhanced recycling of GFP-ABCG25 from early endosomes to the plasma membrane and higher accumulation of GFP-ABCG25. Phenotypically, snx2a and snx2b plants were highly sensitive to exogenous ABA and displayed enhanced ABA-mediated inhibition of inward K⁺ currents and ABA-mediated activation of slow anion currents in guard cells, resulting in an increased tolerance to drought stress. Based on these results, we propose that SNX2 proteins play a crucial role in stomatal movement and tolerance to drought stress by modulating the endosomal trafficking of ABCG25 and thus cellular ABA levels.

Blocked at the Stomatal Gate, a Key Step of Wheat Stb16q-Mediated Resistance to Zymoseptoria tritici

Septoria tritici blotch (STB), caused by the fungus Zymoseptoria tritici, is among the most threatening wheat diseases in Europe. Genetic resistance remains one of the main environmentally sustainable strategies to efficiently control STB. However, the molecular and physiological mechanisms underlying resistance are still unknown, limiting the implementation of knowledge-driven management strategies. Among the 22 known major resistance genes (Stb), the recently cloned Stb16q gene encodes a cysteine-rich receptor-like kinase conferring a full broad-spectrum resistance against Z. tritici. Here, we showed that an avirulent Z. tritici inoculated on Stb16q quasi near isogenic lines (NILs) either by infiltration into leaf tissues or by brush inoculation of wounded tissues partially bypasses Stb16q-mediated resistance. To understand this bypass, we monitored the infection of GFP-labeled avirulent and virulent isolates on Stb16q NILs, from germination to pycnidia formation. This quantitative cytological analysis revealed that 95% of the penetration attempts were unsuccessful in the Stb16q incompatible interaction, while almost all succeeded in compatible interactions. Infectious hyphae resulting from the few successful penetration events in the Stb16q incompatible interaction were arrested in the sub-stomatal cavity of the primary-infected stomata. These results indicate that Stb16q-mediated resistance mainly blocks the avirulent isolate during its stomatal penetration into wheat tissue. Analyses of stomatal aperture of the Stb16q NILs during infection revealed that Stb16q triggers a temporary stomatal closure in response to an avirulent isolate. Finally, we showed that infiltrating avirulent isolates into leaves of the Stb6 and Stb9 NILs also partially bypasses resistances, suggesting that arrest during stomatal penetration might be a common major mechanism for Stb-mediated resistances.

Phytohormonal Regulation Through Protein S-Nitrosylation Under Stress

The liaison between Nitric oxide (NO) and phytohormones regulates a myriad of physiological processes at the cellular level. The interaction between NO and phytohormones is mainly influenced by NO-mediated post-translational modifications (PTMs) under basal as well as induced conditions. Protein S-nitrosylation is the most prominent and widely studied PTM among others. It is the selective but reversible redox-based covalent addition of a NO moiety to the sulfhydryl group of cysteine (Cys) molecule(s) on a target protein to form S-nitrosothiols. This process may involve either direct S-nitrosylation or indirect S-nitrosylation followed by transfer of NO group from one thiol to another (transnitrosylation). During S-nitrosylation, NO can directly target Cys residue (s) of key genes involved in hormone signaling thereby regulating their function. The phytohormones regulated by NO in this manner includes abscisic acid, auxin, gibberellic acid, cytokinin, ethylene, salicylic acid, jasmonic acid, brassinosteroid, and strigolactone during various metabolic and physiological conditions and environmental stress responses. S-nitrosylation of key proteins involved in the phytohormonal network occurs during their synthesis, degradation, or signaling roles depending upon the response required to maintain cellular homeostasis. This review presents the interaction between NO and phytohormones and the role of the canonical NO-mediated post-translational modification particularly, S-nitrosylation of key proteins involved in the phytohormonal networks under biotic and abiotic stresses.

Stomata facilitate foliar sorption of silver nanoparticles by Arabidopsis thaliana

Application of nanopesticides may substantially increase surface attachment and internalization of engineered nanoparticles (ENPs) in food crops. This study investigated the role of stomata in the internalization of silver nanoparticles (Ag NPs) using abscisic acid (ABA)-responsive ecotypes (Ler and Col-7) and ABA-insensitive mutants (ost1-2 and scord7) of Arabidopsis thaliana in batch sorption experiments, in combination with microscopic visualization. Compared with those of the ABA-free control, stomatal apertures were significantly smaller for the Ler and Col-7 ecotypes (p ˂ 0.05) but remained unchanged for the ost1-2 and scord7 mutants, after exposure to 10 μM ABA for 1 h. Generally Ag NP sorption to the leaves of the Ler and Col-7 ecotypes treated with 10 μM ABA was lower than that in the ABA-free control, mainly due to ABA-induced stomatal closure. The difference in Ag NP sorption with and without ABA was less pronounced for Col-7 than for Ler, suggesting different sorption behaviors between these two ecotypes. In contrast, there was no significant difference in foliar sorption of Ag NPs by the ost1-2 and scord7 mutants with and without ABA treatment. Ag NPs were widely attached to the Arabidopsis leaf surface, and found at cell membrane, cytoplasm, and plasmodesmata, as revealed by scanning electron microscopy and transmission electron microscopy, respectively. These results highlight the important role of stomata in the internationalization of ENPs in plants and may have broad implications in foliar application of nanopesticides and minimizing contamination of food crops by ENPs.

Moss stomata do not respond to light and CO2 concentration but facilitate carbon uptake by sporophytes: a gas exchange, stomatal aperture, and 13 C-labelling study

Stomata exert control on fluxes of CO₂ and water (H₂ O) in the majority of vascular plants and thus are pivotal for planetary fluxes of carbon and H₂ O. However, in mosses, the significance and possible function of the sporophytic stomata are not well understood, hindering understanding of the ancestral function and evolution of these key structures of land plants. Infrared gas analysis and ¹³ CO₂ labelling, with supporting data from gravimetry and optical and scanning electron microscopy, were used to measure CO₂ assimilation and water exchange on young, green, ± fully expanded capsules of 11 moss species with a range of stomatal numbers, distributions, and aperture sizes. Moss sporophytes are effectively homoiohydric. In line with their open fixed apertures, moss stomata, contrary to those in tracheophytes, do not respond to light and CO₂ concentration. Whereas the sporophyte cuticle is highly impermeable to gases, stomata are the predominant sites of ¹³ CO₂ entry and H₂ O loss in moss sporophytes, and CO₂ assimilation is closely linked to total stomatal surface areas. Higher photosynthetic autonomy of moss sporophytes, consequent on the presence of numerous stomata, may have been the key to our understanding of evolution of large, gametophyte-independent sporophytes at the onset of plant terrestrialization.

Comparison of Different Semi-Automated Bioreactors for In Vitro Propagation of Taro (Colocasia esculenta L. Schott)

Taro is important for its nutritional content, medicinal use, and bioethanol production. The aim of the present study was to compare different semi-automated bioreactors (SABs) during in vitro multiplication of C. esculenta. The SABs used were temporary immersion bioreactors (TIBs), SETIS™ bioreactors and ebb-and-flow bioreactors; semi-solid culture medium was used as a control treatment. At 30 d of culture, different developmental variables, determination of chlorophyll, stomatal content, and survival percentage during acclimatization were evaluated. SABs increased the shoot multiplication rate relative to the semi-solid medium; however, the SETIS™ bioreactor showed the highest shoot production, with 36 shoots per explant, and the highest chlorophyll content. The stomatal index was higher in the semi-solid medium compared to the SABs, while the percentage of closed stomata was higher in the SABs than in the semi-solid culture medium. The survival rate during acclimatization showed no differences among the culture systems assessed, obtaining survival rates higher than 99%. In conclusion, the SETIS™ bioreactor showed the highest multiplication rate; however, other bioreactor alternatives are available for semi-automation and cost reduction for micropropagation of C. esculenta.

Abscisic Acid-Enemy or Savior in the Response of Cereals to Abiotic and Biotic Stresses?

Abscisic acid (ABA) is well-known phytohormone involved in the control of plant natural developmental processes, as well as the stress response. Although in wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) its role in mechanism of the tolerance to most common abiotic stresses, such as drought, salinity, or extreme temperatures seems to be fairly well recognized, not many authors considered that changes in ABA content may also influence the sensitivity of cereals to adverse environmental factors, e.g., by accelerating senescence, lowering pollen fertility, and inducing seed dormancy. Moreover, recently, ABA has also been regarded as an element of the biotic stress response; however, its role is still highly unclear. Many studies connect the susceptibility to various diseases with increased concentration of this phytohormone. Therefore, in contrast to the original assumptions, the role of ABA in response to biotic and abiotic stress does not always have to be associated with survival mechanisms; on the contrary, in some cases, abscisic acid can be one of the factors that increases the susceptibility of plants to adverse biotic and abiotic environmental factors.

Role of Stomatal Conductance in Modifying the Dose Response of Stress-Volatile Emissions in Methyl Jasmonate Treated Leaves of Cucumber (Cucumis sativa)

Treatment by volatile plant hormone methyl jasmonate (MeJA) leads to release of methanol and volatiles of lipoxygenase pathway (LOX volatiles) in a dose-dependent manner, but how the dose dependence is affected by stomatal openness is poorly known. We studied the rapid (0-60 min after treatment) response of stomatal conductance (G_s), net assimilation rate (A), and LOX and methanol emissions to varying MeJA concentrations (0.2-50 mM) in cucumber (Cucumis sativus) leaves with partly open stomata and in leaves with reduced G_s due to drought and darkness. Exposure to MeJA led to initial opening of stomata due to an osmotic shock, followed by MeJA concentration-dependent reduction in G_s, whereas A initially decreased, followed by recovery for lower MeJA concentrations and time-dependent decline for higher MeJA concentrations. Methanol and LOX emissions were elicited in a MeJA concentration-dependent manner, whereas the peak methanol emissions (15-20 min after MeJA application) preceded LOX emissions (20-60 min after application). Furthermore, peak methanol emissions occurred earlier in treatments with higher MeJA concentration, while the opposite was observed for LOX emissions. This difference reflected the circumstance where the rise of methanol release partly coincided with MeJA-dependent stomatal opening, while stronger stomatal closure at higher MeJA concentrations progressively delayed peak LOX emissions. We further observed that drought-dependent reduction in G_s ameliorated MeJA effects on foliage physiological characteristics, underscoring that MeJA primarily penetrates through the stomata. However, despite reduced G_s, dark pretreatment amplified stress-volatile release upon MeJA treatment, suggesting that increased leaf oxidative status due to sudden illumination can potentiate the MeJA response. Taken together, these results collectively demonstrate that the MeJA dose response of volatile emission is controlled by stomata that alter MeJA uptake and volatile release kinetics and by leaf oxidative status in a complex manner.

Molecular Evolution and Interaction of Membrane Transport and Photoreception in Plants

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

Hydrogen peroxide plays an important role in PERK4-mediated abscisic acid-regulated root growth in Arabidopsis

Abscisic acid (ABA) is a crucial factor that affects primary root tip growth in plants. Previous research suggests that reactive oxygen species (ROS), especially hydrogen peroxide, are important regulators of ABA signalling in root growth of Arabidopsis. PROLINE-RICH EXTENSIN-LIKE RECEPTOR KINASE 4 (PERK4) plays an important role in ABA responses. Arabidopsis perk4 mutants display attenuated sensitivity to ABA, especially in primary root growth. To gain insights into the mechanism(s) of PERK4-associated ABA inhibition of root growth, in this study we investigated the involvement of ROS in this process. Normal ROS accumulation in the primary root in response to exogenous ABA treatment was not observed in perk4 mutants. PERK4 deficiency prohibits ABA-induced expression of RESPIRATORY BURST OXIDASE HOMOLOGUE (RBOH) genes, therefore the perk4-1 mutant showed decreased production of ROS in the root. The perk4-1/rbohc double mutant displayed the same phenotype as the perk4 and rbohc single mutants in response to exogenous ABA treatment. The results suggest that PERK4-stimulated ROS accumulation during ABA-regulated primary root growth may be mediated by RBOHC.

Label-free quantitative proteomic analysis revealed a positive effect of ectopic over-expression of PeaT1 from Alternaria tenuissima on rice (Oryza sativa) response to drought

The protein elicitor PeaT1 was found in Alternaria tenuissima and exerted broad spectrum resistance in wheat, cotton, and rice. Recently, we found that overexpressing PeaT1 rice (OE) could enhance plant drought tolerance. Elucidating some elevated drought stress-related proteins and associated mechanisms is inevitable for improving drought tolerance in rice. In this study, combining a label-free quantitative proteomic method, multiple proteins were differentially accumulated in OE plants. Among these, a total of 57 significant changed proteins (including 32 up-regulated and 25 down-regulated) were mainly involved in metabolic, cellular, biological progress, and stress response. Using the RT-qPCR assay, 18 proteins' relative abundance was detected mostly consistent with the proteins abundance in proteomic data. Specially, proteins involved in abiotic stress, such as OsSKIPa and OsPP2C, which were significantly induced in early after dehydration treatment in transgenic rice, and the other stress response genes (prohibitin protein, PsbP protein, msrB Protein) also changed in PeaT1 OE lines. Taken together, these results suggested that these differential proteins would be helpful for understanding the functional molecular mechanism of PeaT1 in rice.

Plant Hormone Signaling Crosstalks between Biotic and Abiotic Stress Responses

In the natural environment, plants are often bombarded by a combination of abiotic (such as drought, salt, heat or cold) and biotic (necrotrophic and biotrophic pathogens) stresses simultaneously. It is critical to understand how the various response pathways to these stresses interact with one another within the plants, and where the points of crosstalk occur which switch the responses from one pathway to another. Calcium sensors are often regarded as the first line of response to external stimuli to trigger downstream signaling. Abscisic acid (ABA) is a major phytohormone regulating stress responses, and it interacts with the jasmonic acid (JA) and salicylic acid (SA) signaling pathways to channel resources into mitigating the effects of abiotic stresses versus defending against pathogens. The signal transduction in these pathways are often carried out via GTP-binding proteins (G-proteins) which comprise of a large group of proteins that are varied in structures and functions. Deciphering the combined actions of these different signaling pathways in plants would greatly enhance the ability of breeders to develop food crops that can thrive in deteriorating environmental conditions under climate change, and that can maintain or even increase crop yield.

Balancing Strength and Flexibility: How the Synthesis, Organization, and Modification of Guard Cell Walls Govern Stomatal Development and Dynamics

Guard cells are pairs of epidermal cells that control gas diffusion by regulating the opening and closure of stomatal pores. Guard cells, like other types of plant cells, are surrounded by a three-dimensional, extracellular network of polysaccharide-based wall polymers. In contrast to the walls of diffusely growing cells, guard cell walls have been hypothesized to be uniquely strong and elastic to meet the functional requirements of withstanding high turgor and allowing for reversible stomatal movements. Although the walls of guard cells were long underexplored as compared to extensive studies of stomatal development and guard cell signaling, recent research has provided new genetic, cytological, and physiological data demonstrating that guard cell walls function centrally in stomatal development and dynamics. In this review, we highlight and discuss the latest evidence for how wall polysaccharides are synthesized, deposited, reorganized, modified, and degraded in guard cells, and how these processes influence stomatal form and function. We also raise open questions and provide a perspective on experimental approaches that could be used in the future to shed light on the composition and architecture of guard cell walls.

Glucose triggers stomatal closure mediated by basal signaling through HXK1 and PYR/RCAR receptors in Arabidopsis

Sugars play important roles in regulating plant growth, development, and stomatal movement. Here, we found that glucose triggered stomatal closure in a dose- and time-dependent manner in Arabidopsis. Pharmacological data showed that glucose-induced stomatal closure was greatly inhibited by catalase [CAT; a reactive oxygen species (ROS) scavenger], diphenyleneiodonium chloride (DPI; an NADPH oxidase inhibitor), lanthanum chloride (LaCl3; a Ca2+ channel blocker), EGTA (a Ca2+ chelator), and two nitrate reductase (NR) inhibitors, tungstate and sodium azide (NaN3), while it was not affected by salicylhydroxamic acid (SHAM; a peroxidase inhibitor). Moreover, glucose induced ROS and nitric oxide (NO) production in guard cells of Arabidopsis. The ROS production was almost completely removed by CAT, strongly restricted by DPI, and was not affected by SHAM. NO production was partially suppressed by tungstate and NaN3, and the levels of NO were significantly reduced in the nia1-1nia2-5 mutant. Additionally, glucose-triggered stomatal closure was significantly impaired in gin1-1, gin2-1, pyr1pyl1pyl2pyl4, abi1-1, ost1, slac1-4, cpk6-1, and nia1-1nia2-5 mutants. Likewise, the reductions in leaf stomatal conductance (gs) and transpiration rate (E) caused by glucose were reversed in the above mutants. These results suggest that glucose-triggered stomatal closure may be dependent on basal signaling through PYR/RCAR receptors and hexokinase1 (HXK1).

Noncanonical Function of a Small-Molecular Virulence Factor Coronatine against Plant Immunity: An In Vivo Raman Imaging Approach

Coronatine (1), a small-molecular virulence factor produced by plant-pathogenic bacteria, promotes bacterial infection by inducing the opening of stomatal pores, the major route of bacterial entry into the plant, via the jasmonate-mediated COI1-JAZ signaling pathway. However, this pathway is also important for multiple plant functions, including defense against wounding by herbivorous insects. Thus, suppression of the COI1-JAZ signaling pathway to block bacterial infection would concomitantly impair plant defense against herbivorous wounding. Here, we report additional, COI1-JAZ-independent, action of 1 in Arabidopsis thaliana guard cells. First, we found that a stereoisomer of 1 regulates the movement of Arabidopsis guard cells without affecting COI1-JAZ signaling. Second, we found using alkyne-tagged Raman imaging (ATRI) that 1 is localized to the endoplasmic reticulum (ER) of living guard cells of Arabidopsis. The use of arc6 mutant lacking chloroplast formation was pivotal to circumvent the issue of autofluorescence during ATRI. These findings indicate that 1 has an ER-related action on Arabidopsis stomata that bypasses the COI1-JAZ signaling module. It may be possible to suppress the action of 1 on stomata without impairing plant defense responses against herbivores.

NO and H2O2 contribute to SO2 toxicity via Ca2+ signaling in Vicia faba guard cells

NO and H₂O₂ have been implicated as important signals in biotic and abiotic stress responses of plants to the environment. Previously, we have shown that SO₂ exposure increased the levels of NO and H₂O₂ in plant cells. We hypothesize that, as signaling molecules, NO and H₂O₂ mediate SO₂-caused toxicity. In this paper, we show that SO₂ hydrates caused guard cell death in a concentration-dependent manner in the concentration range of 0.25 to 6 mmol L^-1, which was associated with elevation of intracellular NO, H₂O₂, and Ca²⁺ levels in Vicia faba guard cells. NO donor SNP enhanced SO₂ toxicity, while NO scavenger c-PTIO and NO synthesis inhibitors L-NAME and tungstate significantly prevented SO₂ toxicity. ROS scavenger ascorbic acid (AsA) and catalase (CAT), Ca²⁺ chelating agent EGTA, and Ca²⁺ channel inhibitor LaCl₃ also markedly blocked SO₂ toxicity. In addition, both c-PTIO and AsA could completely block SO₂-induced elevation of intracellular Ca²⁺ level. Moreover, c-PTIO efficiently blocked SO₂-induced H₂O₂ elevation, and AsA significantly blocked SO₂-induced NO elevation. These results indicate that extra NO and H₂O₂ are produced and accumulated in SO₂-treated guard cells, which further activate Ca²⁺ signaling to mediate SO₂ toxicity. Our findings suggest that both NO and H₂O₂ contribute to SO₂ toxicity via Ca²⁺ signaling.

Overexpression of the PeaT1 Elicitor Gene from Alternaria tenuissima Improves Drought Tolerance in Rice Plants via Interaction with a Myo-Inositol Oxygenase

Abiotic stresses, especially drought, seriously threaten cereal crops yields and quality. In this study, we observed that the rice plants of overexpression the Alternariatenuissima PeaT1 gene showed enhanced drought stress tolerance and increased the survival rate following a drought treatment. In PeaT1-overexpressing (PeaT1OE) plants, abscisic acid and chlorophyll content significantly increased, while the malondialdehyde (MDA) content decreased compared with the wild-type plants. Additionally, we confirmed that the transcript levels of drought-responsive genes, including OsAM1, OsLP2, and OsDST, were prominently lower in the PeaT1OE plants. In contrast, expression levels of genes encoding positive drought stress regulators including OsSKIPa, OsCPK9, OsNAC9, OSEREBP1, and OsTPKb were upregulated in PeaT1OE plants. Furthermore, combing the yeast two-hybrid assay, we found that PeaT1 could interact with amyo-inositol oxygenase (OsMIOX), which was verified by pull-down assay. Interestingly, OsMIOX was highly expressed in PeaT1OE plants during the drought treatment. Additionally, the OsMIOX-GFP fusion protein co-localized with the endoplasmic reticulum (ER) marker in tobacco protoplasts, suggesting OsMIOX performs its function in ER. Therefore, our results are useful for elucidating the molecular mechanism underlying the improvement of drought tolerance by PeaT1.

Using Clear Nail Polish to Make Arabidopsis Epidermal Impressions for Measuring the Change of Stomatal Aperture Size in Immune Response

Plant stomata are an essential route for bacterial pathogens to entry inside host tissue and cause diseases. As an important defense mechanism, plant stomata can actively restrict bacterial invasion by dynamically regulating the opening, closing, and reopening of stomatal guard cells. Therefore, accurately measuring the stomatal aperture size during the bacterial pathogenesis is an important approach to study the stomata related immunity. Several methods have been developed for stomatal aperture measurement. Here, we described a detailed protocol of using clear nail polish to make Arabidopsis epidermal impressions for investigate the change of stomatal aperture size in plant immune response. The application of this approach can instantly fix the status of stomatal guard cells, and provides clear, stable, and almost permanent slides of epidermal impressions for measurement of stomatal aperture size.

Polychromatic Supplemental Lighting from underneath Canopy Is More Effective to Enhance Tomato Plant Development by Improving Leaf Photosynthesis and Stomatal Regulation

Light insufficient stress caused by canopy interception and mutual shading is a major factor limiting plant growth and development in intensive crop cultivation. Supplemental lighting can be used to give light to the lower canopy leaves and is considered to be an effective method to cope with low irradiation stress. Leaf photosynthesis, stomatal regulation, and plant growth and development of young tomato plants were examined to estimate the effects of supplemental lighting with various composite spectra and different light orientations. Light-emitting diodes (LEDs) of polychromatic light quality, red + blue (R/B), white + red + blue (W/R/B), white + red + far-red (W/R/FR), and white + blue (W/B) were assembled from the underneath canopy or from the inner canopy as supplemental lighting resources. The results showed that the use of supplemental lighting significantly increased the photosynthetic efficiency, and reduced stomatal closure while promoting plant growth. Among all supplemental lighting treatments, the W/R/B and W/B from the underneath canopy had best performance. The different photosynthetic performances among the supplemental lighting treatments are resulted from variations in CO2 utilization. The enhanced blue light fraction in the W/R/B and W/B could better stimulate stomatal opening and promote photosynthetic electron transport activity, thus better improving photosynthetic rate. Compared with the inner canopy treatment, the supplemental lighting from the underneath canopy could better enhance the carbon dioxide assimilation efficiency and excessive energy dissipation, leading to an improved photosynthetic performance. Stomatal morphology was highly correlated to leaf photosynthesis and plant development, and should thus be an important determinant for the photosynthesis and the growth of greenhouse tomatoes.

A Role for Barley Calcium-Dependent Protein Kinase CPK2a in the Response to Drought

Increasing the drought tolerance of crops is one of the most challenging goals in plant breeding. To improve crop productivity during periods of water deficit, it is essential to understand the complex regulatory pathways that adapt plant metabolism to environmental conditions. Among various plant hormones and second messengers, calcium ions are known to be involved in drought stress perception and signaling. Plants have developed specific calcium-dependent protein kinases that convert calcium signals into phosphorylation events. In this study we attempted to elucidate the role of a calcium-dependent protein kinase in the drought stress response of barley (Hordeum vulgare L.), one of the most economically important crops worldwide. The ongoing barley genome project has provided useful information about genes potentially involved in the drought stress response, but information on the role of calcium-dependent kinases is still limited. We found that the gene encoding the calcium-dependent protein kinase HvCPK2a was significantly upregulated in response to drought. To better understand the role of HvCPK2a in drought stress signaling, we generated transgenic Arabidopsis plants that overexpressed the corresponding coding sequence. Overexpressing lines displayed drought sensitivity, reduced nitrogen balance index (NBI), an increase in total chlorophyll content and decreased relative water content. In addition, in vitro kinase assay experiments combined with mass spectrometry allowed HvCPK2a autophosphorylation sites to be identified. Our results suggest that HvCPK2a is a dual-specificity calcium-dependent protein kinase that functions as a negative regulator of the drought stress response in barley.

Overexpression of AtABCG25 enhances the abscisic acid signal in guard cells and improves plant water use efficiency

In addition to improving drought tolerance, improvement of water use efficiency is a major challenge in plant physiology. Due to their trade-off relationships, it is generally considered that achieving stress tolerance is incompatible with maintaining stable growth. Abscisic acid (ABA) is a key phytohormone that regulates the balance between intrinsic growth and environmental responses. Previously, we identified AtABCG25 as a cell-membrane ABA transporter that export ABA from the inside to the outside of cells. AtABCG25-overexpressing plants showed a lower transpiration phenotype without any growth retardation. Here, we dissected this useful trait using precise phenotyping approaches. AtABCG25 overexpression stimulated a local ABA response in guard cells. Furthermore, AtABCG25 overexpression enhanced drought tolerance, probably resulting from maintenance of water contents over the common threshold for survival after drought stress treatment. Finally, we observed enhanced water use efficiency by overexpression of AtABCG25, in addition to drought tolerance. These results were consistent with the function of AtABCG25 as an ABA efflux transporter. This unique trait may be generally useful for improving the water use efficiency and drought tolerance of plants.

Abscisic Acid and Abiotic Stress Tolerance in Crop Plants

Abiotic stress is a primary threat to fulfill the demand of agricultural production to feed the world in coming decades. Plants reduce growth and development process during stress conditions, which ultimately affect the yield. In stress conditions, plants develop various stress mechanism to face the magnitude of stress challenges, although that is not enough to protect them. Therefore, many strategies have been used to produce abiotic stress tolerance crop plants, among them, abscisic acid (ABA) phytohormone engineering could be one of the methods of choice. ABA is an isoprenoid phytohormone, which regulates various physiological processes ranging from stomatal opening to protein storage and provides adaptation to many stresses like drought, salt, and cold stresses. ABA is also called an important messenger that acts as the signaling mediator for regulating the adaptive response of plants to different environmental stress conditions. In this review, we will discuss the role of ABA in response to abiotic stress at the molecular level and ABA signaling. The review also deals with the effect of ABA in respect to gene expression.

Abscisic Acid and Abiotic Stress Tolerance in Crop Plants

Abiotic stress is a primary threat to fulfill the demand of agricultural production to feed the world in coming decades. Plants reduce growth and development process during stress conditions, which ultimately affect the yield. In stress conditions, plants develop various stress mechanism to face the magnitude of stress challenges, although that is not enough to protect them. Therefore, many strategies have been used to produce abiotic stress tolerance crop plants, among them, abscisic acid (ABA) phytohormone engineering could be one of the methods of choice. ABA is an isoprenoid phytohormone, which regulates various physiological processes ranging from stomatal opening to protein storage and provides adaptation to many stresses like drought, salt, and cold stresses. ABA is also called an important messenger that acts as the signaling mediator for regulating the adaptive response of plants to different environmental stress conditions. In this review, we will discuss the role of ABA in response to abiotic stress at the molecular level and ABA signaling. The review also deals with the effect of ABA in respect to gene expression.

Climate Change and Defense against Pathogens in Plants

Most reviews of climate change are epidemiological, focusing on impact assessment and risk mapping. However, there are many reports of the effects of environmental stress factors on defense mechanisms in plants against pathogens. We review those representative of key climate change-related stresses to determine whether there are any patterns or trends in adaptation responses. We recognize the complexity of climate change itself and the multitrophic nature of the complex biological interactions of plants, microbes, soil, and the environment and, therefore, the difficulty of reductionist dissection approaches to resolving the problems. We review host defense genes, germplasm, and environmental interactions in different types of organisms but find no significant group-specific trends. Similarly, we review by host defense mechanism type and by host-pathogen trophic relationship but identify no dominating mechanism for stress response. However, we do identify core stress response mechanisms playing key roles in multiple response pathways whether to biotic or abiotic stress. We suggest that these should be central to mechanistic climate change plant defense research. We also recognize biodiversity, heterogeneity, and the need for understanding stress in a true systems biology approach as being essential components of progressing our understanding of and response to climate change.

Glucose- and mannose-induced stomatal closure is mediated by ROS production, Ca(2+) and water channel in Vicia faba

Sugars act as vital signaling molecules that regulate plant growth, development and stress responses. However, the effects of sugars on stomatal movement have been unclear. In our study, we explored the effects of monosaccharides such as glucose and mannose on stomatal aperture. Here, we demonstrate that glucose and mannose trigger stomatal closure in a dose- and time-dependent manner in epidermal peels of broad bean (Vicia faba). Pharmacological studies revealed that glucose- and mannose-induced stomatal closure was almost completely inhibited by two reactive oxygen species (ROS) scavengers, catalase (CAT) and reduced glutathione (GSH), was significantly abolished by an NADPH oxidase inhibitor, diphenylene iodonium chloride (DPI), whereas they were hardly affected by a peroxidase inhibitor, salicylhydroxamic acid (SHAM). Furthermore, glucose- and mannose-induced stomatal closure was strongly inhibited by a Ca(2+) channel blocker, LaCl3 , a Ca(2+) chelator, ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) and two water channel blockers, HgCl2 and dimethyl sulfoxide (DMSO); whereas the inhibitory effects of the water channel blockers were essentially abolished by the reversing agent β-mercaptoethanol (β-ME). These results suggest that ROS production mainly via NADPH oxidases, Ca(2+) and water channels are involved in glucose- and mannose-induced stomatal closure.

Preparation of Epidermal Peels and Guard Cell Protoplasts for Cellular, Electrophysiological, and -Omics Assays of Guard Cell Function

Bioassays are commonly used to study stomatal phenotypes. There are multiple options in the choice of plant materials and species used for observation of stomatal and guard cell responses in vivo. Here, detailed procedures for bioassays of stomatal responses to abscisic acid (ABA) in Arabidopsis thaliana are described, including ABA promotion of stomatal closure, ABA inhibition of stomatal opening, and ABA promotion of reaction oxygen species (ROS) production in guard cells. We also include an example of a stomatal bioassay for the guard cell CO2 response using guard cell-enriched epidermal peels from Brassica napus. Highly pure preparations of guard cell protoplasts can be produced, which are also suitable for studies on guard cell signaling, as well as for studies on guard cell ion transport. Small-scale and large-scale guard cell protoplast preparations are commonly used for electrophysiological and -omics studies, respectively. We provide a procedure for small-scale guard cell protoplasting from A. thaliana. Additionally, a general protocol for large-scale preparation of guard cell protoplasts, with specifications for three different species, A. thaliana, B. napus, and Vicia faba is also provided.

Coronatine Facilitates Pseudomonas syringae Infection of Arabidopsis Leaves at Night

In many land plants, the stomatal pore opens during the day and closes during the night. Thus, periods of darkness could be effective in decreasing pathogen penetration into leaves through stomata, the primary sites for infection by many pathogens. Pseudomonas syringae pv. tomato (Pst) DC3000 produces coronatine (COR) and opens stomata, raising an intriguing question as to whether this is a virulence strategy to facilitate bacterial infection at night. In fact, we found that (a) biological concentration of COR is effective in opening dark-closed stomata of Arabidopsis thaliana leaves, (b) the COR defective mutant Pst DC3118 is less effective in infecting Arabidopsis in the dark than under light and this difference in infection is reduced with the wild type bacterium Pst DC3000, and (c) cma, a COR biosynthesis gene, is induced only when the bacterium is in contact with the leaf surface independent of the light conditions. These findings suggest that Pst DC3000 activates virulence factors at the pre-invasive phase of its life cycle to infect plants even when environmental conditions (such as darkness) favor stomatal immunity. This functional attribute of COR may provide epidemiological advantages for COR-producing bacteria on the leaf surface.

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.

The phytotoxin coronatine is a multifunctional component of the virulence armament of Pseudomonas syringae

Plant pathogens deploy an array of virulence factors to suppress host defense and promote pathogenicity. Numerous strains of Pseudomonas syringae produce the phytotoxin coronatine (COR). A major aspect of COR function is its ability to mimic a bioactive jasmonic acid (JA) conjugate and thus target the JA-receptor COR-insensitive 1 (COI1). Biological activities of COR include stimulation of JA-signaling and consequent suppression of SA-dependent defense through antagonistic crosstalk, antagonism of stomatal closure to allow bacterial entry into the interior of plant leaves, contribution to chlorotic symptoms in infected plants, and suppression of plant cell wall defense through perturbation of secondary metabolism. Here, we review the virulence function of COR, including updates on these established activities as well as more recent findings revealing COI1-independent activity of COR and shedding light on cooperative or redundant defense suppression between COR and type III effector proteins.

Comparative transcriptome sequencing of tolerant rice introgression line and its parents in response to drought stress

BACKGROUND

Rice (Oryza sativa. L) is more sensitive to drought stress than other cereals, and large genotypic variation in drought tolerance (DT) exists within the cultivated rice gene pool and its wild relatives. Selective introgression of DT donor segments into a drought-sensitive (DS) elite recurrent parent by backcrossing is an effective way to improve drought stress tolerance in rice. To dissect the molecular mechanisms underlying DT in rice, deep transcriptome sequencing was used to investigate transcriptome differences among a DT introgression line H471, the DT donor P28, and the drought-sensitive, recurrent parent HHZ under drought stress.

RESULTS

The results revealed constitutively differential gene expression before stress and distinct global transcriptome reprogramming among the three genotypes under a time series of drought stress, consistent with their different genotypes and DT phenotypes. A set of genes with higher basal expression in both H471 and P28 compared with HHZ were functionally enriched in oxidoreductase and lyase activities, implying their positive role in intrinsic DT. Gene Ontology analysis indicated that common up-regulated genes in all three genotypes under mild drought stress were enriched in signaling transduction and transcription regulation. Meanwhile, diverse functional categories were characterized for the commonly drought-induced genes in response to severe drought stress. Further comparative transcriptome analysis between H471 and HHZ under drought stress found that introgression caused wide-range gene expression changes; most of the differentially expressed genes (DEGs) in H471 relative to HHZ under drought were beyond the identified introgressed regions, implying that introgression resulted in novel changes in expression. Co-expression analysis of these DEGs represented a complex regulatory network, including the jasmonic acid and gibberellin pathway, involved in drought stress tolerance in H471.

CONCLUSIONS

Comprehensive gene expression profiles revealed that genotype-specific drought induced genes and genes with higher expression in the DT genotype under normal and drought conditions contribute jointly to DT improvement. The molecular genetic pathways of drought stress tolerance uncovered in this study, as well as the DEGs co-localized with DT-related QTLs and introgressed intervals, will serve as useful resources for further functional dissection of the molecular mechanisms of drought stress response in rice.

NRGA1, a putative mitochondrial pyruvate carrier, mediates ABA regulation of guard cell ion channels and drought stress responses in Arabidopsis

Abscisic acid (ABA) regulates ion channel activity and stomatal movements in response to drought and other stresses. Here, we show that the Arabidopsis thaliana gene NRGA1 is a putative mitochondrial pyruvate carrier which negatively regulates ABA-induced guard cell signaling. NRGA1 transcript was abundant in the A. thaliana leaf and particularly in the guard cells, and its product was directed to the mitochondria. The heterologous co-expression of NRGA1 and AtMPC1 in yeast complemented a loss-of-function mitochondrial pyruvate carrier (MPC) mutant. The nrga1 loss-of-function mutant was very sensitive to the presence of ABA in the context of stomatal movements, and exhibited a heightened tolerance to drought stress. Disruption of NRGA1 gene resulted in increased ABA inhibition of inward K(+) currents and ABA activation of slow anion currents in guard cells. The nrga1/NRGA1 functional complementation lines restored the mutant's phenotypes. Furthermore, transgenic lines of constitutively overexpressing NRGA1 showed opposite stomatal responses, reduced drought tolerance, and ABA sensitivity of guard cell inward K(+) channel inhibition and anion channel activation. Our findings highlight a putative role for the mitochondrial pyruvate carrier in guard cell ABA signaling in response to drought.

Copper amine oxidase and phospholipase D act independently in abscisic acid (ABA)-induced stomatal closure in Vicia faba and Arabidopsis

Recent evidence has demonstrated that both copper amine oxidase (CuAO; EC 1.4.3.6) and phospholipase D (PLD; EC 3.1.4.4) are involved in abscisic acid (ABA)-induced stomatal closure. In this study, we investigated the interaction between CuAO and PLD in the ABA response. Pretreatment with either CuAO or PLD inhibitors alone or that with both additively led to impairment of ABA-induced H2O2 production and stomatal closure in Vicia faba. ABA-stimulated PLD activation could not be inhibited by the CuAO inhibitor, and CuAO activity was not affected by the PLD inhibitor. These data suggest that CuAO and PLD act independently in the ABA response. To further examine PLD and CuAO activities in ABA responses, we used the Arabidopsis mutants cuaoζ and pldα1. Ablation of guard cell-expressed CuAOζ or PLDα1 gene retarded ABA-induced H2O2 generation and stomatal closure. As a product of PLD, phosphatidic acid (PA) substantially enhanced H2O2 production and stomatal closure in wide type, pldα1, and cuaoζ. Moreover, putrescine (Put), a substrate of CuAO as well as an activator of PLD, induced H2O2 production and stomatal closure in WT but not in both mutants. These results suggest that CuAO and PLD act independently in ABA-induced stomatal closure.

Intertissue signal transfer of abscisic acid from vascular cells to guard cells

Abscisic acid (ABA) is a phytohormone that responds to environmental stresses, such as water deficiency. Recent studies have shown that ABA biosynthetic enzymes are expressed in the vascular area under both nonstressed and water-stressed growth conditions. However, specific cells in the vasculature involved in ABA biosynthesis have not been identified. Here, we detected the expression of two genes encoding ABA biosynthetic enzymes, ABSCISIC ACID DEFICIENT2 and ABSCISIC ALDEHYDE OXIDASE3, in phloem companion cells in vascular tissues. Furthermore, we identified an ATP-binding cassette transporter, Arabidopsis thaliana ABCG25 (AtABCG25), expressed in the same cells. Additionally, AtABCG25-expressing Spodoptera frugiperda9 culture cells showed an ABA efflux function. Finally, we observed that enhancement of ABA biosynthesis in phloem companion cells induced guard cell responses, even under normal growth conditions. These results show that ABA is synthesized in specific cells and can be transported to target cells in different tissues.

ABA induces H2O2 production in guard cells, but does not close the stomata on Vicia faba leaves developed at high air humidity

Plants developed under constant high (> 85%) relative air humidity (RH) have larger stomata that are unable to close completely. One of the hypotheses for the less responsive stomata is that the plants have reduced sensitivity to abscisic acid (ABA). Both ABA and darkness are signals for stomatal closure and induce the production of the secondary messenger hydrogen peroxide (H2O2). In this study, the ability of Vicia faba plants developed in moderate or high RH to close the stomata in response to darkness, ABA and H2O2 was investigated. Moreover, the ability of the plants to produce H2O2 when treated with ABA or transferred to darkness was also assessed. Our results show that the ABA concentration in moderate RH is not increased during darkness even though the stomata are closing. This indicates that stomatal closure in V. faba during darkness is independent of ABA production. ABA induced both H2O2 production and stomatal closure in stomata formed at moderate RH. H2O2 production, as a result of treatment with ABA, was also observed in stomata formed at high RH, though the closing response was considerably smaller as compared with moderate RH. In either RH, leaf ABA concentration was not affected by darkness. Similarly to ABA treatment, darkness elicited both H2O2 production and stomatal closure following plant cultivation at moderate RH. Contrary to this, neither H2O2 production nor stomatal closure took place when stomata were formed at high RH. These results suggest that the reduced stomatal response in plants developed in continuous high RH is caused by one or more factors downstream of H2O2 in the signaling pathway toward stomatal closure.

Phosphorylation of BLUS1 kinase by phototropins is a primary step in stomatal opening

Opening of stomata in the plant facilitates photosynthetic CO2 fixation and transpiration. Blue-light perception by phototropins (phot1, phot2) activates the plasma membrane H(+)-ATPase, causing stomata to open. Here we describe a regulator that connects these components, a Ser/Thr protein kinase, BLUS1 (BLUE LIGHT SIGNALING1), which mediates a primary step for phototropin signalling in guard cells. blus1 mutants identified by infrared thermography result in a loss of blue light-dependent stomatal opening. BLUS1 encodes a protein kinase that is directly phosphorylated by phot1 in vitro and in vivo at Ser-348 within its C-terminus. Both phosphorylation of Ser-348 and BLUS1 kinase activity are essential for activation of the H(+)-ATPase. blus1 mutants show lower stomatal conductance and CO2 assimilation than wild-type plants under decreased ambient CO2. Together, our analyses demonstrate that BLUS1 functions as a phototropin substrate and primary regulator of stomatal control to enhance photosynthetic CO2 assimilation under natural light conditions.

Algorithmic Perspectives of Network Transitive Reduction Problems and their Applications to Synthesis and Analysis of Biological Networks

In this survey paper, we will present a number of core algorithmic questions concerning several transitive reduction problems on network that have applications in network synthesis and analysis involving cellular processes. Our starting point will be the so-called minimum equivalent digraph problem, a classic computational problem in combinatorial algorithms. We will subsequently consider a few non-trivial extensions or generalizations of this problem motivated by applications in systems biology. We will then discuss the applications of these algorithmic methodologies in the context of three major biological research questions: synthesizing and simplifying signal transduction networks, analyzing disease networks, and measuring redundancy of biological networks.

Differential role of ethylene and hydrogen peroxide in dark-induced stomatal closure

Regulation of stomatal aperture is crucial in terrestrial plants for controlling water loss and gaseous exchange with environment. While much is known of signaling for stomatal opening induced by blue light and the role of hormones, little is known about the regulation of stomatal closing in darkness. The present study was aimed to verify their role in stomatal regulation in darkness. Epidermal peelings from the leaves of Commelina benghalensis were incubated in a defined medium in darkness for 1 h followed by a 1 h incubation in different test solutions [H2O2, propyl gallate, ethrel (ethylene), AgNO3, sodium orthovanadate, tetraethyl ammonium chloride, CaCl2, LaCl3, separately and in combination] before stomatal apertures were measured under the microscope. In the dark stomata remained closed under treatments with ethylene and propyl gallate but opened widely in the presence of H2O2 and AgNO3. The opening effect was largely unaffected by supplementing the treatment with Na-vanadate (PM H+ ATPase inhibitor) and tetraethyl ammonium chloride (K(+)-channel inhibitor) except that opening was significantly inhibited by the latter in presence of H2O2. On the other hand, H2O2 could not override the closing effect of ethylene at any concentrations while a marginal opening of stomata was found when Ag NO3 treatment was given together with propyl gallate. CaCl2 treatment opened stomata in the darkness while LaCl3 maintained stomata closed. A combination of LaCl3 and propyl gallate strongly promoted stomatal opening. A probable action of ethylene in closing stomata of Commelina benghalensis in dark has been proposed.

Leaf water dynamics of Arabidopsis thaliana monitored in-vivo using terahertz time-domain spectroscopy

The declining water availability for agriculture is becoming problematic for many countries. Therefore the study of plants under water restriction is acquiring extraordinary importance. Botanists currently follow the dehydration of plants comparing the fresh and dry weight of excised organs, or measuring their osmotic or water potentials; these are destructive methods inappropriate for in-vivo determination of plants' hydration dynamics. Water is opaque in the terahertz band, while dehydrated biological tissues are partially transparent. We used terahertz spectroscopy to study the water dynamics of Arabidopsis thaliana by comparing the dehydration kinetics of leaves from plants under well-irrigated and water deficit conditions. We also present measurements of the effect of dark-light cycles and abscisic acid on its water dynamics. The measurements we present provide a new perspective on the water dynamics of plants under different external stimuli and confirm that terahertz can be an excellent non-contact probe of in-vivo tissue hydration.

Identification of cyclic GMP-activated nonselective Ca2+-permeable cation channels and associated CNGC5 and CNGC6 genes in Arabidopsis guard cells

Cytosolic Ca(2+) in guard cells plays an important role in stomatal movement responses to environmental stimuli. These cytosolic Ca(2+) increases result from Ca(2+) influx through Ca(2+)-permeable channels in the plasma membrane and Ca(2+) release from intracellular organelles in guard cells. However, the genes encoding defined plasma membrane Ca(2+)-permeable channel activity remain unknown in guard cells and, with some exceptions, largely unknown in higher plant cells. Here, we report the identification of two Arabidopsis (Arabidopsis thaliana) cation channel genes, CNGC5 and CNGC6, that are highly expressed in guard cells. Cytosolic application of cyclic GMP (cGMP) and extracellularly applied membrane-permeable 8-Bromoguanosine 3',5'-cyclic monophosphate-cGMP both activated hyperpolarization-induced inward-conducting currents in wild-type guard cells using Mg(2+) as the main charge carrier. The cGMP-activated currents were strongly blocked by lanthanum and gadolinium and also conducted Ba(2+), Ca(2+), and Na(+) ions. cngc5 cngc6 double mutant guard cells exhibited dramatically impaired cGMP-activated currents. In contrast, mutations in CNGC1, CNGC2, and CNGC20 did not disrupt these cGMP-activated currents. The yellow fluorescent protein-CNGC5 and yellow fluorescent protein-CNGC6 proteins localize in the cell periphery. Cyclic AMP activated modest inward currents in both wild-type and cngc5cngc6 mutant guard cells. Moreover, cngc5 cngc6 double mutant guard cells exhibited functional abscisic acid (ABA)-activated hyperpolarization-dependent Ca(2+)-permeable cation channel currents, intact ABA-induced stomatal closing responses, and whole-plant stomatal conductance responses to darkness and changes in CO2 concentration. Furthermore, cGMP-activated currents remained intact in the growth controlled by abscisic acid2 and abscisic acid insensitive1 mutants. This research demonstrates that the CNGC5 and CNGC6 genes encode unique cGMP-activated nonselective Ca(2+)-permeable cation channels in the plasma membrane of Arabidopsis guard cells.

Ethylene limits abscisic acid- or soil drying-induced stomatal closure in aged wheat leaves

The mechanism of age-induced decreased stomatal sensitivity to abscisic acid (ABA) and soil drying has been explored here. Older, fully expanded leaves partly lost their ability to close stomata in response to foliar ABA sprays, and soil drying which stimulated endogenous ABA production, while young fully expanded leaves closed their stomata more fully. However, ABA- or soil drying-induced stomatal closure of older leaves was partly restored by pretreating plants with 1-methylcyclopropene (1-MCP), which can antagonize ethylene receptors, or by inoculating soil around the roots with the rhizobacterium Variovorax paradoxus 5C-2, which contains 1-aminocyclopropane-1-carboxylic acid (ACC)-deaminase. ACC (the immediate biosynthetic precursor of ethylene) sprays revealed higher sensitivity of stomata to ethylene in older leaves than younger leaves, despite no differences in endogenous ACC concentrations or ethylene emission. Taken together, these results indicate that the relative insensitivity of stomatal closure to ABA and soil drying in older leaves is likely due to altered stomatal sensitivity to ethylene, rather than ethylene production. To our knowledge, this is the first study to mechanistically explain diminished stomatal responses to soil moisture deficit in older leaves, and the associated reduction in leaf water-use efficiency.

The role of ABA and MAPK signaling pathways in plant abiotic stress responses

As sessile organisms, plants have developed specific mechanisms that allow them to rapidly perceive and respond to stresses in the environment. Among the evolutionarily conserved pathways, the ABA (abscisic acid) signaling pathway has been identified as a central regulator of abiotic stress response in plants, triggering major changes in gene expression and adaptive physiological responses. ABA induces protein kinases of the SnRK family to mediate a number of its responses. Recently, MAPK (mitogen activated protein kinase) cascades have also been shown to be implicated in ABA signaling. Therefore, besides discussing the role of ABA in abiotic stress signaling, we will also summarize the evidence for a role of MAPKs in the context of abiotic stress and ABA signaling.

Can prolonged exposure to low VPD disturb the ABA signalling in stomatal guard cells?

The response of stomata to many environmental factors is well documented. Multiple signalling pathways for abscisic acid (ABA)-induced stomatal closure have been proposed over the last decades. However, it seems that exposure of a leaf for a long time (several days) to some environmental conditions generates a sort of memory in the guard cells that results in the loss of suitable responses of the stomata to closing stimuli, such as desiccation and ABA. In this review paper we discuss changes in the normal pattern of signal transduction that could account for disruption of guard cell signalling after long-term exposure to some environmental conditions, with special emphasis on long-term low vapour pressure deficit (VPD).