The stomatal response to reduced relative humidity requires guard cell-autonomous ABA synthesis

Insights into ABA-mediated regulation of guard cell primary metabolism revealed by systems biology approaches

Despite the fact that guard cell abscisic acid (ABA) signalling pathway is well documented, our understanding concerning how and to which extent ABA regulates guard cell metabolism remains fragmentary. Here we have adopted different systems approaches to investigate how ABA modulates guard cell central metabolism by providing genes that are possibly ABA-regulated. By using previous published Arabidopsis guard cell transcript profiling data, we carried out an extensive co-expression network analysis using ABA-related genes and those related to the metabolism and transport of sugars, starch and organic acids. Next, we investigated the presence of ABA responsive elements (ABRE) in the promoter of genes that are highly expressed in guard cells, responsive to ABA and co-expressed with ABA-related genes. Together, these analyses indicated that 44 genes are likely regulated by ABA and 8 of them are highly expressed in guard cells in both the presence and absence of ABA, including genes of the tricarboxylic acid cycle and those related to sucrose and hexose transport and metabolism. It seems likely that ABA may modulate both sucrose transport through guard cell plasma membrane and sucrose metabolism within guard cells. In this context, genes associated with sucrose synthase, sucrose phosphate synthase, trehalose-6-phosphate, invertase, UDP-glucose epimerase/pyrophosphorylase and different sugar transporters contain ABRE in their promoter and are thus possibly ABA regulated. Although validation experiments are required, our study highlights the importance of systems biology approaches to drive new hypothesis and to unravel genes and pathways that are regulated by ABA in guard cells.

Arabidopsis MATE45 antagonizes local abscisic acid signaling to mediate development and abiotic stress responses

Anthocyanins provide ideal visual markers for the identification of mutations that disrupt molecular responses to abiotic stress. We screened Arabidopsis mutants of ABC (ATP-Binding Cassette) and MATE (Multidrug And Toxic compound Extrusion) transporter genes under nutritional stress and identified four genes (ABCG25,ABCG9,ABCG5, and MATE45) required for normal anthocyanin pigmentation. ABCG25 was previously demonstrated to encode a vascular-localized cellular exporter of abscisic acid (ABA). Our results show that MATE45 encodes an aerial meristem- and a vascular-localized transporter associated with the trans-Golgi, and that it plays an important role in controlling the levels and distribution of ABA in growing aerial meristems and non-meristematic tissues. MATE45 promoter-GUS reporter fusions revealed the activity localized to the leaf and influorescence meristems and the vasculature. Loss-of-function mate45 mutants exhibited accelerated rates of aerial organ initiation suggesting at least partial functional conservation with the maize ortholog bige1. The aba2-1 mutant, which is deficient in ABA biosynthesis, exhibited a number of phenotypes that were rescued in the mate45-1 aba2-1 double mutant. mate45 exhibited enhanced the seed dormancy, and germination was hypersensitive to ABA. Enhanced frequency of leaf primordia growth in mate45 seedlings grown in nutrient imbalance stress was ABA-dependent. The ABA signaling reporter construct pRD29B::GUS revealed elevated levels of ABA signaling in the true leaf primordia of mate45 seedlings grown under nutritional stress, and gradually reduced signaling in surrounding cotyledon and hypocotyl tissues concomitant with reduced expressions of ABCG25. Our results suggest a role of MATE45 in reducing meristematic ABA and in maintaining ABA distribution in adjacent non-meristematic tissues.

Understanding the Molecular Basis of Salt Sequestration in Epidermal Bladder Cells of Chenopodium quinoa

Soil salinity is destroying arable land and is considered to be one of the major threats to global food security in the 21st century. Therefore, the ability of naturally salt-tolerant halophyte plants to sequester large quantities of salt in external structures, such as epidermal bladder cells (EBCs), is of great interest. Using Chenopodium quinoa, a pseudo-cereal halophyte of great economic potential, we have shown previously that, upon removal of salt bladders, quinoa becomes salt sensitive. In this work, we analyzed the molecular mechanism underlying the unique salt dumping capabilities of bladder cells in quinoa. The transporters differentially expressed in the EBC transcriptome and functional electrophysiological testing of key EBC transporters in Xenopus oocytes revealed that loading of Na⁺ and Cl^- into EBCs is mediated by a set of tailored plasma and vacuole membrane-based sodium-selective channel and chloride-permeable transporter.

Phytomelatonin receptor PMTR1-mediated signaling regulates stomatal closure in Arabidopsis thaliana

Melatonin has been detected in plants in 1995; however, the function and signaling pathway of this putative phytohormone are largely undetermined due to a lack of knowledge about its receptor. Here, we discovered the first phytomelatonin receptor (CAND2/PMTR1) in Arabidopsis thaliana and found that melatonin governs the receptor-dependent stomatal closure. The application of melatonin induced stomatal closure through the heterotrimeric G protein α subunit-regulated H₂ O₂ and Ca²⁺ signals. The Arabidopsis mutant lines lacking AtCand2 that encodes a candidate G protein-coupled receptor were insensitive to melatonin-induced stomatal closure. Accordingly, the melatonin-induced H₂ O₂ production and Ca²⁺ influx were completely abolished in cand2. CAND2 is a membrane protein that interacts with GPA1 and the expression of AtCand2 was tightly regulated by melatonin in various organs and guard cells. CAND2 showed saturable and specific ¹²⁵ I-melatonin binding, with apparent K_d (dissociation constant) of 0.73 ± 0.10 nmol/L (r²  = .99), demonstrating this protein is a phytomelatonin receptor (PMTR1). Our results suggest that the phytomelatonin regulation of stomatal closure is dependent on its receptor CAND2/PMTR1-mediated H₂ O₂ and Ca²⁺ signaling transduction cascade.

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

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

ABA Transport and Plant Water Stress Responses

To understand the integrative networks of signaling molecules, the sites of their biosynthesis and action must be clarified, particularly for phytohormones such as abscisic acid (ABA). The relationship between the sites of ABA biosynthesis and transport has been discussed extensively in the context of guard cells and stomatal regulation. However, guard cells are not the only site of ABA action. Recent studies have reported multiple sites of ABA biosynthesis and multiple ABA transporters, indicating that ABA transport regulation is not unidirectional but rather forms complex networks. Therefore, it is important to determine how multiple ABA sources coordinately contribute to individual biological processes under various physiological conditions.

Advances and current challenges in calcium signaling

Content Summary 414 I. Introduction 415 II. Ca²⁺ importer and exporter in plants 415 III. The Ca²⁺ decoding toolkit in plants 415 IV. Mechanisms of Ca²⁺ signal decoding 417 V. Immediate Ca²⁺ signaling in the regulation of ion transport 418 VI. Ca²⁺ signal integration into long-term ABA responses 419 VII Integration of Ca²⁺ and hormone signaling through dynamic complex modulation of the CCaMK/CYCLOPS complex 420 VIII Ca²⁺ signaling in mitochondria and chloroplasts 422 IX A view beyond recent advances in Ca²⁺ imaging 423 X Modeling approaches in Ca²⁺ signaling 424 XI Conclusions: Ca²⁺ signaling a still young blooming field of plant research 424 Acknowledgements 425 ORCID 425 References 425 SUMMARY: Temporally and spatially defined changes in Ca²⁺ concentration in distinct compartments of cells represent a universal information code in plants. Recently, it has become evident that Ca²⁺ signals not only govern intracellular regulation but also appear to contribute to long distance or even organismic signal propagation and physiological response regulation. Ca²⁺ signals are shaped by an intimate interplay of channels and transporters, and during past years important contributing individual components have been identified and characterized. Ca²⁺ signals are translated by an elaborate toolkit of Ca²⁺ -binding proteins, many of which function as Ca²⁺ sensors, into defined downstream responses. Intriguing progress has been achieved in identifying specific modules that interconnect Ca²⁺ decoding proteins and protein kinases with downstream target effectors, and in characterizing molecular details of these processes. In this review, we reflect on recent major advances in our understanding of Ca²⁺ signaling and cover emerging concepts and existing open questions that should be informative also for scientists that are currently entering this field of ever-increasing breath and impact.

Leaves, not roots or floral tissue, are the main site of rapid, external pressure-induced ABA biosynthesis in angiosperms

Rapid biosynthesis of abscisic acid (ABA) in the leaf, triggered by a decrease in cell volume, is essential for a functional stomatal response. However, it is not known whether rapid biosynthesis of ABA is also triggered in other plant tissues. Through the application of external pressure to flower, root, and leaf tissues, we test whether a reduction in cell volume can trigger rapid increases in ABA levels across the plant body in two species, Solanum lycopersicum and Passiflora tarminiana. Our results show that, in contrast to rapid ABA synthesis in the leaf, flower and root tissue did not show a significant, increase in ABA level in response to a drop in cell volume over a short time frame, suggesting that rapid ABA biosynthesis occurs only in leaf, not in flower or root tissues. A gene encoding the key, rate-limiting carotenoid cleavage enzyme (9-cis-epoxycarotenoid dioxygenase, NCED) in the ABA biosynthetic pathway in S. lycopersicum, NCED1, was upregulated to a lesser degree in flowers and roots compared with leaves in response to applied pressure. In both species, floral tissues contained substantially lower levels of the NCED substrate 9'-cis-neoxanthin than leaves, and this ABA precursor could not be detected in roots. Slow and minimal ABA biosynthesis was detected after 2 h in petals, indicating that floral tissue is capable of synthesizing ABA in response to sustained water deficit. Our results indicate that rapid ABA biosynthesis predominantly occurs in the leaves, and not in other tissues.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

The Asparagine-Rich Protein NRP Facilitates the Degradation of the PP6-type Phosphatase FyPP3 to Promote ABA Response in Arabidopsis

The phytohormone abscisic acid (ABA) plays critical roles in abiotic stress responses and plant development. In germinating seeds, the phytochrome-associated protein phosphatase, FyPP3, negatively regulates ABA signaling by dephosphorylating the transcription factor ABI5. However, whether and how FyPP3 is regulated at the posttranscriptional level remains unclear. Here, we report that an asparagine-rich protein, NRP, interacts with FyPP3 and tethers FyPP3 to SYP41/61-positive endosomes for subsequent degradation in the vacuole. Upon ABA treatment, the expression of NRP was induced and NRP-mediated FyPP3 turnover was accelerated. Consistently, ABA-induced FyPP3 turnover was abolished in an nrp null mutant. On the other hand, FyPP3 can dephosphorylate NRP in vitro, and overexpression of FyPP3 reduced the half-life of NRP in vivo. Genetic analyses showed that NRP has a positive role in ABA-mediated seed germination and gene expression, and that NRP is epistatic to FyPP3. Taken together, our results identify a new regulatory circuit in the ABA signaling network, which links the intracellular trafficking with ABA signaling.

Heterologous expression of rice 9-cis-epoxycarotenoid dioxygenase 4 (OsNCED4) in Arabidopsis confers sugar oversensitivity and drought tolerance

BACKGROUND

The 9-cis-epoxycarotenoid dioxygenases OsNCED4 was cloned from rice in conjunction with OsNCED 1-3 and 5, of which 3 has been shown to function in ABA biosynthesis and alteration of leaf morphology. In higher plants, NCEDs have been shown to be key enzymes controlling ABA biosynthesis and belong to a differentially expressed gene family. Aside from OsNCED3, it remains largely unknown if other OsNCED genes are involved in ABA biosynthesis in rice. Thus, transgenic Arabidopsis plants overexpressing OsNCED4 were generated in the 129B08/nced3 mutant background to explore OsNCED4 function in ABA biosynthesis.

RESULTS

Heterologous expression of OsNCED4 in Arabidopsis increased ABA levels and altered plant size and leaf shape, delayed seed germination, caused sugar oversensitivity in post-germination growth, and enhanced tolerance to drought. The native OsNCED3 and OsNCED4 promoters were expressed in an overlapping pattern in rice seeds and young seedlings, suggesting possible functional redundancy between OsNCED3 and OsNCED4. At the one-leaf stage, similar regulation of OsNCED3 and OsNCED4 gene expression in roots or leaves in response to moderate salt stress (150 mM NaCl) was observed.

CONCLUSION

Like OsNCED3, OsNCED4 is functionally active in ABA biosynthesis in rice. OsNCED3 and OsNCED4 might play redundant roles in controlling ABA biosynthesis in rice, as suggested by GUS staining assay, but this should be further analyzed through complementation of rice NCED knockout mutants.

Early response to salt ions in maize (Zea mays L.)

Abscisic acid (ABA) regulates leaf growth and transpiration rate of plants exposed to salt stress. Despite the known fact that cell dehydration is instrumental for the modulation of ABA concentrations when NaCl is high in the external environment, it was never tested as to whether sodium (Na) or chlorine (Cl) also modulate ABA concentrations. To answer this question, a hydroponic study on maize (Zea mays) was established, by exposing plants to 50mM of sodium glucosamide or glucosamine chloride. The effect of both ions on ABA was investigated in an early stage before (i) the salt ions accumulated to toxic tissue concentrations and before (ii) cells dehydrated. This allowed studying early responses to Na and Cl separately, well before plants were stressed by these ions. Gas chromatography-mass spectrometry analysis was used to quantify ABA concentrations in roots and in leaves after a period of 2h after ion application. The transcript abundance of the key regulatory enzyme of the biosynthesis of ABA in maize, the 9-cis-epoxycarotenoid dioxygenase gene viviparous 14, was quantified via real-time quantitative-reverse-transcriptase-polymerase-chain-reaction. The results reveal that Cl and Na induce the increase of leaf tissue ABA concentrations at two hours after plants were exposed to 50mM of the ions. Surprisingly, this effect was more pronounced in response to the Cl component. The increase in the guard-cell regulating ABA concentration correlated with a reduced transpiration. Mainly because of this result we suggest that the early accumulation of ABA is useful in maintaining cell turgor.

Regulatory Gene Networks in Drought Stress Responses and Resistance in Plants

Plant responses to drought stress have been analyzed extensively to reveal complex regulatory gene networks, including the detection of water deficit signals, as well as the physiological, cellular, and molecular responses. Plants recognize water deficit conditions at their roots and transmit this signal to their shoots to synthesize abscisic acid (ABA) in their leaves. ABA is a key phytohormone that regulates physiological and molecular responses to drought stress, such as stomatal closure, gene expression, and the accumulation of osmoprotectants and stress proteins. ABA transporters function as the first step for propagating synthesized ABA. To prevent water loss, ABA influx in guard cells is detected by several protein kinases, such as SnRK2s and MAPKs that regulate stomatal closure. ABA mediates a wide variety of gene expression machineries with stress-responsive transcription factors, including DREBs and AREBs, to acquire drought stress resistance in whole tissues. In this chapter, we summarize recent advances in drought stress signaling, focusing on gene networks in cellular and intercellular stress responses and drought resistance.

Specialized stomatal humidity responses underpin ecological diversity in C3 bromeliads

The Neotropical Bromeliaceae display an extraordinary level of ecological variety, with species differing widely in habit, photosynthetic pathway and growth form. Divergences in stomatal structure and function, hitherto understudied in treatments of bromeliad evolutionary physiology, could have been critical to the generation of variety in ecophysiological strategies among the bromeliads. Because humidity is a key factor in bromeliad niches, we focussed on stomatal responses to vapour pressure deficit (VPD). We measured the sensitivity of stomatal conductance and assimilation rate to VPD in eight C₃ bromeliad species of contrasting growth forms and ecophysiological strategies and parameterised the kinetics of stomatal responses to a step change in VPD. Notably, three tank-epiphyte species displayed low conductance, high sensitivity and fast kinetics relative to the lithophytes, while three xeromorphic terrestrial species showed high conductance and sensitivity but slow stomatal kinetics. An apparent feedforward response of transpiration to VPD occurred in the tank epiphytes, while water-use efficiency was differentially impacted by stomatal closure depending on photosynthetic responses. Differences in stomatal responses to VPD between species of different ecophysiological strategies are closely linked to modifications of stomatal morphology, which we argue has been a pivotal component of the evolution of high diversity in this important plant family.

Metabolism within the specialized guard cells of plants

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

Surviving a Dry Future: Abscisic Acid (ABA)-Mediated Plant Mechanisms for Conserving Water under Low Humidity

Angiosperms are able to respond rapidly to the first sign of dry conditions, a decrease in air humidity, more accurately described as an increase in the vapor pressure deficit between the leaf and the atmosphere (VPD), by abscisic acid (ABA)-mediated stomatal closure. The genes underlying this response offer valuable candidates for targeted selection of crop varieties with improved drought tolerance, a critical goal for current plant breeding programs, to maximize crop production in drier and increasingly marginalized environments, and meet the demands of a growing population in the face of a changing climate. Here, we review current understanding of the genetic mechanisms underpinning ABA-mediated stomatal closure, a key means for conserving water under dry conditions, examine how these mechanisms evolved, and discuss what remains to be investigated.

The spatio-temporal specificity of PYR1/PYL/RCAR ABA receptors in response to developmental and environmental cues

From the different functions ABA exerted between the aboveground and belowground, seed and vegetative tissues, primary root and lateral root, stimulating stomatal closure and inhibiting stomatal opening, between young and senescence leaves in stomatal movement, among different cells in plasma membrane water permeability, we addressed the organ-, tissue-, cell-, physiological processes-, and development stage specificities of PYR1/PYL/RCAR ABA receptors. This specificity may reflect the spatio-temporal properties of water potentials as well as the endogenous ABA levels in detail context, which plus the various affinities among this receptor families, resulted in the specificity of the transcripts as well as genes functions. PYR1/PYL/RCAR ABA receptors may integrate the message of ABA resource (local signaling or long distance signaling) and concentration, thus fine-tuning ABA response to environmental- and developmental cues. It also evolutionally affording land plants sophisticated mechanism to survival adverse environments.

Non-26S Proteasome Endomembrane Trafficking Pathways in ABA Signaling

The phytohormone abscisic acid (ABA) is a vital endogenous messenger that regulates diverse physiological processes in plants. The regulation of ABA signaling has been well studied at both the transcriptional and translational levels. Post-translational modification of key regulators in ABA signaling by the 26S ubiquitin proteasome pathway is well known. Recently, increasing evidence demonstrates that atypical turnover of key regulators by the endocytic trafficking pathway and autophagy also play vital roles in ABA perception, signaling, and action. We summarize and synthesize here recent findings in the field of ABA signaling.

Plant hormone transporters: what we know and what we would like to know

Hormone transporters are crucial for plant hormone action, which is underlined by severe developmental and physiological impacts caused by their loss-of-function mutations. Here, we summarize recent knowledge on the individual roles of plant hormone transporters in local and long-distance transport. Our inventory reveals that many hormones are transported by members of distinct transporter classes, with an apparent dominance of the ATP-binding cassette (ABC) family and of the Nitrate transport1/Peptide transporter family (NPF). The current need to explore further hormone transporter regulation, their functional interaction, transport directionalities, and substrate specificities is briefly reviewed.

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

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

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

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

Stomate-based defense and environmental cues

Environmental conditions play crucial roles in modulating immunity and disease in plants. For instance, many bacterial disease outbreaks occur after periods of high humidity and rain. A critical step in bacterial infection is entry into the plant interior through wounds or natural openings, such as stomata. Bacterium-triggered stomatal closure is an integral part of the plant immune response to reduce pathogen invasion. Recently, we found that high humidity compromises stomatal defense, which is accompanied by regulation of the salicylic acid and jasmonic acid pathways in guard cells. Periods of darkness, when most stomata are closed, are effective in decreasing pathogen penetration into leaves. However, coronatine produced by Pseudomonas syringae pv. tomato (Pst) DC3000 cells can open dark-closed stomata facilitating infection. Thus, a well-known disease-promoting environmental condition (high humidity) acts in part by suppressing stomatal defense, whereas an anti-stomatal defense factor such as coronatine, may provide epidemiological advantages to ensure bacterial infection when environmental conditions (darkness and insufficient humidity) favor stomatal defense.

Resolving the central metabolism of Arabidopsis guard cells

Photosynthesis and water use efficiency, key factors affecting plant growth, are directly controlled by microscopic and adjustable pores in the leaf-the stomata. The size of the pores is modulated by the guard cells, which rely on molecular mechanisms to sense and respond to environmental changes. It has been shown that the physiology of mesophyll and guard cells differs substantially. However, the implications of these differences to metabolism at a genome-scale level remain unclear. Here, we used constraint-based modeling to predict the differences in metabolic fluxes between the mesophyll and guard cells of Arabidopsis thaliana by exploring the space of fluxes that are most concordant to cell-type-specific transcript profiles. An independent 13C-labeling experiment using isolated mesophyll and guard cells was conducted and provided support for our predictions about the role of the Calvin-Benson cycle in sucrose synthesis in guard cells. The combination of in silico with in vivo analyses indicated that guard cells have higher anaplerotic CO2 fixation via phosphoenolpyruvate carboxylase, which was demonstrated to be an important source of malate. Beyond highlighting the metabolic differences between mesophyll and guard cells, our findings can be used in future integrated modeling of multi-cellular plant systems and their engineering towards improved growth.

Isolation of Guard-cell Enriched Tissue for RNA Extraction

This is a protocol for isolation of guard cell enriched samples from Arabidopsis thaliana plants for RNA extraction. Leaves are blended in ice-water and filtered through nylon mesh to obtain guard cell enriched fragments. With guard cell enriched samples, gene expression analysis can be done, e.g., comparing different gene expression levels in guard cells versus whole leaf to determine if a gene of interest is predominantly expressed in guard cells. It can also be used to study the effect of treatments or different genetic backgrounds in the regulation of the guard cell expressed genes.

Ascophyllum nodosum Seaweed Extract Alleviates Drought Stress in Arabidopsis by Affecting Photosynthetic Performance and Related Gene Expression

Drought represents one of the most relevant abiotic stress affecting growth and yield of crop plants. In order to improve the agricultural productivity within the limited water and land resources, it is mandatory to increase crop yields in presence of unfavorable environmental stresses. The use of biostimulants, often containing seaweed extracts, represents one of the options for farmers willing to alleviate abiotic stress consequences on crops. In this work, we investigated the responses of Arabidopsis plants treated with an extract from the brown alga Ascophyllum nodosum (ANE), under drought stress conditions, demonstrating that ANE positively influences Arabidopsis survival. Pre-treatment with ANE induced a partial stomatal closure, associated with changes in the expression levels of genes involved in ABA-responsive and antioxidant system pathways. The pre-activation of these pathways results in a stronger ability of ANE-treated plants to maintain a better photosynthetic performance compared to untreated plants throughout the dehydration period, combined with a higher capacity to dissipate the excess of energy as heat in the reaction centers of photosystem II. Our results suggest that drought stressed plants treated with ANE are able to maintain a strong stomatal control and relatively higher values of both water use efficiency (WUE) and mesophyll conductance during the last phase of dehydration. Simultaneously, the activation of a pre-induced antioxidant defense system, in combination with a more efficient energy dissipation mechanism, prevents irreversible damages to the photosynthetic apparatus. In conclusion, pre-treatment with ANE is effective to acclimate plants to the incoming stress, promoting an increased WUE and dehydration tolerance.

ABA-Mediated Stomatal Response in Regulating Water Use during the Development of Terminal Drought in Wheat

End-of-season drought or "terminal drought," which occurs after flowering, is considered the most significant abiotic stress affecting crop yields. Wheat crop production in Mediterranean-type environments is often exposed to terminal drought due to decreasing rainfall and rapid increases in temperature and evapotranspiration during spring when wheat crops enter the reproductive stage. Under such conditions, every millimeter of extra soil water extracted by the roots benefits grain filling and yield and improves water use efficiency (WUE). When terminal drought develops, soil dries from the top, exposing the top part of the root system to dry soil while the bottom part is in contact with available soil water. Plant roots sense the drying soil and produce signals, which on transmission to shoots trigger stomatal closure to regulate crop water use through transpiration. However, transpiration is linked to crop growth and productivity and limiting transpiration may reduce potential yield. While an early and high degree of stomatal closure affects photosynthesis and hence biomass production, a late and low degree of stomatal closure exhausts available soil water rapidly which results in yield losses through a reduction in post-anthesis water use. The plant hormone abscisic acid (ABA) is considered the major chemical signal involved in stomatal regulation. Wheat genotypes differ in their ability to produce ABA under drought and also in their stomatal sensitivity to ABA. In this viewpoint article we discuss the possibilities of exploiting genotypic differences in ABA response to soil drying in regulating the use of water under terminal drought. Root density distribution in the upper drying layers of the soil profile is identified as a candidate trait that can affect ABA accumulation and subsequent stomatal closure. We also examine whether leaf ABA can be designated as a surrogate characteristic for improved WUE in wheat to sustain grain yield under terminal drought. Ease of collecting leaf samples to quantify ABA compared to extracting xylem sap will facilitate rapid screening of a large number of germplasm for drought tolerance.

The effect of blue light on stomatal oscillations and leaf turgor pressure in banana leaves

Stomatal oscillations are cyclic opening and closing of stomata, presumed to initiate from hydraulic mismatch between leaf water supply and transpiration rate. To test this assumption, mismatches between water supply and transpiration were induced using manipulations of vapour pressure deficit (VPD) and light spectrum in banana (Musa acuminata). Simultaneous measurements of gas exchange with changes in leaf turgor pressure were used to describe the hydraulic mismatches. An increase of VPD above a certain threshold caused stomatal oscillations with variable amplitudes. Oscillations in leaf turgor pressure were synchronized with stomatal oscillations and balanced only when transpiration equaled water supply. Surprisingly, changing the light spectrum from red and blue to red alone at constant VPD also induced stomatal oscillations - while the addition of blue (10%) to red light only ended oscillations. Blue light is known to induce stomatal opening and thus should increase the hydraulic mismatch, reduce the VPD threshold for oscillations and increase the oscillation amplitude. Unexpectedly, blue light reduced oscillation amplitude, increased VPD threshold and reduced turgor pressure loss. These results suggest that additionally, to the known effect of blue light on the hydroactive opening response of stomata, it can also effect stomatal movement by increased xylem-epidermis water supply.

An Integrated Hydraulic-Hormonal Model of Conifer Stomata Predicts Water Stress Dynamics

A simple model combining leaf hydraulics and abscisic acid sensitivity can predict stomatal dynamics to short-term changes in plant water status in a conifer.

Differences and commonalities of plant responses to single and combined stresses

In natural or agricultural environments, plants are constantly exposed to a wide range of biotic and abiotic stresses. Given the forecasted global climate changes, plants will cope with heat waves, drought periods and pathogens at the same time or consecutively. Heat and drought cause opposing physiological responses, while pathogens may or may not profit from climate changes depending on their lifestyle. Several studies have been conducted to find stress-specific signatures or stress-independent commonalities. Previously this has been done by comparing different single stress treatments. This approach has been proven difficult since most studies, comparing single and combined stress conditions, have come to the conclusion that each stress treatment results in specific transcriptional changes. Although transcriptional changes at the level of individual genes are highly variable and stress-specific, central metabolic and signaling responses seem to be common, often leading to an overall reduced plant growth. Understanding how specific transcriptional changes are linked to stress adaptations and identifying central hubs controlling this interaction will be the challenge for the coming years. In this review, we will summarize current knowledge on plant responses to different individual and combined stresses and try to find a common thread potentially underlying these responses. We will begin with a brief summary of known physiological, metabolic, transcriptional and hormonal responses to individual stresses, elucidate potential commonalities and conflicts and finally we will describe results obtained during combined stress experiments. Here we will concentrate on simultaneous application of stress conditions but we will also touch consequences of sequential stress treatments.

Xylem and stomata, coordinated through time and space

Land plants exhibit a degree of homeostasis in leaf water content to protect against damage to photosynthetic and xylem tissues, and to maintain an efficient allocation of resources. This is achieved by a strong coordination between the systems regulating water delivery (xylem) and water loss (stomata). This review discusses evolution in xylem and stomatal function, specifically focussing on the interactions between them.

Water potential regulation, stomatal behaviour and hydraulic transport under drought: deconstructing the iso/anisohydric concept

In this review, we address the relationship between stomatal behaviour, water potential regulation and hydraulic transport in plants, focusing on the implications for the iso/anisohydric classification of plant drought responses at seasonal timescales. We first revise the history of the isohydric concept and its possible definitions. Then, we use published data to answer two main questions: (1) is greater stomatal control in response to decreasing water availability associated with a tighter regulation of leaf water potential (Ψ_L ) across species? and (2) is there an association between tighter Ψ_L regulation (~isohydric behaviour) and lower leaf conductance over time during a drought event? These two questions are addressed at two levels: across species growing in different sites and comparing only species coexisting at a given site. Our analyses show that, across species, a tight regulation of Ψ_L is not necessarily associated with greater stomatal control or with more constrained assimilation during drought. Therefore, iso/anisohydry defined in terms of Ψ_L regulation cannot be used as an indicator of a specific mechanism of drought-induced mortality or as a proxy for overall plant vulnerability to drought.

Up-regulation of NCED3 and ABA biosynthesis occur within minutes of a decrease in leaf turgor but AHK1 is not required

A major environmental signal influencing day-time stomatal aperture is the vapour pressure deficit between the leaf and atmosphere (VPD). In angiosperms, increased VPD triggers biosynthesis of abscisic acid (ABA), prompting rapid stomatal closure. Altered cell turgor has been proposed as the trigger for ABA biosynthesis, but the timing and nature of the genetic signals linking these processes have remained uncertain. We investigated this in Arabidopsis by examining changes induced by a decrease in leaf turgor, simulating a natural increase in VPD. We found that the rate-limiting gene within the de novo ABA biosynthesis pathway, 9-cis-epoxycarotenoid dioxygenase 3 (NCED3), was induced and ABA levels increased within just 5 min of decreased leaf turgor. This rapid induction matches the time-frame for initiation of stomatal closure in response to a doubling in VPD. We further examined Arabidopsis histidine kinase1 (AHK1) as the most likely candidate for the turgor-sensing receptor involved, but found no significant difference between wild-type and an ahk1 null mutant in the induction of ABA-biosynthetic genes, ABA production, or stomatal behaviour. We show that decreased leaf turgor triggers de novo ABA biosynthesis within the time-frame of the stomatal response to VPD, but that AHK1 does not fulfil a critical role as a turgor-sensing receptor within this pathway.

Reduced abscisic acid content is responsible for enhanced sucrose accumulation by potassium nutrition in vegetable soybean seeds

In order to understand the physiological mechanism of potassium (K) application in enhancing sugar content of vegetable soybean seeds, pot experiments were conducted in 2014 and 2015 with two vegetable soybean (Glycine max L. Merr.) cultivars (c.v. Zhongkemaodou 1 and c.v. 121) under normal rate of nitrogen and phosphorus application. Three potassium (K) fertilization treatments were imposed: No K application (K0), 120 kg K2SO4 ha-1 at seeding (K1), and 120 kg K2SO4 ha-1 at seedling + 1% K2SO4 foliar application at flowering (K2). Contents of indole-3-acetic acid (IAA), gibberellins (GA), cytokinins (ZR) and abscisic acid (ABA) in seeds were determined from 4 to 8 weeks after flowering. K fertilization increased the contents of IAA, GA, ZR, soluble sugar, sucrose and fresh pod yield, but reduced ABA content consistently. When the contents of soluble sugar and sucrose reached the highest level at 7 weeks after flowering for the 2 cultivars, the contents of IAA、GA、ZR all reached the lowest level in general. The content of ABA in seed was negatively correlated with the sucrose content (P < 0.01, r = -0.749**, -0.768** in 2014 and -0.535**, -0.791** in 2015 for c.v.121 and c.v. Zhongkemaodou 1 respectively). The changes in ratio of the ABA to (IAA + GA + ZR) from 4 to 8 weeks after flowering affected by K application were coincident to the changes of sucrose accumulation. The reduced ratio of ABA/(IAA + GA + ZR) affected by K nutrition particularly reduced abscisic acid content plays a critical role in enhancing sucrose content, which might be a partial mechanism involved in K nutrition to improve the quality of vegetable soybean.

Does ozone increase ABA levels by non-enzymatic synthesis causing stomata to close?

Reactive oxygen species (ROS) are widely recognized as important regulators of stomatal aperture and plant gas exchange. The pathways through which stomata perceive ROS share many common linkages with the well characterized signalling pathway for the hormone abscisic acid (ABA), a major driver of stomatal closure. Given reports that ABA receptor mutants have no stomatal response to ozone-triggered ROS production, as well as evidence that all steps in the ABA biosynthetic pathway can be non-enzymatically converted by ROS, here we investigated the possibility that ozone closes stomata by directly converting ABA precursors to ABA. In plants where stomata were responsive to ozone, we found that foliar ABA levels rapidly increased upon exposure to ozone. Recovery of gas exchange post-exposure occurred only when ABA levels declined. Our data suggest that stomatal closure in response to ozone exposure occurs as a result of direct oxidation of ABA precursors leading to ABA production, but the importance of this ROS interaction remains uncertain under normal photosynthetic conditions.

Characterization of Soybean Genetically Modified for Drought Tolerance in Field Conditions

Drought is one of the most stressful environmental factor causing yield and economic losses in many soybean-producing regions. In the last decades, transcription factors (TFs) are being used to develop genetically modified plants more tolerant to abiotic stresses. Dehydration responsive element binding (DREB) and ABA-responsive element-binding (AREB) TFs were introduced in soybean showing improved drought tolerance, under controlled conditions. However, these results may not be representative of the way in which plants behave over the entire season in the real field situation. Thus, the objectives of this study were to analyze agronomical traits and physiological parameters of AtDREB1A (1Ab58), AtDREB2CA (1Bb2193), and AtAREB1 (1Ea2939) GM lines under irrigated (IRR) and non-irrigated (NIRR) conditions in a field experiment, over two crop seasons and quantify transgene and drought-responsive genes expression. Results from season 2013/2014 revealed that line 1Ea2939 showed higher intrinsic water use and leaf area index. Lines 1Ab58 and 1Bb2193 showed a similar behavior to wild-type plants in relation to chlorophyll content. Oil and protein contents were not affected in transgenic lines in NIRR conditions. Lodging, due to plentiful rain, impaired yield from the 1Ea2939 line in IRR conditions. qPCR results confirmed the expression of the inserted TFs and drought-responsive endogenous genes. No differences were identified in the field experiment performed in crop season 2014/2015, probably due to the optimum rainfall volume during the cycle. These field screenings showed promising results for drought tolerance. However, additional studies are needed in further crop seasons and other sites to better characterize how these plants may outperform the WT under field water deficit.

Don't ignore the green light: exploring diverse roles in plant processes

The pleasant green appearance of plants, caused by their reflectance of wavelengths in the 500-600 nm range, might give the impression that green light is of minor importance in biology. This view persists to an extent. However, there is strong evidence that these wavelengths are not only absorbed but that they also drive and regulate physiological responses and anatomical traits in plants. This review details the existing evidence of essential roles for green wavelengths in plant biology. Absorption of green light is used to stimulate photosynthesis deep within the leaf and canopy profile, contributing to carbon gain and likely crop yield. In addition, green light also contributes to the array of signalling information available to leaves, resulting in developmental adaptation and immediate physiological responses. Within shaded canopies this enables optimization of resource-use efficiency and acclimation of photosynthesis to available irradiance. In this review, we suggest that plants may use these wavelengths not just to optimize stomatal aperture but also to fine-tune whole-canopy efficiency. We conclude that all roles for green light make a significant contribution to plant productivity and resource-use efficiency. We also outline the case for using green wavelengths in applied settings such as crop cultivation in LED-based agriculture and horticulture.

What are the evolutionary origins of stomatal responses to abscisic acid in land plants?

The evolution of active stomatal closure in response to leaf water deficit, mediated by the hormone abscisic acid (ABA), has been the subject of recent debate. Two different models for the timing of the evolution of this response recur in the literature. A single-step model for stomatal control suggests that stomata evolved active, ABA-mediated control of stomatal aperture, when these structures first appeared, prior to the divergence of bryophyte and vascular plant lineages. In contrast, a gradualistic model for stomatal control proposes that the most basal vascular plant stomata responded passively to changes in leaf water status. This model suggests that active ABA-driven mechanisms for stomatal responses to water status instead evolved after the divergence of seed plants, culminating in the complex, ABA-mediated responses observed in modern angiosperms. Here we review the findings that form the basis for these two models, including recent work that provides critical molecular insights into resolving this intriguing debate, and find strong evidence to support a gradualistic model for stomatal evolution.

Bioenergy Sorghum Crop Model Predicts VPD-Limited Transpiration Traits Enhance Biomass Yield in Water-Limited Environments

Bioenergy sorghum is targeted for production in water-limited annual cropland therefore traits that improve plant water capture, water use efficiency, and resilience to water deficit are necessary to maximize productivity. A crop modeling framework, APSIM, was adapted to predict the growth and biomass yield of energy sorghum and to identify potentially useful traits for crop improvement. APSIM simulations of energy sorghum development and biomass accumulation replicated results from field experiments across multiple years, patterns of rainfall, and irrigation schemes. Modeling showed that energy sorghum's long duration of vegetative growth increased water capture and biomass yield by ~30% compared to short season crops in a water-limited production region. Additionally, APSIM was extended to enable modeling of VPD-limited transpiration traits that reduce crop water use under high vapor pressure deficits (VPDs). The response of transpiration rate to increasing VPD was modeled as a linear response until a VPD threshold was reached, at which the slope of the response decreases, representing a range of responses to VPD observed in sorghum germplasm. Simulation results indicated that the VPD-limited transpiration trait is most beneficial in hot and dry regions of production where crops are exposed to extended periods without rainfall during the season or to a terminal drought. In these environments, slower but more efficient transpiration increases biomass yield and prevents or delays the exhaustion of soil water and onset of leaf senescence. The VPD-limited transpiration responses observed in sorghum germplasm increased biomass accumulation by 20% in years with lower summer rainfall, and the ability to drastically reduce transpiration under high VPD conditions could increase biomass by 6% on average across all years. This work indicates that the productivity and resilience of bioenergy sorghum grown in water-limited environments could be further enhanced by development of genotypes with optimized VPD-limited transpiration traits and deployment of these crops in water limited growing environments. The energy sorghum model and VPD-limited transpiration trait implementation are made available to simulate performance in other target environments.

Microcompartmentation of cytosolic aldolase by interaction with the actin cytoskeleton in Arabidopsis

Evidence is accumulating for molecular microcompartments formed when proteins interact in localized domains with the cytoskeleton, organelle surfaces, and intracellular membranes. To understand the potential functional significance of protein microcompartmentation in plants, we studied the interaction of the glycolytic enzyme fructose bisphosphate aldolase with actin in Arabidopsis thaliana. Homology modelling of a major cytosolic isozyme of aldolase, FBA8, suggested that the tetrameric holoenzyme has two actin binding sites and could therefore act as an actin-bundling protein, as was reported for animal aldolases. This was confirmed by in vitro measurements of an increase in viscosity of F-actin polymerized in the presence of recombinant FBA8. Simultaneously, interaction with F-actin caused non-competitive inhibition of aldolase activity. We did not detect co-localization of an FBA8-RFP fusion protein, expressed in an fba8-knockout background, with the actin cytoskeleton using confocal laser-scanning microscopy. However, we did find evidence for a low level of interaction using FRET-FLIM analysis of FBA8-RFP co-expressed with the actin-binding protein GFP-Lifeact. Furthermore, knockout of FBA8 caused minor alterations of guard cell actin cytoskeleton morphology and resulted in a reduced rate of stomatal closure in response to decreased humidity. We conclude that cytosolic aldolase can be microcompartmented in vivo by interaction with the actin cytoskeleton and may subtly modulate guard cell behaviour as a result.

Pre-mRNA splicing repression triggers abiotic stress signaling in plants

Alternative splicing (AS) of precursor RNAs enhances transcriptome plasticity and proteome diversity in response to diverse growth and stress cues. Recent work has shown that AS is pervasive across plant species, with more than 60% of intron-containing genes producing different isoforms. Mammalian cell-based assays have discovered various inhibitors of AS. Here, we show that the macrolide pladienolide B (PB) inhibits constitutive splicing and AS in plants. Also, our RNA sequencing (RNA-seq) data revealed that PB mimics abiotic stress signals including salt, drought and abscisic acid (ABA). PB activates the abiotic stress- and ABA-responsive reporters RD29A::LUC and MAPKKK18::uidA in Arabidopsis thaliana and mimics the effects of ABA on stomatal aperture. Genome-wide analysis of AS by RNA-seq revealed that PB perturbs the splicing machinery and leads to a striking increase in intron retention and a reduction in other forms of AS. Interestingly, PB treatment activates the ABA signaling pathway by inhibiting the splicing of clade A PP2C phosphatases while still maintaining to some extent the splicing of ABA-activated SnRK2 kinases. Taken together, our data establish PB as an inhibitor and modulator of splicing and a mimic of abiotic stress signals in plants. Thus, PB reveals the molecular underpinnings of the interplay between stress responses, ABA signaling and post-transcriptional regulation in plants.

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.

Isolation of an ABA Transporter-Like 1 Gene from Arachis hypogaea That Affects ABA Import and Reduces ABA Sensitivity in Arabidopsis

Abscisic acid (ABA) transporters are essential for the transport of ABA from its sites of synthesis to its multiple sites of action within plants and are key players in plant stress responses. Despite their importance, there is limited information on ABA transporters in crop plants. In this study, we isolated and characterized an ABA transporter-like 1 (AhATL1) gene from peanut (Arachis hypogaea L.) whose cognate protein, AhATL1, is a member of the ATP-binding cassette transporter G subfamily and localizes to the plasma membrane. The expression of both the AhATL1 transcript and the corresponding protein were upregulated by water stress and treatment with exogenous ABA. Overexpression of AhATL1 in ecotype Columbia (Col) Arabidopsis (AhATL1-OX) plants reduced ABA sensitivity. When AhATL1-OX and Arabidopsis Col plants were subjected to dehydration stress, the expression of 9-cis-epoxycarotenoid dioxygenase 3 (AtNCED3) and responsive to desiccation 29 A (AtRD29A) accumulated rapidly in rosette leaves of both lines. In contrast, while expression of ATP-binding cassette G 40 (AtABCG40) was increased in Col rosette leaves, there was no change in expression of AtABCG40 in AhATL1-OX leaves. Similarly, water loss from detached leaves of AhATL1-OX plants was more rapid than from Col leaves. Therefore, we suggest that the function of AhATL1 is probably to modulate ABA sensitivity by specifically influencing ABA import into cells.

Abscisic acid signaling is controlled by a BRANCHED1/HD-ZIP I cascade in Arabidopsis axillary buds

Shoot-branching patterns determine key aspects of plant life and are important targets for crop breeding. However, we are still largely ignorant of the genetic networks controlling locally the most important decision during branch development: whether the axillary bud, or branch primordium, grows out to give a lateral shoot or remains dormant. Here we show that, inside the buds, the TEOSINTE BRANCHED1, CYCLOIDEA, PCF (TCP) transcription factor BRANCHED1 (BRC1) binds to and positively regulates the transcription of three related Homeodomain leucine zipper protein (HD-ZIP)-encoding genes: HOMEOBOX PROTEIN 21 (HB21), HOMEOBOX PROTEIN 40 (HB40), and HOMEOBOX PROTEIN 53 (HB53). These three genes, together with BRC1, enhance 9-CIS-EPOXICAROTENOID DIOXIGENASE 3 (NCED3) expression, lead to abscisic acid accumulation, and trigger hormone response, thus causing suppression of bud development. This TCP/HD-ZIP genetic module seems to be conserved in dicot and monocotyledonous species to prevent branching under light-limiting conditions.

Survival strategies of citrus rootstocks subjected to drought

Two citrus rootstocks, Rangpur lime (RL) and Sunki Maravilha mandarin (SM), were analyzed either ungrafted or grafted with their reciprocal graft combinations or with shoot scions of two commercial citrus varieties: Valencia orange (VO) and Tahiti acid lime (TAL). All graft combinations were subjected to distinct watering regimes: well-watered, severe drought and rehydration. Growth and water relation parameters, gas exchange as well as sugar and hormone profiles were determined. Data indicated that RL adopted a dehydration avoidance strategy and maintained growth, whereas SM adopted a dehydration tolerance strategy focused on plant survival. Compared with RL, the leaves and roots of SM exhibited higher concentrations of abscisic acid and salicylic acid, which induced drought tolerance, and accumulation of carbohydrates such as trehalose and raffinose, which are important reactive oxygen species scavengers. SM rootstocks were able to transfer their survival strategy to the grafted shoot scions (RL, VO, TAL). Because of their contrasting survival strategies, RL reached the permanent wilting point more quickly than SM whereas SM recovered from prolonged droughts more efficiently than RL. This is one of the most complete studies of drought tolerance mechanisms in citrus crops and is the first to use reciprocal grafting to clarify scion/rootstock interactions.

Long-distance transport of phytohormones through the plant vascular system

Phytohormones are a group of low abundance molecules that activate various metabolic and developmental processes in response to environmental and endogenous signals. Like animal hormones, plant hormones often have distinct source and target tissues, hence ensuring long-range communication at the whole-plant level. Plants rely on various hormone distribution mechanisms depending on the distance and the direction of the transport. Here, we highlight the recent findings on the long-distance movement of plant hormones within the vasculature, from the physiological role to the molecular mechanism of the transport.

Abscisic acid and other plant hormones: Methods to visualize distribution and signaling

The exploration of plant behavior on a cellular scale in a minimal invasive manner is key to understanding plant adaptations to their environment. Plant hormones regulate multiple aspects of growth and development and mediate environmental responses to ensure a successful life cycle. To monitor the dynamics of plant hormone actions in intact tissue, we need qualitative and quantitative tools with high temporal and spatial resolution. Here, we describe a set of biological instruments (reporters) for the analysis of the distribution and signaling of various plant hormones. Furthermore, we provide examples of their utility for gaining novel insights into plant hormone action with a deeper focus on the drought hormone abscisic acid.

Wheat bHLH-type transcription factor gene TabHLH1 is crucial in mediating osmotic stresses tolerance through modulating largely the ABA-associated pathway

KEY MESSAGE

Wheat bHLH family gene TabHLH1 is responsive to drought and salt stresses, and it acts as one crucial regulator in mediating tolerance to aforementioned stresses largely through an ABA-associated pathway. Osmotic stresses are adverse factors for plant growth and crop productivity. In this study, we characterized TabHLH1, a gene encoding wheat bHLH-type transcription factor (TF) protein, in mediating plant adaptation to osmotic stresses. TabHLH1 protein contains a conserved basic-helix-loop-helix (bHLH) domain shared by its plant counterparts. Upon PEG-simulated drought stress, salt stress, and exogenous abscisic acid (ABA), the TabHLH1 transcripts in roots and leaves were induced. Under PEG-simulated drought stress and salt stress treatments, the tobacco seedlings with TabHLH1 overexpression exhibited improved growth and osmotic stress-associated traits, showing increased biomass and reduced leaf water loss rate (WLR) relative to wild type (WT). The transgenic lines also possessed promoted stomata closure under drought stress, salt stress, and exogenous ABA and increased proline and soluble sugar contents and reduced hydrogen peroxide (H2O2) amount under osmotic stress conditions, indicating that TabHLH1-mediated osmolyte accumulation and cellular ROS homeostasis contributed to the drought stress and salt stress tolerance. NtPYL12 and NtSAPK2;1, the genes encoding ABA receptor and SnRK2 family kinase, respectively, showed up-regulated expression in lines overexpressing TabHLH1 under osmotic stress and exogenous ABA conditions; overexpression of them conferred plants modified stomata movement, leaf WLR, and growth feature under drought and high salinity, suggesting that these ABA-signaling genes are mediated by wheat TabHLH1 gene and involved in regulating plant responses to simulated drought and salt stresses. Our investigation indicates that the TabHLH1 gene plays critical roles in plant tolerance to osmotic stresses largely through an ABA-dependent pathway.

The site of water stress governs the pattern of ABA synthesis and transport in peanut

Abscisic acid (ABA) is one of the most important phytohormones involved in stress responses in plants. However, knowledge of the effect on ABA distribution and transport of water stress at different sites on the plant is limited. In this study, water stress imposed on peanut leaves or roots by treatment with PEG 6000 is termed “leaf stress” or “root stress”, respectively. Immunoenzyme localization technolony was first used to detect ABA distribution in peanut. Under root stress, ABA biosynthesis and distribution level were all more pronounced in root than in leaf. However, ABA transport and the ability to induce stomatal closure were still better in leaf than in root during root stress; However, ABA biosynthesis initially increased in leaf, then rapidly accumulated in the vascular cambium of leaves and induced stomatal closure under leaf stress; ABA produced in root tissues was also transported to leaf tissues to maintain stomatal closure. The vascular system was involved in the coordination and integration of this complex regulatory mechanism for ABA signal accumulation. Water stress subject to root or leaf results in different of ABA biosynthesis and transport ability that trigger stoma close in peanut.