Abscisic Acid Transport and Homeostasis in the Context of Stomatal Regulation

A review on strategies for crop improvement against drought stress through molecular insights

The demand for food goods is rising along with the world population growth, which is directly related to the yield of agricultural crops around the world. However, a number of environmental factors, including floods, salinity, moisture, and drought, have a detrimental effect on agricultural production around the world. Among all of these stresses, drought stress (DS) poses a constant threat to agricultural crops and is a significant impediment to global agricultural productivity. Its potency and severity are expected to increase in the future years. A variety of techniques have been used to generate drought-resistant plants in order to get around this restriction. Different crop plants exhibit specific traits that contribute to drought resistance (DR), such as early flowering, drought escape (DE), and leaf traits. We are highlighting numerous methods that can be used to overcome the effects of DS in this review. Agronomic methods, transgenic methods, the use of sufficient fertilizers, and molecular methods such as clustered regularly interspaced short palindromic repeats (CRISPRs)-associated nuclease 9 (Cas9), virus-induced gene silencing (VIGS), quantitative trait loci (QTL) mapping, microRNA (miRNA) technology, and OMICS-based approaches make up the majority of these techniques. CRISPR technology has rapidly become an increasingly popular choice among researchers exploring natural tolerance to abiotic stresses although, only a few plants have been produced so far using this technique. In order to address the difficulties imposed by DS, new plants utilizing the CRISPR technology must be developed.

Precision phenotyping of a barley diversity set reveals distinct drought response strategies

Plants exhibit an array of drought responses and adaptations, where the trade-off between water loss and CO2 uptake for growth is mediated by regulation of stomatal aperture in response to soil water content (SWC), among other factors. For crop yield stability, the question is how drought timing and response patterns relate to post-drought growth resilience and vigor. We earlier identified, in a few reference varieties of barley that differed by the SWC at which transpiration was curtailed, two divergent water use strategies: water-saving ("isohydric") and water-spending ("anisohydric"). We proposed that an isohydric strategy may reduce risk from spring droughts in climates where the probability of precipitation increases during the growing season, whereas the anisohydric is consistent with environments having terminal droughts, or with those where dry periods are short and not seasonally progressive. Here, we have examined drought response physiology in an 81-line barley (Hordeum vulgare L.) diversity set that spans 20th century European breeding and identified several lines with a third, dynamic strategy. We found a strong positive correlation between vigor and transpiration, the dynamic group being highest for both. However, these lines curtailed daily transpiration at a higher SWC than the isohydric group. While the dynamic lines, particularly cv Hydrogen and Baronesse, were not the most resilient in terms of restoring initial growth rates, their strong initial vigor and high return to initial transpiration rates meant that their growth nevertheless surpassed more resilient lines during recovery from drought. The results will be of use for defining barley physiological ideotypes suited to future climate scenarios.

Identification and functional characterization of ABC transporters for selenium accumulation and tolerance in soybean

ATP-binding cassette (ABC) transporters were crucial for various physiological processes like nutrition, development, and environmental interactions. Selenium (Se) is an essential micronutrient for humans, and its role in plants depends on applied dosage. ABC transporters are considered to participate in Se translocation in plants, but detailed studies in soybean are still lacking. We identified 196 ABC genes in soybean transcriptome under Se exposure using next-generation sequencing and single-molecule real-time sequencing technology. These proteins fell into eight subfamilies: 8 GmABCA, 51 GmABCB, 39 GmABCC, 5 GmABCD, 1 GmABCE, 10 GmABCF, 74 GmABCG, and 8 GmABCI, with amino acid length 121-3022 aa, molecular weight 13.50-341.04 kDa, and isoelectric point 4.06-9.82. We predicted a total of 15 motifs, some of which were specific to certain subfamilies (especially GmABCB, GmABCC, and GmABCG). We also found predicted alternative splicing in GmABCs: 60 events in selenium nanoparticles (SeNPs)-treated, 37 in sodium selenite (Na2SeO3)-treated samples. The GmABC genes showed differential expression in leaves and roots under different application of Se species and Se levels, most of which are belonged to GmABCB, GmABCC, and GmABCG subfamilies with functions in auxin transport, barrier formation, and detoxification. Protein-protein interaction and weighted gene co-expression network analysis suggested functional gene networks with hub ABC genes, contributing to our understanding of their biological functions. Our results illuminate the contributions of GmABC genes to Se accumulation and tolerance in soybean and provide insight for a better understanding of their roles in soybean as well as in other plants.

Abscisic acid root-to-shoot translocation by transporter AtABCG25 mediates stomatal movements in Arabidopsis

The phytohormone abscisic acid (ABA) plays a central role in regulating stomatal movements under drought conditions. The root-derived peptide CLAVATA3/EMBRYO SURROUNDING REGION-RELATED 25 (CLE25) moves from the root to shoot for activating ABA biosynthesis under drought conditions. However, the root-to-shoot translocation of root-derived ABA and its regulation of stomatal movements in the shoot remain to be clarified. Here, we reveal that the ABA transporter ATP-binding cassette subfamily G member 25 (AtABCG25) mediates root-to-shoot translocation of ABA and ABA-glucosyl ester (ABA-GE) in Arabidopsis (Arabidopsis thaliana). Isotope-labeled ABA tracer experiments and hormone quantification in xylem sap showed that the root-to-shoot translocation of ABA and ABA-GE was substantially impaired in the atabcg25 mutant under nondrought and drought conditions. However, the contents of ABA and ABA-GE in the leaves were lower in the atabcg25 mutant than in the wild type (WT) under nondrought but similar under drought conditions. Consistently, the stomatal closure was suppressed in the atabcg25 mutant under nondrought but not under drought conditions. The transporter activity assays showed that AtABCG25 directly exported ABA and ABA-GE in planta and in yeast (Saccharomyces cerevisiae) cells. Thus, we proposed a working model in which root-derived ABA transported by AtABCG25 via xylem mediates stomatal movements in the shoot under nondrought conditions but might exhibit little effect on stomatal movements under drought conditions. These findings extend the functions of AtABCG25 and provide insights into the long-distance translocation of ABA and its role in stomatal movements.

Overexpression of PtoMYB99 diminishes poplar tolerance to osmotic stress by suppressing ABA and JA biosynthesis

Drought poses a serious challenge to sustained plant growth and crop yields in the context of global climate change. Drought tolerance in poplars and their underlying mechanisms still remain largely unknown. In this article, we investigated the overexpression of PtoMYB99 - both a drought and abscisic acid (ABA) induced gene constraining drought tolerance in poplars (as compared with wild type poplars). First, we found that PtoMYB99-OE lines exhibited increased stomatal opening and conductance, higher transpiration and photosynthetic rates, as well as reduced levels of ABA and jasmonic acid (JA). Second, PtoMYB99-OE lines accumulated more reactive oxygen species (ROS), including H2O2 and O2-, as well as malonaldehyde (MDA), proline, and soluble sugar under osmotic stress; conversely, the activity of antioxidant enzymes (SOD, POD, and CAT), was weakened in the PtoMYB99-OE lines. Third, the expression of ABA biosynthetic genes, PtoNCED3.1 and PtoNCED3.2, as well as JA biosynthetic genes, PtoOPR3.1 and PtoOPR3.2, was significantly reduced in the PtoMYB99-OE lines under both normal conditions and osmotic stress. Based on our results, we conclude that the overexpression of PtoMYB99 compromises tolerance to osmotic stress in poplar. These findings contribute to the understanding of the role of the MYB genes in drought stress and the biosynthesis of ABA and JA.

Identification and validation of seed dormancy loci and candidate genes and construction of regulatory networks by WGCNA in maize introgression lines

KEY MESSAGE

Seventeen PHS-QTLs and candidate genes were obtained, including eleven major loci, three under multiple environments and two with co-localization by the other mapping methods; The functions of three candidate genes were validated using mutants; nine target proteins and five networks were filtered by joint analysis of GWAS and WGCNA. Seed dormancy (SD) and pre-harvest sprouting (PHS) affect yield, as well as grain and hybrid quality in seed production. Therefore, identification of genetic and regulatory pathways underlying PHS and SD is key to gene function analysis, allelic variation mining and genetic improvement. In this study, 78,360 SNPs by SLAF-seq of 230 maize chromosome segment introgression lines (ILs), PHS under five environments were used to conduct GWAS (genome wide association study) (a threshold of 1/n), and seventeen unreported PHS QTLs were obtained, including eleven QTLs with PVE > 10% and three QTLs under multiple environments. Two QTL loci were co-located between the other two genetic mapping methods. Using differential gene expression analyses at two stages of grain development, gene functional analysis of Arabidopsis mutants, and gene functional analysis in the QTL region, seventeen PHS QTL-linked candidate genes were identified, and their five molecular regulatory networks constructed. Based on the Arabidopsis T-DNA mutations, three candidate genes were shown to regulate for SD and PHS. Meanwhile, using RNA-seq of grain development, the weighted correlation network analysis (WGCNA) was performed, deducing five regulatory pathways and target genes that regulate PHS and SD. Based on the conjoint analysis of GWAS and WGCNA, four pathways, nine target proteins and target genes were revealed, most of which regulate cell wall metabolism, cell proliferation and seed dehydration tolerance. This has important theoretical and practical significance for elucidating the genetic basis of maize PHS and SD, as well as mining of genetic resources and genetic improvement of traits.

Metabolic pathways engineering for drought or/and heat tolerance in cereals

Drought (D) and heat (H) are the two major abiotic stresses hindering cereal crop growth and productivity, either singly or in combination (D/+H), by imposing various negative impacts on plant physiological and biochemical processes. Consequently, this decreases overall cereal crop production and impacts global food availability and human nutrition. To achieve global food and nutrition security vis-a-vis global climate change, deployment of new strategies for enhancing crop D/+H stress tolerance and higher nutritive value in cereals is imperative. This depends on first gaining a mechanistic understanding of the mechanisms underlying D/+H stress response. Meanwhile, functional genomics has revealed several stress-related genes that have been successfully used in target-gene approach to generate stress-tolerant cultivars and sustain crop productivity over the past decades. However, the fast-changing climate, coupled with the complexity and multigenic nature of D/+H tolerance suggest that single-gene/trait targeting may not suffice in improving such traits. Hence, in this review-cum-perspective, we advance that targeted multiple-gene or metabolic pathway manipulation could represent the most effective approach for improving D/+H stress tolerance. First, we highlight the impact of D/+H stress on cereal crops, and the elaborate plant physiological and molecular responses. We then discuss how key primary metabolism- and secondary metabolism-related metabolic pathways, including carbon metabolism, starch metabolism, phenylpropanoid biosynthesis, γ-aminobutyric acid (GABA) biosynthesis, and phytohormone biosynthesis and signaling can be modified using modern molecular biotechnology approaches such as CRISPR-Cas9 system and synthetic biology (Synbio) to enhance D/+H tolerance in cereal crops. Understandably, several bottlenecks hinder metabolic pathway modification, including those related to feedback regulation, gene functional annotation, complex crosstalk between pathways, and metabolomics data and spatiotemporal gene expressions analyses. Nonetheless, recent advances in molecular biotechnology, genome-editing, single-cell metabolomics, and data annotation and analysis approaches, when integrated, offer unprecedented opportunities for pathway engineering for enhancing crop D/+H stress tolerance and improved yield. Especially, Synbio-based strategies will accelerate the development of climate resilient and nutrient-dense cereals, critical for achieving global food security and combating malnutrition.

Drought and recovery in barley: key gene networks and retrotransposon response

Introduction

During drought, plants close their stomata at a critical soil water content (SWC), together with making diverse physiological, developmental, and biochemical responses.

Methods

Using precision-phenotyping lysimeters, we imposed pre-flowering drought on four barley varieties (Arvo, Golden Promise, Hankkija 673, and Morex) and followed their physiological responses. For Golden Promise, we carried out RNA-seq on leaf transcripts before and during drought and during recovery, also examining retrotransposon BARE1expression. Transcriptional data were subjected to network analysis.

Results

The varieties differed by their critical SWC (ϴ_crit), Hankkija 673 responding at the highest and Golden Promise at the lowest. Pathways connected to drought and salinity response were strongly upregulated during drought; pathways connected to growth and development were strongly downregulated. During recovery, growth and development pathways were upregulated; altogether, 117 networked genes involved in ubiquitin-mediated autophagy were downregulated.

Discussion

The differential response to SWC suggests adaptation to distinct rainfall patterns. We identified several strongly differentially expressed genes not earlier associated with drought response in barley. BARE1 transcription is strongly transcriptionally upregulated by drought and downregulated during recovery unequally between the investigated cultivars. The downregulation of networked autophagy genes suggests a role for autophagy in drought response; its importance to resilience should be further investigated.

High abscisic acid and low root hydraulic conductivity may explain low leaf hydration in 'Mandarin' lime exposed to aluminum

The first symptom of aluminum (Al) toxicity is the inhibition of root growth, which has been associated with low leaf hydration, with negative consequences for leaf gas exchange including stomatal conductance (gs) observed in many plant species. Here we asked whether low leaf hydration occurs before or after the inhibition of root growth of Citrus × limonia Osbeck ('Mandarin' lime) cultivated for 60 days in nutrient solution with 0 and 1480 μM Al. The length, diameter, surface area and biomass of roots of plants exposed to Al were lower than control plants only at 30 days after treatments (DAT). Until the end of the study, estimated gs (measured by sap flow techniques) was lower than in control plants from 3 DAT, total plant transpiration (Eplant) and root hydraulic conductivity (Lpr) at 7 DAT, and midday leaf water potential (Ψmd) and relative leaf water content at 15 DAT. Abscisic acid (ABA) in leaves was twofold higher in Al-exposed plants 1 DAT, and in roots a twofold higher peak was observed at 15 DAT. As ABA in leaves approached values of control plants after 15 DAT, we propose that low gs of plants exposed to Al is primarily caused by ABA, and the maintenance of low gs could be ascribed to the low Lpr from 7 DAT until the end of the study. Therefore, the low leaf hydration in 'Mandarin' lime exposed to Al does not seem to be caused by root growth inhibition or by a simple consequence of low water uptake due to a stunted root system.

The Arabidopsis D27-LIKE1 is a cis/cis/trans-β-carotene isomerase that contributes to Strigolactone biosynthesis and negatively impacts ABA level

The enzyme DWARF27 (D27) catalyzes the reversible isomerization of all-trans- into 9-cis-β-carotene, initiating strigolactone (SL) biosynthesis. Genomes of higher plants encode two D27-homologs, D27-like1 and -like2, with unknown functions. Here, we investigated the enzymatic activity and biological function of the Arabidopsis D27-like1. In vitro enzymatic assays and expression in Synechocystis sp. PCC6803 revealed an unreported 13-cis/15-cis/9-cis- and a 9-cis/all-trans-β-carotene isomerization. Although disruption of AtD27-like1 did not cause SL deficiency phenotypes, overexpression of AtD27-like1 in the d27 mutant restored the more-branching phenotype, indicating a contribution of AtD27-like1 to SL biosynthesis. Accordingly, generated d27 d27like1 double mutants showed a more pronounced branching phenotype compared to d27. The contribution of AtD27-like1 to SL biosynthesis is likely a result of its formation of 9-cis-β-carotene that was present at higher levels in AtD27-like1 overexpressing lines. By contrast, AtD27-like1 expression correlated negatively with the content of 9-cis-violaxanthin, a precursor of ABA, in shoots. Consistently, ABA levels were higher in shoots and also in dry seeds of the d27like1 and d27 d27like1 mutants. Transgenic lines expressing GUS driven by the AtD27LIKE1 promoter and transcript analysis of hormone-treated Arabidopsis seedlings revealed that AtD27LIKE1 is expressed in different tissues and affects ABA and auxin. Taken together, our work reports a cis/cis-β-carotene isomerase that affects the content of both cis-carotenoid-derived plant hormones, ABA and SLs.

Terpenoid Transport in Plants: How Far from the Final Picture?

Contrary to the biosynthetic pathways of many terpenoids, which are well characterized and elucidated, their transport inside subcellular compartments and the secretion of reaction intermediates and final products at the short- (cell-to-cell), medium- (tissue-to-tissue), and long-distance (organ-to-organ) levels are still poorly understood, with some limited exceptions. In this review, we aim to describe the state of the art of the transport of several terpene classes that have important physiological and ecological roles or that represent high-value bioactive molecules. Among the tens of thousands of terpenoids identified in the plant kingdom, only less than 20 have been characterized from the point of view of their transport and localization. Most terpenoids are secreted in the apoplast or stored in the vacuoles by the action of ATP-binding cassette (ABC) transporters. However, little information is available regarding the movement of terpenoid biosynthetic intermediates from plastids and the endoplasmic reticulum to the cytosol. Through a description of the transport mechanisms of cytosol- or plastid-synthesized terpenes, we attempt to provide some hypotheses, suggestions, and general schemes about the trafficking of different substrates, intermediates, and final products, which might help develop novel strategies and approaches to allow for the future identification of terpenoid transporters that are still uncharacterized.

An ABCG-Type Transporter Facilitates ABA Influx and Regulates Camptothecin Biosynthesis in Camptotheca acuminata

Camptothecin (CPT) and its derivatives from Camptotheca acuminata have antitumor effects as a DNA topoisomerase I inhibitor. Previous studies have shown that application of exogenous abscisic acid (ABA) significantly promoted the accumulation level of CPT and induced the expression of CPT biosynthetic genes, which revealed that ABA signaling is effectively involved in regulating CPT biosynthesis in C. acuminata. In this study, an ABA transporter, CaABAT, which encodes a plasma membrane protein belonging to the ABCG subfamily, was identified in C. acuminata, and its ABA import activity was confirmed by transport assay in yeast cells. Real-time PCR analysis showed that CaABAT was predominately expressed in C. acuminata leaves and its expression could be significantly upregulated by exogenous ABA treatment. Silencing of CaABAT down-regulated the expression of ABA response genes, which indicated that translocation of ABA by CaABAT should initiate changes in plant physiological status in response to ABA signaling, thus leading to decreased expression of CPT biosynthesis pathway genes and low accumulation levels of CPT in C. acuminata.

COP1 mutation causes low leaf temperature under various abiotic stresses in Arabidopsis thaliana

Stomata are microscopic pores on epidermal cells of leaves and stems that regulate water loss and gas exchange between the plant and its environment. Constitutive photomorphogenic 1 (COP1) is an E3 ubiquitin ligase that is involved in plant growth and development and multiple abiotic stress responses by regulating the stability of various target proteins. However, little is known about how COP1 controls stomatal aperture and leaf temperature under various environmental conditions. Here, we show that COP1 participates in leaf temperature and stomatal closure regulation under normal and stress conditions in Arabidopsis. Leaf temperature of cop1 mutants was relatively lower than that of wild type (WT) under drought, salt, and heat stress and after abscisic acid (ABA), CaCl₂, and H₂O₂ treatments. However, leaf temperature was generally higher in both WT and cop1 mutants after abiotic stress and chemical treatment than that of untreated WT and cop1 mutants. Stomatal aperture was wider in cop1 mutants than that in WT under all conditions tested, although the extent of stomatal closure varied between WT and cop1 mutants. Under dark conditions, leaf temperature was also lower in cop1 mutants than that in WT. Expression of the genes encoding ABA receptors, ABA biosynthesis proteins, positive regulators of stomatal closure and heat tolerance, and ABA-responsive proteins was lower in cop1 mutants that that in WT. In addition, expression of respiration-related genes was lower in cop1 mutants that that in WT. Taken together, the data provide evidence that mutations in COP1 lead to wider stomatal aperture and higher leaf temperature under normal and stress conditions, indicating that leaf temperature is highly correlated with stomatal aperture.

Climate change impedes plant immunity mechanisms

Rapid climate change caused by human activity is threatening global crop production and food security worldwide. In particular, the emergence of new infectious plant pathogens and the geographical expansion of plant disease incidence result in serious yield losses of major crops annually. Since climate change has accelerated recently and is expected to worsen in the future, we have reached an inflection point where comprehensive preparations to cope with the upcoming crisis can no longer be delayed. Development of new plant breeding technologies including site-directed nucleases offers the opportunity to mitigate the effects of the changing climate. Therefore, understanding the effects of climate change on plant innate immunity and identification of elite genes conferring disease resistance are crucial for the engineering of new crop cultivars and plant improvement strategies. Here, we summarize and discuss the effects of major environmental factors such as temperature, humidity, and carbon dioxide concentration on plant immunity systems. This review provides a strategy for securing crop-based nutrition against severe pathogen attacks in the era of climate change.

Transcriptome analysis of response strategy in Hemerocallis fulva under drought stress

BACKGROUND

Hemerocallis fulva is an important ground cover plant widely used in urban greening. The analysis of the molecular mechanism underlying the drought response of H. fulva can lay a foundation for improving its adaptability and expanding its planting area.

OBJECTIVE

To reveal the drought response mechanisms of H. fulva, identify candidate unigenes associated with drought response, and lay a foundation for further unigenes functional study and drought resistance improvement of H. fulva via genetic engineering.

METHODS

RNA was isolated from H. fulva under different experimental conditions. De novo transcriptomic analysis of the samples was performed to screen drought response unigenes. The transcriptional changes of candidate drought response unigenes were verified by quantitative real-time PCR.

RESULTS

The differentially expressed unigenes and their functions were analyzed after H. fulva treated by PEG-simulated drought stress and rewatering. The candidate unigenes, associated with H. fulva drought response, were identified after transcriptome analysis. Then, the transcription level of drought response unigenes of H. fulva under different conditions was further verified. Abscisic acid, protein phosphorylation, sterol biosynthesis and ion transport were involved in drought response with quick restore in H. fulva. The response unigenes, involved in hormone (ABA, JA, CK and GA) signaling pathways, defense response, high light response, karrikin response and leaf shaping, can maintain at changed expression levels even after stress withdraw.

CONCLUSION

Hemerocallis fulva has unique drought response mechanism. Negative regulation mechanism may play more important roles in drought response of H. fulva. The analysis of candidate unigenes, associated with drought response, lays a foundation for further drought resistance improvement of H. fulva.

Transcriptional Stress Memory and Transgenerational Inheritance of Drought Tolerance in Plants

Plants respond to drought stress by producing abscisic acid, a chemical messenger that regulates gene expression and thereby expedites various physiological and cellular processes including the stomatal operation to mitigate stress and promote tolerance. To trigger or suppress gene transcription under drought stress conditions, the surrounding chromatin architecture must be converted between a repressive and active state by epigenetic remodeling, which is achieved by the dynamic interplay among DNA methylation, histone modifications, loop formation, and non-coding RNA generation. Plants can memorize chromatin status under drought conditions to enable them to deal with recurrent stress. Furthermore, drought tolerance acquired during plant growth can be transmitted to the next generation. The epigenetically modified chromatin architectures of memory genes under stressful conditions can be transmitted to newly developed cells by mitotic cell division, and to germline cells of offspring by overcoming the restraints on meiosis. In mammalian cells, the acquired memory state is completely erased and reset during meiosis. The mechanism by which plant cells overcome this resetting during meiosis to transmit memory is unclear. In this article, we review recent findings on the mechanism underlying transcriptional stress memory and the transgenerational inheritance of drought tolerance in plants.

Abscisic acid: Metabolism, transport, crosstalk with other plant growth regulators, and its role in heavy metal stress mitigation

Heavy metal (HM) stress is threatening agricultural crops, ecological systems, and human health worldwide. HM toxicity adversely affects plant growth, physiological processes, and crop productivity by disturbing cellular ionic balance, metabolic balance, cell membrane integrity, and protein and enzyme activities. Plants under HM stress intrinsically develop mechanisms to counter the adversities of HM but not prevent them. However, the exogenous application of abscisic acid (ABA) is a strategy for boosting the tolerance capacity of plants against HM toxicity by improving osmolyte accumulation and antioxidant machinery. ABA is an essential plant growth regulator that modulates various plant growth and metabolic processes, including seed development and germination, vegetative growth, stomatal regulation, flowering, and leaf senescence under diverse environmental conditions. This review summarizes ABA biosynthesis, signaling, transport, and catabolism in plant tissues and the adverse effects of HM stress on crop plants. Moreover, we describe the role of ABA in mitigating HM stress and elucidating the interplay of ABA with other plant growth regulators.

Neophaseic acid catabolism in the 9'-hydroxylation pathway of abscisic acid in Arabidopsis thaliana

Abscisic acid (ABA) hydroxylation is an important pathway for ABA inactivation and homeostasis maintenance. Here, we discover a new downstream catabolite of neophaseic acid (neoPA) in the ABA 9'-hydroxyl pathway and identify it as epi-neodihydrophaseic acid (epi-neoDPA) by comparing its accurate mass, retention time, and MSⁿ spectra with those of our chemically synthesized epi-neoDPA. Analyses of Arabidopsis seed germination and ABA-related gene expression reveal that neoPA rather than epi-neoDPA possesses ABA-like hormonal activity. In vitro enzyme activity tests of prokaryotic recombinant protein reveal that NeoPAR1 (neoPA reductase 1) identified from Arabidopsis converts neoPA into epi-neoDPA. Site-directed mutation at Tyr163 in the conserved motif of NeoPAR1 abolishes the catalytic activity of NeoPAR1. Accelerated seed germination was observed in NeoPAR1 knockdown and knockout mutants, whereas retarded seed germination and the accumulation of epi-neoDPA and ABA were observed in NeoPAR1 overexpression lines, suggesting that NeoPAR1 is involved in seed germination and maintenance of ABA homeostasis.

OsWRKY114 Negatively Regulates Drought Tolerance by Restricting Stomatal Closure in Rice

The WRKY family of transcription factors plays a pivotal role in plant responses to biotic and abiotic stress. The WRKY Group III transcription factor OsWRKY114 is a positive regulator of innate immunity against Xanthomonas oryzae pv. oryzae; however, its role in abiotic stress responses is largely unknown. In this study, we showed that the abundant OsWRKY114 transcripts present in transgenic rice plants are reduced under drought conditions. The overexpression of OsWRKY114 significantly increased drought sensitivity in rice, which resulted in a lower survival rate after drought stress. Moreover, we showed that stomatal closure, which is a strategy to save water under drought, is restricted in OsWRKY114-overexpressing plants compared with wild-type plants. The expression levels of PYR/PYL/RCAR genes, such as OsPYL2 and OsPYL10 that confer drought tolerance through stomatal closure, were also markedly lower in the OsWRKY114-overexpressing plants. Taken together, these results suggest that OsWRKY114 negatively regulates plant tolerance to drought stress via inhibition of stomatal closure, which would otherwise prevent water loss in rice.

Screening and Identification of Potential Abscisic Acid Catabolites by Chemical Labeling-Assisted Ultrahigh-Performance Liquid Chromatography Coupled with High-Resolution Mass Spectrometry

In this study, a screening strategy was established based on ultrahigh-performance liquid chromatography coupled with high-resolution mass spectrometry assisted by chemical isotope labeling (CIL-UPLC-HRMS) for screening and identifying abscisic acid (ABA) catabolites. Based on the structures of known ABA catabolites, this strategy first proposed the structures of catabolites to be discovered. Afterward, a pair of isotope reagents N,N-2-dimethylaminoethylamine (DMED) and d₄-DMED were used as labeling reagents to label the carboxyl groups in ABA and its catabolites. Then, the mass-to-charge ratio (m/z) of DMED- and d₄-DMED-labeled ABA catabolites was calculated based on the labeling schema. In light of the characteristic fragmentation patterns of the DMED-labeled standards of ABA and its catabolites, screening criteria were formulated. Using our strategy, ABA, t-ABA, and 18 ABA catabolites were identified from seven plant samples. Of the identified catabolites, 16 were known, and to our knowledge, 2 were previously unidentified. Our findings contribute to ABA catabolic network improvement.

Characterization of Organellar-Specific ABA Responses during Environmental Stresses in Tobacco Cells and Arabidopsis Plants

Abscisic acid (ABA) is a critical phytohormone involved in multifaceted processes in plant metabolism and growth under both stressed and nonstressed conditions. Its accumulation in various tissues and cells has long been established as a biomarker for plant stress responses. To date, a comprehensive understanding of ABA distribution and dynamics at subcellular resolution in response to environmental cues is still lacking. Here, we modified the previously developed ABA sensor ABAleon2.1_Tao3 (Tao3) and targeted it to different organelles including the endoplasmic reticulum (ER), chloroplast/plastid, and nucleus through the addition of corresponding signal peptides. Together with the cytosolic Tao3, we show distinct ABA distribution patterns in different tobacco cells with the chloroplast showing a lower level of ABA in both cell types. In a tobacco mesophyll cell, organellar ABA displayed specific alterations depending on osmotic stimulus, with ABA levels being generally enhanced under a lower and higher concentration of salt and mannitol treatment, respectively. In Arabidopsis roots, cells from both the meristem and elongation zone accumulated ABA considerably in the cytoplasm upon mannitol treatment, while the plastid and nuclear ABA was generally reduced dependent upon specific cell types. In Arabidopsis leaf tissue, subcellular ABA seemed to be less responsive when stressed, with notable increases of ER ABA in epidermal cells and a reduction of nuclear ABA in guard cells. Together, our results present a detailed characterization of stimulus-dependent cell type-specific organellar ABA responses in tobacco and Arabidopsis plants, supporting a highly coordinated regulatory network for mediating subcellular ABA homeostasis during plant adaptation processes.

Arabidopsis ABCG27 plays an essential role in flower and leaf development by modulating abscisic acid content

Abscisic acid (ABA) is a phytohormone that mediates stress responses and regulates plant development. Several ATP-binding cassette (ABC) transporters in the G subfamily of ABC (ABCG) proteins have been reported to transport ABA. We investigated whether there are any other ABCG proteins that mediate plant developmental processes regulated by ABA in Arabidopsis (Arabidopsis thaliana). The ABCG27 gene was upregulated in response to exogenous ABA treatment. The abcg27 knockout mutant exhibited two developmental defects: epinastic leaves and abnormally long pistils, which reduced fertility and silique length. ABCG27 expression was induced threefold when flower buds were exposed to exogenous ABA, and the promoter of ABCG27 had two ABA-responsive elements. ABA content in the pistil and true leaves were increased in the abcg27 knockout mutant. Detached abcg27 pistils exposed to exogenous ABA grew longer than those of the wild-type control. ABCG27 fused to GFP localized to the plasma membrane when expressed in Arabidopsis mesophyll protoplasts. A transcriptome analysis of the pistils and true leaves of the wild type and abcg27 knockout mutant revealed that the expression of organ development-related genes changed in the knockout mutant. In particular, the expression of trans-acting small interference (ta-si) RNA processing enzyme genes, which regulate flower and leaf development, was low in the knockout mutant. Together, these results suggest that ABCG27 most likely function as an ABA transporter at the plasma membrane, modulating ABA levels and thereby regulating the development of the pistils and leaves under normal, non-stressed conditions.

ABA is important not only under stress - revealed by the discovery of new ABA transporters

Recent reports by Qin et al. and Zhang et al. cast light on long-distance abscisic acid (ABA) transport and demonstrate that ABA has a major impact on plant growth and physiology under both non-stress and stress conditions. Insights into ABA transport may provide a means to modulate the response of plants to climate change.

Mechanisms of Abscisic Acid-Mediated Drought Stress Responses in Plants

Drought is one of the major constraints to rain-fed agricultural production, especially under climate change conditions. Plants evolved an array of adaptive strategies that perceive stress stimuli and respond to these stress signals through specific mechanisms. Abscisic acid (ABA) is a premier signal for plants to respond to drought and plays a critical role in plant growth and development. ABA triggers a variety of physiological processes such as stomatal closure, root system modulation, organizing soil microbial communities, activation of transcriptional and post-transcriptional gene expression, and metabolic alterations. Thus, understanding the mechanisms of ABA-mediated drought responses in plants is critical for ensuring crop yield and global food security. In this review, we highlighted how plants adjust ABA perception, transcriptional levels of ABA- and drought-related genes, and regulation of metabolic pathways to alter drought stress responses at both cellular and the whole plant level. Understanding the synergetic role of drought and ABA will strengthen our knowledge to develop stress-resilient crops through integrated advanced biotechnology approaches. This review will elaborate on ABA-mediated drought responses at genetic, biochemical, and molecular levels in plants, which is critical for advancement in stress biology research.

An alternative, zeaxanthin epoxidase-independent abscisic acid biosynthetic pathway in plants

Abscisic acid (ABA) is an important carotenoid-derived phytohormone that plays essential roles in plant response to biotic and abiotic stresses as well as in various physiological and developmental processes. In Arabidopsis, ABA biosynthesis starts with the epoxidation of zeaxanthin by the ABA DEFICIENT 1 (ABA1) enzyme, leading to epoxycarotenoids; e.g., violaxanthin. The oxidative cleavage of 9-cis-epoxycarotenoids, a key regulatory step catalyzed by 9-CIS-EPOXYCAROTENOID DIOXYGENASE, forms xanthoxin, which is converted in further reactions mediated by ABA DEFICIENT 2 (ABA2), ABA DEFICIENT 3 (ABA3), and ABSCISIC ALDEHYDE OXIDASE 3 (AAO3) into ABA. By combining genetic and biochemical approaches, we unravel here an ABA1-independent ABA biosynthetic pathway starting upstream of zeaxanthin. We identified the carotenoid cleavage products (i.e., apocarotenoids, β-apo-11-carotenal, 9-cis-β-apo-11-carotenal, 3-OH-β-apo-11-carotenal, and 9-cis-3-OH-β-apo-11-carotenal) as intermediates of this ABA1-independent ABA biosynthetic pathway. Using labeled compounds, we showed that β-apo-11-carotenal, 9-cis-β-apo-11-carotenal, and 3-OH-β-apo-11-carotenal are successively converted into 9-cis-3-OH-β-apo-11-carotenal, xanthoxin, and finally into ABA in both Arabidopsis and rice. When applied to Arabidopsis, these β-apo-11-carotenoids exert ABA biological functions, such as maintaining seed dormancy and inducing the expression of ABA-responsive genes. Moreover, the transcriptomic analysis revealed a high overlap of differentially expressed genes regulated by β-apo-11-carotenoids and ABA, suggesting that β-apo-11-carotenoids exert ABA-independent regulatory activities. Taken together, our study identifies a biological function for the common plant metabolites, β-apo-11-carotenoids, extends our knowledge about ABA biosynthesis, and provides new insights into plant apocarotenoid metabolic networks.

Structural and Functional Insights into the Role of Guard Cell Ion Channels in Abiotic Stress-Induced Stomatal Closure

A stomatal pore is formed by a pair of specialized guard cells and serves as a major gateway for water transpiration and atmospheric CO₂ influx for photosynthesis in plants. These pores must be tightly controlled, as inadequate CO₂ intake and excessive water loss are devastating for plants. When the plants are exposed to extreme weather conditions such as high CO₂ levels, O₃, low air humidity, and drought, the turgor pressure of the guard cells exhibits an appropriate response against these stresses, which leads to stomatal closure. This phenomenon involves a complex network of ion channels and their regulation. It is well-established that the turgor pressure of guard cells is regulated by ions transportation across the membrane, such as anions and potassium ions. In this review, the guard cell ion channels are discussed, highlighting the structure and functions of key ion channels; the SLAC1 anion channel and KAT1 potassium channel, and their regulatory components, emphasizing their significance in guard cell response to various stimuli.

HSP70-16 and VDAC3 jointly inhibit seed germination under cold stress in Arabidopsis

Abscisic acid (ABA) transport plays a crucial role in seed germination under unfavourable conditions such as cold stress. Both heat shock protein 70 (HSP70) and voltage-dependent anion channel (VDAC) protein are involved in cold stress responses in Arabidopsis. However, their roles in seed germination with regard to ABA signaling remain unknown. Here we demonstrated that Arabidopsis HSP70-16 and VDAC3 jointly suppress seed germination under cold stress conditions. At 4°C, both HSP70-16 and VDAC3 facilitated the efflux of ABA from the endosperm to the embryo and thus inhibited seed germination. HSP70-16 interacted with VDAC3 on the plasma membrane and in the nucleus, and the interplay between HSP70-16 and VDAC3 activated the opening of the VDAC3 ion channel. Our work established a novel function of HSP70-16 in seed germination under cold stress and a possible association of VDAC3 activity with ABA transportation from endosperm to embryo under cold stress conditions. This study reveals that HSP70-16 interacts with VDAC3 and facilitates the opening of the VDAC3 ion channel, which influences ABA efflux from endosperm to embryo, thus negatively regulates seed germination under cold stress.

ABA homeostasis and long-distance translocation are redundantly regulated by ABCG ABA importers

The effects of abscisic acid (ABA) on plant growth, development, and response to the environment depend on local ABA concentrations. Here, we show that in Arabidopsis, ABA homeostasis is regulated by two previously unknown ABA transporters. Adenosine triphosphate–binding cassette subfamily G member 17 (ABCG17) and ABCG18 are localized to the plasma membranes of leaf mesophyll and cortex cells to redundantly promote ABA import, leading to conjugated inactive ABA sinks, thus restricting stomatal closure. ABCG17 and ABCG18 double knockdown revealed that the transporters encoded by these genes not only limit stomatal aperture size, conductance, and transpiration while increasing water use efficiency but also control ABA translocation from the shoot to the root to regulate lateral root emergence. Under abiotic stress conditions, ABCG17 and ABCG18 are transcriptionally repressed, promoting active ABA movement and response. The transport mechanism mediated by ABCG17 and ABCG18 allows plants to maintain ABA homeostasis under normal growth conditions.

Transport mechanisms of plant hormones

Plant growth, development, and response to the environment are mediated by a group of small signaling molecules named hormones. Plants regulate hormone response pathways at multiple levels, including biosynthesis, metabolism, perception, and signaling. In addition, plants exhibit the unique ability to spatially control hormone distribution. In recent years, multiple transporters have been identified for most of the plant hormones. Here we present an updated snapshot of the known transporters for the hormones abscisic acid, auxin, brassinosteroid, cytokinin, ethylene, gibberellin, jasmonic acid, salicylic acid, and strigolactone. We also describe new findings regarding hormone movement and elaborate on hormone substrate specificity and possible genetic redundancy in hormone transport and distribution. Finally, we discuss subcellular, cell-to-cell, and long-distance hormone movement and local hormone sinks that trigger or prevent hormone-mediated responses.

Wood Formation under Severe Drought Invokes Adjustment of the Hormonal and Transcriptional Landscape in Poplar

Drought is a severe environmental stress that exerts negative effects on plant growth. In trees, drought leads to reduced secondary growth and altered wood anatomy. The mechanisms underlying wood stress adaptation are not well understood. Here, we investigated the physiological, anatomical, hormonal, and transcriptional responses of poplar to strong drought. Drought-stressed xylem was characterized by higher vessel frequencies, smaller vessel lumina, and thicker secondary fiber cell walls. These changes were accompanied by strong increases in abscisic acid (ABA) and antagonistic changes in salicylic acid in wood. Transcriptional evidence supported ABA biosynthesis and signaling in wood. Since ABA signaling activates the fiber-thickening factor NST1, we expected upregulation of the secondary cell wall (SCW) cascade under stress. By contrast, transcription factors and biosynthesis genes for SCW formation were down-regulated, whereas a small set of cellulose synthase-like genes and a huge array of genes involved in cell wall modification were up-regulated in drought-stressed wood. Therefore, we suggest that ABA signaling monitors normal SCW biosynthesis and that drought causes a switch from normal to "stress wood" formation recruiting a dedicated set of genes for cell wall biosynthesis and remodeling. This proposition implies that drought-induced changes in cell wall properties underlie regulatory mechanisms distinct from those of normal wood.

OsIAA20, an Aux/IAA protein, mediates abiotic stress tolerance in rice through an ABA pathway

In plants, auxin and ABA play significant roles in conferring tolerance to environmental abiotic stresses. Earlier studies have been shown that some Aux/IAA genes, with important signaling factors in the auxin pathway, were induced in response to drought and other abiotic stresses. However, the mechanistic links between Aux/IAA expression and general drought response remain largely unknown. In this study, OsIAA20, a rice Aux/IAA protein, shown with important roles in abiotic stress. Phenotypic analyses revealed that OsIAA20 RNAi transgenic rice reduced drought and salt tolerance; whereas, OsIAA20 overexpression plants displayed the opposite phenotype. Physiological analyses of OsIAA20 RNAi rice grown under drought or salt stress showed that proline and chlorophyll content significantly decreased, while malondialdehyde content and the ratio of Na⁺/ K⁺ significantly increased. In addition, OsIAA20down-regulation reduced stomatal closure and increased the rate of water loss, while transgenic plants overexpressing OsIAA20 exhibited the opposite physiological responses. Furthermore, an ABA-responsive gene, OsRab21, was down-regulated in OsIAA20 RNAi rice lines and upregulated in OsIAA20 overexpression plants. Those results means OsIAA20 played an important role in plant drought and salt stress responses, by an ABA dependent mechanism, and it will be a candidate target gene used to breed abiotic stress tolerance.

Metabolism of abscisic acid in two contrasting rice genotypes submitted to recurrent water deficit

Drought is the main constrain for crops worldwide, however, the effects of recurrent water deficit remain still hidden. We analysed two rice genotypes, 'BRS-Querência' (lowlands) and 'AN-Cambará' (uplands), after 7 days of recurrent drought followed by 24 h of rehydration, hypothesising that genotypes grown in regions with different water availabilities respond differently to water deficits, and that a previous exposure to stress could alter abscisic acid (ABA) metabolism. The results showed that both genotypes reduced stomatal conductance and increased ABA concentration. After rehydration, the ABA levels decreased, mainly in the plants of BRS-Querência subjected to recurrent stress. However, the levels of ABA were higher in plants in recurrent water deficit compared to non-recurrent stress plants in both genotypes. Remarkably in the lowland genotype, the ABA glucosyl-ester (ABA-GE) concentration increased after recovery in the plants under recurrent stress. Regarding of gene expression, the genes associated in ABA biosynthesis with the highest expression levels were NCED2, NCED3, NCED4 and AAO2. However, 'AN-Cambará' showed less transcriptional activation. Taking into account the genes involved in ABA catabolism, ABAH1 appears to play an important role related to the recurrent stress in upland plants. These results indicate that one of the factors that can promote greater tolerance for the upland genotype is the tradeoff between ABA and ABA-GE when plants are subjected to water deficits. In addition, they indicate that abscisic acid metabolism is altered due to the genotype (upland or lowland) and pre-exposure to stress can also modify adaptive responses in rice varieties (recurrent stress).

Stomatal responses to carbon dioxide and light require abscisic acid catabolism in Arabidopsis

In plants, stomata control water loss and CO₂ uptake. The aperture and density of stomatal pores, and hence the exchange of gases between the plant and the atmosphere, are controlled by internal factors such as the plant hormone abscisic acid (ABA) and external signals including light and CO₂. In this study, we examine the importance of ABA catabolism in the stomatal responses to CO₂ and light. By using the ABA 8'-hydroxylase-deficient Arabidopsis thaliana double mutant cyp707a1 cyp707a3, which is unable to break down and instead accumulates high levels of ABA, we reveal the importance of the control of ABA concentration in mediating stomatal responses to CO₂ and light. Intriguingly, our experiments suggest that endogenously produced ABA is unable to close stomata in the absence of CO₂. Furthermore, we show that when plants are grown in short day conditions ABA breakdown is required for the modulation of both elevated [CO₂]-induced stomatal closure and elevated [CO₂]-induced reductions in leaf stomatal density. ABA catabolism is also required for the stomatal density response to light intensity, and for the full range of light-induced stomatal opening, suggesting that ABA catabolism is critical for the integration of stomatal responses to a range of environmental stimuli.

Signaling mechanisms in abscisic acid-mediated stomatal closure

The plant hormone abscisic acid (ABA) plays a central role in the regulation of stomatal movements under water-deficit conditions. The identification of ABA receptors and the ABA signaling core consisting of PYR/PYL/RCAR ABA receptors, PP2C protein phosphatases and SnRK2 protein kinases has led to studies that have greatly advanced our knowledge of the molecular mechanisms mediating ABA-induced stomatal closure in the past decade. This review focuses on recent progress in illuminating the regulatory mechanisms of ABA signal transduction, and the physiological importance of basal ABA signaling in stomatal regulation by CO2 and, as hypothesized here, vapor-pressure deficit. Furthermore, advances in understanding the interactions of ABA and other stomatal signaling pathways are reviewed here. We also review recent studies investigating the use of ABA signaling mechanisms for the manipulation of stomatal conductance and the enhancement of drought tolerance and water-use efficiency of plants.

Transcriptional Regulation of Protein Phosphatase 2C Genes to Modulate Abscisic Acid Signaling

The plant hormone abscisic acid (ABA) triggers cellular tolerance responses to osmotic stress caused by drought and salinity. ABA controls the turgor pressure of guard cells in the plant epidermis, leading to stomatal closure to minimize water loss. However, stomatal apertures open to uptake CO₂ for photosynthesis even under stress conditions. ABA modulates its signaling pathway via negative feedback regulation to maintain plant homeostasis. In the nuclei of guard cells, the clade A type 2C protein phosphatases (PP2Cs) counteract SnRK2 kinases by physical interaction, and thereby inhibit activation of the transcription factors that mediate ABA-responsive gene expression. Under osmotic stress conditions, PP2Cs bind to soluble ABA receptors to capture ABA and release active SnRK2s. Thus, PP2Cs function as a switch at the center of the ABA signaling network. ABA induces the expression of genes encoding repressors or activators of PP2C gene transcription. These regulators mediate the conversion of PP2C chromatins from a repressive to an active state for gene transcription. The stress-induced chromatin remodeling states of ABA-responsive genes could be memorized and transmitted to plant progeny; i.e., transgenerational epigenetic inheritance. This review focuses on the mechanism by which PP2C gene transcription modulates ABA signaling.

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

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

The modulation of stomatal conductance and photosynthetic parameters is involved in Fusarium head blight resistance in wheat

Fusarium head blight (FHB) is one of the most devastating fungal diseases affecting grain crops and Fusarium graminearum is the most aggressive causal species. Several evidences shown that stomatal closure is involved in the first line of defence against plant pathogens. However, there is very little evidence to show that photosynthetic parameters change in inoculated plants. The aim of the present study was to study the role of stomatal regulation in wheat after F. graminearum inoculation and explore its possible involvement in FHB resistance. RT-qPCR revealed that genes involved in stomatal regulation are induced in the resistant Sumai3 cultivar but not in the susceptible Rebelde cultivar. Seven genes involved in the positive regulation of stomatal closure were up-regulated in Sumai3, but it is most likely, that two genes, TaBG and TaCYP450, involved in the negative regulation of stomatal closure, were strongly induced, suggesting that FHB response is linked to cross-talk between the genes promoting and inhibiting stomatal closure. Increasing temperature of spikes in the wheat genotypes and a decrease in photosynthetic efficiency in Rebelde but not in Sumai3, were observed, confirming the hypothesis that photosynthetic parameters are related to FHB resistance.

Evasion of Plant Innate Defense Response by Salmonella on Lettuce

To establish host association, the innate immune system, which is one of the first lines of defense against infectious disease, must be circumvented. Plants encounter enteric foodborne bacterial pathogens under both pre- and post-harvest conditions. Human enteric foodborne pathogens can use plants as temporary hosts. This unique interaction may result in recalls and illness outbreaks associated with raw agricultural commodities. The purpose of this study was to determine if Salmonella enterica Typhimurium applied to lettuce leaves can suppress the innate stomatal defense in lettuce and utilization of UD1022 as a biocontrol against this ingression. Lettuce leaves were spot inoculated with S. Typhimurium wild type and its mutants. Bacterial culture and confocal microscopy analysis of stomatal apertures were used to support findings of differences in S. Typhimurium mutants compared to wild type. The persistence and internalization of these strains on lettuce was compared over a 7-day trial. S. Typhimurium may bypass the innate stomatal closure defense response in lettuce. Interestingly, a few key T3SS components in S. Typhimurium were involved in overriding stomatal defense response in lettuce for ingression. We also show that the T3SS in S. Typhimurium plays a critical role in persistence of S. Typhimurium in planta. Salmonella populations were significantly reduced in all UD1022 groups by day 7 with the exception of fliB and invA mutants. Salmonella internalization was not detected in plants after UD1022 treatment and had significantly higher stomatal closure rates (aperture width = 2.34 μm) by day 1 compared to controls (8.5 μm). S. Typhimurium SPI1 and SPI2 mutants showed inability to reopen stomates in lettuce suggesting the involvement of key T3SS components in suppression of innate response in plants. These findings impact issues of contamination related to plant performance and innate defense responses for plants.

Transcriptional analyses of two soybean cultivars under salt stress

Soybean is an economically important plant, and its production is affected in soils with high salinity levels. It is important to understand the adaptive mechanisms through which plants overcome this kind of stress and to identify potential genes for improving abiotic stress tolerance. RNA-Seq data of two Glycine max cultivars, a drought-sensitive (C08) and a tolerant (Conquista), subjected to different periods of salt stress were analyzed. The transcript expression profile was obtained using a transcriptogram approach, comparing both cultivars and different times of treatment. After 4 h of salt stress, Conquista cultivar had 1400 differentially expressed genes, 647 induced and 753 repressed. Comparative expression revealed that 719 genes share the same pattern of induction or repression between both cultivars. Among them, 393 genes were up- and 326 down-regulated. Salt stress also modified the expression of 54 isoforms of miRNAs in Conquista, by the maturation of 39 different pre-miRNAs. The predicted targets for 12 of those mature miRNAs also have matches with 15 differentially expressed genes from our analyses. We found genes involved in important pathways related to stress adaptation. Genes from both ABA and BR signaling pathways were modulated, with possible crosstalk between them, and with a likely post-transcriptional regulation by miRNAs. Genes related to ethylene biosynthesis, DNA repair, and plastid translation process were those that could be regulated by miRNA.

Auxin-Abscisic Acid Interactions in Plant Growth and Development

Plant hormones regulate many aspects of plant growth, development, and response to biotic and abiotic stress. Much research has gone into our understanding of individual plant hormones, focusing primarily on their mechanisms of action and the processes that they regulate. However, recent research has begun to focus on a more complex problem; how various plant hormones work together to regulate growth and developmental processes. In this review, we focus on two phytohormones, abscisic acid (ABA) and auxin. We begin with brief overviews of the hormones individually, followed by in depth analyses of interactions between auxin and ABA, focusing on interactions in individual tissues and how these interactions are occurring where possible. Finally, we end with a brief discussion and future prospects for the field.

Secreted Peptide PIP1 Induces Stomatal Closure by Activation of Guard Cell Anion Channels in Arabidopsis

Plant stomata which consist of a pair of guard cells, are not only finely controlled to balance water loss as transpiration and CO₂ absorption for photosynthesis, but also serve as the major sites to defend against pathogen attack, thus allowing plants to respond appropriately to abiotic and biotic stress conditions. The regulatory signaling network for stomatal movement is complex in nature, and plant peptides have been shown to be involved in signaling processes. Arabidopsis secreted peptide PIP1 was previously identified as an endogenous elicitor, which induced immune response through its receptor, RLK7. PIP1-RLK7 can activate stomatal immunity against the bacterial strain Pst DC3118. However, the molecular mechanism of PIP1 in stomatal regulation is still unclear and additional new factors need to be discovered. In this study, we further clarified that PIP1 could function as an important regulator in the induction of stomatal closure. The results showed that PIP1 could promote stomata to close in a certain range of concentrations and response time. In addition, we uncovered that PIP1-RLK7 signaling regulated stomatal response by activating S-type anion channel SLAC1. PIP1-induced stomatal closure was impaired in bak1, mpk3, and mpk6 mutants, indicating that BAK1 and MPK3/MPK6 were required for PIP1-regulated stomatal movement. Our research further deciphered that OST1 which acts as an essential ABA-signaling component, also played a role in PIP1-induced stomatal closure. In addition, ROS participated in PIP1-induced stomatal closure and PIP1 could activate Ca²⁺ permeable channels. In conclusion, we reveal the role of peptide PIP1 in triggering stomatal closure and the possible mechanism of PIP1 in the regulation of stomatal apertures. Our findings improve the understanding of the role of PIP1 in stomatal regulation and immune response.

Seasonal change in response of stomatal conductance to vapor pressure deficit and three phytohormones in three tree species

Stomatal behavior under global climate change is a central topic of plant ecophysiological research. Vapor pressure deficit (VPD) and phytohormones can affect stomata of leaves which can affect gas exchange characteristics of plant. The role of VPD in regulating leaf gas exchange of three tree species was investigated in Jinan, China. Experiments were performed in June, August, and October. Levels of three phytohormones (GA₃, IAA, ABA) in the leaves of the three trees were determined by high-performance liquid chromatography in three seasons. The responses of stomatal conductance (g_s) to an increasing VPD in the leaves of the three trees had peak curves under different seasons, which differed from the prevailing response pattern of g_s to VPD in most literature. The peak curve could be fitted with a Log-Normal Model (R² = 0.838-0.995). The VPD/RH values of the corresponding maximum of g_s (g_s-max-VPD/RH) could be calculated by fitted models. The g_s-max-RH could be affected by environmental conditions, because of positive correlation between g_s-max-RH and the mean monthly temperature in 2010 (R² > 0.81). Two typical stomatal models (the Leuning model and the optimal stomatal behavior model) were used to estimate g_s values, but they poorly predicted g_s in the three trees. The concentration of ABA was positively correlated to sensitivity in response of stomatal conductance to VPD in the leaves of the tree species during the different seasons.

Implantable Organic Electronic Ion Pump Enables ABA Hormone Delivery for Control of Stomata in an Intact Tobacco Plant

Electronic control of biological processes with bioelectronic devices holds promise for sophisticated regulation of physiology, for gaining fundamental understanding of biological systems, providing new therapeutic solutions, and digitally mediating adaptations of organisms to external factors. The organic electronic ion pump (OEIP) provides a unique means for electronically-controlled, flow-free delivery of ions, and biomolecules at cellular scale. Here, a miniaturized OEIP device based on glass capillary fibers (c-OEIP) is implanted in a biological organism. The capillary form factor at the sub-100 µm scale of the device enables it to be implanted in soft tissue, while its hyperbranched polyelectrolyte channel and addressing protocol allows efficient delivery of a large aromatic molecule. In the first example of an implantable bioelectronic device in plants, the c-OEIP readily penetrates the leaf of an intact tobacco plant with no significant wound response (evaluated up to 24 h) and effectively delivers the hormone abscisic acid (ABA) into the leaf apoplast. OEIP-mediated delivery of ABA, the phytohormone that regulates plant's tolerance to stress, induces closure of stomata, the microscopic pores in leaf's epidermis that play a vital role in photosynthesis and transpiration. Efficient and localized ABA delivery reveals previously unreported kinetics of ABA-induced signal propagation.

Apocarotenoids Involved in Plant Development and Stress Response

Carotenoids are isoprenoid pigments synthesized by all photosynthetic organisms and many heterotrophic microorganisms. They are equipped with a conjugated double-bond system that builds the basis for their role in harvesting light energy and in protecting the cell from photo-oxidation. In addition, the carotenoids polyene makes them susceptible to oxidative cleavage, yielding carbonyl products called apocarotenoids. This oxidation can be catalyzed by carotenoid cleavage dioxygenases or triggered nonenzymatically by reactive oxygen species. The group of plant apocarotenoids includes important phytohormones, such as abscisic acid and strigolactones, and signaling molecules, such as β-cyclocitral. Abscisic acid is a key regulator of plant's response to abiotic stress and is involved in different developmental processes, such as seed dormancy. Strigolactone is a main regulator of plant architecture and an important signaling molecule in the plant-rhizosphere communication. β-Cyclocitral, a volatile derived from β-carotene oxidation, mediates the response of cells to singlet oxygen stress. Besides these well-known examples, recent research unraveled novel apocarotenoid growth regulators and suggests the presence of yet unidentified ones. In this review, we describe the biosynthesis and biological functions of established regulatory apocarotenoids and touch on the recently identified anchorene and zaxinone, with emphasis on their role in plant growth, development, and stress response.

Preliminary Classification of the ABC Transporter Family in Betula halophila and Expression Patterns in Response to Exogenous Phytohormones and Abiotic Stresses

ATP-binding cassette (ABC) transporters comprise a transport system superfamily which is ubiquitous in eukaryotic and prokaryotic cells. In plants, ABC transporters play important roles in hormone transport and stress tolerance. In this study, 15 BhABC transporters encoded by genes identified from the transcriptome of Betula halophila were categorized into four subfamilies (ABCB, ABCF, ABCG, and ABCI) using structural domain and phylogenetic analyses. Upon B. halophila exposure to exogenous phytohormones and abiotic stressors, gene expression patterns and transcriptional responses for each subfamily of genes were obtained using semi-quantitative RT-PCR analysis. The results demonstrated that expression of most genes belonging to ABCB and ABCG subfamilies changed in response to exogenous phytohormone exposures and abiotic stress. These results suggest that BhABC genes may participate in hormone transport and that their expression may be influenced by ABA-dependent signaling pathways involved in abiotic stress responses to various stressors.

ABA-glucose ester hydrolyzing enzyme ATBG1 and PHYB antagonistically regulate stomatal development

The integration of conflicting signals in response to environmental constraints is essential to efficient plant growth and development. The light-dependent and the stress hormone abscisic acid (ABA)-dependent signaling pathways play opposite roles in many aspects of plant development. While these pathways have been extensively studied, the complex nature of their molecular dialogue is still obscure. When mobilized by the Arabidopsis thaliana β-glucosidase 1 (AtBG1), the glucose ester-conjugated inactive form of ABA has proven to be a source of the active hormone that is essential for the adaptation of the plant to water deficit, as evidenced by the impaired stomatal closure of atbg1 mutants in response to water stress. In a suppressor screen designed to identify the molecular components of AtBG1-associated physiological and developmental mechanisms, we identified the mutation variant of AtBG1 traits (vat1), a new mutant allele of the red light/far-red light photoreceptor PHYTOCHROME B (PHYB). Our study reveals that atbg1 plants harbor increased stomatal density in addition to impaired stomatal closure. We also provide evidence that the vat1/phyb mutation can restore the apparent transpiration of the atbg1 mutant by decreasing stomatal aperture and restoring a stomatal density similar to wild-type plants. Expression of key regulators of stomatal development showed a crosstalk between AtBG1-mediated ABA signaling and PHYB-mediated stomatal development. We conclude that the AtBG1-dependent regulation of ABA homeostasis and the PHYB-mediated light signaling pathways act antagonistically in the control of stomatal development.

Research Progress and Perspective on Drought Stress in Legumes: A Review

Climate change, food shortage, water scarcity, and population growth are some of the threatening challenges being faced in today's world. Drought stress (DS) poses a constant challenge for agricultural crops and has been considered a severe constraint for global agricultural productivity; its intensity and severity are predicted to increase in the near future. Legumes demonstrate high sensitivity to DS, especially at vegetative and reproductive stages. They are mostly grown in the dry areas and are moderately drought tolerant, but severe DS leads to remarkable production losses. The most prominent effects of DS are reduced germination, stunted growth, serious damage to the photosynthetic apparatus, decrease in net photosynthesis, and a reduction in nutrient uptake. To curb the catastrophic effect of DS in legumes, it is imperative to understand its effects, mechanisms, and the agronomic and genetic basis of drought for sustainable management. This review highlights the impact of DS on legumes, mechanisms, and proposes appropriate management approaches to alleviate the severity of water stress. In our discussion, we outline the influence of water stress on physiological aspects (such as germination, photosynthesis, water and nutrient uptake), growth parameters and yield. Additionally, mechanisms, various management strategies, for instance, agronomic practices (planting time and geometry, nutrient management), plant growth-promoting Rhizobacteria and arbuscular mycorrhizal fungal inoculation, quantitative trait loci (QTLs), functional genomics and advanced strategies (CRISPR-Cas9) are also critically discussed. We propose that the integration of several approaches such as agronomic and biotechnological strategies as well as advanced genome editing tools is needed to develop drought-tolerant legume cultivars.

MtABCG20 is an ABA exporter influencing root morphology and seed germination of Medicago truncatula

Abscisic acid (ABA) integrates internal and external signals to coordinate plant development, growth and architecture. It plays a central role in stomatal closure, and prevents germination of freshly produced seeds and germination of non-dormant seeds under unfavorable circumstances. Here, we describe a Medicago truncatula ATP-binding cassette (ABC) transporter, MtABCG20, as an ABA exporter present in roots and germinating seeds. In seeds, MtABCG20 was found in the hypocotyl-radicle transition zone of the embryonic axis. Seeds of mtabcg20 plants were more sensitive to ABA upon germination, due to the fact that ABA translocation within mtabcg20 embryos was impaired. Additionally, the mtabcg20 produced fewer lateral roots and formed more nodules compared with wild-type plants in conditions mimicking drought stress. Heterologous expression in Arabidopsis thaliana provided evidence that MtABCG20 is a plasma membrane protein that is likely to form homodimers. Moreover, export of ABA from Nicotiana tabacum BY2 cells expressing MtABCG20 was faster than in the BY2 without MtABCG20. Our results have implications both in legume crop research and determination of the fundamental molecular processes involved in drought response and germination.

Expression Pattern and Function Analysis of AtPPRT1, a Novel Negative Regulator in ABA and Drought Stress Responses in Arabidopsis

Abscisic acid (ABA) plays a fundamental role in plant growth and development, as well as in the responses to abiotic stresses. Previous studies have revealed that many components in ABA and drought stress signaling pathways are ubiquitinated by E3 ligases. In this study, AtPPRT1, a putative C3HC4 zinc-finger ubiquitin E3 ligase, was explored for its role in abiotic stress response in Arabidopsis thaliana. The expression of AtPPRT1 was induced by ABA. In addition, the β-glucuronidase (GUS) gene driven by the AtPPRT1 promoter was more active in the root hair zone and root tips of primary and major lateral roots of young seedlings in the presence of ABA. The assays for seed germination, stomatal aperture, root length, and water deficit demonstrated that the AtPPRT1-overexpressing Arabidopsis was insensitive to ABA and sensitive to drought stress compared with wild-type (WT) plants. The analysis by quantitative real-time PCR (qRT-PCR) revealed that the expression of three stress-inducible genes (AtRAB18, AtERD10, and AtKIN1) were upregulated in the atpprt1 mutant and downregulated in AtPPRT1-overexpressing plants, while two ABA hydrolysis genes (AtCYP707A1 and AtCYP707A3) were downregulated in the atpprt1 mutant and upregulated in AtPPRT1-overexpressing plants in the presence of ABA. AtPPRT1 was localized in the mitochondria. Our findings indicate that AtPPRT1 plays a negative role in ABA and drought stress responses.

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

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