The Arabidopsis thaliana MYB60 promoter provides a tool for the spatio-temporal control of gene expression in stomatal guard cells

Genome-wide study of the R2R3-MYB gene family and analysis of HhMYB111r-induced salt tolerance in Hibiscus hamabo Sieb. et Zucc

Hibiscus hamabo Sieb. et Zucc. (H. hamabo) is a semi-mangrove plant with excellent stress tolerance that plays a crucial role in the ecological restoration of saline and alkaline areas. It is an ideal candidate species for studying the mechanisms involved in stress tolerance. Although the MYB gene family has preliminarily been characterized in H. hamabo, the specific functions and action mechanisms of the R2R3-MYB genes in this species have not fully been elucidated. In this study, 190 R2R3-MYB genes were identified at the genomic level using bioinformatics methods. The genes were divided into 26 subgroups based on their evolutionary relationships and found to be distributed randomly on 46 chromosomes. RNA sequencing data and subsequent real-time quantitative PCR analysis of 12 differentially expressed R2R3-HhMYB genes showed HhMYB111r to be highly expressed under various abiotic stress conditions. Self-activation and subcellular localization results showed that the intact HhMYB111r had strong self-activation activity and located in both the nucleus and cytoplasm. Overexpression in Arabidopsis significantly improved salt tolerance, and silencing HhMYB111r reduced the tolerance of H. hamabo to salt stress, indicating that HhMYB111r positively regulates the salt stress response. In this first analysis of the R2R3-MYB gene family in H. hamabo, we identified a key salt stress response gene, HhMYB111r, enriching the understanding of MYB function and laying a foundation for exploring the abiotic stress response of plants.

Genome-wide investigation of MYB gene family in Areca catechu and potential roles of AcTDF in transgenic Arabidopsis

BACKGROUND

MYB protein, a crucial transcription factor, holds crucial importance in plant growth, development, stress responses, and secondary metabolite regulation. While MYB proteins have been extensively studied, research on MYBs within the palm family, particularly in Areca catechu, remains limited.

METHODS AND RESULTS

This study identified 259 MYB genes in Areca catechu, including 105 1R-MYBs, 150 R2R3-MYBs, 3 3R-MYBs, and 1 4R-MYBs. Physicochemical properties, collinearity, and gene structure of these genes were analyzed. The AcMYB is distributed across 16 chromosomes of A.catechu and has 119 and 195 homologs in Arabidopsis and rice, respectively. Cis-acting elements in the promoter region suggest roles in plant hormones, growth, development, and stress. R2R3-MYB genes were divided into eight groups based on tissue expression profiles. The flowering-related gene AcTDF is highly expressed in male flowers. Overexpression of AcTDF in Arabidopsis promotes early flowering, upregulates AtSOC1 and AtFUL, and enhances tolerance to drought and salt stress.

CONCLUSIONS

These results provide valuable insights for the identification and analysis of the MYB gene family in Areca catechu and offer a basis for the subsequent verification of its related functions and the role and significance of its role in the evolution of palms.

AcMYB96 promotes anthocyanin accumulation in onion (Allium cepa L) without forming the MBW complex

Anthocyanins are major flavonoid compounds with established health benefits. Although the molecular mechanisms of MYB transcription factors (TFs) within the MYB-basic helix-loop-helix (bHLH)-WD-repeat protein (MBW) complex in anthocyanin biosynthesis have been revealed, the functions of other MYB TFs that are unable to form the MBW complex in this process remain unclear. In this study, we uncovered and extensively characterized an R2R3-MYB TF in onion (Allium cepa L.), named AcMYB96, which was identified as a potential anthocyanin activator. AcMYB96 was classified into subgroup 1 of the R2R3-MYB TF family and lacked the conserved sequences required for interactions with bHLH IIIf TFs. Consistently, yeast two-hybrid assays showed that AcMYB96 did not interact with any bHLH IIIf TFs examined, including AcB2 and AtTT8. The transcription pattern of AcMYB96 correlated with the level of anthocyanin accumulation, and its role in activating anthocyanin biosynthesis was confirmed through overexpression in the epithelial cells of onion bulbs and Arabidopsis. Yeast one-hybrid, electrophoretic mobility shift, and promoter transactivation assays further demonstrated that AcMYB96 promoted anthocyanin biosynthesis by binding to the promoters of the chalcone synthase (AcCHS1), anthocyanidin synthase (AcANS), and UDP-glucose-flavonoid 3-O-glucosyltransferase (AcUFGT) genes, thereby activating their expression independent of bHLH IIIf TFs. These results demonstrate that AcMYB96 activates anthocyanin biosynthesis without forming the MBW complex, providing a theoretical foundation to further enrich the gene resources for promoting anthocyanin accumulation and breeding red onions with high anthocyanin content.

Molecular and physiological responses of two quinoa genotypes to drought stress

Quinoa is an important economic food crop. However, quinoa seedlings are susceptible to drought stress, and the molecular mechanism of drought tolerance remains unclear. In this study, we compared transcriptomic and physiological analyses of drought-tolerant (L1) and susceptible (HZ1) genotypes exposed to 20% PEG for 3 and 9 days at seedling stage. Compared with HZ1, drought stress had less damage to photosynthetic system, and the contents of SOD, POD and CAT were higher and the contents of H2O2 and O2 -were lower in L1 leaves. Based on the RNA-seq method, we identified 2423, 11856, 1138 and 3903 (HZ1-C3-VS-T3, HZ1-C9-vs-T9, L1-C3-vs-T3 and L1-C9-vs-T9) annotated DEGs. Go enrichment was shown in terms of Biological Process: DEGs involved in biological processes such as metabolic process, cellular process, and single-organism process were most abundant in all four comparison treatments. In Molecular Function: the molecular functions of catalytic activity, binding and transporter activity have the most DEGs in all four processes. Cellular Component: membrane, membrane part, and cell have the most DEGs in each of the four processes. These DEGs include AP2/ERF, MYB, bHLH, b-ZIP, WRKY, HD-ZIP, NAC, C3h and MADS, which encode transcription factors. In addition, the MAPK pathway, starch and sucrose metabolism, phenylpropanoid biosynthesis and plant hormone signal transduction were significantly induced under drought stress, among them, G-hydrolases-66, G-hydrolases-81, G-hydrolases-78, Su-synthase-02, Su-synthase-04, Su-synthase-06, BRI1-20 and bHLH17 were all downregulated at two drought stress points in two genotypes, PP2C01, PP2C03, PP2C05-PP2C07, PP2C10, F-box01 and F-box02 were upregulated at two drought stress points in two genotypes. These results agree with the physiological responses and RNA-seq results. Collectively, these findings may lead to a better understanding of drought tolerance, and some of the important DEGs detected in this study could be targeted for future research. And our results will provide a comprehensive basis for the molecular network that mediates drought tolerance in quinoa seedlings and promote the breeding of drought-resistant quinoa varieties.

Genome-wide association analyses using multilocus models on bananas (Musa spp.) reveal candidate genes related to morphology, fruit quality, and yield

Bananas (Musa spp.) are an essential fruit worldwide and rank as the fourth most significant food crop for addressing malnutrition due to their rich nutrients and starch content. The potential of their genetic diversity remains untapped due to limited molecular breeding tools. Our study examined a phenotypically diverse group of 124 accessions from the Colombian Musaceae Collection conserved in AGROSAVIA. We assessed 12 traits categorized into morphology, fruit quality, and yield, alongside sequence data. Our sequencing efforts provided valuable insights, with an average depth of about 7× per accession, resulting in 187,133 single-nucleotide polymorphisms (SNPs) against Musa acuminata (A genome) and 220,451 against Musa balbisiana (B genome). Population structure analysis grouped samples into four and five clusters based on the reference genome. By using different association models, we identified marker-trait associations (MTAs). The mixed linear model revealed four MTAs, while the Bayesian-information and linkage-disequilibrium iteratively nested keyway and fixed and random model for circulating probability unification models identified 82 and 70 MTAs, respectively. We identified 38 and 40 candidate genes in linkage proximity to significant MTAs for the A genome and B genome, respectively. Our findings provide insights into the genetic underpinnings of morphology, fruit quality, and yield. Once validated, the SNP markers and candidate genes can potentially drive advancements in genomic-guided breeding strategies to enhance banana crop improvement.

Genome-Wide Analysis of MYB Genes in Primulina eburnea (Hance) and Identification of Members in Response to Drought Stress

Due to periodic water deficiency in karst environments, Primulina eburnea experiences sporadic drought stress in its habitat. Despite being one of the largest gene families and functionally diverse in terms of plant growth and development, MYB transcription factors in P. eburnea have not been studied. Here, a total of 230 MYB genes were identified in P. eburnea, including 67 1R-MYB, 155 R2R3-MYB, six 3R-MYB, and two 4R-MYB genes. The R2R3-type PebMYB genes could be classified into 16 subgroups, while the remaining PebMYB genes (1R-MYB, 3R-MYB, and 4R-MYB genes) were divided into 10 subgroups. Notably, the results of the phylogenetic analysis were further supported by the motif and gene structure analysis, which showed that individuals in the same subgroup had comparable motif and structure organization. Additionally, gene duplication and synteny analyses were performed to better understand the evolution of PebMYB genes, and 291 pairs of segmental duplicated genes were found. Moreover, RNA-seq analysis revealed that the PebMYB genes could be divided into five groups based on their expression characteristics. Furthermore, 11 PebMYB genes that may be involved in drought stress response were identified through comparative analysis with Arabidopsis thaliana. Notably, seven of these genes (PebMYB3, PebMYB13, PebMYB17, PebMYB51, PebMYB142, PebMYB69, and PebMYB95) exhibited significant differences in expression between the control and drought stress treatments, suggesting that they may play important roles in drought stress response. These findings clarified the characteristics of the MYB gene family in P. eburnea, augmenting our comprehension of their potential roles in drought stress adaptation.

Single-cell RNA sequencing reveals a hierarchical transcriptional regulatory network of terpenoid biosynthesis in cotton secretory glandular cells

Plants can synthesize a wide range of terpenoids in response to various environmental cues. However, the specific regulatory mechanisms governing terpenoid biosynthesis at the cellular level remain largely elusive. In this study, we employed single-cell RNA sequencing to comprehensively characterize the transcriptome profile of cotton leaves and established a hierarchical transcriptional network regulating cell-specific terpenoid production. We observed substantial expression levels of genes associated with the biosynthesis of both volatile terpenes (such as β-caryophyllene and β-myrcene) and non-volatile gossypol-type terpenoids in secretory glandular cells. Moreover, two novel transcription factors, namely GoHSFA4a and GoNAC42, are identified to function downstream of the Gossypium PIGMENT GLAND FORMATION genes. Both transcription factors could directly regulate the expression of terpenoid biosynthetic genes in secretory glandular cells in response to developmental and environmental stimuli. For convenient retrieval of the single-cell RNA sequencing data generated in this study, we developed a user-friendly web server . Our findings not only offer valuable insights into the precise regulation of terpenoid biosynthesis genes in cotton leaves but also provide potential targets for cotton breeding endeavors.

Epicuticular wax accumulation and regulation of wax pathway gene expression during bioenergy Sorghum stem development

Bioenergy sorghum is a drought-tolerant high-biomass C4 grass targeted for production on annual cropland marginal for food crops due primarily to abiotic constraints. To better understand the overall contribution of stem wax to bioenergy sorghum's resilience, the current study characterized sorghum stem cuticular wax loads, composition, morphometrics, wax pathway gene expression and regulation using vegetative phase Wray, R07020, and TX08001 genotypes. Wax loads on sorghum stems (~103-215 µg/cm2) were much higher than Arabidopsis stem and leaf wax loads. Wax on developing sorghum stem internodes was enriched in C28/30 primary alcohols (~65%) while stem wax on fully developed stems was enriched in C28/30 aldehydes (~80%). Scanning Electron Microscopy showed minimal wax on internodes prior to the onset of elongation and that wax tubules first appear associated with cork-silica cell complexes when internode cell elongation is complete. Sorghum homologs of genes involved in wax biosynthesis/transport were differentially expressed in the stem epidermis. Expression of many wax pathway genes (i.e., SbKCS6, SbCER3-1, SbWSD1, SbABCG12, SbABCG11) is low in immature apical internodes then increases at the onset of stem wax accumulation. SbCER4 is expressed relatively early in stem development consistent with accumulation of C28/30 primary alcohols on developing apical internodes. High expression of two SbCER3 homologs in fully elongated internodes is consistent with a role in production of C28/30 aldehydes. Gene regulatory network analysis aided the identification of sorghum homologs of transcription factors that regulate wax biosynthesis (i.e., SbSHN1, SbWRI1/3, SbMYB94/96/30/60, MYS1) and other transcription factors that could regulate and specify expression of the wax pathway in epidermal cells during cuticle development.

Defining Mechanisms of C3 to CAM Photosynthesis Transition toward Enhancing Crop Stress Resilience

Global climate change and population growth are persistently posing threats to natural resources (e.g., freshwater) and agricultural production. Crassulacean acid metabolism (CAM) evolved from C3 photosynthesis as an adaptive form of photosynthesis in hot and arid regions. It features the nocturnal opening of stomata for CO2 assimilation, diurnal closure of stomata for water conservation, and high water-use efficiency. To cope with global climate challenges, the CAM mechanism has attracted renewed attention. Facultative CAM is a specialized form of CAM that normally employs C3 or C4 photosynthesis but can shift to CAM under stress conditions. It not only serves as a model for studying the molecular mechanisms underlying the CAM evolution, but also provides a plausible solution for creating stress-resilient crops with facultative CAM traits. This review mainly discusses the recent research effort in defining the C3 to CAM transition of facultative CAM plants, and highlights challenges and future directions in this important research area with great application potential.

GhMYB44 enhances stomatal closure to confer drought stress tolerance in cotton and Arabidopsis

MYB genes play crucial roles in plant response to abiotic stress. However, the function of MYB genes in cotton during abiotic stress is less well elucidated. Here, we found an R2R3-type MYB gene, GhMYB44, was induced by simulated drought (PEG6000) and ABA in three cotton varieties. After drought stress, the GhMYB44-silenced plants showed substantial changes at the physiological level, including significantly increased malondialdehyde content and decreased SOD activity. Silencing the GhMYB44 gene increased stomatal aperture and water loss rate, reduced plant drought tolerance. Transgenic Arabidopsis thaliana over-expressed GhMYB44 (GhMYB44-OE) enhanced resistance to mannitol-simulated osmotic stress. The stomatal aperture of the GhMYB44-OE Arabidopsis was significantly smaller than those of the wild type (WT), and the GhMYB44-OE Arabidopsis increased tolerance to drought stress. Transgenic Arabidopsis had higher germination rate under ABA treatment compared to WT, and the transcript levels of AtABI1, AtPP2CA and AtHAB1 were suppressed in GhMYB44-OE plants, indicating a potential role of GhMYB44 in the ABA signal pathway. These results showed that GhMYB44 acts as a positive regulator in plant response to drought stress, potentially useful for engineering drought-tolerant cotton.

A MYB Transcription Factor Atlas Provides Insights into the Evolution of Environmental Adaptations in Plants

The MYB transcription factor superfamily includes key regulators of plant development and responses to environmental changes. The diversity of lifestyles and morphological characteristics exhibited by plants are potentially associated with the genomic dynamics of the MYB superfamily. With the release of the plant genomes, a comprehensive phylogenomic analysis of the MYB superfamily across Viridiplantae is allowed. The present study performed phylogenetic, phylogenomic, syntenic, horizontal gene transfer, and neo/sub-functionalization analysis of the MYB superfamily to explore the evolutionary contributions of MYB members to species diversification, trait formation, and environmental adaptation in 437 different plant species. We identified major changes in copy number variation and genomic context within subclades across lineages. Multiple MYB subclades showed highly conserved copy number patterns and synteny across flowering plants, whereas others were more dynamic and showed lineage-specific patterns. As examples of lineage-specific morphological divergence, we hypothesize that the gain of a MYB orthogroup associated with flower development and environmental responses and an orthogroup associated with auxin and wax biosynthesis in angiosperms were correlated with the emergence of flowering plants, unbiased neo-/sub-functionalization of gene duplicates contributed to environmental adaptation, and species-specific neo-/sub-functionalization contributed to phenotype divergence between species. Transposable element insertion in promoter regions may have facilitated the sub-/neo-functionalization of MYB genes and likely played a tissue-specific role contributing to sub-/neo-functionalization in plant root tissues. This study provides new insights into the evolutionary divergence of the MYB superfamily across major flowering and non-flowering lineages and emphasizes the need for lineage-/tissue-specific characterization to further understand trait variability and environmental adaptation.

How Changes in ABA Accumulation and Signaling Influence Tomato Drought Responses and Reproductive Development

Water deficit conditions trigger the production of a chemical signal, the phytohormone abscisic acid (ABA), which coordinates multiple responses at different temporal and spatial scales. Despite the complexity of natural drought conditions, the modulation of ABA signaling could be harnessed to ameliorate the drought performances of crops in the face of increasingly challenging climate conditions. Based on recent studies, increasing ABA sensitivity can lead to genotypes with improved drought resistance traits, with sustained biomass production in water-limiting environments and little or no costs with respect to biomass production under optimal conditions. However, variations in ABA production and sensitivity lead to changes in various aspects of reproductive development, including flowering time. Here we provide an updated summary of the literature on ABA-related genes in tomato and discuss how their manipulation can impact water-deficit-related responses and/or other developmental traits. We suggest that a better understanding of specific ABA components’ function or their expression may offer novel tools to specifically engineer drought resistance without affecting developmental traits.

GsMYB7 encoding a R2R3-type MYB transcription factor enhances the tolerance to aluminum stress in soybean (Glycine max L.)

Background

MYB transcription factor (TF) is one of the largest families of TFs in plants and play essential roles in plant growth and development, and is involved in responses to biological and abiotic stress. However, there are few reports on GsMYB7 gene in soybean under aluminum acid stress, and its regulatory mechanism remains unclear.

Results

The GsMYB7 protein is localized in the nucleus and has transcriptional activation ability. Quantitative real-time PCR (qRT-PCR) results showed that GsMYB7 held a constitutive expression pattern rich in roots. When AlCl₃ concentration was 25 µM, the total root surface area (SA) of GsMYB7 transgenic lines were 34.97% higher than that of wild-type Huachun 6 (HC6). While the accumulation of Al³⁺ in root tip of transgenic plants after aluminum treatment was 17.39% lower than that of wild-type. RNA-sequencing analysis indicated that over 1181 genes were regulated by GsMYB7 and aluminum stress. Among all the regulated genes, the expression levels of glutathione peroxidase, protein kinase, cytochrome and other genes in the transgenic lines were significantly higher than those in wild type by acidic aluminum stress. The bioinformatics and qRT-PCR results showed that 9 candidate genes were induced under the treatments of acidic aluminum stress which were indirectly and/or directly regulated by GsMYB7. After AlCl₃ treatments, the transcripts of these genes in GsMYB7 transgenic seedlings were significantly higher than those of wide-type HC6.

Conclusions

The results suggested that GsMYB7 may enhance soybean tolerance to acidic aluminum stress by regulating the downstream genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08744-w.

Cloning and Functional Analysis of NoMYB60 Gene Involved in Flavonoid Biosynthesis in Watercress (Nasturtium officinale R. Br.)

The MYB60 gene belongs to the R2R3-MYB subfamily, which includes the MYB31/30/96/94 genes. Although these genes have been shown to respond to heat and drought stresses, their role in flavonoid synthesis remains unclear. In this study, NoMYB60 was cloned from watercress and its structure and function were analyzed. Sequence structure analysis showed that NoMYB60 had a highly conserved R2R3 DNA-binding region at the N-terminus. Under the treatment of ABA, SA or MeJA, the expression level of NoMYB60 first significantly increased and then decreased, indicating that ABA, SA and MeJA positively regulated NoMYB60. The subcellular localization of NoMYB60-GFP indicated that NoMYB60 was localized in the nuclear region, which is consistent with the molecular characterization of the transcription factor. Gene silencing experiments were also performed to further test the function of NoMYB60. The result showed that virus-induced silencing of NoMYB60 affected the expression of enzyme genes in flavonoid synthesis pathways and promoted the synthesis of flavonoids. Moreover, we discovered that NoMYB60 interacts with NoBEH1/2. In this study, provides a reference for research on the regulation mechanism of flavonoid synthesis in Cruciferae and other crops.

A tonoplast-localized magnesium transporter is crucial for stomatal opening in Arabidopsis under high Mg2+ conditions

Plant stomata play an important role in CO₂ uptake for photosynthesis and transpiration, but the mechanisms underlying stomatal opening and closing under changing environmental conditions are still not completely understood. Through large-scale genetic screening, we isolated an Arabidopsis mutant (closed stomata2 (cst2)) that is defective in stomatal opening. We cloned the causal gene (MGR1/CST2) and functionally characterized this gene. The mutant phenotype was caused by a mutation in a gene encoding an unknown protein with similarities to the human magnesium (Mg²⁺ ) efflux transporter ACDP/CNNM. MGR1/CST2 was localized to the tonoplast and showed transport activity for Mg²⁺ . This protein was constitutively and highly expressed in guard cells. Knockout of this gene resulted in stomatal closing, decreased photosynthesis and growth retardation, especially under high Mg²⁺ conditions, while overexpression of this gene increased stomatal opening and tolerance to high Mg²⁺ concentrations. Furthermore, guard cell-specific expression of MGR1/CST2 in the mutant partially restored its stomatal opening. Our results indicate that MGR1/CST2 expression in the leaf guard cells plays an important role in maintaining cytosolic Mg²⁺ concentrations through sequestering Mg²⁺ into vacuoles, which is required for stomatal opening, especially under high Mg²⁺ conditions.

How do plants remember drought?

Plants develop both short-term and transgenerational memory of drought stress through epigenetic regulation of transcription for a better response to subsequent exposure. Recurrent spells of droughts are more common than a single drought, with intermittent moist recovery intervals. While the detrimental effects of the first drought on plant structure and physiology are unavoidable, if survived, plants can memorize the first drought to present a more robust response to the following droughts. This includes a partial stomatal opening in the watered recovery interval, higher levels of osmoprotectants and ABA, and attenuation of photosynthesis in the subsequent exposure. Short-term drought memory is regulated by ABA and other phytohormone signaling with transcriptional memory behavior in various genes. High levels of methylated histones are deposited at the drought-tolerance genes. During the recovery interval, the RNA polymerase is stalled to be activated by a pause-breaking factor in the subsequent drought. Drought leads to DNA demethylation near drought-response genes, with genetic control of the process. Progenies of the drought-exposed plants can better adapt to drought owing to the inheritance of particular methylation patterns. However, a prolonged watered recovery interval leads to loss of drought memory, mediated by certain demethylases and chromatin accessibility factors. Small RNAs act as critical regulators of drought memory by altering transcript levels of drought-responsive target genes. Further studies in the future will throw more light on the genetic control of drought memory and the interplay of genetic and epigenetic factors in its inheritance. Plants from extreme environments can give queues to understanding robust memory responses at the ecosystem level.

Evolution and functional diversification of R2R3-MYB transcription factors in plants

R2R3-MYB genes (R2R3-MYBs) form one of the largest transcription factor gene families in the plant kingdom, with substantial structural and functional diversity. However, the evolutionary processes leading to this amazing functional diversity have not yet been clearly established. Recently developed genomic and classical molecular technologies have provided detailed insights into the evolutionary relationships and functions of plant R2R3-MYBs. Here, we review recent genome-level and functional analyses of plant R2R3-MYBs, with an emphasis on their evolution and functional diversification. In land plants, this gene family underwent a large expansion by whole genome duplications and small-scale duplications. Along with this population explosion, a series of functionally conserved or lineage-specific subfamilies/groups arose with roles in three major plant-specific biological processes: development and cell differentiation, specialized metabolism, and biotic and abiotic stresses. The rapid expansion and functional diversification of plant R2R3-MYBs are highly consistent with the increasing complexity of angiosperms. In particular, recently derived R2R3-MYBs with three highly homologous intron patterns (a, b, and c) are disproportionately related to specialized metabolism and have become the predominant subfamilies in land plant genomes. The evolution of plant R2R3-MYBs is an active area of research, and further studies are expected to improve our understanding of the evolution and functional diversification of this gene family.

Single-cell transcriptomics sheds light on the identity and metabolism of developing leaf cells

As the main photosynthetic instruments of vascular plants, leaves are crucial and complex plant organs. A strict organization of leaf mesophyll and epidermal cell layers orchestrates photosynthesis and gas exchange. In addition, water and nutrients for leaf growth are transported through the vascular tissue. To establish the single-cell transcriptomic landscape of these different leaf tissues, we performed high-throughput transcriptome sequencing of individual cells isolated from young leaves of Arabidopsis (Arabidopsis thaliana) seedlings grown in two different environmental conditions. The detection of approximately 19,000 different transcripts in over 1,800 high-quality leaf cells revealed 14 cell populations composing the young, differentiating leaf. Besides the cell populations comprising the core leaf tissues, we identified subpopulations with a distinct identity or metabolic activity. In addition, we proposed cell-type-specific markers for each of these populations. Finally, an intuitive web tool allows for browsing the presented dataset. Our data present insights on how the different cell populations constituting a developing leaf are connected via developmental, metabolic, or stress-related trajectories.

The AtMYB60 transcription factor regulates stomatal opening by modulating oxylipin synthesis in guard cells

Stomata are epidermal pores formed by pairs of specialized guard cells, which regulate gas exchanges between the plant and the atmosphere. Modulation of transcription has emerged as an important level of regulation of stomatal activity. The AtMYB60 transcription factor was previously identified as a positive regulator of stomatal opening, although the details of its function remain unknown. Here, we propose a role for AtMYB60 as a negative modulator of oxylipins synthesis in stomata. The atmyb60-1 mutant shows reduced stomatal opening and accumulates increased levels of 12-oxo-phytodienoic acid (12-OPDA), jasmonic acid (JA) and jasmonoyl-L-isoleucine (JA-Ile) in guard cells. We provide evidence that 12-OPDA triggers stomatal closure independently of JA and cooperatively with abscisic acid (ABA) in atmyb60-1. Our study highlights the relevance of oxylipins metabolism in stomatal regulation and indicates AtMYB60 as transcriptional integrator of ABA and oxylipins responses in guard cells.

Harnessing tissue-specific genome editing in plants through CRISPR/Cas system: current state and future prospects

In a nutshell, tissue-specific CRISPR/Cas genome editing is the most promising approach for crop improvement which can bypass the hurdle associated with constitutive GE such as off target and pleotropic effects for targeted crop improvement. CRISPR/Cas is a powerful genome-editing tool with a wide range of applications for the genetic improvement of crops. However, the constitutive genome editing of vital genes is often associated with pleiotropic effects on other genes, needless metabolic burden, or interference in the cellular machinery. Tissue-specific genome editing (TSGE), on the other hand, enables researchers to study those genes in specific cells, tissues, or organs without disturbing neighboring groups of cells. Until recently, there was only limited proof of the TSGE concept, where the CRISPR-TSKO tool was successfully used in Arabidopsis, tomato, and cotton, laying a solid foundation for crop improvement. In this review, we have laid out valuable insights into the concept and application of TSGE on relatively unexplored areas such as grain trait improvement under favorable or unfavorable conditions. We also enlisted some of the prominent tissue-specific promoters and described the procedure of their isolation with several TSGE promoter expression systems in detail. Moreover, we highlighted potential negative regulatory genes that could be targeted through TSGE using tissue-specific promoters. In a nutshell, tissue-specific CRISPR/Cas genome editing is the most promising approach for crop improvement which can bypass the hurdle associated with constitutive GE such as off target and pleotropic effects for targeted crop improvement.

Arabidopsis NPF4.6 and NPF5.1 Control Leaf Stomatal Aperture by Regulating Abscisic Acid Transport

The plant hormone abscisic acid (ABA) is actively synthesized in vascular tissues and transported to guard cells to promote stomatal closure. Although several transmembrane ABA transporters have been identified, how the movement of ABA within plants is regulated is not fully understood. In this study, we determined that Arabidopsis NPF4.6, previously identified as an ABA transporter expressed in vascular tissues, is also present in guard cells and positively regulates stomatal closure in leaves. We also found that mutants defective in NPF5.1 had a higher leaf surface temperature compared to the wild type. Additionally, NPF5.1 mediated cellular ABA uptake when expressed in a heterologous yeast system. Promoter activities of NPF5.1 were detected in several leaf cell types. Taken together, these observations indicate that NPF5.1 negatively regulates stomatal closure by regulating the amount of ABA that can be transported from vascular tissues to guard cells.

Genome-Wide Comparative Analysis of R2R3 MYB Gene Family in Populus and Salix and Identification of Male Flower Bud Development-Related Genes

The MYB transcription factor (TF) family is one of the largest plant transcription factor gene family playing vital roles in plant growth and development, including defense, cell differentiation, secondary metabolism, and responses to biotic and abiotic stresses. As a model tree species of woody plants, in recent years, the identification and functional prediction of certain MYB family members in the poplar genome have been reported. However, to date, the characterization of the gene family in the genome of the poplar's sister species willow has not been done, nor are the differences and similarities between the poplar and willow genomes understood. In this study, we conducted the first genome-wide investigation of the R2R3 MYB subfamily in the willow, identifying 216 R2R3 MYB gene members, and combined with the poplar R2R3 MYB genes, performed the first comparative analysis of R2R3 MYB genes between the poplar and willow. We identified 81 and 86 pairs of R2R3 MYB paralogs in the poplar and willow, respectively. There were 17 pairs of tandem repeat genes in the willow, indicating active duplication of willow R2R3 MYB genes. A further 166 pairs of poplar and willow orthologs were identified by collinear and synonymous analysis. The findings support the duplication of R2R3 MYB genes in the ancestral species, with most of the R2R3 MYB genes being retained during the evolutionary process. The phylogenetic trees of the R2R3 MYB genes of 10 different species were drawn. The functions of the poplar and willow R2R3 MYB genes were predicted using reported functional groupings and clustering by OrthoFinder. Identified 5 subgroups in general expanded in woody species, three subgroups were predicted to be related to lignin synthesis, and we further speculate that the other two subgroups also play a role in wood formation. We analyzed the expression patterns of the GAMYB gene of subgroup 18 (S18) related to pollen development in the male flower buds of poplar and willow at different developmental stages by qRT-PCR. The results showed that the GAMYB gene was specifically expressed in the male flower bud from pollen formation to maturity, and that the expression first increased and then decreased. Both the specificity of tissue expression specificity and conservation indicated that GAMYB played an important role in pollen development in both poplar and willow and was an ideal candidate gene for the analysis of male flower development-related functions of the two species.

The physiology of plant responses to drought

Drought alone causes more annual loss in crop yield than all pathogens combined. To adapt to moisture gradients in soil, plants alter their physiology, modify root growth and architecture, and close stomata on their aboveground segments. These tissue-specific responses modify the flux of cellular signals, resulting in early flowering or stunted growth and, often, reduced yield. Physiological and molecular analyses of the model plant Arabidopsis thaliana have identified phytohormone signaling as key for regulating the response to drought or water insufficiency. Here we discuss how engineering hormone signaling in specific cells and cellular domains can facilitate improved plant responses to drought. We explore current knowledge and future questions central to the quest to produce high-yield, drought-resistant crops.

The Circadian Clock Influences the Long-Term Water Use Efficiency of Arabidopsis

In plants, water use efficiency (WUE) is a complex trait arising from numerous physiological and developmental characteristics. Here, we investigated the involvement of circadian regulation in long-term WUE in Arabidopsis (Arabidopsis thaliana) under light and dark conditions. Circadian rhythms are generated by the circadian oscillator, which provides a cellular measure of the time of day. In plants, the circadian oscillator contributes to the regulation of many aspects of physiology, including stomatal opening, rate of photosynthesis, carbohydrate metabolism, and developmental processes such as the initiation of flowering. We investigated the impact of the misregulation of numerous genes encoding various components of the circadian oscillator on whole plant, long-term WUE. From this analysis, we identified a role for the circadian oscillator in WUE. It appears that the circadian clock contributes to the control of transpiration and biomass accumulation. We also established that the circadian oscillator within guard cells can contribute to long-term WUE. Our experiments indicate that knowledge of circadian regulation will be important for developing crops with improved WUE.

Phylogenomic Evidence for the Monophyly of Bryophytes and the Reductive Evolution of Stomata

The origin of land plants was accompanied by new adaptations to life on land, including the evolution of stomata-pores on the surface of plants that regulate gas exchange. The genes that underpin the development and function of stomata have been extensively studied in model angiosperms, such as Arabidopsis. However, little is known about stomata in bryophytes, and their evolutionary origins and ancestral function remain poorly understood. Here, we resolve the position of bryophytes in the land plant tree and investigate the evolutionary origins of genes that specify stomatal development and function. Our analyses recover bryophyte monophyly and demonstrate that the guard cell toolkit is more ancient than has been appreciated previously. We show that a range of core guard cell genes, including SPCH/MUTE, SMF, and FAMA, map back to the common ancestor of embryophytes or even earlier. These analyses suggest that the first embryophytes possessed stomata that were more sophisticated than previously envisioned and that the stomata of bryophytes have undergone reductive evolution, including their complete loss from liverworts.

Abscisic acid dynamics, signaling, and functions in plants

Abscisic acid (ABA) is an important phytohormone regulating plant growth, development, and stress responses. It has an essential role in multiple physiological processes of plants, such as stomatal closure, cuticular wax accumulation, leaf senescence, bud dormancy, seed germination, osmotic regulation, and growth inhibition among many others. Abscisic acid controls downstream responses to abiotic and biotic environmental changes through both transcriptional and posttranscriptional mechanisms. During the past 20 years, ABA biosynthesis and many of its signaling pathways have been well characterized. Here we review the dynamics of ABA metabolic pools and signaling that affects many of its physiological functions.

Morpho-Pathological and Global Transcriptomic Analysis Reveals the Robust Nonhost Resistance Responses in Chickpea Interaction with Alternaria brassicae

Alternaria blight, caused by Alternaria brassicae, causes considerable yield loss in Brassica crops. While several blight-resistant varieties have been developed using resistance sources from host germplasm, none of them are entirely successful in imparting durable resistance. This has prompted the exploration of novel gene pools of nonhost plant species. Nonhost resistance (NHR) is a durable form of resistance, comprising pre- and postinvasion layers of defense. We aimed to identify the molecular basis of NHR to A. brassicae and identify the layers of NHR operating in a nonhost, chickpea (Cicer arietinum). To elucidate the layers of NHR operating against A. brassicae, we compared the histopathology and infection patterns of A. brassicae in C. arietinum and Brassica juncea. Delayed conidial germination, impeded hyphal growth, suppressed appressorium formation, and limited hyphal penetration occurred in the nonhost plant compared with the host plant, implying the involvement of the preinvasion layer of NHR in C. arietinum. Next, we investigated the molecular basis of robust NHR, in C. arietinum challenged with A. brassicae, by microarray-based global transcriptome profiling. Genes involved in stomatal closure, cuticular wax biosynthesis, cell-wall modification, and secondary metabolite production (contributing to preinvasion NHR) as well as reactive oxygen species (ROS) and cell death (contributing to postinvasion NHR) were found to be upregulated. Consistent with transcriptomic analysis, the morpho-pathological analysis revealed stomatal closure, ROS accumulation, and localized cell death in C. arietinum as the defense strategies against A. brassicae. Thus, we identified NHR-contributing genes with potential applications in blight resistance gene transfer to B. juncea.

Review: ABA, flavonols, and the evolvability of land plants

There is evidence that the ABA signaling pathway has greatly contributed to increase the complexity of land plants, thereby sustaining their ability to adapt in an ever-changing environment. The regulatory functions of the ABA signaling pathway go well beyond the movements of stomata and the dormancy of seeds. For instance, the ABA signaling regulates the flavonoid biosynthesis, consistent with the high integration of ABA and light signaling pathways, which occurs at the level of key signaling components, such as the bZIP transcription factors HY5 and ABI5. Here we focus on the regulation of 'colorless' (UV-absorbing) flavonol biosynthesis by the ABA signaling and, about how flavonols may regulate, in turn, the ABA signaling network. We discuss very recent findings that quercetin regulates the ABA signaling pathway, and hypothesize this might occur at the level of second messenger and perhaps of primary signaling components as well. We critically review old and recent suggestions of the primary roles played by flavonols, the ancient class of flavonoids already present in bryophytes, in the evolution of terrestrial plants. Our reasoning strongly supports the view that the ABA-flavonol relationship may represent a robust trait of land plants, and might have contributed to their adaptation on land.

A Novel Sweetpotato Transcription Factor Gene IbMYB116 Enhances Drought Tolerance in Transgenic Arabidopsis

Several members of the MYB transcription factor family have been found to regulate growth, developmental processes, metabolism, and biotic and abiotic stress responses in plants. However, the role of MYB116 in plants is still unclear. In this study, a MYB transcription factor gene IbMYB116 was cloned and characterized from the sweetpotato [Ipomoea batatas (L.) Lam.] line Xushu55-2, a line that is considered to be drought resistant. We show here that IbMYB116 is a nuclear protein and that it possesses a transactivation domain at the C terminus. This gene exhibited a high expression level in the leaf tissues of Xushu55-2 and was strongly induced by PEG6000 and methyl-jasmonate (MeJA). The IbMYB116-overexpressing Arabidopsis plants showed significantly enhanced drought tolerance, increased MeJA content, and a decreased H₂O₂ level under drought stress. The overexpression of IbMYB116 in Arabidopsis systematically upregulated jasmonic acid (JA) biosynthesis genes and activated the JA signaling pathway as well as reactive oxygen species (ROS)-scavenging system genes under drought stress conditions. The overall results suggest that the IbMYB116 gene might enhance drought tolerance by activating a ROS-scavenging system through the JA signaling pathway in transgenic Arabidopsis. These findings reveal, for the first time, the crucial role of IbMYB116 in the drought tolerance of plants.

COP1 plays a prominent role in drought stress tolerance in Arabidopsis and Pea

Constitutively photomorphogenic 1 (COP1) is an E3 ubiquitin ligase that has been studied extensively in the photomorphogenesis- and light-related processes in Arabidopsis. However, the possible role of COP1 in plant drought stress response remains unknown. Hence, in the present study, the stomatal behavior as one of the key elements in plant dehydration response was investigated in Arabidopsis cop1-4 and pea light-independent photomorphogenesis (lip1) mutants. We observed that water loss rate in the cop1-4 and lip1 detached leaves was significantly much faster than wild-type, resulting from failing to reduce the stomatal aperture by the mutants. But, interestingly, the cop1-4 and lip1 isolated leaves treated with abscisic acid (ABA) as well as cop1-4 and lip1 soil-grown under drought stress could close their stomata as wild-type. Hence, COP1 plays a fundamental role in the regulation of stomatal movements in response to dehydration and its function was conserved during evolution in both Arabidopsis and pea. Further evaluations showed the cop1-4 mutant was not significantly damaged from the oxidative stress derived from soil water limiting conditions when compared to wild-type. Similarly, the up-regulation level of several key stress-responsive genes was relatively lower in cop1-4 than wild-type. Therefore, COP1 might be considered as a potential key regulator of both short-and long-term dehydration response. Multiple stress-related cis-elements were also detected in the COP1 promoter region, which supported its up-regulation in response to drought, salt, and cold stresses. Besides, we figured out the constitutively open stomata of cop1-4 in darkness can be as a result of the reduced AtMYB61 expression.

A wheat MYB transcriptional repressor TaMyb1D regulates phenylpropanoid metabolism and enhances tolerance to drought and oxidative stresses in transgenic tobacco plants

MYB transcription factors are involved in the regulation of plant development and response to biotic and abiotic stress. In this study, TaMyb1D, a novel subgroup 4 gene of the R2R3-MYB subfamily, was cloned from wheat (Triticum aestivum L.). TaMyb1D was localized in the nucleus and functioned as a transcriptional repressor. The overexpression of TaMyb1D in tobacco (Nicotiana tabacum) plants repressed the expression of genes related to phenylpropanoid metabolism and down-regulated the accumulation of lignin in stems and flavonoids in leaves. These changes affected plant development under normal conditions. The expression of TaMyb1D was ubiquitous and up-regulated by PEG6000 and H2O2 treatments in wheat. TaMyb1D-overexpressing transgenic tobacco plants exhibited higher relative water content and lower water loss rate during drought stress, as well as higher chlorophyll content in leaves during oxidative stress. The transgenic plants showed a lower leakage of ions as well as reduced malondialdehyde and H2O2 levels during conditions of drought and oxidative stresses. In addition, TaMyb1D up-regulated the expression levels of ROS- and stress-related genes in response to drought stress. Therefore, the overexpression of TaMyb1D enhanced tolerance to drought and oxidative stresses in tobacco plants. Our study demonstrates that TaMyb1D functions as a negative regulator of phenylpropanoid metabolism and a positive regulator of plant tolerance to drought and oxidative stresses.

GUS Staining of Guard Cells to Identify Localised Guard Cell Gene Expression

Determination of a gene expression in guard cells is essential for studying stomatal movements. GUS staining is one means of detecting the localization of a gene expression in guard cells. If a gene is specially expressed in guard cells, the whole cotyledons or rosette leaf can be used for GUS staining. However, if a gene is expressed in both mesophyll and guard cells, it is hard to exhibit a clear expression of the gene in guard cells by a GUS staining image from leaf. To gain a clear guard cell GUS image of small G protein ROP7, a gene expressed in both mesophyll and guard cells, we peeled the epidermal strips from the leaf of 3-4 week-old plants. After removing the mesophyll cells, the epidermal strips were used for GUS staining. We compared the GUS staining images from epidermal strips or leaf of small G protein ROP7 and RopGEF4, a gene specifically expressed in guard cells, and found that GUS staining of epidermal strips provided a good method to show the guard cell expression of a gene expressed in both mesophyll and guard cells. This protocol is applicable for any genes that are expressed in guard cells of Arabidopsis, or other plants that epidermal strips can be easily peeled from the leaf.

The Solanum tuberosum KST1 partial promoter as a tool for guard cell expression in multiple plant species

To date, guard cell promoters have been examined in only a few species, primarily annual dicots. A partial segment of the potato (Solanum tuberosum) KST1 promoter (KST1 partial promoter, KST1ppro) has previously been shown to confer guard cell expression in potato, tomato (Solanum lycopersicum), citrus [Troyer citrange (C. sinensis×Poncirus trifoliata)], and Arabidopsis (Arabidopsis thaliana). Here, we describe an extensive analysis of the expression pattern of KST1ppro in eight (previously reported, as well as new) species from five different angiosperm families, including the Solanaceae and the Cucurbitaceae, Arabidopsis, the monocot barley (Hordeum vulgare), and two perennial species: grapevine (Vitis vinifera) and citrus. Using confocal imaging and three-dimensional movies, we demonstrate that KST1ppro drives guard cell expression in all of these species, making it the first dicot-originated guard cell promoter shown to be active in a monocot and the first promoter reported to confer guard cell expression in barley and cucumber (Cucumis sativus). The results presented here indicate that KST1ppro can be used to drive constitutive guard cell expression in monocots and dicots and in both annual and perennial plants. In addition, we show that the KST1ppro is active in guard cells shortly after the symmetric division of the guard mother cell and generates stable expression in mature guard cells. This allows us to follow the spatial and temporal distribution of stomata in cotyledons and true leaves.

Two Chloroplast Proteins Suppress Drought Resistance by Affecting ROS Production in Guard Cells

Chloroplast as the site for photosynthesis is an essential organelle in plants, but little is known about its role in stomatal regulation and drought resistance. In this study, we show that two chloroplastic proteins essential for thylakoid formation negatively regulate drought resistance in Arabidopsis (Arabidopsis thaliana). By screening a mutant pool with T-DNA insertions in nuclear genes encoding chloroplastic proteins, we identified an HCF106 knockdown mutant exhibiting increased resistance to drought stress. The hcf106 mutant displayed elevated levels of reactive oxygen species (ROS) in guard cells, improved stomatal closure, and reduced water loss under drought conditions. The HCF106 protein was found to physically interact with THF1, a previously identified chloroplastic protein crucial for thylakoid formation. The thf1 mutant phenotypically resembled the hcf106 mutant and displayed more ROS accumulation in guard cells, increased stomatal closure, reduced water loss, and drought resistant phenotypes compared to the wild type. The hcf106thf1 double mutant behaved similarly as the thf1 single mutant. These results suggest that HCF106 and THF1 form a complex to modulate chloroplast function and that the complex is important for ROS production in guard cells and stomatal control in response to environmental stresses. Our results also suggest that modulating chloroplastic proteins could be a way for improving drought resistance in crops.

Transcription Factors and Plants Response to Drought Stress: Current Understanding and Future Directions

Increasing vulnerability of plants to a variety of stresses such as drought, salt and extreme temperatures poses a global threat to sustained growth and productivity of major crops. Of these stresses, drought represents a considerable threat to plant growth and development. In view of this, developing staple food cultivars with improved drought tolerance emerges as the most sustainable solution toward improving crop productivity in a scenario of climate change. In parallel, unraveling the genetic architecture and the targeted identification of molecular networks using modern "OMICS" analyses, that can underpin drought tolerance mechanisms, is urgently required. Importantly, integrated studies intending to elucidate complex mechanisms can bridge the gap existing in our current knowledge about drought stress tolerance in plants. It is now well established that drought tolerance is regulated by several genes, including transcription factors (TFs) that enable plants to withstand unfavorable conditions, and these remain potential genomic candidates for their wide application in crop breeding. These TFs represent the key molecular switches orchestrating the regulation of plant developmental processes in response to a variety of stresses. The current review aims to offer a deeper understanding of TFs engaged in regulating plant's response under drought stress and to devise potential strategies to improve plant tolerance against drought.

Transcriptional regulatory networks in Arabidopsis thaliana during single and combined stresses

Differentially evolved responses to various stress conditions in plants are controlled by complex regulatory circuits of transcriptional activators, and repressors, such as transcription factors (TFs). To understand the general and condition-specific activities of the TFs and their regulatory relationships with the target genes (TGs), we have used a homogeneous stress gene expression dataset generated on ten natural ecotypes of the model plant Arabidopsis thaliana, during five single and six combined stress conditions. Knowledge-based profiles of binding sites for 25 stress-responsive TF families (187 TFs) were generated and tested for their enrichment in the regulatory regions of the associated TGs. Condition-dependent regulatory sub-networks have shed light on the differential utilization of the underlying network topology, by stress-specific regulators and multifunctional regulators. The multifunctional regulators maintain the core stress response processes while the transient regulators confer the specificity to certain conditions. Clustering patterns of transcription factor binding sites (TFBS) have reflected the combinatorial nature of transcriptional regulation, and suggested the putative role of the homotypic clusters of TFBS towards maintaining transcriptional robustness against cis-regulatory mutations to facilitate the preservation of stress response processes. The Gene Ontology enrichment analysis of the TGs reflected sequential regulation of stress response mechanisms in plants.

Plant MYB Transcription Factors: Their Role in Drought Response Mechanisms

Water scarcity is one of the major causes of poor plant performance and limited crop yields worldwide and it is the single most common cause of severe food shortage in developing countries. Several molecular networks involved in stress perception, signal transduction and stress responses in plants have been elucidated so far. Transcription factors are major players in water stress signaling. In recent years, different MYB transcription factors, mainly in Arabidopsis thaliana (L.) Heynh. but also in some crops, have been characterized for their involvement in drought response. For some of them there is evidence supporting a specific role in response to water stress, such as the regulation of stomatal movement, the control of suberin and cuticular waxes synthesis and the regulation of flower development. Moreover, some of these genes have also been characterized for their involvement in other abiotic or biotic stresses, an important feature considering that in nature, plants are often simultaneously subjected to multiple rather than single environmental perturbations. This review summarizes recent studies highlighting the role of the MYB family of transcription factors in the adaptive responses to drought stress. The practical application value of MYBs in crop improvement, such as stress tolerance engineering, is also discussed.

Physiological and transcriptional memory in guard cells during repetitive dehydration stress

Arabidopsis plants subjected to a daily dehydration stress and watered recovery cycle display physiological and transcriptional stress memory. Previously stressed plants have stomatal apertures that remain partially closed during a watered recovery period, facilitating reduced transpiration during a subsequent dehydration stress. Guard cells (GCs) display transcriptional memory that is similar to that in leaf tissues for some genes, but display GC-specific transcriptional memory for other genes. The rate-limiting abscisic acid (ABA) biosynthetic genes NINE-CIS-EPOXYCAROTENOID DIOXYGENASE 3 (NCED3) and ALDEHYDE OXIDASE 3 (AAO3) are expressed at much higher levels in GCs, particularly during the watered recovery interval, relative to their low levels in leaves. A genetic analysis using mutants in the ABA signaling pathway indicated that GC stomatal memory is ABA-dependent, and that ABA-dependent SNF1-RELATED PROTEIN KINASE 2.2 (SnRK2.2), SnRK2.3 and SnRK2.6 have distinguishable roles in the process. SnRK2.6 is more important for overall stomatal control, while SnRK2.2 and SnRK2.3 are more important for implementing GC stress memory in the subsequent dehydration response. Collectively, our results support a model of altered ABA production in GCs that maintains a partially closed stomatal aperture during an overnight watered recovery period.

Mesophyll photosynthesis and guard cell metabolism impacts on stomatal behaviour

Stomata control gaseous fluxes between the internal leaf air spaces and the external atmosphere. Guard cells determine stomatal aperture and must operate to ensure an appropriate balance between CO2 uptake for photosynthesis (A) and water loss, and ultimately plant water use efficiency (WUE). A strong correlation between A and stomatal conductance (gs ) is well documented and often observed, but the underlying mechanisms, possible signals and metabolites that promote this relationship are currently unknown. In this review we evaluate the current literature on mesophyll-driven signals that may coordinate stomatal behaviour with mesophyll carbon assimilation. We explore a possible role of various metabolites including sucrose and malate (from several potential sources; including guard cell photosynthesis) and new evidence that improvements in WUE have been made by manipulating sucrose metabolism within the guard cells. Finally we discuss the new tools and techniques available for potentially manipulating cell-specific metabolism, including guard and mesophyll cells, in order to elucidate mesophyll-derived signals that coordinate mesophyll CO2 demands with stomatal behaviour, in order to provide a mechanistic understanding of these processes as this may identify potential targets for manipulations in order to improve plant WUE and crop yield.

Chimeric promoter mediates guard cell-specific gene expression in tobacco under water deficit

The engineering of stomatal activity under water deficit through guard cell-specific gene regulation is an effective approach to improve drought tolerance of crops but it requires an appropriate promoter(s) inducible by water deficit in guard cells. We report that a chimeric promoter can induce guard cell-specific gene expression under water deficit. A chimeric promoter, p4xKST82-rd29B, was constructed using a tetramer of the 82 bp guard cell-specific regulatory region of potato KST1 promoter (4xKST82) and Arabidopsis dehydration-responsive rd29B promoter. Transgenic tobacco plants carrying p4xKST82-rd29B:mGFP-GUS exhibited GUS expression in response to water deficit. GUS enzyme activity of p4xKST82-rd29B:mGFP-GUS transgenic plants increased ~300 % by polyethylene glycol treatment compared to that of control plant but not by abscisic acid (ABA), indicating that the p4xKST82-rd29B chimeric promoter can be used to induce the guard cell-specific expression of genes of interest in response to water deficit in an ABA-independent manner.

Stomatal size, speed, and responsiveness impact on photosynthesis and water use efficiency

The control of gaseous exchange between the leaf and bulk atmosphere by stomata governs CO₂ uptake for photosynthesis and transpiration, determining plant productivity and water use efficiency. The balance between these two processes depends on stomatal responses to environmental and internal cues and the synchrony of stomatal behavior relative to mesophyll demands for CO₂. Here we examine the rapidity of stomatal responses with attention to their relationship to photosynthetic CO₂ uptake and the consequences for water use. We discuss the influence of anatomical characteristics on the velocity of changes in stomatal conductance and explore the potential for manipulating the physical as well as physiological characteristics of stomatal guard cells in order to accelerate stomatal movements in synchrony with mesophyll CO₂ demand and to improve water use efficiency without substantial cost to photosynthetic carbon fixation. We conclude that manipulating guard cell transport and metabolism is just as, if not more likely to yield useful benefits as manipulations of their physical and anatomical characteristics. Achieving these benefits should be greatly facilitated by quantitative systems analysis that connects directly the molecular properties of the guard cells to their function in the field.

The transcriptional regulatory network in the drought response and its crosstalk in abiotic stress responses including drought, cold, and heat

Drought negatively impacts plant growth and the productivity of crops around the world. Understanding the molecular mechanisms in the drought response is important for improvement of drought tolerance using molecular techniques. In plants, abscisic acid (ABA) is accumulated under osmotic stress conditions caused by drought, and has a key role in stress responses and tolerance. Comprehensive molecular analyses have shown that ABA regulates the expression of many genes under osmotic stress conditions, and the ABA-responsive element (ABRE) is the major cis-element for ABA-responsive gene expression. Transcription factors (TFs) are master regulators of gene expression. ABRE-binding protein and ABRE-binding factor TFs control gene expression in an ABA-dependent manner. SNF1-related protein kinases 2, group A 2C-type protein phosphatases, and ABA receptors were shown to control the ABA signaling pathway. ABA-independent signaling pathways such as dehydration-responsive element-binding protein TFs and NAC TFs are also involved in stress responses including drought, heat, and cold. Recent studies have suggested that there are interactions between the major ABA signaling pathway and other signaling factors in stress responses. The important roles of these TFs in crosstalk among abiotic stress responses will be discussed. Control of ABA or stress signaling factor expression can improve tolerance to environmental stresses. Recent studies using crops have shown that stress-specific overexpression of TFs improves drought tolerance and grain yield compared with controls in the field.