The guard cell as a single-cell model towards understanding drought tolerance and abscisic acid action

Unlocking dynamic root phenotypes for simultaneous enhancement of water and phosphorus uptake

Phosphorus (P) and water are crucial for plant growth, but their availability is challenged by climate change, leading to reduced crop production and global food security. In many agricultural soils, crop productivity is confronted by both water and P limitations. The diminished soil moisture decreases available P due to reduced P diffusion, and inadequate P availability diminishes tissue water status through modifications in stomatal conductance and a decrease in root hydraulic conductance. P and water display contrasting distributions in the soil, with P being concentrated in the topsoil and water in the subsoil. Plants adapt to water- and P-limited environments by efficiently exploring localized resource hotspots of P and water through the adaptation of their root system. Thus, developing cultivars with improved root architecture is crucial for accessing and utilizing P and water from arid and P-deficient soils. To meet this goal, breeding towards multiple advantageous root traits can lead to better cultivars for water- and P-limited environments. This review discusses the interplay of P and water availability and highlights specific root traits that enhance the exploration and exploitation of optimal resource-rich soil strata while reducing metabolic costs. We propose root ideotype models, including 'topsoil foraging', 'subsoil foraging', and 'topsoil/subsoil foraging' for maize (monocot) and common bean (dicot). These models integrate beneficial root traits and guide the development of water- and P-efficient cultivars for challenging environments.

Overexpression of a Ramie (Boehmaeria nivea L. Gaud) Group I WRKY Gene, BnWRKY49, Increases Drought Resistance in Arabidopsis thaliana

Plants face multiple stresses in their natural habitats. WRKY transcription factors (TFs) play an important regulatory role in plant stress signaling, regulating the expression of multiple stress-related genes to improve plant stress resistance. In this study, we analyzed the expression profiles of 25 BnWRKY genes in three stages of ramie growth (the seedling stage, the rapid-growth stage, and the fiber maturity stage) and response to abiotic stress through qRT-PCR. The results indicated that 25 BnWRKY genes play a role in different growth stages of ramie and were induced by salt and drought stress in the root and leaf. We selected BnWRKY49 as a candidate gene for overexpression in Arabidopsis. BnWRKY49 was localized in the nucleus. Overexpression of BnWRKY49 affected root elongation under drought and salt stress at the Arabidopsis seedling stage and exhibited increased tolerance to drought stress. Further research found that BnWRKY49-overexpressing lines showed decreased stomatal size and increased cuticular wax deposition under drought compared with wild type (WT). Antioxidant enzyme activities of SOD, POD, and CAT were higher in the BnWRKY49-overexpressing lines than the WT. These findings suggested that the BnWRKY49 gene played an important role in drought stress tolerance in Arabidopsis and laid the foundation for further research on the functional analysis of the BnWRKYs in ramie.

Modulation of potassium transport to increase abiotic stress tolerance in plants

Potassium is the major cation responsible for the maintenance of the ionic environment in plant cells. Stable potassium homeostasis is indispensable for virtually all cellular functions, and, concomitantly, viability. Plants must cope with environmental changes such as salt or drought that can alter ionic homeostasis. Potassium fluxes are required to regulate the essential process of transpiration, so a constraint on potassium transport may also affect the plant's response to heat, cold, or oxidative stress. Sequencing data and functional analyses have defined the potassium channels and transporters present in the genomes of different species, so we know most of the proteins directly participating in potassium homeostasis. The still unanswered questions are how these proteins are regulated and the nature of potential cross-talk with other signaling pathways controlling growth, development, and stress responses. As we gain knowledge regarding the molecular mechanisms underlying regulation of potassium homeostasis in plants, we can take advantage of this information to increase the efficiency of potassium transport and generate plants with enhanced tolerance to abiotic stress through genetic engineering or new breeding techniques. Here, we review current knowledge of how modifying genes related to potassium homeostasis in plants affect abiotic stress tolerance at the whole plant level.

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

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

ALDH2C4 regulates cuticle thickness and reduces water loss to promote drought tolerance

In Arabidopsis thaliana, ALDH2C4 encodes coniferaldehyde dehydrogenase, which oxidizes coniferaldehyde to ferulic acid. Drought stress is one of the important abiotic stresses affecting plant growth. However, the role of ferulic acid in drought resistance is unknown. To investigate the contribution of ferulic acid to cuticle composition and drought resistance, we used two Arabidopsis aldh2c4 mutant lines. Compared with wild-type (WT) leaves, ferulic acid contents were significantly lower (by more than 50 %) in mutants. The mutants also had lower amounts of cutin and wax, primarily due to reductions in C_18:2 dioic acid and alkanes, respectively. Furthermore, the leaves of the mutant plants exhibited greater rates of water loss and released chlorophyll faster than WT leaves when immersed in 80 % ethanol, indicating a defective cuticle barrier. The growth of aldh2c4 mutants was severely inhibited, and their leaves showed a higher degree of wilting relative to the WT plants under drought conditions. In aldh2c4 complementation lines, the growth inhibition of the mutant plants under drought stress was alleviated. Taken together, our results demonstrate that ferulic acid plays an important role in the composition and structural properties of the cuticle and that a ferulic acid deficiency in the cutin leads to reduced drought tolerance.

Transcriptome profiling of Arabidopsis slac1-3 mutant reveals compensatory alterations in gene expression underlying defective stomatal closure

Plants adjust their stomatal aperture for regulating CO₂ uptake and transpiration. S-type anion channel SLAC1 (slow anion channel-associated 1) is required for stomatal closure in response to various stimuli such as abscisic acid, CO₂, and light/dark transitions etc. Arabidopsis slac1 mutants exhibited defects in stimulus-induced stomatal closure, reduced sensitivity to darkness, and faster water loss from detached leaves. The global transcriptomic response of a plant with defective stimuli-induced stomatal closure (particularly because of defects in SLAC1) remains to be explored. In the current research we attempted to address the same biological question by comparing the global transcriptomic changes in Arabidopsis slac1-3 mutant and wild-type (WT) under dark, and dehydration stress, using RNA-sequencing. Abscisic acid (ABA)- and dark-induced stomatal closure was defective in Arabidopsis slac1-3 mutants, consequently the mutants had cooler leaf temperature than WT. Next, we determined the transcriptomic response of the slac1-3 mutant and WT under dark and dehydration stress. Under dehydration stress, the molecular response of slac1-3 mutant was clearly distinct from WT; the number of differentially expressed genes (DEGs) was significantly higher in mutant than WT. Dehydration induced DEGs in mutant were related to hormone signaling pathways, and biotic and abiotic stress response. Although, overall number of DEGs in both genotypes was not different under dark, however, the expression pattern was very much distinct; whereas majority of DEGs in WT were found to be downregulated, in slac1-3 majority were upregulated under dark. Further, a set 262 DEGs was identified with opposite expression pattern between WT and mutant under light-darkness transition. Amongst these, DEGs belonging to stress hormone pathways, and biotic and abiotic stress response were over-represented. To sum up, we have reported gene expression reprogramming underlying slac1-3 mutation and resultantly defective stomatal closure in Arabidopsis. Moreover, the induction of biotic and abiotic response in mutant under dehydration and darkness could be suggestive of the role of stomata as a switch in triggering these responses. To summarize, the data presented here provides useful insights into the gene expression reprogramming underlying slac1-3 mutation and resultant defects in stomatal closure.

The key clock component ZEITLUPE (ZTL) negatively regulates ABA signaling by degradation of CHLH in Arabidopsis

Ubiquitination-mediated protein degradation plays important roles in ABA signal transduction and delivering responses to chloroplast stress signals in plants, but additional E3 ligases of protein ubiquitination remain to be identified to understand the complex signaling network. Here we reported that ZEITLUPE (ZTL), an F-box protein, negatively regulates abscisic acid (ABA) signaling during ABA-inhibited early seedling growth and ABA-induced stomatal closure in Arabidopsis thaliana. Using molecular biology and biochemistry approaches, we demonstrated that ZTL interacts with and ubiquitinates its substrate, CHLH/ABAR (Mg-chelatase H subunit/putative ABA receptor), to modulate CHLH stability via the 26S proteasome pathway. CHLH acts genetically downstream of ZTL in ABA and drought stress signaling. Interestingly, ABA conversely induces ZTL phosphorylation, and high levels of ABA also induce CHLH proteasomal degradation, implying that phosphorylated ZTL protein may enhance the affinity to CHLH, leading to the increased degradation of CHLH after ABA treatment. Taken together, our results revealed a possible mechanism of reciprocal regulation between ABA signaling and the circadian clock, which is thought to be essential for plant fitness and survival.

Pepper SUMO E3 ligase CaDSIZ1 enhances drought tolerance by stabilizing the transcription factor CaDRHB1

Small ubiquitin-like modifier (SUMO) conjugation (SUMOylation) is a reversible post-translational modification associated with protein stability and activity, and modulates hormone signaling and stress responses in plants. Previously, we reported that the pepper dehydration-responsive homeobox domain transcription factor CaDRHB1 acts as a positive modulator of drought response. Here, we show that CaDRHB1 protein stability is enhanced by SUMO E3 ligase Capsicum annuum DRHB1-interacting SAP and Miz domain (SIZ1) (CaDSIZ1)-mediated SUMOylation in response to drought, thereby positively modulating abscisic acid (ABA) signaling and drought responses. Substituting lysine (K) 138 of CaDRHB1 with arginine reduced CaDSIZ1-mediated SUMOylation, indicating that K138 is the principal site for SUMO conjugation. Virus-induced silencing of CaDSIZ1 promoted CaDRHB1 degradation, suggesting that CaDSIZ1 is involved in drought-induced SUMOylation of CaDRHB1. CaDSIZ1 interacted with and facilitated SUMO conjugation of CaDRHB1. CaDRHB1, mainly localized in the nucleus, but also in the cytoplasm in the SUMOylation mimic state, suggesting that SUMOylation of CaDRHB1 promotes its nuclear export, leading to cytoplasmic accumulation. Moreover, CaDSIZ1-silenced pepper plants were less sensitive to ABA and considerably sensitive to drought stress, whereas CaDSIZ1-overexpressing plants displayed ABA-hypersensitive and drought-tolerant phenotypes. Collectively, our data indicate that CaDSIZ1-mediated SUMOylation of CaDRHB1 functions in ABA-mediated drought tolerance.

Transcriptomic analysis of methyl jasmonate treatment reveals gene networks involved in drought tolerance in pearl millet

Water deficit stress at the early stage of development is one of the main factors limiting pearl millet production. One practice to counteract this limitation would be to resort to the application of hormones to stimulate plant growth and development at critical stages. Exogenous methyl jasmonate (MeJA) can improve drought tolerance by modulating signaling, metabolism, and photosynthesis pathways, therefore, we assumed that can occur in pearl millet during the early stage of development. To decipher the molecular mechanisms controlling these pathways, RNAseq was conducted in two pearl millet genotypes, drought-sensitive SosatC88 and drought-tolerant Souna3, in response to 200 μM of MeJA. Pairwise comparison between the MeJA-treated and non-treated plants revealed 3270 differentially expressed genes (DEGs) among 20,783 transcripts in SosatC88 and 127 DEGs out of 20,496 transcripts in Souna3. Gene ontology (GO) classification assigned most regulated DEGs in SosatC88 to heme binding, oxidation-reduction process, response to oxidative stress and membrane, and in Souna3 to terpene synthase activity, lyase activity, magnesium ion binding, and thylakoid. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis reveals that DEGs in SosatC88 are related to the oxidation-reduction process, the biosynthesis of other secondary metabolites, the signal transduction, and the metabolism of terpenoids, while in Souna3, DEGs are related to the metabolism of terpenoids and the energy metabolism. Two genes encoding a diterpenoid biosynthesis-related (Pgl_GLEAN_10009413) and a Glutathione S transferase T3 (Pgl_GLEAN_10034098) were contra-regulated between SosatC88 and Souna3. Additionally, five random genes differentially expressed by RNAseq were validated using qPCR, therefore, they are potential targets for the development of novel strategies breeding schemes for plant growth under water deficit stress. These insights into the molecular mechanisms of pearl millet genotype tolerance at the early stage of development contribute to the understanding of the role of hormones in adaptation to drought-prone environments.

Genome-Wide Analysis of the Soybean TIFY Family and Identification of GmTIFY10e and GmTIFY10g Response to Salt Stress

TIFY proteins play crucial roles in plant abiotic and biotic stress responses. Our transcriptome data revealed several TIFY family genes with significantly upregulated expression under drought, salt, and ABA treatments. However, the functions of the GmTIFY family genes are still unknown in abiotic stresses. We identified 38 GmTIFY genes and found that TIFY10 homologous genes have the most duplication events, higher selection pressure, and more obvious response to abiotic stresses compared with other homologous genes. Expression pattern analysis showed that GmTIFY10e and GmTIFY10g genes were significantly induced by salt stress. Under salt stress, GmTIFY10e and GmTIFY10g transgenic Arabidopsis plants showed higher root lengths and fresh weights and had significantly better growth than the wild type (WT). In addition, overexpression of GmTIFY10e and GmTIFY10g genes in soybean improved salt tolerance by increasing the PRO, POD, and CAT contents and decreasing the MDA content; on the contrary, RNA interference plants showed sensitivity to salt stress. Overexpression of GmTIFY10e and GmTIFY10g in Arabidopsis and soybean could improve the salt tolerance of plants, while the RNAi of GmTIFY10e and GmTIFY10g significantly increased sensitivity to salt stress in soybean. Further analysis demonstrated that GmTIFY10e and GmTIFY10g genes changed the expression levels of genes related to the ABA signal pathway, including GmSnRK2, GmPP2C, GmMYC2, GmCAT1, and GmPOD. This study provides a basis for comprehensive analysis of the role of soybean TIFY genes in stress response in the future.

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.

Potassium physiology from Archean to Holocene: A higher-plant perspective

In this paper, we discuss biological potassium acquisition and utilization processes over an evolutionary timescale, with emphasis on modern vascular plants. The quintessential osmotic and electrical functions of the K⁺ ion are shown to be intimately tied to K⁺-transport systems and membrane energization. Several prominent themes in plant K⁺-transport physiology are explored in greater detail, including: (1) channel mediated K⁺ acquisition by roots at low external [K⁺]; (2) K⁺ loading of root xylem elements by active transport; (3) variations on the theme of K⁺ efflux from root cells to the extracellular environment; (4) the veracity and utility of the "affinity" concept in relation to transport systems. We close with a discussion of the importance of plant-potassium relations to our human world, and current trends in potassium nutrition from farm to table.

Structure and activity of SLAC1 channels for stomatal signaling in leaves

Stomata in leaves regulate gas exchange between the plant and its atmosphere. Various environmental stimuli elicit abscisic acid (ABA); ABA leads to phosphoactivation of slow anion channel 1 (SLAC1); SLAC1 activity reduces turgor pressure in aperture-defining guard cells; and stomatal closure ensues. We used electrophysiology for functional characterizations of Arabidopsis thaliana SLAC1 (AtSLAC1) and cryoelectron microscopy (cryo-EM) for structural analysis of Brachypodium distachyon SLAC1 (BdSLAC1), at 2.97-Å resolution. We identified 14 phosphorylation sites in AtSLAC1 and showed nearly 330-fold channel-activity enhancement with 4 to 6 of these phosphorylated. Seven SLAC1-conserved arginines are poised in BdSLAC1 for regulatory interaction with the N-terminal extension. This BdSLAC1 structure has its pores closed, in a basal state, spring loaded by phenylalanyl residues in high-energy conformations. SLAC1 phosphorylation fine-tunes an equilibrium between basal and activated SLAC1 trimers, thereby controlling the degree of stomatal opening.

Overexpression of HvAKT1 improves drought tolerance in barley by regulating root ion homeostasis and ROS and NO signaling

Potassium (K+) is the major cationic inorganic nutrient utilized for osmotic regulation, cell growth, and enzyme activation in plants. Inwardly rectifying K+ channel 1 (AKT1) is the primary channel for root K+ uptake in plants, but the function of HvAKT1 in barley plants under drought stress has not been fully elucidated. In this study, we conducted evolutionary bioinformatics, biotechnological, electrophysiological, and biochemical assays to explore molecular mechanisms of HvAKT1 in response to drought in barley. The expression of HvAKT1 was significantly up-regulated by drought stress in the roots of XZ5-a drought-tolerant wild barley genotype. We isolated and functionally characterized the plasma membrane-localized HvAKT1 using Agrobacterium-mediated plant transformation and Barley stripe mosaic virus-induced gene silencing of HvAKT1 in barley. Evolutionary bioinformatics indicated that the K+ selective filter in AKT1 originated from streptophyte algae and is evolutionarily conserved in land plants. Silencing of HvAKT1 resulted in significantly decreased biomass and suppressed K+ uptake in root epidermal cells under drought treatment. Disruption of HvAKT1 decreased root H+ efflux, H+-ATPase activity, and nitric oxide (NO) synthesis, but increased hydrogen peroxide (H2O2) production in the roots under drought stress. Furthermore, we observed that overexpression of HvAKT1 improves K+ uptake and increases drought resistance in barley. Our results highlight the importance of HvAKT1 for root K+ uptake and its pleiotropic effects on root H+-ATPase, and H2O2 and NO in response to drought stress, providing new insights into the genetic basis of drought tolerance and K+ nutrition in barley.

Plant Immune Mechanisms: From Reductionistic to Holistic Points of View

After three decades of the amazing progress made on molecular studies of plant-microbe interactions (MPMI), we have begun to ask ourselves "what are the major questions still remaining?" as if the puzzle has only a few pieces missing. Such an exercise has ultimately led to the realization that we still have many more questions than answers. Therefore, it would be an impossible task for us to project a coherent "big picture" of the MPMI field in a single review. Instead, we provide our opinions on where we would like to go in our research as an invitation to the community to join us in this exploration of new MPMI frontiers.

The Sucrose Non-Fermenting 1-Related Protein Kinase 2 (SnRK2) Genes Are Multifaceted Players in Plant Growth, Development and Response to Environmental Stimuli

Reversible protein phosphorylation orchestrated by protein kinases and phosphatases is a major regulatory event in plants and animals. The SnRK2 subfamily consists of plant-specific protein kinases in the Ser/Thr protein kinase superfamily. Early observations indicated that SnRK2s are mainly involved in response to abiotic stress. Recent evidence shows that SnRK2s are multifarious players in a variety of biological processes. Here, we summarize the considerable knowledge of SnRK2s, including evolution, classification, biological functions and regulatory mechanisms at the epigenetic, post-transcriptional and post-translation levels.

The Shaker Type Potassium Channel, GORK, Regulates Abscisic Acid Signaling in Arabidopsis

Evolution of adaptive mechanisms to abiotic stress is essential for plant growth and development. Plants adapt to stress conditions by activating the abscisic acid (ABA) signaling pathway. It has been suggested that the ABA receptor, clade A protein phosphatase, SnRK2 type kinase, and SLAC1 anion channel are important components of the ABA signaling pathway. In this study, we report that the shaker type potassium (K⁺) channel, GORK, modulates plant responses to ABA and abiotic stresses. Our results indicate that the full length of PP2CA is needed to interact with the GORK C-terminal region. We identified a loss of function allele in gork that displayed ABA-hyposensitive phenotype. gork and pp2ca mutants showed opposite responses to ABA in seed germination and seedling growth. Additionally, gork mutant was tolerant to the NaCl and mannitol treatments, whereas pp2ca mutant was sensitive to the NaCl and mannitol treatments. Thus, our results indicate that GORK enhances the sensitivity to ABA and negatively regulates the mechanisms involved in high salinity and osmotic stresses via PP2CA-mediated signals.

The cotton endocycle-involved protein SPO11-3 functions in salt stress via integrating leaf stomatal response, ROS scavenging and root growth

The SPORULATION 11 (SPO11) proteins are among eukaryotic the topoisomerase VIA (Topo VIA) homologs involved in modulating various important biological processes, such as growth, development and stress response via endoreduplication in plants, but the underlying mechanism response to stress remains largely unknown under salt treatment. Here, we attempted to characterize a homolog of TOP VIA in upland cotton (Gossypium hirsutum L.), designated as GhSPO11-3. The silencing of GhSPO11-3 in cotton plants resulted in a dwarf phenotype with a failure of cell endoreduplication and a phase shift in the ploidy levels. The GhSPO11-3-silenced plants also showed substantial changes including accumulated malondialdehyde, significantly reduced chlorophyll and proline contents and decreased antioxidative enzyme activity after salt treatment. In addition, transgenic Arabidopsis lines overexpressing GhSPO11-3 accelerated both leaf and root growth with cell expansion and endopolyploidy. Both leaf stomatal density and aperture were markedly decreased, and the transgenic Arabidopsis lines were more tolerant with expression of stress-responsive genes under salinity stress. Furthermore, consistent with the reduced reactive oxygen species (ROS), the expression of ROS scavenging-related genes was largely reinforced, and antioxidant enzyme activities were accordingly significantly enhanced in transgenic Arabidopsis lines under salt stress. In general, these results indicated that GhSPO11-3 likely respond to salt stress by positively regulating root growth, stomatal response, ROS production and the expression of stress-related genes to cope with adverse conditions in plants.

The Copper Amine Oxidase AtCuAOδ Participates in Abscisic Acid-Induced Stomatal Closure in Arabidopsis

Plant copper amine oxidases (CuAOs) are involved in wound healing, defense against pathogens, methyl-jasmonate-induced protoxylem differentiation, and abscisic acid (ABA)-induced stomatal closure. In the present study, we investigated the role of the Arabidopsis thaliana CuAOδ (AtCuAOδ; At4g12290) in the ABA-mediated stomatal closure by genetic and pharmacological approaches. Obtained data show that AtCuAOδ is up-regulated by ABA and that two Atcuaoδ T-DNA insertional mutants are less responsive to this hormone, showing reduced ABA-mediated stomatal closure and H₂O₂ accumulation in guard cells as compared to the wild-type (WT) plants. Furthermore, CuAO inhibitors, as well as the hydrogen peroxide (H₂O₂) scavenger N,N¹-dimethylthiourea, reversed most of the ABA-induced stomatal closure in WT plants. Consistently, AtCuAOδ over-expressing transgenic plants display a constitutively increased stomatal closure and increased H₂O₂ production compared to WT plants. Our data suggest that AtCuAOδ is involved in the H₂O₂ production related to ABA-induced stomatal closure.

Two Clade A Phosphatase 2Cs Expressed in Guard Cells Physically Interact With Abscisic Acid Signaling Components to Induce Stomatal Closure in Rice

BACKGROUND

The core ABA signaling components functioning in stomatal closure/opening, namely ABA receptors, phosphatases, SnRK2s and SLAC1, are well characterized in Arabidopsis, but their functions in guard cells of rice have not been extensively studied.

RESULTS

In this study, we confirmed that OsSLAC1, the rice homolog of AtSLAC1, is specifically expressed in rice guard cells. Among the rice SAPKs, SAPK10 was specifically expressed in guard cells. In addition, SAPK10 phosphorylated OsSLAC1 in vitro and transgenic rice overexpressing SAPK10 or OsSLAC1 showed significantly less water loss than control. Thus, those might be major positive signaling components to close stomata in rice. We identified that only OsPP2C50 and OsPP2C53 among 9 OsPP2CAs might be related with stomatal closure/opening signaling based on guard cell specific expression and subcellular localization. Transgenic rice overexpressing OsPP2C50 and OsPP2C53 showed significantly higher water loss than control. We also characterized the interaction networks between OsPP2C50 and OsPP2C53, SAPK10 and OsSLAC1 and found two interaction pathways among those signaling components: a hierarchical interaction pathway that consisted of OsPP2C50 and OsPP2C53, SAPK10 and OsSLAC1; and a branched interaction pathway wherein OsPP2C50 and OsPP2C53 interacted directly with OsSLAC1.

CONCLUSION

OsPP2C50 and OsPP2C53 is major negative regulators of ABA signaling regarding stomata closing in rice. Those can regulate the OsSLAC1 directly or indirectly thorough SAPK10.

Synthesis of All Stereoisomers of RK460 and Evaluation of Their Activity and Selectivity as Abscisic Acid Receptor Antagonists

The PYR/PYL/RCAR protein families have recently emerged as receptors of the phytohormone abscisic acid (ABA, 1), which regulates plant responses to environmental stress. These families have multiple members with different physiological actions, and so selective agonists or antagonists are needed both as tools to elucidate functional differences and as lead compounds for agrochemicals. We previously identified RK460 (rac-3 a) as a PYR1-selective antagonist, and showed that it possesses five stereocenters on a 6,5-cis-bicyclo skeleton. Here, we synthesized all the stereoisomers of RK460 and evaluated their activity towards a panel of receptors. Relative stereochemistry as well as absolute stereochemistry was important for selective action.

Editing a Stomatal Developmental Gene in Rice with CRISPR/Cpf1

CRISPR has arguably been the fastest growing genome editing tool so far. CRISPR/Cas9 (Cas9) has been proved to be efficient and precise in genome editing. However Cas9 has certain limitations. CRISPR/Cpf1 (Cpf1) has been discovered as an alternate approach that can overcome some of those limitations. Cpf1 allows targeting in AT-rich region, creating a staggered cleavage, and cutting at the distal end to the PAM (Protospacer Adjacent Motif) regions. We have successfully tested the efficiency of Cpf1 system in rice using OsEPFL9 which is a developmental gene known to regulate the stomatal density in leaf. Regulation of stomatal density and patterning is an important factor in regulating plant physiology, especially in improving the plant water use efficiency. We targeted the Exon1 of OsEPFL9 and the knockout lines were studied for several generations for establishment of stabilized editing, as well as transmission and segregation of edits through generations. The usage of Cpf1 as a genome editing tool to manipulate stomatal patterning may further help us gain more insight of the physiology of rice in stress conditions.

Controversies in Midday Water Potential Regulation and Stomatal Behavior Might Result From the Environment, Genotype, and/or Rootstock: Evidence From Carménère and Syrah Grapevine Varieties

Controversies exist regarding the iso/anisohydric continuum for classifying plant water-use strategies. Isohydricity has been argued to result from plant-environment interaction rather than it being an intrinsic property of the plant itself. Discrepancies remain regarding the degree of isohydricity (σ) of plants and their threshold for physiological responses and resistance to drought. Thus, the aim of this study was to evaluate the isohydricity of the grapevine varieties Syrah and Carménère under a non-lethal water deficit progression from veraison from two different locations, the Cachapoal Valley (CV) and Maipo Valley (MV), in central Chile and with different rootstock only in Syrah. For this purpose, the midday stem water potential (Ψ_mds) regulation and stomatal responses to drought, leaf traits related to pressure-volume curves, stomatal sensitivity to ABA, cavitation threshold, and photosynthetic responses were assessed. A higher atmospheric water demand was observed in the CV compared to the MV, with lower Ψ_mds values in the former for both varieties. Also, the σ values in Carménère were 1.11 ± 0.14 MPa MPa^-1 and 0.68 ± 0.18 MPa MPa^-1 in the CV and MV, respectively, and in Syrah they were 1.10 ± 0.07 MPa MPa^-1 in the CV and 0.60 ± 0.10 MPa MPa^-1 in the MV. Even though similar variations in σ between locations in both varieties were evident, Carménère plants showed a conserved stomatal response to Ψ_mds in both study sites, while those of Syrah resulted in a higher stomatal sensitivity to Ψ_mds in the site of lower σ. Besides the differences in seasonal weather conditions, it is likely that the different rootstock and clonal variability of each season in Syrah were able to induce coordinated changes in σ, Ψ_gs12, and osmotic potential at full turgor (π₀). On the other hand, irrespective of the σ, and given the similarity between the π₀ and Ψ_gs12 in leaves before drought, it seems that π₀ could be a convenient tool for assessing the Ψ_mds threshold values posing a risk to the plants in order to aid the irrigation decision making in grapevines under controlled water deficit. Finally, water deficits in vineyards might irreversibly compromise the photosynthetic capacity of leaves.

MYB30 regulates root cell elongation under abscisic acid signaling

Reactive oxygen species (ROS) and plant hormones play important roles in regulating plant growth and stress responses as signaling molecules. Abscisic acid (ABA) is known as the key regulator of both abiotic and biotic stress responses. During stress responses, ABA is known to regulate ROS production, indicating that important crosstalk occurs between ROS and ABA signaling. We recently reported that MYB30, an MYB-type transcription factor, regulates root cell elongation under ROS signaling. In this study, we analyzed the molecular interaction between ROS and ABA signal during for root development, which is mediated through MYB30 transcriptional regulation. We showed that MYB30-regulated root cell elongation was mediated by ROS production under ABA signaling. Our findings will provide one piece of evidence of the complex cross talk between ROS and hormone signaling that regulates root development.

Roles of a maize phytochrome-interacting factors protein ZmPIF3 in regulation of drought stress responses by controlling stomatal closure in transgenic rice without yield penalty

ZmPIF3 plays an important role in ABA-mediated regulation of stomatal closure in the control of water loss, and can improve both drought tolerance and did not affect the grain yield in the transgenic rice. Phytochrome-interacting factors (PIFs) are a subfamily of basic helix-loop-helix (bHLH) transcription factors and play important roles in regulating plant growth and development. In our previous study, overexpression of a maize PIFs family gene, ZmPIF3, improved drought tolerance in transgenic rice. In this study, measurement of water loss rate, transpiration rate, stomatal conductance, guard cell aperture, density and length of ZmPIF3 transgenic plants showed that ZmPIF3 can enhance water-saving and drought-resistance by decreasing stomatal aperture and reducing transpiration in both transgenic rice and transgenic Arabidopsis. Scrutiny of sensitivity to ABA showed that ZmPIF3 transgenic rice was hypersensitive to ABA, while the endogenous ABA level was not significantly changed. These results indicate that ZmPIF3 plays a major role in the ABA signaling pathway. In addition, DGE results further suggest that ZmPIF3 participates in the ABA signaling pathway and regulates stomatal aperture in rice. Comparison analysis of the phenotype, physiology, and transcriptome of ZmPIF3 transgenic rice compared to control plants further suggests that ZmPIF3 is a positive regulator of ABA signaling and enhances water-saving and drought-resistance traits by reducing stomatal openings to control water loss. Moreover, investigation of the agronomic traits of ZmPIF3 transgenic rice from four cultivating seasons showed that ZmPIF3 expression increased the tiller and panicle number and did not affect the grain yield in the transgenic rice. These results demonstrate that ZmPIF3 is a promising candidate gene in the transgenic breeding of water-saving and drought-resistant rice plants and crop improvement.

Dehydration survival of crop plants and its measurement

Dehydration survival under drought stress is defined in this review as the transition from plant activity into a quiescent state of life preservation, which will be terminated by either recovery or death, depending on the stress regime and the plant's resilience. Dehydration survival is a popular phenotype by which functional genomics attempts to test gene function in drought resistance and survival. The available reports on phenotyping and genotyping of dehydration survival in genomic studies indicate that the measurement of this trait is often biased to the extent that misguided interpretations are likely to occur. This review briefly discusses the physiological basis of dehydration survival in resurrection plants and crop plants, and concludes that in phenotyping dehydration survival there is a need to distinguish between dehydration avoidance and dehydration tolerance (also termed desiccation tolerance) in affecting survival and recovery. Without this distinction, functional genomics studies of the trait might be biased. Survival due to dehydration avoidance is expressed by the capacity to maintain a relatively high plant water status as the plant is desiccated. Survival due to dehydration tolerance is expressed by delayed mortality (mortality at a relatively low plant water status) as affected by the resilience of plant metabolism. The common test of dehydration survival, using the relative recovery after a given number of stress days, is therefore insufficient because it is mainly driven by dehydration avoidance and so ignores a possible role for dehydration tolerance. Conceivable methods for more accurate phenotyping of the two components of dehydration survival are proposed and discussed.

Characterization of increased cuticular wax mutant and analysis of genes involved in wax biosynthesis in Dianthus spiculifolius

Cuticular wax formation on the surface of plant leaves is associated with drought-stress tolerance. The identification of wax biosynthesis-related genes will contribute to the genetic improvement of drought resistance in plants. In this study, we characterize a novel Dianthus spiculifolius mutant with increased cuticular wax. The mutant exhibited stronger drought resistance as indicated by less leaf wilting and death, higher leaf relative water content and water retention capacity, and slower water loss and chlorophyll extraction than did the wild type during drought treatment. In the mutant leaves, 2 730 upregulated and 2 151 downregulated differentially expressed genes (DEGs) were identified by transcriptome sequencing. A wax biosynthesis pathway of the identified DEGs was significantly enriched. Finally, three key genes (DsCER1, DsMAH1, and DsWSD1) involved in wax biosynthesis were identified and verified by qPCR. These results suggest that differential expression of DEGs involved in wax biosynthesis may be associated with the increase in cuticular wax in the mutant. Taken together, our results help elucidate wax formation patterns in D. spiculifolius. Furthermore, the DEGs involved in wax biosynthesis identified here may be valuable genetic resources for improving plant stress tolerance through increased accumulation of cuticular wax.

OsJAZ1 Attenuates Drought Resistance by Regulating JA and ABA Signaling in Rice

Jasmonates (JAs) and abscisic acid (ABA) are phytohormones known play important roles in plant response and adaptation to various abiotic stresses including salinity, drought, wounding, and cold. JAZ (JASMONATE ZIM-domain) proteins have been reported to play negative roles in JA signaling. However, direct evidence is still lacking that JAZ proteins regulate drought resistance. In this study, OsJAZ1 was investigated for its role in drought resistance in rice. Expression of OsJAZ1 was strongly responsive to JA treatment, and it was slightly responsive to ABA, salicylic acid, and abiotic stresses including drought, salinity, and cold. The OsJAZ1-overexpression rice plants were more sensitive to drought stress treatment than the wild-type (WT) rice Zhonghua 11 (ZH11) at both the seedling and reproductive stages, while the jaz1 T-DNA insertion mutant plants showed increased drought tolerance compared to the WT plants. The OsJAZ1-overexpression plants were hyposensitive to MeJA and ABA, whereas the jaz1 mutant plants were hypersensitive to MeJA and ABA. In addition, there were significant differences in shoot and root length between the OsJAZ1 transgenic and WT plants under the MeJA and ABA treatments. A subcellular localization assay indicated that OsJAZ1 was localized in both the nucleus and cytoplasm. Transcriptome profiling analysis by RNA-seq revealed that the expression levels of many genes in the ABA and JA signaling pathways exhibited significant differences between the OsJAZ1-overexpression plants and WT ZH11 under drought stress treatment. Quantitative real-time PCR confirmed the expression profiles of some of the differentially expressed genes, including OsNCED4, OsLEA3, RAB21, OsbHLH006, OsbHLH148, OsDREB1A, OsDREB1B, SNAC1, and OsCCD1. These results together suggest that OsJAZ1 plays a role in regulating the drought resistance of rice partially via the ABA and JA pathways.

Characterization of wheat MYB genes responsive to high temperatures

BACKGROUND

Heat stress is one of the most crucial environmental factors, which reduces crop yield worldwide. In plants, the MYB family is one of the largest families of transcription factors (TFs). Although some wheat stress-related MYB TFs have been characterized, their involvement in response to high-temperature stress has not been properly studied.

RESULTS

Six novel heat-induced MYB genes were identified by comparison with previously established de novo transcriptome sequencing data obtained from wheat plants subjected to heat treatment; genomic and complete coding sequences of these genes were isolated. All six TaMYBs were localized in the nucleus of wheat protoplasts. Transactivation assays in yeast revealed that all six proteins acted as transcriptional activators, and the activation domains were attributed to the C-termini of the six wheat MYB proteins. Phylogenetic analysis of the six TaMYBs and R2R3-MYBs from Arabidopsis revealed that all six proteins were in clades that contained stress-related MYB TFs. The expression profiles of TaMYB genes were different in wheat tissues and in response to various abiotic stresses and exogenous abscisic acid treatment. In transgenic Arabidopsis plants carrying TaMYB80 driven by the CaMV 35S promoter, tolerance to heat and drought stresses increased, which could be attributed to the increased levels of cellular abscisic acid.

CONCLUSIONS

We identified six heat-induced MYB genes in wheat. We performed comprehensive analyses of the cloned MYB genes and their gene products, including gene structures, subcellular localization, transcriptional activation, phylogenetic relationships, and expression patterns in different wheat tissues and under various abiotic stresses. In particular, we showed that TaMYB80 conferred heat and drought tolerance in transgenic Arabidopsis. These results contribute to our understanding of the functions of heat-induced MYB genes and provide the basis for selecting the best candidates for in-depth functional studies of heat-responsive MYB genes in wheat.

ABP9, a maize bZIP transcription factor, enhances tolerance to salt and drought in transgenic cotton

ABP9 , encoding a bZIP transcription factor from maize, enhances tolerance to multiple stresses and may participate in the ABA signaling pathway in transgenic cotton by altering physiological and biochemical processes and stress-related gene expression. Abiotic stresses, such as soil salinity and drought, negatively affect growth, development, and yield in cotton. Gene ABP9, which encodes a bZIP transcription factor, binds to the abscisic acid (ABA)-responsive-element (ABRE2) motif of the maize catalase1 gene. Its expression significantly improves tolerance in Arabidopsis to multiple abiotic stresses, but little is known about its role in cotton. In the present study, the ABP9 gene was introduced into upland cotton (Gossypium hirsutum L.) cultivar R15 by Agrobacterium tumefaciens-mediated transformation, and 12 independent transgenic cotton lines were obtained. Cotton plants over-expressing ABP9 have enhanced tolerance to salt and osmotic stress. Under stress, they developed better root systems in a greenhouse and higher germination, reduced stomatal aperture, and stomatal density in a growth chamber. Under drought conditions, survival rate and relative water content (RWC) of transgenic cotton were higher than those of R15 plants. Under salt and osmotic stresses, chlorophyll, proline, and soluble sugar contents significantly increased in transgenic cotton leaves and the malondialdehyde (MDA) content was lower than in R15. Overexpression of ABP9 also enhanced oxidative stress tolerance, reduced cellular levels of reactive oxygen species (ROS) through increased activities of antioxidative enzymes, and alleviated oxidative damage to cell. Interestingly, ABP9 over-expressing cotton was more sensitive to exogenous ABA than R15 at seed germination, root growth, stomatal aperture, and stomatal density. Moreover, ABP9 overexpression upregulated significantly the transcription levels of stress-related genes such as GhDBP2, GhNCED2, GhZFP1, GhERF1, GhHB1, and GhSAP1 under salt treatment. Conjointly, these results showed that overexpression of ABP9 conferred enhanced tolerance to multiple abiotic stresses in cotton. The stress-tolerant transgenic lines provide valuable resources for cotton breeding.

Mutation in HvCBP20 (Cap Binding Protein 20) Adapts Barley to Drought Stress at Phenotypic and Transcriptomic Levels

CBP20 (Cap-Binding Protein 20) encodes a small subunit of the cap-binding complex (CBC), which is involved in the conserved cell processes related to RNA metabolism in plants and, simultaneously, engaged in the signaling network of drought response, which is dependent on ABA. Here, we report the enhanced tolerance to drought stress of barley mutant in the HvCBP20 gene manifested at the morphological, physiological, and transcriptomic levels. Physiological analyses revealed differences between the hvcbp20.ab mutant and its WT in response to a water deficiency. The mutant exhibited a higher relative water content (RWC), a lower stomatal conductance and changed epidermal pattern compared to the WT after drought stress. Transcriptome analysis using the Agilent Barley Microarray integrated with observed phenotypic traits allowed to conclude that the hvcbp20.ab mutant exhibited better fitness to stress conditions by its much more efficient and earlier activation of stress-preventing mechanisms. The network hubs involved in the adjustment of hvcbp20.ab mutant to the drought conditions were proposed. These results enabled to make a significant progress in understanding the role of CBP20 in the drought stress response.

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

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

BnSIP1-1, a Trihelix Family Gene, Mediates Abiotic Stress Tolerance and ABA Signaling in Brassica napus

The trihelix family genes have important functions in light-relevant and other developmental processes, but their roles in response to adverse environment are largely unclear. In this study, we identified a new gene, BnSIP1-1, which fell in the SIP1 (6b INTERACTING PROTEIN1) clade of the trihelix family with two trihelix DNA binding domains and a fourth amphipathic α-helix. BnSIP1-1 protein specifically targeted to the nucleus, and its expression can be induced by abscisic acid (ABA) and different stresses. Overexpression of BnSIP1-1 improved seed germination under osmotic pressure, salt, and ABA treatments. Moreover, BnSIP1-1 decreased the susceptibility of transgenic seedlings to osmotic pressure and ABA treatments, whereas there was no difference under salt stress between the transgenic and wild-type seedlings. ABA level in the transgenic seedlings leaves was higher than those in the control plants under normal condition. Under exogenous ABA treatment and mannitol stress, the accumulation of ABA in the transgenic plants was higher than that in the control plants; while under salt stress, the difference of ABA content before treatment was gradually smaller with the prolongation of salt treatment time, then after 24 h of treatment the ABA level was similar in transgenic and wild-type plants. The transcription levels of several general stress marker genes (BnRD29A, BnERD15, and BnLEA1) were higher in the transgenic plants than the wild-type plants, whereas salt-responsive genes (BnSOS1, BnNHX1, and BnHKT) were not significantly different or even reduced compared with the wild-type plants, which indicated that BnSIP1-1 specifically exerted different regulatory mechanisms on the osmotic- and salt-response pathways in seedling period. Overall, these findings suggested that BnSIP1-1 played roles in ABA synthesis and signaling, salt and osmotic stress response. To date, information about the involvement of the Brassica napus trihelix gene in abiotic response is scarce. Here, we firstly reported abiotic stress response and possible function mechanisms of a new trihelix gene in B. napus.

Overexpression of AtERF019 delays plant growth and senescence, and improves drought tolerance in Arabidopsis

The transcription factor superfamily, APETALA2/ethylene response factor, is involved in plant growth and development, as well as in environmental stress responses. Here, an uncharacterized gene of this family, AtERF019, was studied in Arabidopsis thaliana under abiotic stress situations. Arabidopsis plants overexpressing AtERF019 showed a delay in flowering time of 7 days and a delay in senescence of 2 weeks when comparison with wild type plants. These plants also showed increased tolerance to water deficiency that could be explained by a lower transpiration rate, owing to their smaller stomata aperture and lower cuticle and cell wall permeability. Furthermore, using a bottom-up proteomic approach, proteins produced in response to stress, namely branched-chain-amino-acid aminotransferase 3 (BCAT3) and the zinc finger transcription factor oxidative stress 2, were only identified in plants overexpressing AtERF019. Additionally, a BCAT3 mutant was more sensitive to water-deficit stress than wild type plants. Predicted gene targets of AtERF019 were oxidative stress 2 and genes related to cell wall metabolism. These data suggest that AtERF019 could play a primary role in plant growth and development that causes an increased tolerance to water deprivation, so strengthening their chances of reproductive success.

The Pepper RING Finger E3 Ligase, CaDIR1, Regulates the Drought Stress Response via ABA-Mediated Signaling

Drought stress from soil or air limits plant growth and development, leading to a reduction in crop productivity. Several E3 ligases positively or negatively regulate the drought stress response. In the present study, we show that the pepper (Capsicum annuum) Drought Induced RING type E3 ligase 1, CaDIR1, regulates the drought stress response via abscisic acid (ABA)-mediated signaling. CaDIR1 contains a C3HC4-type RING finger domain in the N-terminal region; this domain functions during protein degradation via attachment of ubiquitins to the substrate target proteins. The expression levels of the CaDIR1 gene were suppressed and induced by ABA and drought treatments, respectively. We conducted loss-of-function and gain-of function genetic studies to examine the in vivo function of CaDIR1 in response to ABA and drought stress. CaDIR1-silenced pepper plants displayed a drought-tolerant phenotype characterized by a low level of transpirational water loss via increased stomatal closure and elevated leaf temperatures. CaDIR1-overexpressing (OX) Arabidopsis plants exhibited an ABA-hypersensitive phenotype during the germination stage, but an ABA-hyposensitive phenotype-characterized by decreased stomatal closure and reduced leaf temperatures-at the adult stage. Moreover, adult CaDIR1-OX plants exhibited a drought-sensitive phenotype characterized by high levels of transpirational water loss. Our results indicate that CaDIR1 functions as a negative regulator of the drought stress response via ABA-mediated signaling. Our findings provide a valuable insight into the plant defense mechanism that operates during drought stress.

Post-translational control of ABA signalling: the roles of protein phosphorylation and ubiquitination

The plant phytohormone abscisic acid (ABA) plays significant roles in integrating environmental signals with embryogenesis, germination, seedling establishment, the floral transition and the adaptation of plants to stressful environments by modulating stomatal movement and stress-responsive gene expression. ABA signalling consists of ABA perception, signal transduction and ABA-induced responses. ABA receptors such as members of the PYR/PYL family, group A type 2C protein phosphatases (as negative regulators), SnRK2 protein kinases (as positive regulators), bZIP transcription factors and ion channels are key components of ABA signalling. Post-translational modifications, including dephosphorylation, phosphorylation and ubiquitination, play important roles in regulating ABA signalling. In this review, we focus on the roles of post-translational modifications in ABA signalling. The studies presented provide a detailed picture of the ABA signalling network.

Stomatal closure induced by phytosphingosine-1-phosphate and sphingosine-1-phosphate depends on nitric oxide and pH of guard cells in Pisum sativum

MAIN CONCLUSION

Phyto-S1P and S1P induced stomatal closure in epidermis of pea ( Pisum sativum ) by raising the levels of NO and pH in guard cells. Phosphosphingolipids, such as phytosphingosine-1-phosphate (phyto-S1P) and sphingosine-1-phosphate (S1P), are important signaling components during drought stress. The biosynthesis of phyto-S1P or S1P is mediated by sphingosine kinases (SPHKs). Although phyto-S1P and S1P are known to be signaling components in higher plants, their ability to induce stomatal closure has been ambiguous. We evaluated in detail the effects of phyto-S1P, S1P and SPHK inhibitors on signaling events leading to stomatal closure in the epidermis of Pisum sativum. Phyto-S1P or S1P induced stomatal closure, along with a marked rise in nitric oxide (NO) and cytoplasmic pH of guard cells, as in case of ABA. Two SPHK inhibitors, DL-threo dihydrosphingosine and N',N'-dimethylsphingosine, restricted ABA-induced stomatal closure and prevented the increase of NO or pH by ABA. Modulators of NO or pH impaired both stomatal closure and increase in NO or pH by phyto-S1P/S1P. The stomatal closure by phyto-S1P/S1P was mediated by phospholipase D and phosphatidic acid (PA). When present, PA elevated the levels of pH, but not NO of guard cells. Our results demonstrate that stomatal closure induced by phyto-S1P and S1P depends on rise in pH as well as NO of guard cells. A scheme of signaling events initiated by phyto-S1P/S1P, and converging to cause stomatal closure, is proposed.

Xanthan Pyruvilation Is Essential for the Virulence of Xanthomonas campestris pv. campestris

Xanthan, the main exopolysaccharide (EPS) synthesized by Xanthomonas spp., contributes to bacterial stress tolerance and enhances attachment to plant surfaces by helping in biofilm formation. Therefore, xanthan is essential for successful colonization and growth in planta and has also been proposed to be involved in the promotion of pathogenesis by calcium ion chelation and, hence, in the suppression of the plant defense responses in which this cation acts as a signal. The aim of this work was to study the relationship between xanthan structure and its role as a virulence factor. We analyzed four Xanthomonas campestris pv. campestris mutants that synthesize structural variants of xanthan. We found that the lack of acetyl groups that decorate the internal mannose residues, ketal-pyruvate groups, and external mannose residues affects bacterial adhesion and biofilm architecture. In addition, the mutants that synthesized EPS without pyruvilation or without the external mannose residues did not develop disease symptoms in Arabidopsis thaliana. We also observed that the presence of the external mannose residues and, hence, pyruvilation is required for xanthan to suppress callose deposition as well as to interfere with stomatal defense. In conclusion, pyruvilation of xanthan seems to be essential for Xanthomonas campestris pv. campestris virulence.

Long-term effects of abscisic acid (ABA) on the grape berry phenylpropanoid pathway: Gene expression and metabolite content

ABA has been proposed as the main signal triggering the onset of the ripening process in grapes, and modulating the secondary metabolism in grape berry skins. To determine the effect of ABA on secondary metabolism in berries, clusters of Carménère were sprayed with 0 μLL(-1) ABA; 50 μLL(-1) ABA and 100 μLL(-1) ABA during pre-véraison, and the gene expression of the transcription factors and enzymes of the phenylpropanoid pathway were assessed from véraison to 70 days after véraison (DAV). Additionally, flavonols, tannins and anthocyanins were assessed from véraison until harvest (110 DAV). ABA accelerated sugar and anthocyanin accumulation at véraison. The grape transcript abundance of VvDFR, VvANS, VvUFGT and VvMybA1, all peaking around véraison mimicked the concentration of ABA throughout the season. The highest anthocyanin concentration occurred 35 DAV for all treatments, but higher pigment concentrations were observed in ABA-treated berries at véraison and from 60 to 70 DAV to harvest. VvPAL was also increased by treatment at the higher concentration of ABA from véraison to 40 DAV. Regarding flavanol synthesis, VvLAR2 and VvMyb4A decreased from véraison until 40 DAV and then increased again until 70 DAV. Compared to the control, both ABA treatments resulted in a less-than-proportional reduction of the expression of both genes compared to the control and, after 40 DAV, in a more-than-proportional increase compared to the control, suggesting a long-term effect of the pre-véraison ABA spray on the berries. A concomitant increase in flavanols was observed in berries after 40 DAV, and this occurred at a higher extent in berries treated with the highest ABA concentration.

The Cotton Mitogen-Activated Protein Kinase Kinase 3 Functions in Drought Tolerance by Regulating Stomatal Responses and Root Growth

Mitogen-activated protein kinase (MAPK) cascades play critical roles in signal transduction processes in eukaryotes. The MAPK kinases (MAPKKs) that link MAPKK kinases (MAPKKKs) and MAPKs are key components of MAPK cascades. However, the intricate regulatory mechanisms that control MAPKKs under drought stress conditions are not fully understood, especially in cotton (Gossypium hirsutum) Here, we isolated and characterized the cotton group B MAPKK gene GhMKK3 Overexpressing GhMKK3 in Nicotiana benthamiana enhanced tolerance to drought, and the results of RNA sequencing (RNA-seq) and quantitative real-time PCR (qRT-PCR) assays suggest that GhMKK3 plays an important role in responses to abiotic stresses by regulating stomatal responses and root hair growth. Further evidence demonstrated that overexpressing GhMKK3 promoted root growth and ABA-induced stomatal closure. In contrast, silencing GhMKK3 in cotton using virus-induced gene silencing (VIGS) resulted in the opposite phenotypes. More importantly, we identified an ABA- and drought-induced MAPK cascade that is composed of GhMKK3, GhMPK7 and GhPIP1 that compensates for deficiency in the MAPK cascade pathway in cotton under drought stress conditions. Together, these findings significantly improve our understanding of the mechanism by which GhMKK3 positively regulates drought stress responses.

Identification and functional expression of the pepper RING type E3 ligase, CaDTR1, involved in drought stress tolerance via ABA-mediated signalling

Drought negatively affects plant growth and development, thereby leading to loss of crop productivity. Several plant E3 ubiquitin ligases act as positive or negative regulators of abscisic acid (ABA) and thus play important roles in the drought stress response. Here, we show that the C3HC4-type RING finger E3 ligase, CaDTR1, regulates the drought stress response via ABA-mediated signalling. CaDTR1 contains an amino-terminal RING finger motif and two carboxyl-terminal hydrophobic regions; the RING finger motif functions during attachment of ubiquitins to the target proteins, and the carboxyl-terminal hydrophobic regions function during subcellular localisation. The expression of CaDTR1 was induced by ABA, drought, and NaCl treatments. CaDTR1 localised in the nucleus and displayed in vitro E3 ubiquitin ligase activity. CaDTR1-silenced pepper plants exhibited a drought-sensitive phenotype characterised by high levels of transpirational water loss. On the other hand, CaDTR1-overexpressing (OX) Arabidopsis plants exhibited an ABA-hypersensitive phenotype during the germinative and post-germinative growth stages. Moreover, in contrast to CaDTR1-silenced pepper plants, CaDTR1-OX plants exhibited a drought-tolerant phenotype characterised by low levels of transpirational water loss via increased stomatal closure and high leaf temperatures. Our data indicate that CaDTR1 functions as a positive regulator of the drought stress response via ABA-mediated signalling.

Pepper protein phosphatase type 2C, CaADIP1 and its interacting partner CaRLP1 antagonistically regulate ABA signalling and drought response

Abscisic acid (ABA) is a key phytohormone that regulates plant growth and developmental processes, including seed germination and stomatal closing. Here, we report the identification and functional characterization of a novel type 2C protein phosphatase, CaADIP1 (Capsicum annuum ABA and Drought-Induced Protein phosphatase 1). The expression of CaADIP1 was induced in pepper leaves by ABA, drought and NaCl treatments. Arabidopsis plants overexpressing CaADIP1 (CaADIP1-OX) exhibited an ABA-hyposensitive and drought-susceptible phenotype. We used a yeast two-hybrid screening assay to identify CaRLP1 (Capsicum annuum RCAR-Like Protein 1), which interacts with CaADIP1 in the cytoplasm and nucleus. In contrast to CaADIP1-OX plants, CaRLP1-OX plants displayed an ABA-hypersensitive and drought-tolerant phenotype, which was characterized by low levels of transpirational water loss and increased expression of stress-responsive genes relative to those of wild-type plants. In CaADIP1-OX/CaRLP1-OX double transgenic plants, ectopic expression of the CaRLP1 gene led to strong suppression of CaADIP1-induced ABA hyposensitivity during the germinative and post-germinative stages, indicating that CaADIP1 and CaRLP1 act in the same signalling pathway and CaADIP1 functions downstream of CaRLP1. Our results indicate that CaADIP1 and its interacting partner CaRLP1 antagonistically regulate the ABA-dependent defense signalling response to drought stress.

Rhizobacterial Strain Bacillus megaterium BOFC15 Induces Cellular Polyamine Changes that Improve Plant Growth and Drought Resistance

Plant-growth-promoting rhizobacteria can improve plant growth, development, and stress adaptation. However, the underlying mechanisms are still largely unclear. We investigated the effects of Bacillusmegaterium BOFC15 on Arabidopsis plants. BOFC15 produced and secreted spermidine (Spd), a type of polyamine (PA) that plays an important role in plant growth. Moreover, BOFC15 induced changes in the cellular PAs of plants that promoted an increase of free Spd and spermine levels. However, these effects were remarkably abolished by the addition of dicyclohexylamine (DCHA), a Spd biosynthetic inhibitor. Additionally, the inoculation with BOFC15 remarkably increased plant biomass, improved root system architecture, and augmented photosynthetic capacity. Inoculated plants also displayed stronger ability to tolerate drought stress than non-inoculated (control) plants. Abscisic acid (ABA) content was notably higher in the inoculated plants than in the control plants under drought stress and polyethylene glycol (PEG)-induced stress conditions. However, the BOFC15-induced ABA synthesis was markedly inhibited by DCHA. Thus, microbial Spd participated in the modulation of the ABA levels. The Spd-producing BOFC15 improved plant drought tolerance, which was associated with altered cellular ABA levels and activated adaptive responses.

Roles and Transport of Sodium and Potassium in Plants

The two alkali cations Na(+) and K(+) have similar relative abundances in the earth crust but display very different distributions in the biosphere. In all living organisms, K(+) is the major inorganic cation in the cytoplasm, where its concentration (ca. 0.1 M) is usually several times higher than that of Na(+). Accumulation of Na(+) at high concentrations in the cytoplasm results in deleterious effects on cell metabolism, e.g., on photosynthetic activity in plants. Thus, Na(+) is compartmentalized outside the cytoplasm. In plants, it can be accumulated at high concentrations in vacuoles, where it is used as osmoticum. Na(+) is not an essential element in most plants, except in some halophytes. On the other hand, it can be a beneficial element, by replacing K(+) as vacuolar osmoticum for instance. In contrast, K(+) is an essential element. It is involved in electrical neutralization of inorganic and organic anions and macromolecules, pH homeostasis, control of membrane electrical potential, and the regulation of cell osmotic pressure. Through the latter function in plants, it plays a role in turgor-driven cell and organ movements. It is also involved in the activation of enzymes, protein synthesis, cell metabolism, and photosynthesis. Thus, plant growth requires large quantities of K(+) ions that are taken up by roots from the soil solution, and then distributed throughout the plant. The availability of K(+) ions in the soil solution, slowly released by soil particles and clays, is often limiting for optimal growth in most natural ecosystems. In contrast, due to natural salinity or irrigation with poor quality water, detrimental Na(+) concentrations, toxic for all crop species, are present in many soils, representing 6 % to 10 % of the earth's land area. Three families of ion channels (Shaker, TPK/KCO, and TPC) and 3 families of transporters (HAK, HKT, and CPA) have been identified so far as contributing to K(+) and Na(+) transport across the plasmalemma and internal membranes, with high or low ionic selectivity. In the model plant Arabidopsis thaliana, these families gather at least 70 members. Coordination of the activities of these systems, at the cell and whole plant levels, ensures plant K(+) nutrition, use of Na(+) as a beneficial element, and adaptation to saline conditions.

Molecular and cellular characterization of GA-Stimulated Transcripts GASA4 and GASA6 in Arabidopsis thaliana

GA and ABA play antagonistic roles in numerous cellular processes essential for growth, development, and stress responses. GASA4 and GASA6 belong to a family of GA-Stimulated transcripts in Arabidopsis, known as GA-inducible and ABA-repressible. We have found that GASA4 and GASA6 expression is likely mediated through a repressor of GA responses, GA INSENSITIVE (GAI) protein. Moreover, GASA4 and GASA6 are in general up regulated by growth hormones (auxin, BR, cytokinin, and GA) and down regulated by stress hormones (ABA, JA, and SA), indicating a role of GASA4 and GASA6 in hormone crosstalk. Genetic analyses show that suppression of both GASA4 and GASA6 causes late flowering, while over-expression of GASA6 causes early flowering in Arabidopsis. GASA family members encode small polypeptides sharing common structural features: an N-terminal signal peptide, a highly divergent intermediate region, and a conserved C-terminal domain containing 12 conserved cysteines. Despite the presence of a signal peptide, it has not been determined whether or not GASA4 and GASA6 can be processed in vivo. By using imaging and immunological analyses, we show that the N-terminal signal peptide is cleaved as predicted, and the cleavage is important for proper sub-cellular localization of GASA4 and GASA6.

GOLDEN 2-LIKE transcription factors for chloroplast development affect ozone tolerance through the regulation of stomatal movement

Stomatal movements regulate gas exchange, thus directly affecting the efficiency of photosynthesis and the sensitivity of plants to air pollutants such as ozone. The GARP family transcription factors GOLDEN 2-LIKE1 (GLK1) and GLK2 have known functions in chloroplast development. Here, we show that Arabidopsis thaliana (A. thaliana) plants expressing the chimeric repressors for GLK1 and -2 (GLK1/2-SRDX) exhibited a closed-stomata phenotype and strong tolerance to ozone. By contrast, plants that overexpress GLK1/2 exhibited an open-stomata phenotype and higher sensitivity to ozone. The plants expressing GLK1-SRDX had reduced expression of the genes for inwardly rectifying K(+) (K(+) in) channels and reduced K(+) in channel activity. Abscisic acid treatment did not affect the stomatal phenotype of 35S:GLK1/2-SRDX plants or the transcriptional activity for K(+) in channel gene, indicating that GLK1/2 act independently of abscisic acid signaling. Our results indicate that GLK1/2 positively regulate the expression of genes for K(+) in channels and promote stomatal opening. Because the chimeric GLK1-SRDX repressor driven by a guard cell-specific promoter induced a closed-stomata phenotype without affecting chloroplast development in mesophyll cells, modulating GLK1/2 activity may provide an effective tool to control stomatal movements and thus to confer resistance to air pollutants.

Arabidopsis HY1-Modulated Stomatal Movement: An Integrative Hub Is Functionally Associated with ABI4 in Dehydration-Induced ABA Responsiveness

Heme oxygenase (HO; EC 1.14.99.3) has recently been proposed as a novel component in mediating wide ranges of the plant adaptive signaling processes. However, the physiological significance and molecular basis underlying Arabidopsis (Arabidopsis thaliana) HO1 (HY1) functioning in drought tolerance remained unclear. Here, we report that mutation of HY1 promoted, but overexpression of this gene impaired, Arabidopsis drought tolerance. This was attributed to the abscisic acid (ABA)-hypersensitive or -hyposensitive phenotypes, with the regulation of stomatal closure in particular. However, comparative transcriptomic profile analysis showed that the induction of numerous ABA/stress-dependent genes in dehydrated wild-type plants was differentially impaired in the hy1 mutant. In agreement, ABA-induced ABSCISIC ACID-INSENSITIVE4 (ABI4) transcript accumulation was strengthened in the hy1 mutant. Genetic analysis further identified that the hy1-associated ABA hypersensitivity and drought tolerance were arrested in the abi4 background. Moreover, the promotion of ABA-triggered up-regulation of RbohD abundance and reactive oxygen species (ROS) levels in the hy1 mutant was almost fully blocked by the mutation of ABI4, suggesting that the HY1-ABI4 signaling in the wild type involved in stomatal closure was dependent on the RbohD-derived ROS production. However, hy1-promoted stomatal closure was not affected by a nitric oxide scavenger. Correspondingly, ABA-insensitive behaviors in rbohD stomata were not affected by either the mutation of HY1 or its ectopic expression in the rbohD background, both of which responded significantly to exogenous ROS. These data indicate that HY1 functioned negatively and acted upstream of ABI4 in drought signaling, which was casually dependent on the RbohD-derived ROS in the regulation of stomatal closure.

Overexpression of MdSOS2L1, a CIPK protein kinase, increases the antioxidant metabolites to enhance salt tolerance in apple and tomato

Soil salinity hinders the growth of most higher plants and becomes a gradually increasing threat to the agricultural production of such crops as the woody plant apple. In this study, a calcineurin B-like protein (CBL)-interacting protein kinase, MdCIPK24-LIKE1 (named as MdSOS2L1), was identified. Quantitative real-time polymerase chain reaction (qRT-PCR) assay revealed that the expression of MdSOS2L1 was upregulated by CaCl₂ . Yeast two-hybrid (Y2H) assay and transiently transgenic analysis demonstrated that the MdSOS2L1 protein kinase physically interacted with MdCBL1, MdCBL4 and MdCBL10 proteins to increase salt tolerance in apple. Furthermore, iTRAQ proteome combined with liquid chromatography-tandem mass spectrometry (LC/MS) analysis found that several proteins, which are involved in reactive oxygen species (ROS) scavenging, procyanidin biosynthesis and malate metabolism, were induced in MdSOS2L1-overexpressing apple plants. Subsequent studies have shown that MdSOS2L1 increased antioxidant metabolites such as procyanidin and malate to improve salt tolerance in apple and tomato. In summary, our studies provide a mechanism in which SOS2L1 enhances the salt stress tolerance in apple and tomato.

The Arabidopsis AtPP2CA Protein Phosphatase Inhibits the GORK K+ Efflux Channel and Exerts a Dominant Suppressive Effect on Phosphomimetic-activating Mutations

The regulation of the GORK (Guard Cell Outward Rectifying) Shaker channel mediating a massive K(+) efflux in Arabidopsis guard cells by the phosphatase AtPP2CA was investigated. Unlike the gork mutant, the atpp2ca mutants displayed a phenotype of reduced transpiration. We found that AtPP2CA interacts physically with GORK and inhibits GORK activity in Xenopus oocytes. Several amino acid substitutions in the AtPP2CA active site, including the dominant interfering G145D mutation, disrupted the GORK-AtPP2CA interaction, meaning that the native conformation of the AtPP2CA active site is required for the GORK-AtPP2CA interaction. Furthermore, two serines in the GORK ankyrin domain that mimic phosphorylation (Ser to Glu) or dephosphorylation (Ser to Ala) were mutated. Mutations mimicking phosphorylation led to a significant increase in GORK activity, whereas mutations mimicking dephosphorylation had no effect on GORK. In Xenopus oocytes, the interaction of AtPP2CA with "phosphorylated" or "dephosphorylated" GORK systematically led to inhibition of the channel to the same baseline level. Single-channel recordings indicated that the GORK S722E mutation increases the open probability of the channel in the absence, but not in the presence, of AtPP2CA. The dephosphorylation-independent inactivation mechanism of GORK by AtPP2CA is discussed in relation with well known conformational changes in animal Shaker-like channels that lead to channel opening and closing. In plants, PP2C activity would control the stomatal aperture by regulating both GORK and SLAC1, the two main channels required for stomatal closure.

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

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