BRI1-Associated Receptor Kinase 1 Regulates Guard Cell ABA Signaling Mediated by Open Stomata 1 in Arabidopsis

LKS4-mediated SYP121 phosphorylation participates in light-induced stomatal opening in Arabidopsis

By modulating stomatal opening and closure, plants control gas exchange, water loss, and photosynthesis in response to various environmental signals. During light-induced stomatal opening, the transport of ions and solutes across the plasma membrane (PM) of the surrounding guard cells results in an increase in turgor pressure, leading to cell swelling. Simultaneously, vesicles for exocytosis are delivered via membrane trafficking to compensate for the enlarged cell surface area and maintain an appropriate ion-channel density in the PM. In eukaryotic cells, soluble N-ethylmaleimide-sensitive factor adaptor protein receptors (SNAREs) mediate membrane fusion between vesicles and target compartments by pairing the cognate glutamine (Q)- and arginine (R)-SNAREs to form a core SNARE complex. Syntaxin of plants 121 (SYP121) is a known Q-SNARE involved in stomatal movement, which not only facilitates the recycling of K+ channels to the PM but also binds to the channels to regulate their activity. In this study, we found that the expression of a receptor-like cytoplasmic kinase, low-K+ sensitive 4/schengen 1 (LKS4/SGN1), was induced by light; it directly interacted with SYP121 and phosphorylated T270 within the SNARE motif. Further investigation revealed that LKS4-dependent phosphorylation of SYP121 facilitated the interaction between SYP121 and R-SNARE vesicle-associated membrane protein 722 (VAMP722), promoting the assembly of the SNARE complex. Our findings demonstrate that the phosphorylation of SNARE proteins is an important strategy adopted by plants to regulate the SNARE complex assembly as well as membrane fusion. Additionally, we discovered the function of LKS4/SGN1 in light-induced stomatal opening via the phosphorylation of SYP121.

DPY1 as an osmosensor for drought signaling

Drought stress has been extensively studied for its effect on the downstream signaling cascade and stress-responsive gene expression, but understanding the process has remained elusive. Recently, Zhao et al. identified DROOPY LEAF1 (DPY1) as an osmosensor and revealed a novel mechanism of DPY1-STRESS ACTIVATED PROTEIN KINASE6 (SAPK6)-mediated drought stress signaling in higher plants.

Transcriptome Analysis Reveals Key Genes Involved in Trichome Formation in Pepper (Capsicum annuum L.)

Trichomes are specialized organs located in the plant epidermis that play important defense roles against biotic and abiotic stresses. However, the mechanisms regulating the development of pepper epidermal trichomes and the related regulatory genes at the molecular level are not clear. Therefore, we performed transcriptome analyses of A114 (less trichome) and A115 (more trichome) to dig deeper into the genes involved in the regulatory mechanisms of epidermal trichome development in peppers. In this study, the epidermal trichome density of A115 was found to be higher by phenotypic observation and was highest in the leaves at the flowering stage. A total of 39,261 genes were quantified by RNA-Seq, including 11,939 genes not annotated in the previous genome analysis and 18,833 differentially expressed genes. Based on KEGG functional enrichment, it was found that DEGs were mainly concentrated in three pathways: plant-pathogen interaction, MAPK signaling pathway-plant, and plant hormone signal transduction. We further screened the DEGs associated with the development of epidermal trichomes in peppers, and the expression of the plant signaling genes GID1B-like (Capana03g003488) and PR-6 (Capana09g001847), the transcription factors MYB108 (Capana05g002225) and ABR1-like (Capana04g001261), and the plant resistance genes PGIP-like (Capana09g002077) and At5g49770 (Capana08g001721) in the DEGs were higher at A115 compared to A114, and were highly expressed in leaves at the flowering stage. In addition, based on the WGCNA results and the establishment of co-expression networks showed that the above genes were highly positively correlated with each other. The transcriptomic data and analysis of this study provide a basis for the study of the regulatory mechanisms of pepper epidermal trichomes.

Involvement of brassinosteroids and abscisic acid in spikelet degeneration in rice under soil drying during meiosis

Spikelet degeneration in rice (Oryza sativa L.) is a serious physiological defect, and can be regulated by soil moisture status and phytohormones. This study investigated the possibility that brassinosteroids (BRs) in collaboration with abscisic acid (ABA) are involved in mediating the effect of soil drying during meiosis on spikelet degeneration in rice. Three rice cultivars were field grown and three irrigation regimes including well watered (WW), moderate soil drying (MD), and severe soil drying (SD) were imposed during meiosis. MD significantly decreased spikelet degeneration in comparison with WW, due mainly to the alleviation in oxidative damage via enhancing ascorbate-glutathione (AsA-GSH) cycle activity in young panicles, and SD exhibited the opposite effects. Enhanced AsA-GSH cycle strength, decreased oxidative stress, and spikelet degeneration rate were closely associated with the synergistically elevated BR and ABA levels in young panicles in MD. In contrast, low BR and excessive ABA levels led to an increase in spikelet degeneration in SD. The three cultivars exhibited the same tendencies. The intrinsic link among AsA-GSH cycle, oxidative stress, spikelet degeneration rate, and BR and ABA levels was further verified by using transgenic rice lines and chemical regulators. BRs or ABA play a unique role in regulating spikelet degeneration. Synergistically increased BR and ABA levels in MD could work together to strengthen AsA-GSH cycle activity, leading to a reduction in oxidative damage and spikelet degeneration. On the other hand, a severe imbalance between low BR and excessive ABA levels may have contributed to the opposite effects in SD.

Comprehensive analysis of sesame LRR-RLKs: structure, evolution and dynamic expression profiles under Macrophomina phaseolina stress

Leucine-rich repeat receptor-like kinases (LRR-RLKs) can participate in the regulation of plant growth and development, immunity and signal transduction. Sesamum indicum, one of the most important oil crops, has a significant role in promoting human health. In this study, 175 SiLRR-RLK genes were identified in S. indicum, and they were subdivided into 12 subfamilies by phylogenetic analysis. Gene duplication analysis showed that the expansion of the SiLRR-RLK family members in the sesame was mainly due to segmental duplication. Moreover, the gene expansion of subfamilies IV and III contributed to the perception of stimuli under M. phaseolina stress in the sesame. The collinearity analysis with other plant species revealed that the duplication of SiLRR-RLK genes occurred after the differentiation of dicotyledons and monocotyledons. The expression profile analysis and functional annotation of SiLRR-RLK genes indicated that they play a vital role in biotic stress. Furthermore, the protein-protein interaction and coexpression networks suggested that SiLRR-RLKs contributed to sesame resistance to Macrophomina phaseolina by acting alone or as a polymer with other SiLRR-RLKs. In conclusion, the comprehensive analysis of the SiLRR-RLK gene family provided a framework for further functional studies on SiLRR-RLK genes.

Stomatal responses to VPD utilize guard cell intracellular signaling components

Stomatal pores, vital for CO2 uptake and water loss regulation in plants, are formed by two specialized guard cells. Despite their importance, there is limited understanding of how guard cells sense and respond to changes in vapor pressure difference (VPD). This study leverages a selection of CO2 hyposensitive and abscisic acid (ABA) signaling mutants in Arabidopsis, including heterotrimeric G protein mutants and RLK (receptor-like kinase) mutants, along with a variety of canola cultivars to delve into the intracellular signaling mechanisms prompting stomatal closure in response to high VPD. Stomatal conductance response to step changes in VPD was measured using the LI-6800F gas exchange system. Our findings highlight that stomatal responses to VPD utilize intracellular signaling components. VPD hyposensitivity was particularly evident in mutants of the ht1 (HIGH LEAF TEMPERATURE1) gene, which encodes a protein kinase expressed mainly in guard cells, and in gpa1-3, a null mutant of the sole canonical heterotrimeric Gα subunit, previously implicated in stomatal signaling. Consequently, this research identifies a nexus in the intricate relationships between guard cell signal perception, stomatal conductance, environmental humidity, and CO2 levels.

PHB3 interacts with BRI1 and BAK1 to mediate brassinosteroid signal transduction in Arabidopsis and tomato

Brassinosteroids (BRs) are plant hormones that are essential in plant growth and development. BRASSINOSTEROID-INSENSITIVE 1 (BRI1) and BRI1 ASSOCIATED RECEPTOR KINASE 1 (BAK1), which are located on the plasma membrane, function as co-receptors that accept and transmit BR signals. PROHIBITIN 3 (PHB3) was identified in both BRI1 and BAK1 complexes by affinity purification and LC-MS/MS analysis. Biochemical data showed that BRI1/BAK1 interacted with PHB3 in vitro and in vivo. BRI1/BAK1 phosphorylated PHB3 in vitro. When the Thr-80 amino acid in PHB3 was mutated to Ala, the mutant protein was not phosphorylated by BRI1 and the mutant protein interaction with BRI1 was abolished in the yeast two-hybrid assay. BAK1 did not phosphorylate the mutant protein PHB3T54A . The loss-of-function phb3 mutant showed a weaker BR signal than the wild-type. Genetic analyses revealed that PHB3 is a BRI1/BAK1 downstream substrate that participates in BR signalling. PHB3 has five homozygous in tomato, and we named the closest to AtPHB3 as SlPHB3.1. Biochemical data showed that SlBRI1/SlSERK3A/SlSERK3B interacted with SlPHB3.1 and SlPHB3.3. The CRISPR-Cas9 method generated slphb3.1 mutant led to a BR signal stunted relatively in tomatoes. PHB3 is a new component of the BR signal pathway in both Arabidopsis and tomato.

Comparative analysis of BZR1/BES1 family transcription factors in Arabidopsis

Brassinazole Resistant 1 (BZR1) and bri1 EMS Suppressor 1 (BES1) are key transcription factors that mediate brassinosteroid (BR)-responsive gene expression in Arabidopsis. The BZR1/BES1 family is composed of BZR1, BES1, and four BES1/BZR1 homologs (BEH1-BEH4). However, little is known about whether BEHs are regulated by BR signaling in the same way as BZR1 and BES1. We comparatively analyzed the functional characteristics of six BZR1/BES1 family members and their regulatory mechanisms in BR signaling using genetic and biochemical analyses. We also compared their subcellular localizations regulated by the phosphorylation status, interaction with GSK3-like kinases, and heterodimeric combination. We found that all BZR1/BES1 family members restored the phenotypic defects of bri1-5 by their overexpression. Unexpectedly, BEH2-overexpressing plants showed the most distinct phenotype with enhanced BR responses. RNA-Seq analysis indicated that overexpression of both BZR1 and BEH2 regulates BR-responsive gene expression, but BEH2 has a much greater proportion of BR-independent gene expression than BZR1. Unlike BZR1 and BES1, the BR-regulated subcellular translocation of the four BEHs was not tightly correlated with their phosphorylation status. Notably, BEH1 and BEH2 are predominantly localized in the nucleus, which induces the nuclear accumulation of other BZR1/BES1 family proteins through heterodimerization. Altogether, our comparative analyses suggest that BEH1 and BEH2 play an important role in the functional interaction between BZR1/BES1 family transcription factors.

The molecular paradigm of reactive oxygen species (ROS) and reactive nitrogen species (RNS) with different phytohormone signaling pathways during drought stress in plants

Drought is undoubtedly a major environmental constraint that negatively affects agricultural yield and productivity throughout the globe. Plants are extremely vulnerable to drought which imposes several physiological, biochemical and molecular perturbations. Increased generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in different plant organs is one of the inevitable consequences of drought. ROS and RNS are toxic byproducts of metabolic reactions and poise oxidative stress and nitrosative stress that are detrimental for plants. In spite of toxic effects, these potentially active radicals also play a beneficial role in mediating several signal transduction events that lead to plant acclimation and enhanced survival under harsh environmental conditions. The precise understanding of ROS and RNS signaling and their molecular paradigm with different phytohormones, such as auxin, gibberellin, cytokinin, abscisic acid, ethylene, brassinosteroids, strigolactones, jasmonic acid, salicylic acid and melatonin play a pivotal role for maintaining plant fitness and resilience to counteract drought toxicity. Therefore, the present review provides an overview of integrated systemic signaling between ROS, RNS and phytohormones during drought stress based on past and recent advancements and their influential role in conferring protection against drought-induced damages in different plant species. Indeed, it would not be presumptuous to hope that the detailed knowledge provided in this review will be helpful for designing drought-tolerant crop cultivars in the forthcoming times.

The osmotic stress-activated receptor-like kinase DPY1 mediates SnRK2 kinase activation and drought tolerance in Setaria

Plant genomes encode many receptor-like kinases (RLKs) that localize to the cell surface and perceive a wide variety of environmental cues to initiate downstream signaling cascades. Whether these RLKs participate in dehydration stress signaling in plants is largely unknown. DROOPY LEAF1 (DPY1), a leucine-rich repeat (LRR)-RLK, was recently shown to regulate plant architecture by orchestrating early brassinosteroid signaling in foxtail millet (Setaria italica). Here, we show that DPY1 is essential for the acclimation of foxtail millet to drought stress. DPY1 can be phosphorylated and activated in response to osmotic stress and is required for more than half of osmotic stress-induced global phosphorylation events, including the phosphorylation of sucrose nonfermenting kinase 2s (SnRK2s), the central kinases involved in osmotic stress. DPY1 acts upstream of STRESS-ACTIVATED PROTEIN KINASE 6 (SAPK6, a subclass I SnRK2) and is required for full SAPK6 activation, thereby allowing regulation of downstream genes to mount a response against drought stress. These signaling events are largely independent of DPY1-mediated brassinosteroid signaling. The DPY1-SAPK6 module is specific to seed plants and is absent in ancestral nonseed plants. Our findings reveal a dehydration stress-activated RLK that plays an indispensable role in osmotic stress signaling and mediates SnRK2 activation at the cell surface.

Comprehensive Analysis of BR Receptor Expression under Hormone Treatment in the Rubber Tree (Hevea brasiliensis Muell. Arg.)

Brassinosteroids (BRs) are important for plant growth and development, with BRI1 and BAK1 kinases playing an important role in BR signal transduction. Latex from rubber trees is crucial for industry, medicine and defense use. Therefore, it is beneficial to characterize and analyze HbBRI1 and HbBAK1 genes to improve the quality of the resources obtained from Hevea brasiliensis (rubber tree). Based on bioinformatics predictions and rubber tree database, five HbBRI1s with four HbBAK1s were identified and named HbBRI1~HbBRL3 and HbBAK1a~HbBAK1d, respectively, which were clustered in two groups. HbBRI1 genes, except for HbBRL3, exclusively contain introns, which is convenient for responding to external factors, whereas HbBAK1b/c/d contain 10 introns and 11 exons, and HbBAK1a contains eight introns. Multiple sequence analysis showed that HbBRI1s include typical domains of the BRI1 kinase, indicating that HbBRI1s belong to BRI1. HbBAK1s that possess LRR and STK_BAK1_like domains illustrate that HbBAK1s belong to the BAK1 kinase. BRI1 and BAK1 play an important role in regulating plant hormone signal transduction. Analysis of the cis-element of all HbBRI1 and HbBAK1 genes identified hormone response, light regulation and abiotic stress elements in the promoters of HbBRI1s and HbBAK1s. The results of tissue expression patterns indicate that HbBRL1/2/3/4 and HbBAK1a/b/c are highly expressed in the flower, especially HbBRL2-1. The expression of HbBRL3 is extremely high in the stem, and the expression of HbBAK1d is extremely high in the root. Expression profiles with different hormones show that HbBRI1 and HbBAK1 genes are extremely induced by different hormone stimulates. These results provide theoretical foundations for further research on the functions of BR receptors, especially in response to hormone signals in the rubber tree.

Nitric Oxide Acts as a Key Signaling Molecule in Plant Development under Stressful Conditions

Nitric oxide (NO), a colorless gaseous molecule, is a lipophilic free radical that easily diffuses through the plasma membrane. These characteristics make NO an ideal autocrine (i.e., within a single cell) and paracrine (i.e., between adjacent cells) signalling molecule. As a chemical messenger, NO plays a crucial role in plant growth, development, and responses to biotic and abiotic stresses. Furthermore, NO interacts with reactive oxygen species, antioxidants, melatonin, and hydrogen sulfide. It regulates gene expression, modulates phytohormones, and contributes to plant growth and defense mechanisms. In plants, NO is mainly produced via redox pathways. However, nitric oxide synthase, a key enzyme in NO production, has been poorly understood recently in both model and crop plants. In this review, we discuss the pivotal role of NO in signalling and chemical interactions as well as its involvement in the mitigation of biotic and abiotic stress conditions. In the current review, we have discussed various aspects of NO including its biosynthesis, interaction with reactive oxygen species (ROS), melatonin (MEL), hydrogen sulfide, enzymes, phytohormones, and its role in normal and stressful conditions.

Advances in Receptor-like Protein Kinases in Balancing Plant Growth and Stress Responses

Accompanying the process of growth and development, plants are exposed to ever-changing environments, which consequently trigger abiotic or biotic stress responses. The large protein family known as receptor-like protein kinases (RLKs) is involved in the regulation of plant growth and development, as well as in the response to various stresses. Understanding the biological function and molecular mechanism of RLKs is helpful for crop breeding. Research on the role and mechanism of RLKs has recently received considerable attention regarding the balance between plant growth and environmental adaptability. In this paper, we systematically review the classification of RLKs, the regulatory roles of RLKs in plant development (meristem activity, leaf morphology and reproduction) and in stress responses (disease resistance and environmental adaptation). This review focuses on recent findings revealing that RLKs simultaneously regulate plant growth and stress adaptation, which may pave the way for the better understanding of their function in crop improvement. Although the exact crosstalk between growth constraint and plant adaptation remains elusive, a profound study on the adaptive mechanisms for decoupling the developmental processes would be a promising direction for the future research.

CKL2 mediates the crosstalk between abscisic acid and brassinosteroid signaling to promote swift growth recovery after stress in Arabidopsis

Plants must adapt to the constantly changing environment. Adverse environmental conditions trigger various defensive responses, including growth inhibition mediated by phytohormone abscisic acid (ABA). When the stress recedes, plants must transit rapidly from stress defense to growth recovery, but the underlying mechanisms by which plants switch promptly and accurately between stress resistance and growth are poorly understood. Here, using quantitative phosphoproteomics strategy, we discovered that early ABA signaling activates upstream components of brassinosteroid (BR) signaling through CASEIN KINASE 1-LIKE PROTEIN 2 (CKL2). Further investigations showed that CKL2 interacts with and phosphorylates BRASSINOSTEROID INSENSITIVE1 (BRI1), the main BR receptor, to maintain the basal activity of the upstream of BR pathway in plants exposed to continuous stress conditions. When stress recedes, the elevated phosphorylation of BRI1 by CKL2 contributes to the swift reactivation of BR signaling, which results in quick growth recovery. These results suggest that CKL2 plays a critical regulatory role in the rapid switch between growth and stress resistance. Our evidence expands the understanding of how plants modulate stress defense and growth by integrating ABA and BR signaling cascades.

Distinct role of AtCuAOβ- and RBOHD-driven H2O2 production in wound-induced local and systemic leaf-to-leaf and root-to-leaf stomatal closure

Polyamines (PAs) are ubiquitous low-molecular-weight aliphatic compounds present in all living organisms and essential for cell growth and differentiation. The developmentally regulated and stress-induced copper amine oxidases (CuAOs) oxidize PAs to aminoaldehydes producing hydrogen peroxide (H₂O₂) and ammonia. The Arabidopsis thaliana CuAOβ (AtCuAOβ) was previously reported to be involved in stomatal closure and early root protoxylem differentiation induced by the wound-signal MeJA via apoplastic H₂O₂ production, suggesting a role of this enzyme in water balance, by modulating xylem-dependent water supply and stomata-dependent water loss under stress conditions. Furthermore, AtCuAOβ has been shown to mediate early differentiation of root protoxylem induced by leaf wounding, which suggests a whole-plant systemic coordination of water supply and loss through stress-induced stomatal responses and root protoxylem phenotypic plasticity. Among apoplastic ROS generators, the D isoform of the respiratory burst oxidase homolog (RBOH) has been shown to be involved in stress-mediated modulation of stomatal closure as well. In the present study, the specific role of AtCuAOβ and RBOHD in local and systemic perception of leaf and root wounding that triggers stomatal closure was investigated at both injury and distal sites exploiting Atcuaoβ and rbohd insertional mutants. Data evidenced that AtCuAOβ-driven H₂O₂ production mediates both local and systemic leaf-to-leaf and root-to-leaf responses in relation to stomatal movement, Atcuaoβ mutants being completely unresponsive to leaf or root wounding. Instead, RBOHD-driven ROS production contributes only to systemic leaf-to-leaf and root-to-leaf stomatal closure, with rbohd mutants showing partial unresponsiveness in distal, but not local, responses. Overall, data herein reported allow us to hypothesize that RBOHD may act downstream of and cooperate with AtCuAOβ in inducing the oxidative burst that leads to systemic wound-triggered stomatal closure.

Calcium Channels, OST1 and Stomatal Defence: Current Status and Beyond

Stomatal immunity is regulated by pathogen-associated molecular patterns (PAMPs)- and abscisic acid (ABA)-triggered signalling in different ways. Cytoplasmic Ca²⁺ signature in the guard cells plays a vital function in stomatal immunity, but the mechanism of Ca²⁺ import is unknown. It has been very recently established that the hyperosmolality-gated calcium-permeable channels (OSCAs) and cyclic nucleotide-gated channels (CNGCs) are responsible for the influx of Ca²⁺ in the cytoplasm, which are activated after BIK1-mediated phosphorylation and ABA interaction during PAMPs- and ABA-triggered stomatal immunity in plants, respectively. Further, ABA-triggered OPEN STOMATA1 (OST1) causes the disassembly of microtubules in the guard cells besides activation of S-type anion channels (SLAC1) for the efflux of cytoplasmic anions that leads to stomata closure.

The interaction of ABA and ROS in plant growth and stress resistances

The plant hormone ABA (abscisic acid) plays an extremely important role in plant growth and adaptive stress, including but are not limited to seed germination, stomatal closure, pathogen infection, drought and cold stresses. Reactive oxygen species (ROS) are response molecules widely produced by plant cells under biotic and abiotic stress conditions. The production of apoplast ROS is induced and regulated by ABA, and participates in the ABA signaling pathway and its regulated plant immune system. In this review, we summarize ABA and ROS in apoplast ROS production, plant response to biotic and abiotic stresses, plant growth regulation, ABA signal transduction, and the regulatory relationship between ABA and other plant hormones. In addition, we also discuss the effects of protein post-translational modifications on ABA and ROS related factors.

Beyond stress response: OST1 opening doors for plants to grow

The sucrose non-fermenting 1 (SNF1)-related protein kinase 2 (SnRK2) family members have been discovered to regulate abiotic stress response via the abscisic acid (ABA)-independent and dependent signaling pathways. SnRK2.6, also known as Open Stomata 1 (OST1), is a serine/threonine protein kinase that plays critical roles in linking ABA receptor complexes and downstream components such as transcription factors and anion channels to regulate stress response. Asides from its well-known regulatory roles in stomatal movement and cold stress response, OST1 has also been demonstrated recently to modulate major developmental roles of flowering and growth in plants. In this review, we will discuss about the various roles of OST1 as well as the 'doors' that OST1 can 'open' to help plants perform stress adaptation. Therefore, we will address how OST1 can regulate stomata apertures, cold stress tolerance as well as other aspects of its emerging roles such as balancing flowering and root growth in response to drought.

Receptor-like Kinases (LRR-RLKs) in Response of Plants to Biotic and Abiotic Stresses

Plants live under different biotic and abiotic stress conditions, and, to cope with the adversity and severity, plants have well-developed resistance mechanisms. The mechanism starts with perception of the stimuli followed by molecular, biochemical, and physiological adaptive measures. The family of LRR-RLKs (leucine-rich repeat receptor-like kinases) is one such group that perceives biotic and abiotic stimuli and also plays important roles in different biological processes of development. This has been mostly studied in the model plant, Arabidopsis thaliana, and to some extent in other plants, such as Solanum lycopersicum, Nicotiana benthamiana, Brassica napus, Oryza sativa, Triticum aestivum, Hordeum vulgare, Brachypodium distachyon, Medicago truncatula, Gossypium barbadense, Phaseolus vulgaris, Solanum tuberosum, and Malus robusta. Most LRR-RLKs tend to form different combinations of LRR-RLKs-complexes (dimer, trimer, and tetramers), and some of them were observed as important receptors in immune responses, cell death, and plant development processes. However, less is known about the function(s) of LRR-RLKs in response to abiotic and biotic stresses. Here, we give recent updates about LRR-RLK receptors, specifically focusing on their involvement in biotic and abiotic stresses in the model plant, A. thaliana. Furthermore, the recent studies on LRR-RLKs that are homologous in other plants is also reviewed in relation to their role in triggering stress response processes against biotic and abiotic stimuli and/or in exploring their additional function(s). Furthermore, we present the interactions and combinations among LRR-RLK receptors that have been confirmed through experiments. Moreover, based on GENEINVESTIGATOR microarray database analysis, we predict some potential LRR-RLK genes involved in certain biotic and abiotic stresses whose function and mechanism may be explored.

Melatonin as a regulator of plant ionic homeostasis: implications for abiotic stress tolerance

Melatonin is a highly conserved and ubiquitous molecule that operates upstream of a broad array of receptors in animal systems. Since melatonin was discovered in plants in 1995, hundreds of papers have been published revealing its role in plant growth, development, and adaptive responses to the environment. This paper summarizes the current state of knowledge of melatonin's involvement in regulating plant ion homeostasis and abiotic stress tolerance. The major topics covered here are: (i) melatonin's control of H+-ATPase activity and its implication for plant adaptive responses to various abiotic stresses; (ii) regulation of the reactive oxygen species (ROS)-Ca2+ hub by melatonin and its role in stress signaling; and (iii) melatonin's regulation of ionic homeostasis via hormonal cross-talk. We also show that the properties of the melatonin molecule allow its direct scavenging of ROS, thus preventing negative effects of ROS-induced activation of ion channels. The above 'desensitization' may play a critical role in preventing stress-induced K+ loss from the cytosol as well as maintaining basic levels of cytosolic Ca2+ required for optimal cell operation. Future studies should focus on revealing the molecular identity of transporters that could be directly regulated by melatonin and providing a bioinformatic analysis of evolutionary aspects of melatonin sensing and signaling.

Phosphorylation of the plasma membrane H+-ATPase AHA2 by BAK1 is required for ABA-induced stomatal closure in Arabidopsis

Stomatal opening is largely promoted by light-activated plasma membrane-localized proton ATPases (PM H+-ATPases), while their closure is mainly modulated by abscisic acid (ABA) signaling during drought stress. It is unknown whether PM H+-ATPases participate in ABA-induced stomatal closure. We established that BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) interacts with, phosphorylates and activates the major PM Arabidopsis H+-ATPase isoform 2 (AHA2). Detached leaves from aha2-6 single mutant Arabidopsis thaliana plants lost as much water as bak1-4 single and aha2-6 bak1-4 double mutants, with all three mutants losing more water than the wild-type (Columbia-0 [Col-0]). In agreement with these observations, aha2-6, bak1-4, and aha2-6 bak1-4 mutants were less sensitive to ABA-induced stomatal closure than Col-0, whereas the aha2-6 mutation did not affect ABA-inhibited stomatal opening under light conditions. ABA-activated BAK1 phosphorylated AHA2 at Ser-944 in its C-terminus and activated AHA2, leading to rapid H+ efflux, cytoplasmic alkalinization, and reactive oxygen species (ROS) accumulation, to initiate ABA signal transduction and stomatal closure. The phosphorylation-mimicking mutation AHA2S944D driven by its own promoter could largely compensate for the defective phenotypes of water loss, cytoplasmic alkalinization, and ROS accumulation in both aha2-6 and bak1-4 mutants. Our results uncover a crucial role of AHA2 in cytoplasmic alkalinization and ABA-induced stomatal closure during the plant's response to drought stress.

Rapid responses: Receptor-like kinases directly regulate the functions of membrane transport proteins in plants

Receptor-like kinases (RLKs) are a large group of plant-specific transmembrane proteins mainly acting as receptors or co-receptors of various extracellular signals. They usually turn extracellular signals into intracellular responses via altering gene expression profiles. However, recent studies confirmed that many RLKs can physically interact with diverse membrane-localized transport proteins and regulate their activities for speedy responses in limited tissues or cells. In this minireview, we highlight recent discoveries regarding how RLKs can work with membrane transport proteins collaboratively and thereby trigger cellular responses in a precise and rapid manner. It is anticipated that such regulation broadly presents in plants and more examples will be gradually revealed when in-depth analyses are conducted for the functions of RLKs.

BAK1 plays contrasting roles in regulating abscisic acid-induced stomatal closure and abscisic acid-inhibited primary root growth in Arabidopsis

The mechanisms that balance plant growth and stress responses are poorly understood, but they appear to involve abscisic acid (ABA) signaling mediated by protein kinases. Here, to explore these mechanisms, we examined the responses of Arabidopsis thaliana protein kinase mutants to ABA treatment. We found that mutants of BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) were hypersensitive to the effects of ABA on both seed germination and primary root growth. The kinase OPEN STOMATA 1 (OST1) was more highly activated by ABA in bak1 mutant than the wild type. BAK1 was not activated by ABA treatment in the dominant negative mutant abi1-1 or the pyr1 pyl4 pyl5 pyl8 quadruple mutant, but it was more highly activated by this treatment in the abi1-2 abi2-2 hab1-1 loss-of-function triple mutant than the wild type. BAK1 phosphorylates OST1 T146 and inhibits its activity. Genetic analyses suggested that BAK1 acts at or upstream of core components in the ABA signaling pathway, including PYLs, PP2Cs, and SnRK2s, during seed germination and primary root growth. Although the upstream brassinosteroid (BR) signaling components BAK1 and BR INSENSITIVE 1 (BRI1) positively regulate ABA-induced stomatal closure, mutations affecting downstream components of BR signaling, including BRASSINOSTEROID-SIGNALING KINASEs (BSKs) and BRASSINOSTEROID-INSENSITIVE 2 (BIN2), did not affect ABA-mediated stomatal movement. Thus, our study uncovered an important role of BAK1 in negatively regulating ABA signaling during seed germination and primary root growth, but positively modulating ABA-induced stomatal closure, thus optimizing the plant growth under drought stress.

Analysis of Grapevine's Somatic Embryogenesis Receptor Kinase (SERK) Gene Family: VqSERK3/BAK1 Overexpression Enhances Disease Resistance

The somatic embryogenesis receptor kinase (SERK) gene family has been intensively studied in several plant species. Here we confirmed the existence of five SERK genes in grapevine (Chinese wild grapevine Vitis quinquangularis) and named them VqSERK1, VqSERK2, VqSERK3, VqSERK4, and VqSERK5. Analysis of the predicted structures of these SERK proteins revealed they include a signal peptide domain, a leucine zipper domain, a Ser-Pro-Pro domain, a single transmembrane domain, different leucine-rich repeats, and an intracellular kinase activity domain. The SERK genes of grapevine showed different gene expression patterns when treated with powdery mildew (Erysiphe necator) and hormones (salicylic acid, jasmonic acid, abscisic acid, and ethylene). Subcellular localization assays confirmed that VqSERK family proteins localized to the cell membrane. Moreover, we cloned the SERK3/BAK1 gene from the Chinese wild grapevine V. quinquangularis clone 'Shang-24'. Heterologous VqSERK3/BAK1 expression in the Arabidopsis bak1-4 mutant lines restored control of cell death, increased resistance to powdery mildew, and strengthened stomatal immunity. Our work may provide the foundation for further studies of SERK genes for pathogen resistance and hormone treatment in grapevine.

Brassinosteroid-Insensitive 1-Associated Receptor Kinase 1 Modulates Abscisic Acid Signaling by Inducing PYR1 Monomerization and Association With ABI1 in Arabidopsis

Brassinosteroid-Insensitive 1-Associated Receptor Kinase 1 (BAK1) is a versatile kinase involved in many different plant developmental responses. Previously, we showed that BAK1 interacts with open stomata 1 (OST1), a cytoplasmic kinase, to promote abscisic acid (ABA)-induced stomatal closure. ABA is a plant hormone that primarily regulates stress responses and is recognized by the PYRABACTIN RESISTANCE1 (PYR1)/PYR1-LIKE (PYL)/REGULATORY COMPONENT OF ABA RECEPTORS (RCAR), which activates ABA signaling. Here, we demonstrated that BAK1 interacts with PYR1 and phosphorylates PYR1 in response to ABA in plants. We identified T137 and S142 of PYR1 as the phosphosites targeted by BAK1. Using phosphomimetic (PYR1DD) and phospho-dead (PYR1AA) PYR1 compared with wild-type PYR1, we showed that transgenic plants overexpressing a phosphomimetic PYR1 exhibited hypersensitivity to the inhibition of ABA-induced root growth and seed germination and increased ABA-induced stomatal closure and ABA-inducible gene expression. As underlying reasons for these phenomena, we further demonstrated that phosphorylated PYR1 existed in a monomeric form, in which ABA binding was increased, and the degree of complex formation with ABI1 was also increased. These results suggest that BAK1 positively modulates ABA signaling through interaction with PYR1, in addition to OST1.

Abiotic Stresses in Plants and Their Markers: A Practice View of Plant Stress Responses and Programmed Cell Death Mechanisms

Understanding how plants cope with stress and the intricate mechanisms thereby used to adapt and survive environmental imbalances comprise one of the most powerful tools for modern agriculture. Interdisciplinary studies suggest that knowledge in how plants perceive, transduce and respond to abiotic stresses are a meaningful way to design engineered crops since the manipulation of basic characteristics leads to physiological remodeling for plant adaption to different environments. Herein, we discussed the main pathways involved in stress-sensing, signal transduction and plant adaption, highlighting biochemical, physiological and genetic events involved in abiotic stress responses. Finally, we have proposed a list of practice markers for studying plant responses to multiple stresses, highlighting how plant molecular biology, phenotyping and genetic engineering interconnect for creating superior crops.

Insights on Fructans and Resistance of Plants to Drought Stress

Drought, one of the major abiotic stresses affecting plants, is characterized by a decrease of water availability, resulting in a decrease of the water potential (Ψ) of the cells. One of the strategies of plants in resisting to this low Ψ and related stresses is regulating their water-plant relation and the interplay between Ψsolutes and the turgor pressure (Ψp). This regulation avoids the dehydration induced by low Ψ and is resulting from the accumulation of specific molecules which induce higher tolerance to water deficit and also other mechanisms that prevent or repair cell damages. In plants, fructans, the non-structural carbohydrates (NSC), have other physiological functions than carbon reserve. Among these roles, fructans have been implicated in protecting plants against water deficit caused by drought. As an efficient strategy to survive to this abiotic stress, plants synthesize fructans in response to osmotic pressure in order to osmoregulate the cellular flux, therefore, protecting the membrane damage and maintaining Ψp. Although different studies have been conducted to elucidate the mechanisms behind this strategy, still the concept itself is not well-understood and many points remain unclear and need to be elucidated in order to understand the causal relation between water deficit and fructans accumulation during water scarcity. This understanding will be a key tool in developing strategies to enhance crop tolerance to stressful dry conditions, particularly under the changing climate prediction. This review aims to give new insights on the roles of fructans in the response and resistance of plants to water deficit and their fate under this severe environmental condition.

Stomata in a state of emergency: H2O2 is the target locked

Stomatal movements are essential for plants to regulate photosynthesis rate, water status, and immunity. Upon stress stimulation, the production of hydrogen peroxide (H₂O₂) in the apoplasts and its accumulation within the guard cells are among key determinatives for stomatal closure. The regulatory mechanisms of H₂O₂ production and transport under plant-pathogen interaction and drought stress response in stomata are important fields of research. Specifically, the regulation of NADPH oxidases and aquaporins appears to be crucial in H₂O₂-controlled stomatal closure. In this review, we summarize how the calcium-dependent and calcium-independent mechanisms activate RESPIRATORY BURST OXIDASE HOMOLOG (RBOH)D/F NADPH oxidases and the aquaporin PIP2;1 to induce stomatal closure, and highlight how the H₂O₂ production is targeted by pathogen toxins and effectors to counteract plant immunity.

Physiological, Transcriptomic Investigation on the Tea Plant Growth and Yield Motivation by Chitosan Oligosaccharides

Chitosan oligosaccharides (COS) has been abundantly studied for its application on regulating plant growth of many horticultural and agricultural crops. We presented here the effect of COS on tea plant growth and yield by physiological and transcriptomic checking. The results showed that COS treatment can enhance the antioxidant activity of superoxide dismutase (SOD) and peroxidase (POD) and increase the content of chlorophyll and soluble sugar in tea plants. The field trail results show that COS treatment can increase tea buds’ density by 13.81–23.16%, the weight of 100 buds by 15.94–18.15%, and the yield by 14.22–21.08%. Transcriptome analysis found 5409 COS-responsive differentially expressed genes (DEGs), including 3149 up-regulated and 2260 down-regulated genes, and concluded the possible metabolism pathway that responsible for COS promoting tea plant growth. Our results provided fundamental information for better understanding the molecular mechanisms for COS’s acting on tea plant growth and yield promotion and offer academic support for its practical application in tea plant.

OsPSKR15, a phytosulfokine receptor from rice enhances abscisic acid response and drought stress tolerance

Abscisic acid (ABA) is a major phytohormone that acts as stimuli and plays an important role in plant growth, development, and environmental stress responses. Membrane-localized receptor-like kinases (RLKs) help to detect extracellular stimuli and activate downstream signaling responses to modulate a variety of biological processes. Phytosulfokine receptor (PSKR), a Leu-rich repeat (LRR)-RLK, has been characterized for its role in growth, development and biotic stress. Here, we observed that OsPSKR15, a rice PSKR, was upregulated by ABA in Oryza sativa. We demonstrated OsPSKR15 is a positive regulator in plant response to ABA. Ectopic expression of OsPSKR15 in Arabidopsis thaliana increased the sensitivity to ABA during germination, growth and stomatal closure. Consistently, the expression of ABA-inducible genes was significantly upregulated in these plants. OsPSKR15 also regulated reactive oxygen species (ROS)-mediated ABA signaling in guard cells, thereby governing stomatal closure. Furthermore, the constitutive expression of OsPSKR15 enhanced drought tolerance by reducing the transpirational water loss in Arabidopsis. We also reported that OsPSKR15 directly interacts with AtPYL9 and its orthologue OsPYL11 of rice through its kinase domain in the plasma membrane and nucleus. Altogether, these results reveal an important role of OsPSKR15 in plant response toward abiotic stress in an ABA-dependent manner.

The First Line of Defense: Receptor-like Protein Kinase-Mediated Stomatal Immunity

Stomata regulate gas and water exchange between the plant and external atmosphere, which are vital for photosynthesis and transpiration. Stomata are also the natural entrance for pathogens invading into the apoplast. Therefore, stomata play an important role in plants against pathogens. The pattern recognition receptors (PRRs) locate in guard cells to perceive pathogen/microbe-associated molecular patterns (PAMPs) and trigger a series of plant innate immune responses, including rapid closure of stomata to limit bacterial invasion, which is termed stomatal immunity. Many PRRs involved in stomatal immunity are plasma membrane-located receptor-like protein kinases (RLKs). This review focuses on the current research progress of RLK-mediated signaling pathways involved in stomatal immunity, and discusses questions that need to be addressed in future research.

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.

BAK1-induced RPK1 phosphorylation is essential for RPK1-mediated cell death in Arabidopsis

Being sessile, plants must deploy highly exquisite systems to respond to various internal and external signals for modulating growth and development throughout their lifespan. Many studies on Arabidopsis have shown that leucine-rich repeat-containing receptor-like kinases, including BRI1-associated receptor kinase 1 (BAK1) and receptor-like protein kinase 1 (RPK1), are suitable for such pleiotropic demands of plants. Previously, BAK1 and RPK1 were independently proven to be involved in the regulation of premature cell death. BAK1 inhibits spontaneous cell death and promotes defense-induced cell death. Meanwhile, RPK1 mediates reactive oxygen species (ROS) production through complexation with CaM4 and RbohF in an age-dependent manner. In the present study, RPK1-induced cell death and growth retardation were abolished both with respect to the phenotype and ROS production in bak1 mutants. Moreover, BAK1 interacts with RPK1 and mediates its unidirectional phosphorylation in plants. Further, BAK1-mediated RPK1 phosphorylation is indispensable for RPK1-CaM4 interaction, which is vital for ROS production, resulting in cell death. The presence of BAK1 enhanced the expression of cell death- and senescence-related genes, such as ORE1, PR1, SAG12, and SIRK in RPK1-mediated signaling cascades. Overall, in Arabidopsis, in addition to independent cell death regulation by BAK1 and RPK1, multiple-layers control cell death and premature senescence via the coordinated action of BAK1 and RPK1.

Genome-wide association study revealed genetic variations of ABA sensitivity controlled by multiple stress-related genes in rice

Abscisic acid (ABA) is a critical phytohormone that regulates multiple physiological processes including plant growth and stress tolerance. The core ABA signaling pathway has been well established, but genetic variations mediating ABA responses remain largely unknown. In this study, we performed genome-wide association study (GWAS) to identify loci and genes associated with ABA sensitivity (reflected by seed germination inhibition by ABA) in a panel of 425 rice accessions. The seed germination assay revealed that Aus and indica rice had stronger ABA sensitivity than japonica rice. A total of 48 non-redundant association loci were detected in the indica subpopulation and whole population, and 386 genes in these loci were responsive to ABA or abiotic stresses. Eight association loci were overlapped with previously reported loci for yield under drought stress or for drought-indicative image traits. Haplotype analyses of important candidate genes such as OsSAPK6, a key component in the ABA signaling core, were performed to identify key SNPs/InDels that may affect gene functions through promoter activity regulation, amino acid variation, or gene splicing. These results provide insights into the genetic basis of ABA sensitivity related to stress responses.

Abscisic acid receptors are involves in the Jasmonate signaling in Arabidopsis

The phytohormones jasmonates (JAs) act as important molecules of elicitors for the chlorophyll degradation and anthocyanin biosynthesis. JAs do usually not act independently but integrate in complex networks linking to other hormonal signaling transduction. Here, the crosstalk was detected between the JAs (jasmonic acid) and abscisic acid (ABA) signaling pathways in the mediation of chlorophyll degradation and anthocyanin biosynthesis. In this study, we found that the ABA receptor mutants, pyr1pyl1pyl2pyl4 (1124) and pyr1pyl1ply2pyl4pyl5pyl8 (112458) showed less level of chlorophyll and anthocyanin than the wild-type plants, while gain-of-function of RCAR13 transgenic lines inhibited chlorophyll degradation and enhanced anthocyanin accumulation after MeJA treatment. The amidohydrolases, including ILL6 and IAR3 and cytochrome P450 (CYP94B3), encoding JA-Ile catabolism were markedly depressed by ABA receptors. While transcripts of the enzymes for activation of anthocyanin biosynthesis pathway were analyzed, the results indicating that JA biosynthetic genes, including allene oxide synthase (AOS), LOX3 and LOX4 were enhanced by the link of JAs and ABA receptors. Moreover, the ABA receptors are also involved in JAs signal transduction through the regulation of COI, JAZ and MYC2 transcripts. These findings elucidate a connection between a core component of the ABA signaling pathway and JA responses.

The Multifaceted Regulation of SnRK2 Kinases

SNF1-related kinases 2 (SnRK2s) are central regulators of plant responses to environmental cues simultaneously playing a pivotal role in the plant development and growth in favorable conditions. They are activated in response to osmotic stress and some of them also to abscisic acid (ABA), the latter being key in ABA signaling. The SnRK2s can be viewed as molecular switches between growth and stress response; therefore, their activity is tightly regulated; needed only for a short time to trigger the response, it has to be induced transiently and otherwise kept at a very low level. This implies a strict and multifaceted control of SnRK2s in plant cells. Despite emerging new information concerning the regulation of SnRK2s, especially those involved in ABA signaling, a lot remains to be uncovered, the regulation of SnRK2s in an ABA-independent manner being particularly understudied. Here, we present an overview of available data, discuss some controversial issues, and provide our perspective on SnRK2 regulation.

Open stomata 1 exhibits dual serine/threonine and tyrosine kinase activity in regulating abscisic acid signaling

Open Stomata 1 (OST1)/SnRK2.6 is a critical component connecting abscisic acid (ABA) receptor complexes and downstream components, including anion channels and transcription factors. Because OST1 is a serine/threonine kinase, several autophosphorylation sites have been identified, and S175 is known to be critical for its kinase activity. We previously reported that BAK1 interacts with and phosphorylates OST1 to regulate ABA signaling. Here, we mapped additional phosphosites of OST1 generated by autophosphorylation and BAK1-mediated transphosphorylation in Arabidopsis. Many phosphosites serve as both auto- and transphosphorylation sites, especially those clustered in the activation loop region. Phospho-mimetic transgenic plants containing quadruple changes in Y163, S164, S166, and S167 rescued ost1 mutant phenotypes, activating ABA signaling outputs. Moreover, we found that OST1 is an active tyrosine kinase, autophosphorylating the Y182 site. ABA induced tyrosine phosphorylation of Y182 in OST1; this event is catalytically important for OST1 activity in plants. ABA-Insensitive 1 (ABI1) and its homologs ABI2 and HAB1, PP2C serine/threonine phosphatases that are known to dephosphorylate OST1 at S175, function as tyrosine phosphatases acting on the phosphorylated Y182 site. Our results indicate that phosphorylation cycles between OST1 and ABI1, which have dual specificity for tyrosine and serine/threonine, coordinately control ABA signaling in Arabidopsis.

Growth Promotion or Osmotic Stress Response: How SNF1-Related Protein Kinase 2 (SnRK2) Kinases Are Activated and Manage Intracellular Signaling in Plants

Reversible phosphorylation is a major mechanism for regulating protein function and controls a wide range of cellular functions including responses to external stimuli. The plant-specific SNF1-related protein kinase 2s (SnRK2s) function as central regulators of plant growth and development, as well as tolerance to multiple abiotic stresses. Although the activity of SnRK2s is tightly regulated in a phytohormone abscisic acid (ABA)-dependent manner, recent investigations have revealed that SnRK2s can be activated by group B Raf-like protein kinases independently of ABA. Furthermore, evidence is accumulating that SnRK2s modulate plant growth through regulation of target of rapamycin (TOR) signaling. Here, we summarize recent advances in knowledge of how SnRK2s mediate plant growth and osmotic stress signaling and discuss future challenges in this research field.

Genome-Wide Approach to Identify Quantitative Trait Loci for Drought Tolerance in Tetraploid Potato (Solanum tuberosum L.)

Drought represents a major abiotic stress factor negatively affecting growth, yield and tuber quality of potatoes. Quantitative trait locus (QTL) analyses were performed in cultivated potatoes for drought tolerance index DRYM (deviation of relative starch yield from the experimental median), tuber starch content, tuber starch yield, tuber fresh weight, selected transcripts and metabolites under control and drought stress conditions. Eight genomic regions of major interest for drought tolerance were identified, three representing standalone DRYM QTL. Candidate genes, e.g., from signaling pathways for ethylene, abscisic acid and brassinosteroids, and genes encoding cell wall remodeling enzymes were identified within DRYM QTL. Co-localizations of DRYM QTL and QTL for tuber starch content, tuber starch yield and tuber fresh weight with underlying genes of the carbohydrate metabolism were observed. Overlaps of DRYM QTL with metabolite QTL for ribitol or galactinol may indicate trade-offs between starch and compatible solute biosynthesis. Expression QTL confirmed the drought stress relevance of selected transcripts by overlaps with DRYM QTL. Bulked segregant analyses combined with next-generation sequencing (BSAseq) were used to identify mutations in genes under the DRYM QTL on linkage group 3. Future analyses of identified genes for drought tolerance will give a better insight into drought tolerance in potatoes.

Genome-Wide Association Mapping of Salinity Tolerance at the Seedling Stage in a Panel of Vietnamese Landraces Reveals New Valuable QTLs for Salinity Stress Tolerance Breeding in Rice

Rice tolerance to salinity stress involves diverse and complementary mechanisms, such as the regulation of genome expression, activation of specific ion-transport systems to manage excess sodium at the cell or plant level, and anatomical changes that avoid sodium penetration into the inner tissues of the plant. These complementary mechanisms can act synergistically to improve salinity tolerance in the plant, which is then interesting in breeding programs to pyramidize complementary QTLs (quantitative trait loci), to improve salinity stress tolerance of the plant at different developmental stages and in different environments. This approach presupposes the identification of salinity tolerance QTLs associated with different mechanisms involved in salinity tolerance, which requires the greatest possible genetic diversity to be explored. To contribute to this goal, we screened an original panel of 179 Vietnamese rice landraces genotyped with 21,623 SNP markers for salinity stress tolerance under 100 mM NaCl treatment, at the seedling stage, with the aim of identifying new QTLs involved in the salinity stress tolerance via a genome-wide association study (GWAS). Nine salinity tolerance-related traits, including the salt injury score, chlorophyll and water content, and K⁺ and Na⁺ contents were measured in leaves. GWAS analysis allowed the identification of 26 QTLs. Interestingly, ten of them were associated with several different traits, which indicates that these QTLs act pleiotropically to control the different levels of plant responses to salinity stress. Twenty-one identified QTLs colocalized with known QTLs. Several genes within these QTLs have functions related to salinity stress tolerance and are mainly involved in gene regulation, signal transduction or hormone signaling. Our study provides promising QTLs for breeding programs to enhance salinity tolerance and identifies candidate genes that should be further functionally studied to better understand salinity tolerance mechanisms in rice.

Initiation and amplification of SnRK2 activation in abscisic acid signaling

The phytohormone abscisic acid (ABA) is crucial for plant responses to environmental challenges. The SNF1-regulated protein kinase 2s (SnRK2s) are key components in ABA-receptor coupled core signaling, and are rapidly phosphorylated and activated by ABA. Recent studies have suggested that Raf-like protein kinases (RAFs) participate in ABA-triggered SnRK2 activation. In vitro kinase assays also suggest the existence of autophosphorylation of SnRK2s. Thus, how SnRK2 kinases are quickly activated during ABA signaling still needs to be clarified. Here, we show that both B2 and B3 RAFs directly phosphorylate SnRK2.6 in the kinase activation loop. This transphosphorylation by RAFs is essential for SnRK2 activation. The activated SnRK2s then intermolecularly trans-phosphorylate other SnRK2s that are not yet activated to amplify the response. High-order Arabidopsis mutants lacking multiple B2 and B3 RAFs show ABA hyposensitivity. Our findings reveal a unique initiation and amplification mechanism of SnRK2 activation in ABA signaling in higher plants.

Silicon crosstalk with reactive oxygen species, phytohormones and other signaling molecules

Exogenous applications of silicon (Si) can initiate cellular defence pathways to enhance plant resistance to abiotic and biotic stresses. Plant Si accumulation is regulated by several transporters of silicic acid (e.g. Lsi1, Lsi2, and Lsi6), but the precise mechanisms involved in overall Si transport and its beneficial effects remains unclear. In stressed plants, the accumulation of Si leads to a defence mechanism involving the formation of amorphous or hydrated silicic acid caused by their polymerization and interaction with other organic substances. Silicon also regulates plant ionic homeostasis, which involves the nutrient acquisition, availability, and replenishment in the soil through biogeochemical cycles. Furthermore, Si is implicated in modulating ethylene-dependent and jasmonate pathways, as well as other phytohormones, particularly under stress conditions. Crosstalk between Si and phytohormones could lead to improvements in Si-mediated crop growth, especially when plants are exposed to stress. The integration of Si with reactive oxygen species (ROS) metabolism appears to be a part of the signaling cascade that regulates plant phytohormone homeostasis, as well as morphological, biochemical, and molecular responses. This review aims to provide an update on Si interplays with ROS, phytohormones, and other signaling molecules that regulate plant development under stress conditions.

RIC7 plays a negative role in ABA-induced stomatal closure by inhibiting H2O2 production

When plants encounter environmental stresses, phytohormone abscisic acid (ABA) accumulates quickly and efficiently reduces water loss by inducing stomatal closure. Reactive oxygen species (ROS) is an important regulator in ABA-induced stomatal closure, and ROS generation is modulated by multiple components in guard-cell ABA signaling. ROP interactive CRIB-containing protein 7 (RIC7) has been found to negatively regulate ABA-induced stomatal closure. However, the molecular details of the RIC7 function in this process are unclear. Here, by using two RIC7 overexpressing mutants, we confirmed the negative role of RIC7 in ABA-induced stomatal closure and found that guard cells of RIC7 overexpressing mutants generated less H₂O₂ than the wild type with ABA treatment, which were consistent with the reduced expression levels of ROS generation related NADPH oxidase genes AtRBOHD and AtRBOHF, and cytosolic polyamine oxidase genes PAO1 and PAO5 in the RIC7 overexpressing mutants. Furthermore, external applied H₂O₂ failed to rescue the defects of stomatal closure in RIC7 overexpressing mutants. These results suggest that RIC7 affects H₂O₂ generation in guard cells, and the function of H₂O₂ is dependent on RIC7 in ABA-induced stomatal closure, indicative of interdependency between RIC7 and H₂O₂ in ABA guard-cell signaling.

Heterotrimeric G-protein γ subunits regulate ABA signaling in response to drought through interacting with PP2Cs and SnRK2s in mulberry (Morus alba L.)

ABA signaling plays a central role in regulating plants respond to drought. Although much progress has been made in understanding the functions of ABA signaling in drought response, very little information is available regarding woody plants. In this study, the components of ABA signaling pathway were identified in mulberry which has excellent adaptation to drought, including three PYLs, two PP2Cs, two SnRK2s, four ABFs, and an ABA responsive gene MaRD29B. The gene expression of ABA signaling components exhibited significant response to ABA and drought, and their roles in drought response were revealed using a transient transformation system in mulberry seedlings. We discovered the ABA signaling components, MaABI1/2 and MaSnRK2.1/2.4, could directly interact with G-protein γ subunits, MaGγ1 and MaGγ2, which indicated that G-protein γ subunits may mediate the signal crosstalk between G-proteins and ABA signaling. Transient activation assay in tobacco and RNAi silencing assay in mulberry further demonstrated that MaGγ1 and MaGγ2 regulated drought response by enhancing ABA signaling. This study expands the repertoire of ABA signaling controlling drought responses in plants and provides the direct evidence about the crosstalk between ABA signaling and G-proteins for the first time.

Reactive Oxygen Species Link Gene Regulatory Networks During Arabidopsis Root Development

Plant development under altered nutritional status and environmental conditions and during attack from invaders is highly regulated by plant hormones at the molecular level by various signaling pathways. Previously, reactive oxygen species (ROS) were believed to be harmful as they cause oxidative damage to cells; however, in the last decade, the essential role of ROS as signaling molecules regulating plant growth has been revealed. Plant roots accumulate relatively high levels of ROS, and thus, maintaining ROS homeostasis, which has been shown to regulate the balance between cell proliferation and differentiation at the root tip, is important for proper root growth. However, when the balance is disturbed, plants are unable to respond to the changes in the surrounding conditions and cannot grow and survive. Moreover, ROS control cell expansion and cell differentiation processes such as root hair formation and lateral root development. In these processes, the transcription factor-mediated gene expression network is important downstream of ROS. Although ROS can independently regulate root growth to some extent, a complex crosstalk occurs between ROS and other signaling molecules. Hormone signals are known to regulate root growth, and ROS are thought to merge with these signals. In fact, the crosstalk between ROS and these hormones has been elucidated, and the central transcription factors that act as a hub between these signals have been identified. In addition, ROS are known to act as important signaling factors in plant immune responses; however, how they also regulate plant growth is not clear. Recent studies have strongly indicated that ROS link these two events. In this review, we describe and discuss the role of ROS signaling in root development, with a particular focus on transcriptional regulation. We also summarize the crosstalk with other signals and discuss the importance of ROS as signaling molecules for plant root development.

Integration of reactive oxygen species and hormone signaling during abiotic stress

Each year, abiotic stress conditions such as drought, heat, salinity, cold and particularly their different combinations, inflict a heavy toll on crop productivity worldwide. The effects of these adverse conditions on plant productivity are becoming ever more alarming in recent years in light of the increased rate and intensity of global climatic changes. Improving crop tolerance to abiotic stress conditions requires a deep understanding of the response of plants to changes in their environment. This response is dependent on early and late signal transduction events that involve important signaling molecules such as reactive oxygen species (ROS), different plant hormones and other signaling molecules. It is the integration of these signaling events, mediated by an interplay between ROS and different plant hormones that orchestrates the plant response to abiotic stress and drive changes in transcriptomic, metabolic and proteomic networks that lead to plant acclimation and survival. Here we review some of the different studies that address hormone and ROS integration during the response of plants to abiotic stress. We further highlight the integration of ROS and hormone signaling during early and late phases of the plant response to abiotic stress, the key role of respiratory burst oxidase homologs in the integration of ROS and hormone signaling during these phases, and the involvement of hormone and ROS in systemic signaling events that lead to systemic acquired acclimation. Lastly, we underscore the need to understand the complex interactions that occur between ROS and different plant hormones during stress combinations.

Downregulating VAC14 in Guard Cells Causes Drought Hypersensitivity by Inhibiting Stomatal Closure

Stomata are a key land plant innovation that permit the regulation of gaseous exchanges between the plant interior and the surrounding environment. By opening or closing, stomata regulate transpiration of water though the plant; and these actions are coordinated with acquisition of CO₂ for photosynthesis. Stomatal movement is controlled by various environmental and physiological factors and associates with multiple intracellular activities, among which the dynamic remodeling of vacuoles plays a crucial role. Phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂] is critical for dynamic remodeling of vacuoles. Its production requires a PI(3,5)P₂-metabolizing complex consisting of FAB1/PIKfyve kinases, SAC phosphatases, and the scaffolding protein VAC14. Although genetic or pharmacological downregulation of PI(3,5)P₂ causes hyposensitivity to ABA-induced stomatal closure, whether the effect of PI(3,5)P₂ on stomatal movement is cell-autonomous and the physiological consequences of its reduction were unclear. We report that downregulating Arabidopsis VAC14 specifically in guard cells by artificial microRNAs (amiR-VAC14) results in enlarged guard cells and hyposensitivity to ABA- and dark-induced stomatal closure. Vacuolar fission during stomatal closure is compromised by downregulating VAC14 in guard cells. Exogenous application of PI(3,5)P₂ rescued the amiR-VAC14 phenotype whereas PI(3,5)P₂ inhibitor YM201636 caused wild-type plants to have inhibited stomatal closure. We further show that downregulating VAC14 specifically in guard cells impairs drought tolerance, suggestive of a key role of guard cell-produced PI(3,5)P₂ in plant fitness.

Decoding ABA and osmostress signalling in plants from an evolutionary point of view

The plant hormone abscisic acid (ABA) is fundamental for land plant adaptation to water-limited conditions. Osmostress, such as drought, induces ABA accumulation in angiosperms, triggering physiological responses such as stomata closure. The core components of angiosperm ABA signalling are soluble ABA receptors, group A protein phosphatase type 2C and SNF1-related protein kinase2 (SnRK2). ABA also has various functions in non-angiosperms, however, suggesting that its role in adaptation to land may not have been angiosperm-specific. Indeed, among land plants, the core ABA signalling components are evolutionarily conserved, implying their presence in a common ancestor. Results of ongoing functional genomics studies of ABA signalling components in bryophytes and algae have expanded our understanding of the evolutionary role of ABA signalling, with genome sequencing uncovering the ABA core module even in algae. In this review, we describe recent discoveries involving the ABA core module in non-angiosperms, tracing the footprints of how ABA evolved as a phytohormone. We also cover the latest findings on Raf-like kinases as upstream regulators of the core ABA module component SnRK2. Finally, we discuss the origin of ABA signalling from an evolutionary perspective.