Expression Of ABA-responsive Genes Research Articles

The CDPK-related protein kinase HvCRK2 and HvCRK4 interact with HvCML32 to negatively regulate drought tolerance in transgenic Arabidopsis thaliana

Calcium-dependent protein kinase (CDPK) as one of calcium sensors were play important roles in stress responses. CDPK-related protein kinase (CRK) was identified as subgroup III of CDPK has been characterized in many plants, but the members and functions of CRK genes in hulless barley (Hordeum vulgare L.) has not been clarified. Here, we identified four HvCRK genes and named HvCRK1-4 according to chromosomes localization. Moreover, the physiological function of highly induced genes of HvCRK2 and HvCRK4 were investigated in drought stress tolerance by examining their overexpression transgenic lines functions generated in Arabidopsis thaliana. Under drought stress, both overexpression HvCRK2 and HvCRK4 displayed reduced drought resistance, and accompanied by higher accumulation levels of ROS. Notably, overexpression of HvCRK2 and HvCRK4 reduced sensitivity to exogenous ABA, meanwhile the expression of ABA-responsive genes in transgenic plants were down-regulated compared to the wild type in response to drought stress. Furthermore, the physically interaction of HvCRK2 and HvCRK4 with calmodulin (CaM) and calmodulin-like (CML) proteins were determined in vivo, the further results showed that HvCML32 binds to HvCRK2/4 S_TKC structural domains and negatively regulates drought tolerance. In summary, this study identified HvCRK members and indicated that HvCRK2 and HvCRK4 genes play negative roles in drought tolerance, and provide insight into potential molecular mechanism of HvCRK2 and HvCRK4 in response to drought stress.

BcABF1 Plays a Role in the Feedback Regulation of Abscisic Acid Signaling via the Direct Activation of BcPYL4 Expression in Pakchoi.

Abscisic acid-responsive element-binding factor 1 (ABF1), a key transcription factor in the ABA signal transduction process, regulates the expression of downstream ABA-responsive genes and is involved in modulating plant responses to abiotic stress and developmental processes. However, there is currently limited research on the feedback regulation of ABF1 in ABA signaling. This study delves into the function of BcABF1 in Pakchoi. We observed a marked increase in BcABF1 expression in leaves upon ABA induction. The overexpression of BcABF1 not only spurred Arabidopsis growth but also augmented the levels of endogenous IAA. Furthermore, BcABF1 overexpression in Arabidopsis significantly decreased leaf water loss and enhanced the expression of genes associated with drought tolerance in the ABA pathway. Intriguingly, we found that BcABF1 can directly activate BcPYL4 expression, a critical receptor in the ABA pathway. Similar to BcABF1, the overexpression of BcPYL4 in Arabidopsis also reduces leaf water loss and promotes the expression of drought and other ABA-responsive genes. Finally, our findings suggested a novel feedback regulation mechanism within the ABA signaling pathway, wherein BcABF1 positively amplifies the ABA signal by directly binding to and activating the BcPYL4 promoter.

Characterization of the effects of exogenous abscisic acid (ABA) application on the expression of ABA-responsive genes in Abies koreana

Characterization of the effects of exogenous abscisic acid (ABA) application on the expression of ABA-responsive genes in Abies koreana

Reconstitution of phytochrome A-mediated light modulation of the ABA signaling pathways in yeast

Abscisic acid (ABA), a classical plant hormone, plays an essential role in plant adaptation to environmental stresses. The ABA signaling mechanisms have been extensively investigated, and it was shown that the PYR1 (PYRABACTIN RESISTANCE1)/PYL (PYR1-LIKE)/RCAR (REGULATORY COMPONENT OF ABA RECEPTOR) ABA receptors, the PP2C coreceptors, and the SnRK2 protein kinases constitute the core ABA signaling module responsible for ABA perception and initiation of downstream responses. We recently showed that ABA signaling is modulated by light signals, but the underlying molecular mechanisms remain largely obscure. In this study, we established a system in yeast cells that was not only successful in reconstituting a complete ABA signaling pathway, from hormone perception to ABA-responsive gene expression, but also suitable for functionally characterizing the regulatory roles of additional factors of ABA signaling. Using this system, we analyzed the roles of several light signaling components, including the red and far-red light photoreceptors phytochrome A (phyA) and phyB, and the photomorphogenic central repressor COP1, in the regulation of ABA signaling. Our results showed that both phyA and phyB negatively regulated ABA signaling, whereas COP1 positively regulated ABA signaling in yeast cells. Further analyses showed that photoactivated phyA interacted with the ABA coreceptors ABI1 and ABI2 to decrease their interactions with the ABA receptor PYR1. Together, data from our reconstituted yeast ABA signaling system provide evidence that photoactivated photoreceptors attenuate ABA signaling by directly interacting with the key components of the core ABA signaling module, thus conferring enhanced ABA tolerance to light-grown plants.

Constitutive expression of nucleotide-binding and leucine-rich repeat gene AtRPS2 enhanced drought and salt tolerance in rice

Drought and salt are major abiotic stresses that severely restricts plant growth and development, leading to serious losses in agricultural production. Therefore, improving crop tolerance to drought and salt stresses is an urgent issue. A previous study showed that overexpression of Arabidopsis NLR gene AtRPS2 conferred broad-spectrum disease resistance in rice. In this study, we demonstrated that constitutive expression of AtRPS2 increased abscisic acid (ABA) sensitivity during seedling stage, the shoot length of transgenic plants were shorter than wild type plants. Exogenous application of ABA markedly induced the expression of stress-related genes and promoted stomatal close in transgenic plants. Overexpression of AtRPS2 also enhanced drought and salt tolerance in rice, transgenic plants exhibited higher survival rates under drought and salt conditions than wild type plants. The activities of catalase (CAT) and superoxide dismutase (SOD) were higher in AtRPS2 transgenic rice than wild type plants. In addition, the expression of stress-related genes and ABA-responsive genes were significantly upregulated in AtRPS2 transgenic plants than wild type plants under drought and salt treatments. Besides, exogenous application of ABA could facilitate drought and salt tolerance in AtRPS2 transgenic plants. Taken together, this study indicated that AtRPS2 could improve drought and salt tolerance in rice, and this phenomenon is likely to be regulated through ABA signaling pathways.

The SCL30a SR protein regulates ABA-dependent seed traits and germination under stress.

SR proteins are conserved RNA-binding proteins best known as splicing regulators that have also been implicated in other steps of gene expression. Despite mounting evidence for a role in plant development and stress responses, the molecular pathways underlying SR protein regulation of these processes remain poorly understood. Here we show that the plant-specific SCL30a SR protein negatively regulates ABA signaling to control seed traits and stress responses during germination in Arabidopsis. Transcriptome-wide analyses revealed that loss of SCL30a function barely affects splicing, but largely induces ABA-responsive gene expression and genes repressed during germination. Accordingly, scl30a mutant seeds display delayed germination and hypersensitivity to ABA and high salinity, while transgenic plants overexpressing SCL30a exhibit reduced ABA and salt stress sensitivity. An ABA biosynthesis inhibitor rescues the enhanced mutant seed stress sensitivity, and epistatic analyses confirm that this hypersensitivity requires a functional ABA pathway. Finally, seed ABA levels are unchanged by altered SCL30a expression, indicating that the gene promotes seed germination under stress by reducing sensitivity to the phytohormone. Our results reveal a new player in ABA-mediated control of early development and stress response.

ASR1 and ASR2, Two Closely Related ABA-Induced Serine-Rich Transcription Repressors, Function Redundantly to Regulate ABA Responses in Arabidopsis.

The plant hormone abscisic acid (ABA) is able to regulate the expression of ABA-responsive genes via signaling transduction, and thus plays an important role in regulating plant responses to abiotic stresses. Hence, characterization of unknown ABA response genes may enable us to identify novel regulators of ABA and abiotic stress responses. By using RT-PCR analysis, we found that the expression levels of ABA-induced Serine-rich Repressor 1 (ASR1)and ASR2, two closely related unknown function genes, were increased in response to ABA treatment. Amino acid sequence analyses show that ASR1 contains an L×L×L motif and both ASR1 and ASR2 are enriched in serine. Transfection assays in Arabidopsis leaf protoplasts show that ASR1 and ASR2 were predominantly localized in the nucleus and were able to repress the expression of the reporter gene. The roles of ASRs in regulating ABA responses were examined by generating transgenic Arabidopsis plants over-expressing ASR1 and ASR2, respectively, and CRISPR/Cas9 gene-edited single and double mutants for ASR1 and ASR2. In both the seed germination and cotyledon greening assays, ABA sensitivity remained largely unchanged in the over-expression transgenic plants and the single mutants of ASR1 and ASR2, but greatly increased ABA sensitivity was observed in the asr1 asr2 double mutants. In root elongation assays, however, decreased ABA sensitivity was observed in the 35S:ASR1 and 35S:ASR2 transgenic plants, whereas increased ABA sensitivity was observed in the asr1 and asr2 single mutants, and ABA sensitivity was further increased in the asr1 asr2 double mutants. Transcriptome analysis show that the differentially expressed genes (DEGs) down-regulated in the 35S:ASR1 transgenic plant seedlings, but up-regulated in the asr1 asr2 double mutant seedlings were highly enriched in processes including responses to plant hormones and stress stimuli. Taken together, our results show that ASR1 and ASR2 are closely related ABA response genes, ASR1 and ASR2 are serine-rich novel transcription repressors, and they negatively regulate ABA responses in Arabidopsis in a redundant manner.

Jasmonic Acid-Induced β-Cyclocitral Confers Resistance to Bacterial Blight and Negatively Affects Abscisic Acid Biosynthesis in Rice.

Jasmonic acid (JA) regulates the production of several plant volatiles that are involved in plant defense mechanisms. In this study, we report that the JA-responsive volatile apocarotenoid, β-cyclocitral (β-cyc), negatively affects abscisic acid (ABA) biosynthesis and induces a defense response against Xanthomonas oryzae pv. oryzae (Xoo), which causes bacterial blight in rice (Oryza sativa L.). JA-induced accumulation of β-cyc was regulated by OsJAZ8, a repressor of JA signaling in rice. Treatment with β-cyc induced resistance against Xoo and upregulated the expression of defense-related genes in rice. Conversely, the expression of ABA-responsive genes, including ABA-biosynthesis genes, was downregulated by JA and β-cyc treatment, resulting in a decrease in ABA levels in rice. β-cyc did not inhibit the ABA-dependent interactions between OsPYL/RCAR5 and OsPP2C49 in yeast cells. Furthermore, we revealed that JA-responsive rice carotenoid cleavage dioxygenase 4b (OsCCD4b) was localized in the chloroplast and produced β-cyc both in vitro and in planta. These results suggest that β-cyc plays an important role in the JA-mediated resistance against Xoo in rice.

AtbZIP62 Acts as a Transcription Repressor to Positively Regulate ABA Responses in Arabidopsis.

The basic region/leucine zipper (bZIP) transcription factor AtbZIP62 is involved in the regulation of plant responses to abiotic stresses, including drought and salinity stresses, NO3 transport, and basal defense in Arabidopsis. It is unclear if it plays a role in regulating plant responses to abscisic acid (ABA), a phytohormone that can regulate plant abiotic stress responses via regulating downstream ABA-responsive genes. Using RT-PCR analysis, we found that the expression level of AtbZIP62 was increased in response to exogenously applied ABA. Protoplast transfection assays show that AtbZIP62 is predominantly localized in the nucleus and functions as a transcription repressor. To examine the roles of AtbZIP62 in regulating ABA responses, we generated transgenic Arabidopsis plants overexpressing AtbZIP62 and created gene-edited atbzip62 mutants using CRISPR/Cas9. We found that in both ABA-regulated seed germination and cotyledon greening assays, the 35S:AtbZIP62 transgenic plants were hypersensitive, whereas atbzip62 mutants were hyposensitive to ABA. To examine the functional mechanisms of AtbZIP62 in regulating ABA responses, we generated Arabidopsis transgenic plants overexpressing 35S:AtbZIP62-GR, and performed transcriptome analysis to identify differentially expressed genes (DEGs) in the presence and absence of DEX, and found that DEGs are highly enriched in processes including response to abiotic stresses and response to ABA. Quantitative RT-PCR results further show that AtbZIP62 may regulate the expression of several ABA-responsive genes, including USP, ABF2, and SnRK2.7. In summary, our results show that AtbZIP62 is an ABA-responsive gene, and AtbZIP62 acts as a transcription repressor to positively regulate ABA responses in Arabidopsis.

ABA promotes sulfite stress tolerance by ABF4-mediated upregulation of SOX expression

With the development of industry, atmospheric SO 2 is rapidly increased, seriously affecting plant growth and crop production. However, the mechanism of how plants respond to high SO 2 -induced sulfite stress is poorly characterized. Here we report that ABA signaling is involved in plant tolerance to sulfite stress. Our results indicate that the sulfite-stressed plants accumulated higher ABA with promoted expression of ABA-responsive genes compared with unstressed control and the ABA-biosynthetic mutant ABA DEFICIENT2 ( aba2–1 ) had less sulfite tolerance than wild-type plants, supporting a role of ABA in plant sulfite stress response. ABA functions through its downstream transcription factor ABF4 because the abf4–1 mutant was more sensitive but the 35 S::ABF4 plants more tolerant to the sulfite stress than wild-type plants. Further, ABF4 upregulates SOX expression by directly binding to its promoter and overexpression of SOX can rescue increased stress sensitivity of abf4–1 mutant, showing that SOX acts downstream of ABF4 . Together, our study reveals that the sulfite-stressed plants can activate ABA/ABF4 signaling to enhance SOX expression in plant sulfite stress tolerance. • Sulfite stress induces the accumulation of ABA. • ABA protects plants from sulfite stress. • Sulfite stress promotes expression of transcription factor ABF4. • ABF4 increases plant sulfite stress tolerance by modulating SOX expression.

CycC1;1 negatively modulates ABA signaling by interacting with and inhibiting ABI5 during seed germination.

Regulation of seed germination is important for plant survival and propagation. ABSCISIC ACID (ABA) INSENSITIVE5 (ABI5), the central transcription factor in the ABA signaling pathway, plays a fundamental role in the regulation of ABA-responsive gene expression during seed germination; however, how ABI5 transcriptional activation activity is regulated remains to be elucidated. Here, we report that C-type Cyclin1;1 (CycC1;1) is an ABI5-interacting partner affecting the ABA response and seed germination in Arabidopsis (Arabidopsis thaliana). The CycC1;1 loss-of-function mutant is hypersensitive to ABA, and this phenotype was rescued by mutation of ABI5. Moreover, CycC1;1 suppresses ABI5 transcriptional activation activity for ABI5-targeted genes including ABI5 itself by occupying their promoters and disrupting RNA polymerase II recruitment; thus the cycc1;1 mutant shows increased expression of ABI5 and genes downstream of ABI5. Furthermore, ABA reduces the interaction between CycC1;1 and ABI5, while phospho-mimic but not phospho-dead mutation of serine-42 in ABI5 abolishes CycC1;1 interaction with ABI5 and relieves CycC1;1 inhibition of ABI5-mediated transcriptional activation of downstream target genes. Together, our study illustrates that CycC1;1 negatively modulates the ABA response by interacting with and inhibiting ABI5, while ABA relieves the CycC1;1 interaction with and inhibition of ABI5 to activate ABI5 activity for the ABA response, thereby inhibiting seed germination.

The exodomain of the impaired oomycete susceptibility 1 receptor mediates both endoplasmic reticulum stress responses and abscisic acid signalling during downy mildew infection of Arabidopsis

The phytohormone abscisic acid (ABA) regulates cell growth and plant development, and contributes to defence responses to pathogens. We previously showed that the Arabidopsis malectin-like domain leucine-rich repeat receptor-like kinase (MLD-LRR-RLK) impaired oomycete susceptibility 1 (IOS1) attenuates ABA signalling during infection with the oomycete downy mildew pathogen Hyaloperonospora arabidopsidis. The exodomain of IOS1 with its MLD retains the receptor in the endoplasmic reticulum (ER), where it interacts with the ribophorin HAP6 to dampen a pathogen-induced ER stress response called the unfolded protein response (UPR). The down-regulation of both ABA and UPR signalling probably provides the pathogen with an advantage for infection. Here, we show that ABA-related phenotypes of the ios1-1 mutant, such as up-regulated expression of ABA-responsive genes and hypersensitivity to exogenous ABA application, were reverted by expression of the IOS1 exodomain in the mutant background. Furthermore, knockdown mutants for ER-resident HAP6 showed similarly reduced UPR and ABA signalling, indicating that HAP6 positively regulates both pathways. Our data suggest that the IOS1 exodomain and HAP6 contribute in the ER to the IOS1-mediated interference with ABA and UPR signalling.

OsMLP423 Is a Positive Regulator of Tolerance to Drought and Salt Stresses in Rice.

Rice (Oryza sativa L.) is one of the main food crops for human survival, and its yield is often restricted by abiotic stresses. Drought and soil salinity are among the most damaging abiotic stresses affecting today’s agriculture. Given the importance of abscisic acid (ABA) in plant growth and abiotic stress responses, it is very important to identify new genes involved in ABA signal transduction. We screened a drought-inducing gene containing about 158 amino acid residues from the transcriptome library of rice exposed to drought treatment, and we found ABA-related cis-acting elements and multiple drought-stress-related cis-acting elements in its promoter sequence. The results of real-time PCR showed that OsMLP423 was strongly induced by drought and salt stresses. The physiological and biochemical phenotype analysis of transgenic plants confirmed that overexpression of OsMLP423 enhanced the tolerance to drought and salt stresses in rice. The expression of OsMLP423-GFP fusion protein indicated that OsMLP423 was located in both the cell membrane system and nucleus. Compared with the wild type, the overexpressed OsMLP423 showed enhanced sensitivity to ABA. Physiological analyses showed that the overexpression of OsMLP423 may regulate the water loss efficiency and ABA-responsive gene expression of rice plants under drought and salt stresses, and it reduces membrane damage and the accumulation of reactive oxygen species. These results indicate that OsMLP423 is a positive regulator of drought and salinity tolerance in rice, governing the tolerance of rice to abiotic stresses through an ABA-dependent pathway. Therefore, this study provides a new insight into the physiological and molecular mechanisms of OsMLP423-mediated ABA signal transduction participating in drought and salt stresses.

Abscisic acid negatively regulates the Polycomb-mediated H3K27me3 through the PHD-finger protein, VIL1.

Polycomb dictates developmental programs in higher eukaryotes, including flowering plants. A phytohormone, abscisic acid (ABA), plays a pivotal role in seed and seedling development and mediates responses to multiple environmental stresses, such as salinity and drought. In this study, we show that ABA affects the Polycomb Repressive Complex 2 (PRC2)-mediated Histone H3 Lys 27 trimethylation (H3K27me3) through VIN3-LIKE1/VERNALIZATION 5 (VIL1/VRN5) to fine-tune the timely repression of ABSCISIC ACID INSENSITIVE 3 (ABI3) and ABSCISIC ACID INSENSITIVE 4 (ABI4) in Arabidopsis thaliana. vil1 mutants exhibit hypersensitivity to ABA during early seed germination and show enhanced drought tolerance. Our study revealed that the ABA signaling pathway utilizes a facultative component of the chromatin remodeling complex to demarcate the level of expression of ABA-responsive genes.

Abscisic acid-induced cytoplasmic translocation of constitutive photomorphogenic 1 enhances reactive oxygen species accumulation through the HY5-ABI5 pathway to modulate seed germination.

Seed germination is a physiological process regulated by multiple factors. Abscisic acid (ABA) can inhibit seed germination to improve seedling survival under conditions of abiotic stress, and this process is often regulated by light signals. Constitutive photomorphogenic 1 (COP1) is an upstream core repressor of light signals and is involved in several ABA responses. Here, we demonstrate that COP1 is a negative regulator of the ABA‐mediated inhibition of seed germination. Disruption of COP1 enhanced Arabidopsis seed sensitivity to ABA and increased reactive oxygen species (ROS) levels. In seeds, ABA induced the translocation of COP1 to the cytoplasm, resulting in enhanced ABA‐induced ROS levels. Genetic evidence indicated that HY5 and ABI5 act downstream of COP1 in the ABA‐mediated inhibition of seed germination. ABA‐induced COP1 cytoplasmic localization increased HY5 and ABI5 protein levels in the nucleus, leading to increased expression of ABI5 target genes and ROS levels in seeds. Together, our results reveal that ABA‐induced cytoplasmic translocation of COP1 activates the HY5‐ABI5 pathway to promote the expression of ABA‐responsive genes and the accumulation of ROS during ABA‐mediated inhibition of seed germination. These findings enhance the role of COP1 in the ABA signal transduction pathway.

AtUSP17 negatively regulates salt stress tolerance through modulation of multiple signaling pathways in Arabidopsis.

AtUSP17 is a multiple stress-inducible gene that encodes a universal stress protein (USP) in Arabidopsis thaliana. In the present study, we functionally characterized AtUSP17 using its knock-down mutant, Atusp17, and AtUSP17-overexpression lines (WTOE). The overexpression of AtUSP17 in wild-type and Atusp17 mutant Arabidopsis plants resulted in higher sensitivity to salt stress during seed germination than WT and Atusp17 mutant lines. In addition, the WTOE and FC lines exhibited higher abscisic acid (ABA) sensitivity than Atusp17 mutant during germination. The exogenous application of ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) was able to rescue the salt hypersensitive phenotype of WTOE lines. In contrast, AgNO3 , an ethylene action inhibitor, further blocked the effect of ACC during germination. The addition of ACC under salt stress resulted in reduced reactive oxygen species (ROS) accumulation, expression of ABA-responsive genes, improved proline synthesis, increased expression of positive regulators of ethylene signaling and antioxidant defense genes with enhanced antioxidant enzyme activities. The WTOE lines exhibited salt sensitivity even at the adult plant stage, while Atusp17 mutant exhibited higher salt tolerance with higher chlorophyll, relative water content and lower electrolyte leakage as compared with WT. The BAR interaction viewer database and available literature mining identified AtUSP17-interacting proteins, which include RGS1, RACK1C and PRN1 involved in G-protein signaling, which play a crucial role in salt stress responses. Based on the present study and available literature, we proposed a model in which AtUSP17 negatively mediates salt tolerance in Arabidopsis through modulation of ethylene, ABA, ROS, and G-protein signaling and responses.

Salt Stress Promotes Abscisic Acid Accumulation to Affect Cell Proliferation and Expansion of Primary Roots in Rice.

The primary root is the basic component of the root system and plays a key role in early seedling growth in rice. Its growth is easily affected by environmental cues, such as salt stress. Abscisic acid (ABA) plays an essential role in root development, but the molecular mechanism underlying ABA-regulated root growth in response to salt stress remains poorly understood. In this study, we report that salt stress inhibits primary root elongation and promotes primary root swelling. Moreover, salt stress induces the expression of ABA-responsive genes and ABA accumulation in the primary root, revealing that ABA plays an essential role in salt-modulated root growth. Transgenic lines of OsSAPK10-OE and OsABIL2-OE, which constitutively express OsSAPK10 or OsABIL2, with enhanced or attenuated ABA signaling, show increased and decreased sensitivity to salt, correspondingly. Microscopic analysis indicates that salt and ABA inhibits cell proliferation and promotes cell expansion in the root apical meristem. Transcriptome analysis showed that ABA induces the expression of EXPANSIN genes. Further investigations indicate that ABA exerts these effects largely through ABA signaling. Thus, our findings deepen our understanding of the role of ABA in controlling primary root growth in response to salt stress, and this knowledge can be used by breeders to cultivate rice varieties suitable for saline–alkali land.

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

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

The Arabidopsis circadian clock protein PRR5 interacts with and stimulates ABI5 to modulate abscisic acid signaling during seed germination.

Seed germination and postgerminative growth require the precise coordination of multiple intrinsic and environmental signals. The phytohormone abscisic acid (ABA) suppresses these processes in Arabidopsis thaliana and the circadian clock contributes to the regulation of ABA signaling. However, the molecular mechanism underlying circadian clock-mediated ABA signaling remains largely unknown. Here, we found that the core circadian clock proteins PSEUDO-RESPONSE REGULATOR5 (PRR5) and PRR7 physically associate with ABSCISIC ACID-INSENSITIVE5 (ABI5), a crucial transcription factor of ABA signaling. PRR5 and PRR7 positively modulate ABA signaling redundantly during seed germination. Disrupting PRR5 and PRR7 simultaneously rendered germinating seeds hyposensitive to ABA, whereas the overexpression of PRR5 enhanced ABA signaling to inhibit seed germination. Consistent with this, the expression of several ABA-responsive genes is upregulated by PRR proteins. Genetic analysis demonstrated that PRR5 promotes ABA signaling mainly dependently on ABI5. Further mechanistic investigation revealed that PRR5 stimulates the transcriptional function of ABI5 without affecting its stability. Collectively, our results indicate that these PRR proteins function synergistically with ABI5 to activate ABA responses during seed germination, thus providing a mechanistic understanding of how ABA signaling and the circadian clock are directly integrated through a transcriptional complex involving ABI5 and central circadian clock components.

Wheat TaPUB1 protein mediates ABA response and seed development through ubiquitination

Abscisic acid (ABA) is an important regulator of plant growth, development, and biotic and abiotic stress responses. Ubiquitination plays important roles in regulating ABA signaling. E3 ligase, a key member in ubiquitination, actively participates in the regulation of biosynthesis, de-repression, and activation of ABA response and degradation of signaling components. In this study, we found that that overexpression of wheat E3 ligase TaPUB1 decreased the sensitivity of wheat seedlings to ABA, whereas TaPUB1-RNA interference (TaPUB1-RNAi) lines increased wheat sensitivity to ABA during germination, root growth, and stomatal opening. TaPUB1 influenced the expression of several ABA-responsive genes, and also interacted with TaPYL4 and TaABI5, which are involved in ABA signal transduction, and promoted their degradation. Additionally, we observed that TaPUB1-OE lines resulted in lower single-split grain numbers, larger seed size, and higher thousand kernel weight, when compared with the WT lines. Contrasting results were obtained for TaPUB1-RNAi lines. It suggests that TaPUB1 acts as a negative regulator in the ABA signaling pathway by interacting with TaPYL4 and TaABI5, subsequently affecting seed development in wheat. In addition, the enhanced abiotic tolerance of overexpression lines due to enhanced photosynthesis and root development may be related to the degradation of TaABI5 by TaPUB1.