Abscisic Acid Insensitivity Research Articles

Loss-of-Function Mutation of ACTIN-RELATED PROTEIN 6 (ARP6) Impairs Root Growth in Response to Salinity Stress.

H2A.Z-containing nucleosomes have been found to function in various developmental programs in Arabidopsis (e.g., floral transition, warm ambient temperature, and drought stress responses). The SWI2/SNF2-Related 1 Chromatin Remodeling (SWR1) complex is known to control the deposition of H2A.Z, and it has been unraveled that ACTIN-RELATED PROTEIN 6 (ARP6) is one component of this SWR1 complex. Previous studies showed that the arp6 mutant exhibited some distinguished phenotypes such as early flowering, leaf serration, elongated hypocotyl, and reduced seed germination rate in response to osmotic stress. In this study, we aimed to investigate the changes of arp6 mutant when the plants were grown in salt stress condition. The phenotypic observation showed that the arp6 mutant was more sensitive to salt stress than the wild type. Upon salt stress condition, this mutant exhibited attenuated root phenotypes such as shorter primary root length and fewer lateral root numbers. The transcript levels of stress-responsive genes, ABA INSENSITIVE 1 (ABI1) and ABI2, were found to be impaired in the arp6 mutant in comparison with wild-type plants in response to salt stress. In addition, a meta-analysis of published data indicated a number of genes involved in auxin response were induced in arp6 mutant grown in non-stress condition. These imply that the loss of H2A.Z balance (in arp6 mutant) may lead to change stress and auxin responses resulting in alternative root morphogenesis upon both normal and salinity stress conditions.

Auxin contributes to jasmonate-mediated regulation of abscisic acid signaling during seed germination in Arabidopsis.

Abscisic acid (ABA) represses seed germination and postgerminative growth in Arabidopsis thaliana. Auxin and jasmonic acid (JA) stimulate ABA function; however, the possible synergistic effects of auxin and JA on ABA signaling and the underlying molecular mechanisms remain elusive. Here, we show that exogenous auxin works synergistically with JA to enhance the ABA-induced delay of seed germination. Auxin biosynthesis, perception, and signaling are crucial for JA-promoted ABA responses. The auxin-dependent transcription factors AUXIN RESPONSE FACTOR10 (ARF10) and ARF16 interact with JASMONATE ZIM-DOMAIN (JAZ) repressors of JA signaling. ARF10 and ARF16 positively mediate JA-increased ABA responses, and overaccumulation of ARF16 partially restores the hyposensitive phenotype of JAZ-accumulating plants defective in JA signaling in response to combined ABA and JA treatment. Furthermore, ARF10 and ARF16 physically associate with ABSCISIC ACID INSENSITIVE5 (ABI5), a critical regulator of ABA signaling, and the ability of ARF16 to stimulate JA-mediated ABA responses is mainly dependent on ABI5. ARF10 and ARF16 activate the transcriptional function of ABI5, whereas JAZ repressors antagonize their effects. Collectively, our results demonstrate that auxin contributes to the synergetic modulation of JA on ABA signaling, and explain the mechanism by which ARF10/16 coordinate with JAZ and ABI5 to integrate the auxin, JA, and ABA signaling pathways.

The RNA N6 -methyladenosine demethylase ALKBH9B modulates ABA responses in Arabidopsis.

The mRNA modification N6 -methyladenosine (m6 A) plays vital roles in plant development and biotic and abiotic stress responses. The RNA m6 A demethylase ALKBH9B can remove m6 A in alfalfa mosaic virus RNA and plays roles in alfalfa mosaic virus infection in Arabidopsis. However, it is unknown whether ALKBH9B also exhibits demethylation activity and has a biological role in endogenous plant mRNA. We demonstrated here that mRNA m6 A modification is induced by the phytohormone abscisic acid (ABA) and that ALKBH9B has m6 A demethylation activity on endogenous mRNA. Knocking out ALKBH9B led to hypersensitivity to ABA treatment during seed germination and early seedling development. We further showed that ALKBH9B removes the m6 A modification in the ABA INSENSITIVE 1 (ABI1) and BRI1-EMS-SUPPRESSOR 1 (BES1) transcripts following ABA treatment, affecting the stability of these mRNAs. Furthermore, we determined that ALKBH9B acts genetically upstream of the transcription factors ABI3 and ABI5, and its regulatory function in ABA responses depended on ABI3 and ABI5. Our findings reveal the important roles of the m6 A modification in ABA responses and highlight the role of ALKBH9B-mediated m6 A demethylation in regulating ABA responses post-transcriptionally.

Rose WRKY13 promotes disease protection to Botrytis by enhancing cytokinin content and reducing abscisic acid signaling.

The plant hormones cytokinin (CK) and abscisic acid (ABA) play critical and often opposite roles during plant growth, development, and responses to abiotic and biotic stresses. Rose (Rosa sp.) is an economically important ornamental crop sold as cut flowers. Rose petals are extremely susceptible to gray mold disease caused by the necrotrophic fungal pathogen Botrytis cinerea. The infection of rose petals by B. cinerea leads to tissue collapse and rot, causing severe economic losses. In this study, we showed that CK and ABA play opposite roles in the susceptibility of rose to B. cinerea. Treatment with CK enhanced the disease protection of rose petals to B. cinerea, while ABA promoted disease progression. We further demonstrated that rose flowers activate CK-mediated disease protection via a B. cinerea-induced rose transcriptional repressor, Rosa hybrida (Rh)WRKY13, which is an ortholog of Arabidopsis (Arabidopsis thaliana), AtWRKY40. RhWRKY13 binds to promoter regions of the CK degradation gene CKX3 (RhCKX3) and the ABA-response gene ABA insensitive4 (RhABI4), leading to simultaneous inhibition of their expression in rose petals. The increased CK content and reduced ABA responses result in enhanced protection from B. cinerea. Collectively, these data reveal opposite roles for CK and ABA in the susceptibility of rose petals against B. cinerea infection, which is mediated by B. cinerea-induced RhWRKY13 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.

Lily HD-Zip I Transcription Factor LlHB16 Promotes Thermotolerance by Activating LlHSFA2 and LlMBF1c.

HD-Zip I transcription factors play important roles in plant development and response to abiotic stresses; however, their roles in thermotolerance are largely unknown. Through transcriptome analysis in lily (Lilium longiflorum), we isolated and identified a HD-Zip I gene differentially expressed at high temperatures, LlHB16, which belongs to the β2 subgroup and positively regulates thermotolerance. The expression of LlHB16 was rapidly and continuously activated by heat stress. LlHB16 protein localized to the nucleus and exhibited transactivation activity in both plant and yeast cells, and its C-terminus contributed to its transcriptional activity. Overexpressing LlHB16 in Arabidopsis and lily improved thermotolerance and activated the expression of heat-related genes in both plants, especially that of HSFA2 and MBF1c. In addition, LlHB16 overexpression in Arabidopsis also caused growth defects, delayed flowering and abscisic acid (ABA) insensitivity. Further analysis revealed that LlHB16 directly binds to the promoters of LlHSFA2 and LlMBF1c and activates their expressions. Similarly, the expression of AtHSFA2 and AtMBF1c was also elevated in LlHB16 transgenic Arabidopsis lines. Together, our findings demonstrate that LlHB16 participates in the establishment of thermotolerance involved in activating LlHSFA2 and LlMBF1c, and LlHB16 overexpression resulted in ABA insensitivity in transgenic plants, suggesting that LlHB16 links the basal heat-responsive pathway and ABA signal to collaboratively regulate thermotolerance.

Functional Analysis of ScABI3 from Syntrichia caninervis Mitt. in Medicago sativa L.

ABI3 (ABSCISIC ACID INSENSITIVE 3) is a family of B3 transcription factors essential for regulating the abscisic acid (ABA) signaling pathway involved in various biological processes and abiotic stress. Our previous studies demonstrated that ectopic expression of ScABI3 from a desiccation-tolerant moss (Syntrichia caninervis) into Arabidopsis thaliana enhanced abiotic stress tolerance. However, studies on plant transformation using the ABI3 gene are limited and other possible functions of ScABI3 are not known. Here, we transformed the ScABI3 into alfalfa (Medicago sativa L.) and analyzed the effects on phenotype, photosynthetic efficiency, and nutritional quality. The results showed that the endogenous ABA content of the transgenic plants was significantly higher than WT, and the leaf-stem ratio, leaf area, and branch number increased with ScABI3 overexpression in alfalfa. Further analysis of the gas exchange parameters showed that the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and water-use efficiency (WUE) of the transgenic alfalfa were significantly higher than WT; meanwhile, the protein content of the transgenic lines was higher than the WT, but the crude fat content was lower. Thus, these findings suggest that ScABI3 can be used as a potential candidate gene to manipulate alfalfa’s growth and nutritional quality. This study will provide a theoretical basis for breeding alfalfa varieties and assist in forage production and animal husbandry in the future.

A seed-like proteome in oil-rich tubers.

There are numerous examples of plant organs or developmental stages that are desiccation-tolerant and can withstand extended periods of severe water loss. One prime example are seeds and pollen of many spermatophytes. However, in some plants, also vegetative organs can be desiccation-tolerant. One example are the tubers of yellow nutsedge (Cyperus esculentus), which also store large amounts of lipids similar to seeds. Interestingly, the closest known relative, purple nutsedge (Cyperus rotundus), generates tubers that do not accumulate oil and are not desiccation-tolerant. We generated nanoLC-MS/MS-based proteomes of yellow nutsedge in five replicates of four stages of tuber development and compared them to the proteomes of roots and leaves, yielding 2257 distinct protein groups. Our data reveal a striking upregulation of hallmark proteins of seeds in the tubers. A deeper comparison to the tuber proteome of the close relative purple nutsedge (C. rotundus) and a previously published proteome of Arabidopsis seeds and seedlings indicates that indeed a seed-like proteome was found in yellow but not purple nutsedge. This was further supported by an analysis of the proteome of a lipid droplet-enriched fraction of yellow nutsedge, which also displayed seed-like characteristics. One reason for the differences between the two nutsedge species might be the expression of certain transcription factors homologous to ABSCISIC ACID INSENSITIVE3, WRINKLED1, and LEAFY COTYLEDON1 that drive gene expression in Arabidopsis seed embryos.

Characterizations of a Class-I BASIC PENTACYSTEINE Gene Reveal Conserved Roles in the Transcriptional Repression of Genes Involved in Seed Development.

The developmental regulation of flower organs involves the spatio-temporal regulation of floral homeotic genes. BASIC PENTACYSTEINE genes are plant-specific transcription factors that is involved in many aspects of plant development through gene transcriptional regulation. Although studies have shown that the BPC genes are involved in the developmental regulation of flower organs, little is known about their role in the formation of double-flower due. Here we characterized a Class I BPC gene (CjBPC1) from an ornamental flower—Camellia japonica. We showed that CjBPC1 is highly expressed in the central whorls of flowers in both single and doubled varieties. Overexpression of CjBPC1 in Arabidopsis thaliana caused severe defects in siliques and seeds. We found that genes involved in ovule and seed development, including SEEDSTICK, LEAFY COTYLEDON2, ABSCISIC ACID INSENSITIVE 3 and FUSCA3, were significantly down-regulated in transgenic lines. We showed that the histone 3 lysine 27 methylation levels of these downstream genes were enhanced in the transgenic plants, indicating conserved roles of CjBPC1 in recruiting the Polycomb Repression Complex for gene suppression.

Abscisic acid inhibits primary root growth by impairing ABI4-mediated cell cycle and auxin biosynthesis.

Cell cycle progression and the phytohormones auxin and abscisic acid (ABA) play key roles in primary root growth, but how ABA mediates the transcription of cell cycle-related genes and the mechanism of crosstalk between ABA and auxin requires further research. Here, we report that ABA inhibits primary root growth by regulating the ABA INSENSITIVE4 (ABI4)-CYCLIN-DEPENDENT KINASE B2;2 (CDKB2;2)/CYCLIN B1;1 (CYCB1;1) module-mediated cell cycle as well as auxin biosynthesis in Arabidopsis (Arabidopsis thaliana). ABA induced ABI4 transcription in the primary root tip, and the abi4 mutant showed an ABA-insensitive phenotype in primary root growth. Compared with the wild type (WT), the meristem size and cell number of the primary root in abi4 increased in response to ABA. Further, the transcription levels of several cell-cycle positive regulator genes, including CDKB2;2 and CYCB1;1, were upregulated in abi4 primary root tips. Subsequent chromatin immunoprecipitation (ChIP)-seq, ChIP-qPCR, and biochemical analysis revealed that ABI4 repressed the expression of CDKB2;2 and CYCB1;1 by physically interacting with their promoters. Genetic analysis demonstrated that overexpression of CDKB2;2 or CYCB1;1 fully rescued the shorter primary root phenotype of ABI4-overexpression lines, and consistently, abi4/cdkb2;2-cr or abi4/cycb1;1-cr double mutations largely rescued the ABA-insensitive phenotype of abi4 with regard to primary root growth. The expression levels of DR5promoter-GFP and PIN1promoter::PIN1-GFP in abi4 primary root tips were significantly higher than those in WT after ABA treatment, with these changes being consistent with changes in auxin concentration and expression patterns of auxin biosynthesis genes. Taken together, these findings indicated that ABA inhibits primary root growth through ABI4-mediated cell cycle and auxin-related regulatory pathways.

Integration of ABA, GA, and light signaling in seed germination through the regulation of ABI5

Seed germination is precisely controlled by a variety of signals, among which light signals and the phytohormones abscisic acid (ABA) and gibberellin (GA) play crucial roles. New findings have greatly increased our understanding of the mechanisms by which these three signals regulate seed germination and the close connections between them. Although much work has been devoted to ABA, GA, and light signal interactions, there is still no systematic description of their combination, especially in seed germination. In this review, we integrate ABA, GA, and light signaling in seed germination through the direct and indirect regulation of ABSCISIC ACID INSENSITIVE5 (ABI5), the core transcription factor that represses seed germination in ABA signaling, into our current understanding of the regulatory mechanism of seed germination.

The ABCISIC ACID INSENSITIVE (ABI) 4 Transcription Factor Is Stabilized by Stress, ABA and Phosphorylation.

The Arabidopsis transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4) is a key player in the plant hormone abscisic acid (ABA) signaling pathway and is involved in plant response to abiotic stress and development. Expression of the ABI4 gene is tightly regulated, with low basal expression. Maximal transcript levels occur during the seed maturation and early seed germination stages. Moreover, ABI4 is an unstable, lowly expressed protein. Here, we studied factors affecting the stability of the ABI4 protein using transgenic Arabidopsis plants expressing 35S::HA-FLAG-ABI4-eGFP. Despite the expression of eGFP-tagged ABI4 being driven by the highly active 35S CaMV promoter, low steady-state levels of ABI4 were detected in the roots of seedlings grown under optimal conditions. These levels were markedly enhanced upon exposure of the seedlings to abiotic stress and ABA. ABI4 is degraded rapidly by the 26S proteasome, and we report on the role of phosphorylation of ABI4-serine 114 in regulating ABI4 stability. Our results indicate that ABI4 is tightly regulated both post-transcriptionally and post-translationally. Moreover, abiotic factors and plant hormones have similar effects on ABI4 transcripts and ABI4 protein levels. This double-check mechanism for controlling ABI4 reflects its central role in plant development and cellular metabolism.

Phytochrome interacting factor regulates stomatal aperture by coordinating red light and abscisic acid.

Stomata are crucial valves coordinating the fixation of carbon dioxide by photosynthesis and water loss through leaf transpiration. Phytochrome interacting factors (PIFs) are negative regulators of red light responses that belong to the basic helix-loop-helix family of transcription factors. Here, we show that the rice (Oryza sativa) PIF family gene OsPIL15 acts as a negative regulator of stomatal aperture to control transpiration in rice. OsPIL15 reduces stomatal aperture by activating rice ABSCISIC ACID INSENSITIVE 5 (OsABI5), which encodes a critical positive regulator of ABSCISIC ACID (ABA) signaling in rice. Moreover, OsPIL15 interacts with the NIGT1/HRS1/HHOfamily transcription factor rice HRS1 HOMOLOG 3 (OsHHO3) to possibly enhance the regulation of stomatal aperture. Notably, we discovered that the maize (Zea mays) PIF family genes ZmPIF1 and ZmPIF3, which are homologous to OsPIL15, are also involved in the regulation of stomatal aperture in maize, indicating that PIF-mediated regulation of stomatal aperture may be conserved in the plant lineage. Our findings explain the molecular mechanism by which PIFs play a role in red-light-mediated stomatal opening, and demonstrate that PIFs regulate stomatal aperture by coordinating the red light and ABA signaling pathways.

The transcriptional repressors VAL1 and VAL2 mediate genome-wide recruitment of the CHD3 chromatin remodeler PICKLE in Arabidopsis.

PICKLE (PKL) is a chromodomain helicase DNA-binding domain 3 (CHD3) chromatin remodeler that plays essential roles in controlling the gene expression patterns that determine developmental identity in plants, but the molecular mechanisms through which PKL is recruited to its target genes remain elusive. Here, we define a cis-motif and trans-acting factors mechanism that governs the genomic occupancy profile of PKL in Arabidopsis thaliana. We show that two homologous trans-factors VIVIPAROUS1/ABI3-LIKE1 (VAL1) and VAL2 physically interact with PKL in vivo, localize extensively to PKL-occupied regions in the genome, and promote efficient PKL recruitment at thousands of target genes, including those involved in seed maturation. Transcriptome analysis and genetic interaction studies reveal a close cooperation of VAL1/VAL2 and PKL in regulating gene expression and developmental fate. We demonstrate that this recruitment operates at two master regulatory genes, ABSCISIC ACID INSENSITIVE3 and AGAMOUS-LIKE 15, to repress the seed maturation program and ensure the seed-to-seedling transition. Together, our work unveils a general rule through which the CHD3 chromatin remodeler PKL binds to its target chromatin in plants.

Arabidopsis ABSCISIC ACID INSENSITIVE4 targets PROTEIN L-ISOASPARTYL METHYLTRANSFERASE1 in seed.

Arabidopsis ABSCISIC ACID INSENSITIVE4 (ABI4) positively regulates the protein repairing enzyme (PRE) PROTEIN L-ISOASPARTYL METHYLTRANSFERASE1 (PIMT1) in seed for its implication in seed vigor and longevity. PROTEIN L-ISOASPARTYL METHYLTRANSFERASE (PIMT) is a protein repairing enzyme (PRE) and is implicated in seed vigor and longevity. PIMT has been shown to be induced by ABA, however, its detailed regulation by ABA signaling components is unknown. Herein, we report that ABSCISIC ACID INSENSITIVE4 (ABI4) directly binds to the PIMT1 promoter and regulates its expression in Arabidopsis seeds. AtPIMT1 promoter analysis demonstrated the presence of putative ABI4 binding sites. Our Y1H analysis revealed that AtABI4 transcription factor binds to the AtPIMT1 promoter. Dual luciferase assay also demonstrated the binding of the AtABI4 transcription factor to the AtPIMT1 promoter. Subsequently, we have generated AtPIMT1 promoter GUS lines and revealedthat ABA induced expression of GUS in Arabidopsis thaliana. Expression analyses exhibited reduced accumulation of PIMT1 protein and transcript with significant reduction in total PIMT activity in abi4-1 mutants as compared to that of the wild type. The AtPIMT1 promoter GUS expression in abi4-1 mutants was also found to be severely affected in both the control and ABA treatment. Hence, through molecular and genetic evidences we show that the AtABI4 plays a central role in regulating the expression of AtPIMT1 to impart seed vigor and longevity to orthodox seeds.

Abscisic acid influences ammonium transport via regulation of kinase CIPK23 and ammonium transporters.

Ammonium uptake at plant roots is regulated at the transcriptional, posttranscriptional, and posttranslational levels. Phosphorylation by the protein kinase calcineurin B-like protein (CBL)-interacting protein kinase 23 (CIPK23) transiently inactivates ammonium transporters (AMT1s), but the phosphatases activating AMT1s remain unknown. Here, we identified the PP2C phosphatase abscisic acid (ABA) insensitive 1 (ABI1) as an activator of AMT1s in Arabidopsis (Arabidopsis thaliana). We showed that high external ammonium concentrations elevate the level of the stress phytohormone ABA, possibly by de-glycosylation. Active ABA was sensed by ABI1-PYR1-like () complexes followed by the inactivation of ABI1, in turn activating CIPK23. Under favorable growth conditions, ABI1 reduced AMT1;1 and AMT1;2 phosphorylation, both by binding and inactivating CIPK23. ABI1 further directly interacted with AMT1;1 and AMT1;2, which would be a prerequisite for dephosphorylation of the transporter by ABI1. Thus, ABI1 is a positive regulator of ammonium uptake, coupling nutrient acquisition to abiotic stress signaling. Elevated ABA reduces ammonium uptake during stress situations, such as ammonium toxicity, whereas ABI1 reactivates AMT1s under favorable growth conditions.

Pentatricopeptide repeat protein MITOCHONDRIAL STABILITY FACTOR 3 ensures mitochondrial RNA stability and embryogenesis.

Gene expression in plant mitochondria is predominantly governed at the post-transcriptional level and relies mostly on nuclear-encoded proteins. However, the protein factors involved and the underlying molecular mechanisms are still not well understood. Here, we report on the function of the MITOCHONDRIAL STABILITY FACTOR 3 (MTSF3) protein, previously named EMBRYO DEFECTIVE 2794 (EMB2794), and show that it is essential for accumulation of the mitochondrial NADH dehydrogenase subunit 2 (nad2) transcript in Arabidopsis (Arabidopsis thaliana) but not for splicing of nad2 intron 2 as previously proposed. The MTSF3 gene encodes a pentatricopeptide repeat protein that localizes in the mitochondrion. An MTSF3 null mutation induces embryonic lethality, but viable mtsf3 mutant plants can be generated through partial complementation with the developmentally regulated ABSCISIC ACID INSENSITIVE3 promoter. Genetic analyses revealed growth retardation in rescued mtsf3 plants owing to the specific destabilization of mature nad2 mRNA and a nad2 precursor transcript bearing exons 3 to 5. Biochemical data demonstrate that MTSF3 protein specifically binds to the 3' terminus of nad2. Destabilization of nad2 mRNA induces a substantial decrease in complex I assembly and activity and overexpression of the alternative respiratory pathway. Our results support a role for MTSF3 protein in protecting two nad2 transcripts from degradation by mitochondrial exoribonucleases by binding to their 3' extremities.

Cytochrome c and the transcription factor ABI4 establish a molecular link between mitochondria and ABA-dependent seed germination.

During germination, seed reserves are mobilised to sustain the metabolic and energetic demands of plant growth. Mitochondrial respiration is presumably required to drive germination in several species, but only recently its role in this process has begun to be elucidated. Using Arabidopsis thaliana lines with changes in the levels of the respiratory chain component cytochrome c (CYTc), we investigated the role of this protein in germination and its relationship with hormonal pathways. Cytochrome c deficiency causes delayed seed germination, which correlates with decreased cyanide-sensitive respiration and ATP production at the onset of germination. In addition, CYTc affects the sensitivity of germination to abscisic acid (ABA), which negatively regulates the expression of CYTC-2, one of two CYTc-encoding genes in Arabidopsis. CYTC-2 acts downstream of the transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4), which binds to a region of the CYTC-2 promoter required for repression by ABA and regulates its expression. The results show that CYTc is a main player during seed germination through its role in respiratory metabolism and energy production. In addition, the direct regulation of CYTC-2 by ABI4 and its effect on ABA-responsive germination establishes a link between mitochondrial and hormonal functions during this process.

Mutation of an Essential 60S Ribosome Assembly Factor MIDASIN 1 Induces Early Flowering in Arabidopsis.

Ribosome biogenesis is tightly associated with plant growth and reproduction. Mutations in genes encoding ribosomal proteins (RPs) or ribosome biogenesis factors (RBFs) generally result in retarded growth and delayed flowering. However, the early-flowering phenotype resulting from the ribosome biogenesis defect is rarely reported. We previously identified that the AAA-ATPase MIDASIN 1 (MDN1) functions as a 60S RBF in Arabidopsis. Here, we found that its weak mutant mdn1-1 is early-flowering. Transcriptomic analysis showed that the expression of FLOWERING LOCUS C (FLC) is down-regulated, while that of some autonomous pathway genes and ABSCISIC ACID-INSENSITIVE 5 (ABI5) is up-regulated in mdn1-1. Phenotypic analysis revealed that the flowering time of mdn1-1 is severely delayed by increasing FLC expression, suggesting that the early flowering in mdn1-1 is likely associated with the downregulation of FLC. We also found that the photoperiod pathway downstream of CONSTANTS (CO) and FLOWERING LOCUS T (FT) might contribute to the early flowering in mdn1-1. Intriguingly, the abi5-4 allele completely blocks the early flowering in mdn1-1. Collectively, our results indicate that the ribosome biogenesis defect elicited by the mutation of MDN1 leads to early flowering by affecting multiple flowering regulation pathways.

RAF22, ABI1 and OST1 form a dynamic interactive network that optimizes plant growth and responses to drought stress in Arabidopsis

Plants adapt to their ever-changing environment via positive and negative signals induced by environmental stimuli. Drought stress, for instance, induces accumulation of the plant hormone abscisic acid (ABA), triggering ABA signal transduction. However, the molecular mechanisms for switching between plant growth promotion and stress response remain poorly understood. Here we report that RAF (rapidly accelerated fibrosarcoma)-LIKE MITOGEN-ACTIVATED PROTEIN KINASE KINASE KINASE 22 (RAF22) in Arabidopsis thaliana physically interacts with ABA INSENSITIVE 1 (ABI1) and phosphorylates ABI1 at Ser416 residue to enhance its phosphatase activity. Interestingly, ABI1 can also enhance the activity of RAF22 through dephosphorylation, reciprocally inhibiting ABA signaling and promoting the maintenance of plant growth under normal conditions. Under drought stress, however, the ABA-activated OPEN STOMATA1 (OST1) phosphorylates the Ser81 residue of RAF22 and inhibits its kinase activity, restraining its enhancement of ABI1 activity. Taken together, our study reveals that RAF22, ABI1, and OST1 form a dynamic regulatory network that plays crucial roles in optimizing plant growth and environmental adaptation under drought stress.