Abscisic Acid Insensitivity Research Articles

Genome-Wide Analysis of Somatic Embryogenesis-Related Transcription Factors in Cultivated Strawberry (Fragaria × ananassa) and Evolutionary Relationships among Rosaceae Species

Somatic embryogenesis is a plant regeneration method commonly used in tissue culture. Its molecular mechanisms are well-known in model plants such as Arabidopsis thaliana L. LEAFY COTYLEDON1 (LEC1), LEAFY COTYLEDON2 (LEC2), FUSCA3 (FUS3), ABSCISIC ACID INSENSITIVE3 (ABI3), and BABYBOOM (BBM) genes are considered master regulators in the induction, growth, and maturation of somatic embryos. However, the study of these transcription factors in fruit crops with high agronomic and economic value such as cultivated strawberry (Fragaria × ananassa Duch.) and other Rosaceae species is scarce. The purpose of this study was the in silico characterization of LEC1, ABI3, FUS3, LEC2, and BBM(LAFL-B) genes from F. × ananassa genome and the study of the evolutionary relationships within the Rosaceae family. Synteny analyses and molecular evolutionary rates were performed to analyze the evolution of each transcription factor within the Rosaceae family. Synteny was conserved between F. × ananassa and other Rosaceae genomes, and paralogous genes were selected through negative selection. Additionally, the exon–intron organization and multiple alignments showed that gene structure and DNA-binding domains were conserved in F. × ananassa transcription factors. Finally, phylogenetic trees showed close evolutionary relationships between F. × ananassa and its orthologous proteins in the Rosoideae subfamily. Overall, this research revealed novel insights in the LAFL-B network in F. × ananassa and other species of the Rosaceae family. These results provide useful in silico information and new resources for the establishment of more efficient propagation systems or the study of ploidy effects on somatic embryogenesis.

Leaf vein pattern response to heat and drought requires genes that influence PINFORMED1 localization and is mimicked by ABA treatment

The plasticity of plant development allows acclimation to environmental stresses such as heat and drought. For example, increased vein density with more vein meetings is proposed to provide more efficient and robust water delivery when water availability is reduced. We demonstrate that Arabidopsis leaves respond to short term heat or drought by increasing leaf vein density, which in heat includes a higher density of vein meetings. We find that a set of genes, UNHINGED, FORKED1, FORKED-LIKE1, 2 and 3, which influence PINFORMED1 localization and control vein meeting in developing leaves, is required for the vein pattern response to heat or drought. Comparison of δ13C (‰) and nitrogen (%) under different conditions suggests that while wild type and mutant genotypes enriched δ13C (‰) under drought conditions, only wild type increased total shoot nitrogen suggesting increased photosynthetic assimilation. One explanation is that the mutants’ open vein pattern fails to supply sufficient water to support increased photosynthesis. Finally, treatment with ABA caused changes to leaf vein pattern similar to heat or drought treatment, a response that was eliminated in the abscisic acid insensitive1 but not the forked1 genotype. This suggests that the vein pattern response to reduced water availability may in part be controlled by ABA.

Phosphorylation of Serine 114 of the transcription factor ABSCISIC ACID INSENSITIVE 4 is essential for activity

The transcription factor ABA-INSENSITIVE(ABI)4 has diverse roles in regulating plant growth, including inhibiting germination and reserve mobilization in response to ABA and high salinity, inhibiting seedling growth in response to high sugars, inhibiting lateral root growth, and repressing light-induced gene expression. ABI4 activity is regulated at multiple levels, including gene expression, protein stability, and activation by phosphorylation. Although ABI4 can be phosphorylated at multiple residues by MAPKs, we found that S114 is the preferred site of MPK3. To examine the possible biological role of S114 phosphorylation, we transformed abi4-1 mutant plants with ABI4pro::ABI4 constructs encoding wild type (114S), phosphorylation-null (S114A) or phosphomimetic (S114E) forms of ABI4. Phosphorylation of S114 is necessary for the response to ABA, glucose, salt stress, and lateral root development, where the abi4 phenotype could be complemented by expressing ABI4 (114S) or ABI4 (S114E) but not ABI4 (S114A). Comparison of root transcriptomes in ABA-treated roots of abi4-1 mutant plants transformed with constructs encoding the different phosphorylation-forms of S114 of ABI4 revealed that 85 % of the ABI4-regulated genes whose expression pattern could be restored by expressing ABI4 (114S) are down-regulated by ABI4. Phosphorylation of S114 was required for regulation of 35 % of repressed genes, but only 17 % of induced genes. The genes whose repression requires the phosphorylation of S114 are mainly involved in embryo and seedling development, growth and differentiation, and regulation of gene expression.

Bacterial Quorum-Sensing Signaling-Related drr1 Mutant Influences Abscisic Acid Responsiveness in Arabidopsis thaliana L.

N-acyl-L-homoserine lactones (AHLs) are involved in cell-to-cell communication in Gram-negative bacteria through a process termed quorum-sensing (QS). In this report, we evaluated the response of Arabidopsis thaliana primary roots to abscisic acid (ABA) in wild-type (WT) and decanamide resistant root 1 (drr1) mutant, previously reported to be resistant to N-decanoyl-L-homoserine lactone (C10-HL). When compared to WT seedlings, drr1 mutants were hypersensitive to ABA and had primary roots shorter, which correlated with lower cell division in meristems, a higher concentration of endogenous ABA, and a greater expression of ABI5 gene; and this shortened primary root phenotype was reversed in drr1abi5 double mutants. An analysis of expression of ABSCISIC ACID INSENSITIVE 4 (ABI4) showed an ABA-inducible pattern in primary root tips, which was further increased in drr1 mutant seedlings. Comparison of seed germination in WT, drr1, abi5, and drr1abi5 lines showed higher germination percentages in the following order under control condition: abi5 > drr1abi5 > WT > drr1, while abi5 and drr1abi5 germinated faster and drr1 germinated slower with respect to WT under ABA condition. Taken together, our results suggest that DRR1 is a negative regulator of ABA signaling probably acting upstream of the transcription factors ABI4 and ABI5, which influence ABA responsiveness in primary roots and seed germination.

ABSCISIC ACID INSENSITIVE5 Interacts With RIBOSOMAL S6 KINASE2 to Mediate ABA Responses During Seedling Growth in Arabidopsis.

ABSCISIC ACID INSENSITIVE5 (ABI5) is an important regulator of abscisic acid (ABA) signaling pathway involved in regulating seed germination and postgerminative growth in Arabidopsis, which integrates various phytohormone pathways to balance plant growth and stress responses. However, the transcriptional regulatory mechanisms underlying ABI5 and its interacting proteins remain largely unknown. Here, we found that inhibition of AtTOR could increase ABA content by up-regulating the expression levels of ABA biosynthesis-related genes, and thus activated the expression of ABA-responsive genes. Pharmacological assay showed that abi5-1 mutant was insensitive to TOR inhibitor AZD8055, whereas AtABI5 overexpression lines were hypersensitive to AZD8055 in Arabidopsis. Biochemical interaction assays demonstrated that ABI5 physically interacted with the RIBOSOMAL S6 KINASE2 (S6K2) protein in plant cell. S6K2 positively regulated ABA responses during seedling growth and upregulated ABA-responsive genes expression. Furthermore, genetic and physiological analysis indicated that AtS6K2 overexpression lines enhanced resistance to drought treatment while AtS6K2 interference lines were sensitive to drought. These results indicated that AtABI5 interacted with AtS6K2 to positively modulate ABA responses during seedling growth and shed light on a underlying mechanism of the crosstalk between TOR and ABA signaling pathways in modulating seedling growth in Arabidopsis.

The gibberellin signaling negative regulator RGA-LIKE3 promotes seed storage protein accumulation

Seed storage protein (SSP) acts as one of the main components of seed storage reserves, of which accumulation is tightly mediated by a sophisticated regulatory network. However, whether and how gibberellin (GA) signaling is involved in this important biological event is not fully understood. Here, we show that SSP content in Arabidopsis (Arabidopsis thaliana) is significantly reduced by GA and increased in the GA biosynthesis triple mutant ga3ox1/3/4. Further investigation shows that the DELLA protein RGA-LIKE3 (RGL3), a negative regulator of GA signaling, is important for SSP accumulation. In rgl3 and 35S:RGL3-HA, the expression of SSP genes is down- and upregulated, respectively, compared with that in the wild-type. RGL3 interacts with ABSCISIC ACID INSENSITIVE3 (ABI3), a critical transcription factor for seed developmental processes governing SSP accumulation, both in vivo and in vitro, thus greatly promoting the transcriptional activating ability of ABI3 on SSP genes. In addition, genetic evidence shows that RGL3 and ABI3 regulate SSP accumulation in an interdependent manner. Therefore, we reveal a function of RGL3, a little studied DELLA member, as a coactivator of ABI3 to promote SSP biosynthesis during seed maturation stage. This finding advances the understanding of mechanisms in GA-mediated seed storage reserve accumulation.

Salt stress in Arabidopsis thaliana seedlings: Role of indoleamines in stress alleviation

Salinity is a major environmental stress in agriculture with significantly detrimental effects on crop productivity. The development of strategies to enhance salinity stress tolerance in plants is essential to ensure crop production in saline environments. Melatonin (Mel) and serotonin (Ser) accumulate in response to environmental stresses and are presumed to play protective roles and improve growth of tissues during recovery. In this study, the effects of Mel and Ser were investigated in Arabidopsis under NaCl stress. Exogenous Mel (10 µM) and Ser (10 µM) treatment significantly increased fresh weight, lateral root number, and shoot height in A. thaliana seedlings exposed to NaCl stress (25 mM and 50 mM) compared to the non-treated control seedlings. In order to understand the role of these indoleamines in alleviating salt stress, we investigated the effects of Mel and Ser treatments on the expression of salt stress responsive genes including, transcription factors involved in abscisic acid (ABA) signaling pathway, ABA-INSENSITIVE 3 (ABI3)and ABA-INSENSITIVE 5 (ABI5); ABA responsive gene, RESPONSIVE TO DESSICATION 29B (RD29B), ABA-independent gene, RESPONSIVE TO DESSICATION 29A (RD29A) and Arabidopsis trithorax-like gene (ATX1) which function in stress responses via ABA-dependent and ABA-independent manner. Other genes included, ROS-signaling transcription factor ZAT10 and ZAT12, and the genes encoding ion transporters crucial for maintaining ion homeostasis, HIGH AFFINITY K+ TRANSPORTER 5 (HAK5) and SALT OVERLY SENSITIVE 1 (SOS1). Mel (10 µM) pre-treatment for 24 hrs followed by 50 mM salt treatment up-regulated ABI3, RD29B, ZAT12 and HAK5. The Ser (10 µM) pre-treatment significantly up-regulated ZAT12.These results indicate that indoleamine pre-treatment improved plant growth under salt stress with Mel facilitating salt tolerance via upregulation of ABA responsive genes, mediation of antioxidant defense systems to counteract the salt-induced ROS overproduction as well as controlling ion homeostasis. Although Ser displayed no significant effects on ABA signaling, it was found to increase the expression of antioxidant defense gene, ZAT12. This study demonstrates the importance of indoleamine pathway in mediation of salt stress response and provides the first indication of the involvement of Ser in salt stress tolerance.

ABI5 regulates ABA-induced anthocyanin biosynthesis by modulating the MYB1-bHLH3 complex in apple.

Abscisic acid (ABA) induces anthocyanin biosynthesis in many plant species. However, the molecular mechanism of ABA-regulated anthocyanin biosynthesis remains unclear. As a crucial regulator of ABA signaling, ABSCISIC ACID-INSENSITIVE5 (ABI5) is involved in many aspects of plant growth and development, yet its regulation of anthocyanin biosynthesis has not been elucidated. In this study, we found that MdABI5, the apple homolog of Arabidopsis ABI5, positively regulated ABA-induced anthocyanin biosynthesis. A series of biochemical tests showed that MdABI5 specifically interacts with basic helix-loop-helix 3 (MdbHLH3), a positive regulator of anthocyanin biosynthesis. MdABI5 enhanced the binding of MdbHLH3 to its target genes dihydroflavonol 4-reductase (MdDFR) and UDP flavonoid glucosyl transferase (MdUF3GT). In addition, MdABI5 directly bound to the promoter of MdbHLH3 to activate its expression. Moreover, MdABI5 enhanced ABA-promoted interaction between MdMYB1 and MdbHLH3. Finally, antisense suppression of MdbHLH3 significantly reduced anthocyanin biosynthesis promoted by MdABI5, indicating that MdABI5-promoted anthocyanin biosynthesis was dependent on MdbHLH3. Taken together, our data suggest that MdABI5 plays a positive role in ABA-induced anthocyanin biosynthesis by modulating the MdbHLH3-MdMYB1 complex. Our work broadens the regulatory network of ABA-mediated anthocyanin biosynthesis, providing new insights to further study the transcriptional regulatory mechanisms behind this process.

The ABI4-RbohD/VTC2 regulatory module promotes reactive oxygen species (ROS) accumulation to decrease seed germination under salinity stress.

Salinity stress enhances reactive oxygen species (ROS) accumulation by activating the transcription of NADPH oxidase genes such as RbohD, thus mediating plant developmental processes, including seed germination. However, how salinity triggers the expression of ROS-metabolism-related genes and represses seed germination has not yet been fully addressed. In this study, we show that Abscisic Acid-Insensitive 4 (ABI4), a key component in abscisic acid (ABA) signaling, directly combines with RbohD and Vitamin C Defective 2 (VTC2), the key genes involved in ROS production and scavenging, to modulate ROS metabolism during seed germination under salinity stress. Salinity-induced ABI4 enhances RbohD expression by physically interacting with its promoter, and subsequently promotes ROS accumulation, thus resulting in cell membrane damage and a decrease in seed vigor. Additional genetic evidence indicated that the rbohd mutant largely rescues the salt-hypersensitive phenotype of ABI4 overexpression seeds. Consistently, the abi4/vtc2 double mutant showed the salt-sensitive phenotype, similar to the vtc2 mutant, suggesting that both RbohD and VTC2 are epistatic to ABI4 genetically. Altogether, these results suggest that the salt-induced RbohD transcription and ROS accumulation is dependent on ABI4, and that the ABI4-RbohD/VTC2 regulatory module integrates both ROS metabolism and cell membrane integrity, ultimately repressing seed germination under salinity stress.

The UBC27–AIRP3 ubiquitination complex modulates ABA signaling by promoting the degradation of ABI1 in Arabidopsis

Abscisic acid (ABA) is the key phytohormone in plant drought tolerance and stress adaptation. The clade A protein phosphatase 2Cs (PP2Cs) like ABI1 (ABA-INSENSITIVE 1) work as coreceptors of ABA and regulate multiple ABA responses. Ubiquitination of ABI1 has been proven to play important regulatory roles in ABA signaling. However, the specific ubiquitin conjugating enzyme (E2) involved is unknown. Here, we report that UBC27 is an active E2 that positively regulates ABA signaling and drought tolerance. UBC27 forms the E2-E3 pair with the drought regulator RING E3 ligase AIRP3. Both UBC27 and AIRP3 interact with ABI1 and affect the ubiquitination and degradation of ABI1. ABA activates the expression of UBC27, inhibits the proteasome degradation of UBC27, and enhances the interaction between UBC27 and ABI1 to increase its activity. These findings uncover a regulatory mechanism in ABA signaling and drought response and provide a further understanding of the plant ubiquitination system and ABA signaling pathway.

Molecular Mechanism Underlying the Synergetic Effect of Jasmonate on Abscisic Acid Signaling during Seed Germination in Arabidopsis

Abscisic acid (ABA) is known to suppress seed germination and post-germinative growth of Arabidopsis (Arabidopsis thaliana), and jasmonate (JA) enhances ABA function. However, the molecular mechanism underlying the crosstalk between the ABA and JA signaling pathways remains largely elusive. Here, we show that exogenous coronatine, a JA analog structurally similar to the active conjugate jasmonate-isoleucine, significantly enhances the delayed seed germination response to ABA. Disruption of the JA receptor CORONATINE INSENSITIVE1 or accumulation of the JA signaling repressor JASMONATE ZIM-DOMAIN (JAZ) reduced ABA signaling, while jaz mutants enhanced ABA responses. Mechanistic investigations revealed that several JAZ repressors of JA signaling physically interact with ABSCISIC ACID INSENSITIVE3 (ABI3), a critical transcription factor that positively modulates ABA signaling, and that JAZ proteins repress the transcription of ABI3 and ABI5. Further genetic analyses showed that JA activates ABA signaling and requires functional ABI3 and ABI5. Overexpression of ABI3 and ABI5 simultaneously suppressed the ABA-insensitive phenotypes of the coi1-2 mutant and JAZ-accumulating (JAZ-ΔJas) plants. Together, our results reveal a previously uncharacterized signaling module in which JAZ repressors of the JA pathway regulate the ABA-responsive ABI3 and ABI5 transcription factors to integrate JA and ABA signals during seed germination and post-germinative growth.

The Arabidopsis transcription factor LBD15 mediates ABA signaling and tolerance of water-deficit stress by regulating ABI4 expression.

To survive, sessile plants must adapt to grow and develop when facing water-deficit stress. However, the molecular mechanisms underlying fine-tuning of the antagonistic action between stress response and growth remain to be determined. Here, plants overexpressing Lateral Organ Boundaries Domain 15 (LBD15) showed abscisic acid (ABA) hypersensitivity and tolerance of water-deficit stress, whereas the loss-of-function mutant lbd15 presented decreased sensitivity to ABA and increased sensitivity to water-deficit stress. Further analysis revealed that LBD15 directly binds to the promoter of the ABA signaling pathway gene ABSCISIC ACID INSENSITIVE4 (ABI4) to activate its expression, thereby forming an LBD15-ABI4 cascade to optimally regulate ABA signaling-mediated plant growth and tolerance of water-deficit stress. In addition, drought stress-induced ABA signaling promoted LBD15 expression, which directly activates expression of ABI4 to close stomata. As a result, water loss is reduced, and then water-deficit stress tolerance is increased. The results of this study reveal a molecular mechanism by which LBD15 coordinates and balances plant growth and resistance to water-deficit stress.

Role of Glycine max ABSCISIC ACID INSENSITIVE 3 (GmABI3) in lipid biosynthesis and stress tolerance in soybean.

Soybean is an important oilseed crop and primary dietary protein resource. The limited understanding of soybean oil biosynthesis has become a significant obstacle for the improvement of soybean oil production. A transcription factor ABSCISIC ACID INSENSITIVE 3 (ABI3) is known for its role in plant development and seed dormancy in many crops. The current study was aimed to functionally characterise ABI3 homologue in Glycine max L. For this purpose, the GmABI3 gene was cloned and ectopically expressed in wildtype and abi3 mutant Arabidopsis. The GmABI3 expression in the atabi3 mutant enhanced the triacylglycerol (TAG) content (7.3%) in addition to modified fatty acid composition. The GmABI3 increased eicosenoic acid (20:1) up to 6.5% in genetically complemented Arabidopsis mutant seeds, which is essential for long-chain fatty acid synthesis. The transgenic GmABI3/wildtype seeds contain 34.9% more TAG content compared with wildtype seeds. The results showed that GmABI3 is responsible for seed-specific TAG and long-chain fatty acid biosynthesis in soybean. The exposure to cold and heat stress and exogenous supply of abscisic acid and jasmonic acid altered the level of GmABI3 in treated seeds and leaves. It also concluded that GmABI3 could regulate stress tolerance in soybean, which applies to a wide variety of crops to deal with biological stresses.

Expression of wheat Peptide TRansporter 2.1 (TaPTR2.1) during early seed germination

Peptide TRansporters (PTRs) translocate nitrogen (N)-containing substances across cellular membranes. In Arabidopsis thaliana, the seed-abundant, tonoplast-localized AtPTR2 protein is involved in the regulation of water status during seed germination and is negatively regulated by ABA INSENSITIVE4 (ABI4). A survey of publicly available whole-genome sequencing and RNA-seq data revealed that the genomes of hexaploid wheat (Triticum aestivum L.) harbors three homologs of the AtPTR2 gene, TaPTR2.1, TaPTR2.2, and TaPTR2.3, and transcripts of TaPTR2.1 are detected both in dry and germinating seeds, similar to AtPTR2. However, the functional role of TaPTR2.1 during seed germination has not yet been characterized. In the present study, changes in TaPTR2.1 transcript levels were investigated in the wheat cultivar Keumgang in response to phytohormone, sugar, and osmoticum treatments. A transient increase in the transcript level of TaPTR2.1 during seed germination was significantly lowered by abscisic acid (ABA) and PEG4000 treatments. Further, overexpression of the TaPTR2.1 gene under the control of the Cauliflower mosaic virus 35S promoter in the Arabidopsis ptr2 mutant rescued the delayed seed germination phenotype by restoring the seed water status. Together, these results suggest that TaPTR2.1 regulates the water status of wheat seeds during germination in an ABA-dependent manner.

Mobile TERMINAL FLOWER1 determines seed size in Arabidopsis.

Seed size is a pivotal agronomic trait that links plant sexual reproduction and subsequent seedling establishment, and is affected by the timing of endosperm cellularization following endosperm proliferation after double fertilization. The molecular switch that controls the timing of endosperm cellularization has so far been largely unclear. Here, we report that the Arabidopsis TERMINAL FLOWER1 (TFL1) is a mobile regulator generated in the chalazal endosperm, and moves to the syncytial peripheral endosperm to mediate timely endosperm cellularization and seed size through stabilizing ABSCISIC ACID INSENSITIVE 5. We further show that Ras-related nuclear GTPases interact with TFL1 and regulate its trafficking to the syncytial peripheral endosperm. Our findings reveal TFL1 as an essential molecular switch for regulating endosperm cellularization and seed size. Generation of mobile TFL1 in the chalazal endosperm, which is close to maternal vascular tissues, could provide a hitherto-unknown means to control seed development by mother plants.

Barley ABI5 (Abscisic Acid INSENSITIVE 5) Is Involved in Abscisic Acid-Dependent Drought Response.

ABA INSENSITIVE 5 (ABI5) is a basic leucine zipper (bZIP) transcription factor which acts in the abscisic acid (ABA) network and is activated in response to abiotic stresses. However, the precise role of barley (Hordeum vulgare) ABI5 in ABA signaling and its function under stress remains elusive. Here, we show that HvABI5 is involved in ABA-dependent regulation of barley response to drought stress. We identified barley TILLING mutants carrying different alleles in the HvABI5 gene and we studied in detail the physiological and molecular response to drought and ABA for one of them. The hvabi5.d mutant, carrying G1751A transition, was insensitive to ABA during seed germination, yet it showed the ability to store more water than its parent cv. “Sebastian” (WT) in response to drought stress. The drought-tolerant phenotype of hvabi5.d was associated with better membrane protection, higher flavonoid content, and faster stomatal closure in the mutant under stress compared to the WT. The microarray transcriptome analysis revealed up-regulation of genes associated with cell protection mechanisms in the mutant. Furthermore, HvABI5 target genes: HVA1 and HVA22 showed higher activity after drought, which may imply better adaptation of hvabi5.d to stress. On the other hand, chlorophyll content in hvabi5.d was lower than in WT, which was associated with decreased photosynthesis efficiency observed in the mutant after drought treatment. To verify that HvABI5 acts in the ABA-dependent manner we analyzed expression of selected genes related to ABA pathway in hvabi5.d and its WT parent after drought and ABA treatments. The expression of key genes involved in ABA metabolism and signaling differed in the mutant and the WT under stress. Drought-induced increase of expression of HvNCED1, HvBG8, HvSnRK2.1, and HvPP2C4 genes was 2–20 times higher in hvabi5.d compared to “Sebastian”. We also observed a faster stomatal closure in hvabi5.d and much higher induction of HvNCED1 and HvSnRK2.1 genes after ABA treatment. Together, these findings demonstrate that HvABI5 plays a role in regulation of drought response in barley and suggest that HvABI5 might be engaged in the fine tuning of ABA signaling by a feedback regulation between biosynthetic and signaling events. In addition, they point to different mechanisms of HvABI5 action in regulating drought response and seed germination in barley.

The diverse and unanticipated roles of histone deacetylase 9 in coordinating plant development and environmental acclimation.

Plants tightly control gene transcription to adapt to environmental conditions and steer growth and development. Different types of epigenetic modifications are instrumental in these processes. In recent years, an important role for the chromatin-modifying RPD3/HDA1 class I HDAC HISTONE DEACETYLASE 9 (HDA9) emerged in the regulation of a multitude of plant traits and responses. HDACs are widely considered transcriptional repressors and are typically part of multiprotein complexes containing co-repressors, DNA, and histone-binding proteins. By catalyzing the removal of acetyl groups from lysine residues of histone protein tails, HDA9 negatively controls gene expression in many cases, in concert with interacting proteins such as POWERDRESS (PWR), HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 15 (HOS15), WRKY53, ELONGATED HYPOCOTYL 5 (HY5), ABA INSENSITIVE 4 (ABI4), and EARLY FLOWERING 3 (ELF3). However, HDA9 activity has also been directly linked to transcriptional activation. In addition, following the recent breakthrough discovery of mutual negative feedback regulation between HDA9 and its interacting WRKY-domain transcription factor WRKY53, swift progress in gaining understanding of the biology of HDA9 is expected. In this review, we summarize knowledge on this intriguing versatile-and long under-rated-protein and propose novel leads to further unravel HDA9-governed molecular networks underlying plant development and environmental biology.

To grow or not to grow ….

To grow or not to grow ….

Direct and indirect targets of the arabidopsis seed transcription factor ABSCISIC ACID INSENSITIVE3.

Arabidopsis thaliana ABSCISIC ACID INSENSITIVE3 (ABI3) is a transcription factor in the B3 domain family. ABI3, along with B3 domain transcription factors LEAFY COTYLEDON2 (LEC2) and FUSCA3 (FUS3), and LEC1, a subunit of the CCAAT box-binding complex, form the so-called LAFL network to control various aspects of seed development and maturation. ABI3 also contributes to the abscisic acid (ABA) response. We report on chromatin immunoprecipitation-tiling array experiments to map binding sites for ABI3 globally. We also assessed transcriptomes in response to ABI3 by comparing developing abi3-5 and wild-type seeds and combined this information to ascertain direct and indirect responsive ABI3 target genes. ABI3 can induce and repress its transcription of target genes directly and some intriguing differences exist in cis motifs between these groups of genes. Directly regulated targets reflect the role of ABI3 in seed maturation, desiccation tolerance, entry into a quiescent state and longevity. Interestingly, ABI3 directly represses a gene encoding a microRNA (MIR160B) that targets AUXIN RESPONSE FACTOR (ARF)10 and ARF16 that are involved in establishment of dormancy. In addition, ABI3, like FUS3, regulates genes encoding MIR156 but while FUS3 only induces genes encoding this product, ABI3 induces these genes during the early stages of seed development, but represses these genes during late development. The interplay between ABI3, the other LAFL genes, and the VP1/ABI3-LIKE (VAL) genes, which are involved in the transition to seedling development are examined and reveal complex interactions controlling development.

A Kinase–Phosphatase–Transcription Factor Module Regulates Adventitious Root Emergence in Arabidopsis Root–Hypocotyl Junctions

Adventitious roots form from non-root tissues as part of normal development or in response to stress or wounding. The root primordia form in the source tissue, and during emergence the adventitious roots penetrate the inner cell layers and the epidermis; however, the mechanisms underlying this emergence remain largely unexplored. Here, we report that a regulatory module composed of the AP2/ERF transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4), the MAP kinases MPK3 and MPK6, and the phosphatase PP2C12 plays an important role in the emergence of junction adventitious roots (J-ARs) from the root–hypocotyl junctions in Arabidopsis thaliana. ABI4 negatively regulates J-AR emergence, preventing the accumulation of reactive oxygen species and death of epidermal cells, which would otherwise facilitate J-AR emergence. Phosphorylation by MPK3/MPK6 activates ABI4 and dephosphorylation by PP2C12 inactivates ABI4. MPK3/MPK6 also directly phosphorylate and inactivate PP2C12 during J-AR emergence. We propose that this “double-check” mechanism increases the robustness of MAP kinase signaling and finely regulates the local programmed cell death required for J-AR emergence.