Influence of the transcription factor ABI5 on growth and development in Arabidopsis.

ABA-insensitive 5 (ABI5) belongs to the basic leucine zipper class of transcription factors and is named for being the fifth identified Arabidopsis mutant unresponsive to ABA. To understand the influence of ABI5 in its active state on downstream gene expression and plant growth and development, we overexpressed the full-length ABI5 (A.t.MX-4) and the active forms of ABI5 with deleted transcriptional repression domains (A.t.MX-1, A.t.MX-2, and A.t.MX-3). Compared with the wild type, A.t.MX-1, A.t.MX-2, and A.t.MX-3 exhibited an increase in rosette leaf number and size, earlier flowering, increased thousand-seed weight, and significantly enhanced drought resistance. Thirty-five upregulated/downregulated proteins in the A.t.MX-1 were identified by proteomic analysis, and these proteins were involved in ABA biosynthesis and degradation, abiotic stress, fatty acid synthesis, and energy metabolism. These proteins participate in the regulation of plant drought resistance, flowering timing, and seed size at the levels of transcription and post-translational modification.

The U-Box E3 Ubiquitin Ligase PUB35 Negatively Regulates ABA Signaling through AFP1-mediated Degradation of ABI5.

Abscisic acid (ABA) signaling is crucial for plant responses to various abiotic stresses. The Arabidopsis (Arabidopsis thaliana) transcription factor ABA INSENSITIVE 5 (ABI5) is a central regulator of ABA signaling. ABI5 BINDING PROTEIN 1 (AFP1) interacts with ABI5 and facilitates its 26S-proteasome-mediated degradation, although the detailed mechanism has remained unclear. Here, we report that an ABA-responsive U-box E3 ubiquitin ligase, PLANT U-BOX 35 (PUB35), physically interacts with AFP1 and ABI5. PUB35 directly ubiquitinated ABI5 in a bacterially reconstituted ubiquitination system and promoted ABI5 protein degradation in vivo. ABI5 degradation was enhanced by AFP1 in response to ABA treatment. Phosphorylation of the T201 and T206 residues in ABI5 disrupted the ABI5-AFP1 interaction and affected the ABI5-PUB35 interaction and PUB35-mediated degradation of ABI5 in vivo. Genetic analysis of seed germination and seedling growth showed that pub35 mutants were hypersensitive to ABA as well as to salinity and osmotic stresses, whereas PUB35 overexpression lines were hyposensitive. Moreover, abi5 was epistatic to pub35, whereas the pub35-2 afp1-1 double mutant showed a similar ABA response to the two single mutants. Together, our results reveal a PUB35-AFP1 module involved in fine-tuning ABA signaling through ubiquitination and 26S-proteasome-mediated degradation of ABI5 during seed germination and seedling growth.

SOS2-AFP2 module regulates seed germination by inducing ABI5 degradation in response to salt stress in Arabidopsis.

Soil salinity pose a significant challenge to global agriculture, threatening crop yields and food security. Understanding the salt tolerance mechanisms of plants is crucial for improving their survival under salt stress. AFP2, a negative regulator of ABA signaling, has been shown to play a crucial role in salt stress tolerance during seed germination. Mutations in AFP2 gene lead to increased sensitivity to salt stress. However, the underline mechanisms by which AFP2 regulates seed germination under salt stress remain elusive. In this study, we identified a protein interaction between AFP2 and SOS2, a Ser/Thr protein kinase known to play a critical role in salt stress response. Using a combination of genetic, biochemical, and physiological approaches, we investigated the role of the SOS2-AFP2 module in regulating seed germination under salt stress. Our findings reveal that SOS2 physically interacts with AFP2 and stabilizes it, leading to the degradation of the ABI5 protein, a negative transcription factor in seed germination under salt stress. This study sheds light on previously unknown connections within salt stress and ABA signaling, paving the way for novel strategies to enhance plant resilience against environmental challenges.

ABI3 promotes auxin signalling by regulating SHY2 expression to control primary root growth in response to dehydration stress.

Plants combat dehydration stress through several adaptive measures including root architectural changes. Here we show that when exposed to varying levels of dehydration stress, primary root growth in Arabidopsis is modulated by regulating root meristem activity. ABA in concert with auxin signalling perceives the stress level and adapts primary root growth accordingly. ABI3, the ABA responsive transcription factor stands at the intersection of ABA and auxin signalling and fine tunes primary root growth in response to dehydration stress. Under low ABA or dehydration stress, induction of ABI3 expression promotes auxin signalling by decreasing expression of SHY2, a negative regulator of auxin response. This further enhances the expression of auxin transporter gene PIN1 and cell cycle gene CYCB1;1, resulting in an increase in primary root meristem size and root length. Higher levels of dehydration stress or ABA repress ABI3 expression and promote ABI5 expression. This elevates SHY2 expression, thereby impairing primary root meristem activity and retarding root growth. Notably, ABI5 can promote SHY2 expression only in the absence of ABI3. Such ABA concentration dependent expression of ABI3 therefore functions as a regulatory sensor of dehydration stress levels and orchestrates primary root growth by coordinating its downstream regulon.

Interplay of Light and ABA signaling to modulate plant development.

The exogenous light cues and the phytohormone Abscisic acid (ABA) regulate several aspects of plant growth and development. In recent years, the role of the crosstalk between the light and ABA signaling pathways in regulating different physiological processes has become increasingly evident. This includes the regulation of germination and early seedling development, control of stomatal development and conductance, growth and development of roots, buds, branches, and regulation of flowering. Light and ABA signaling cascades have various convergence points at both DNA and protein levels. The molecular crosstalk involves several light signaling factors like HY5, COP1, PIFs and BBXs that integrate with ABA signaling components like the PYL receptors and ABI5. Especially, ABI5 and PIF4 promoters serve as key "hotspots" for the integration of these two pathways. Plants acquired both light and ABA signaling pathways before they colonized land almost 500 million years ago. In this review, we discuss the recent advances in the interplay of light and ABA signaling regulating plant development and provide an overview of the evolution of these two pathways.

Identification and Characterization of the AREB/ABF Gene Family in Three Orchid Species and Functional Analysis of DcaABI5 in Arabidopsis.

AREB/ABF (ABA response element binding) proteins in plants are essential for stress responses, while our understanding of AREB/ABFs from orchid species, important traditional medicinal and ornamental plants, is limited. Here, twelve AREB/ABF genes were identified within three orchids' complete genomes and classified into three groups through phylogenetic analysis, which was further supported with a combined analysis of their conserved motifs and gene structures. The cis-element analysis revealed that hormone response elements as well as light and stress response elements were widely rich in the AREB/ABFs. A prediction analysis of the orchid ABRE/ABF-mediated regulatory network was further constructed through cis-regulatory element (CRE) analysis of their promoter regions. And it revealed that several dominant transcriptional factor (TF) gene families were abundant as potential regulators of these orchid AREB/ABFs. Expression profile analysis using public transcriptomic data suggested that most AREB/ABF genes have distinct tissue-specific expression patterns in orchid plants. Additionally, DcaABI5 as a homolog of ABA INSENSITIVE 5 (ABI5) from Arabidopsis was selected for further analysis. The results showed that transgenic Arabidopsis overexpressing DcaABI5 could rescue the ABA-insensitive phenotype in the mutant abi5. Collectively, these findings will provide valuable information on AREB/ABF genes in orchids.

ABI5 binding proteins: key players in coordinating plant growth and development.

During the course of terrestrial evolution, plants have developed complex networks that involve the coordination of phytohormone signalling pathways in order to adapt to an ever-changing environment. Transcription factors coordinate these responses by engaging in different protein complexes and exerting both positive and negative effects. ABA INSENSITIVE 5 (ABI5) binding proteins (AFPs), which are closely related to NOVEL INTERACTOR OF JAZ (NINJA)-like proteins, are known for their fundamental role in plants' morphological and physiological growth. Recent studies have shown that AFPs regulate several hormone-signalling pathways, including abscisic acid (ABA) and gibberellic acid (GA). Here, we review the genetic control of AFPs and their crosstalk with plant hormone signalling, and discuss the contributions of AFPs to plants' growth and development.

Receptor for Activated C Kinase 1 counteracts ABSCISIC ACID INSENSITIVE5-mediated inhibition of seed germination and post-germinative growth in Arabidopsis.

ABSCISIC ACID INSENSITIVE5 (ABI5), a key regulator of the abscisic acid (ABA) signalling pathway, plays a fundamental role in seed germination and post-germinative development. However, the molecular mechanism underlying the repression function of ABI5 remains to be elucidated. In this study, we demonstrate that the conserved eukaryotic WD40 repeat protein Receptor for Activated C Kinase 1 (RACK1) is a novel negative regulator of ABI5 in Arabidopsis. The RACK1 loss-of-function mutant is hypersensitive to ABA, while this phenotype is rescued by a mutation in ABI5. Moreover, overexpression of RACK1 suppresses ABI5 transcriptional activation activity for ABI5-targeted genes. RACK1 may also physically interact with ABI5 and facilitate its degradation. Furthermore, we found that RACK1 and the two substrate receptors CUL4-based E3 ligases (DWA1 and DWA2) function together to mediate the turnover of ABI5, thereby efficiently reducing ABA signalling in seed germination and post-germinative growth. In addition, molecular analyses demonstrated that ABI5 may bind to the promoter of RACK1 to repress its expression. Collectively, our findings suggest that RACK1 and ABI5 might form a feedback loop to regulate the homeostasis of ABA signalling in acute seed germination and early plant development.

"Genome-wide identification of bZIP gene family in Pearl millet and transcriptional profiling under abiotic stress, phytohormonal treatments; and functional characterization of PgbZIP9".

Abiotic stresses are major constraints in crop production, and are accountable for more than half of the total crop loss. Plants overcome these environmental stresses using coordinated activities of transcription factors and phytohormones. Pearl millet an important C4 cereal plant having high nutritional value and climate resilient features is grown in marginal lands of Africa and South-East Asia including India. Among several transcription factors, the basic leucine zipper (bZIP) is an important TF family associated with diverse biological functions in plants. In this study, we have identified 98 bZIP family members (PgbZIP) in pearl millet. Phylogenetic analysis divided these PgbZIP genes into twelve groups (A-I, S, U and X). Motif analysis has shown that all the PgbZIP proteins possess conserved bZIP domains and the exon-intron organization revealed conserved structural features among the identified genes. Cis-element analysis, RNA-seq data analysis, and real-time expression analysis of PgbZIP genes suggested the potential role of selected PgbZIP genes in growth/development and abiotic stress responses in pearl millet. Expression profiling of selected PgbZIPs under various phytohormones (ABA, SA and MeJA) treatment showed differential expression patterns of PgbZIP genes. Further, PgbZIP9, a homolog of AtABI5 was found to localize in the nucleus and modulate gene expression in pearl millet under stresses. Our present findings provide a better understanding of bZIP genes in pearl millet and lay a good foundation for the further functional characterization of multi-stress tolerant PgbZIP genes, which could become efficient tools for crop improvement.

ABA functions in low phosphate-induced anthocyanin accumulation through the transcription factor ABI5 in Arabidopsis.

KEY MESSAGE: ABI5 functions in ABA-mediated anthocyanin accumulation in plant response to low phosphate. Low phosphate (LP)-induced anthocyanin biosynthesis and accumulation play an important role in plant adaptive response to phosphate starvation conditions. However, whether and how the stress phytohormone abscisic acid (ABA) participates in LP-induced anthocyanin accumulation remain elusive. Here, we report that ABA is required for LP-induced anthocyanin accumulation in Arabidopsis thaliana. Disrupting ABA DEFICIENT2 (ABA2), a key ABA-biosynthetic gene, or BETA-GLUCOSIDASE1 (BG1), a major gene implicated in converting conjugated ABA to active ABA, significantly impairs LP-induced anthocyanin accumulation, as LP-induced expression of the anthocyanin-biosynthetic genes Chalcone Synthase (CHS) is dampened in the aba2 and bg1 mutant. In addition, LP-induced anthocyanin accumulation is defective in the mutants of ABA signaling pathway, including ABA receptors, ABA Insensitive2, and the transcription factors ABA Insensitive5 (ABI5), suggesting a role of ABI5 in ABA-mediated upregulation of anthocyanin-biosynthetic genes in plant response to LP. Indeed, LP-induced expression of CHS is repressed in the abi5-7 mutant but further promoted in the ABI5-overexpressing plants compared to the wild-type. Moreover, ABI5 can bind to and transcriptionally activate CHS, and the defectiveness of LP-induced anthocyanin accumulation in abi5-7 can be restored by overexpressing CHS. Collectively, our findings illustrates that ABI5 functions in ABA-mediated LP-induced anthocyanin accumulation in Arabidopsis.

Pectin methylesterase 31 is transcriptionally repressed by ABI5 to negatively regulate ABA-mediated inhibition of seed germination.

Pectin methylesterase (PME), a family of enzymes that catalyze the demethylation of pectin, influences seed germination. Phytohormone abscisic acid (ABA) inhibits seed germination. However, little is known about the function of PMEs in response to ABA-mediated seed germination. In this study, we found the role of PME31 in response to ABA-mediated inhibition of seed germination. The expression of PME31 is prominent in the embryo and is repressed by ABA treatment. Phenotype analysis showed that disruption of PME31 increases ABA-mediated inhibition of seed germination, whereas overexpression of PME31 attenuates this effect. Further study found that ABI5, an ABA signaling bZIP transcription factor, is identified as an upstream regulator of PME31. Genetic analysis showed that PME31 functions downstream of ABI5 in ABA-mediated seed germination. Detailed studies showed that ABI5 directly binds to the PME31 promoter and inhibits its expression. In the plants, PME31 expression is reduced by ABI5 in ABA-mediated seed germination. Taken together, PME31 is transcriptionally inhibited by ABI5 and negatively regulates ABA-mediated seed germination inhibition. These findings shed new light on the mechanisms of PMEs in response to ABA-mediated seed germination.

ABA INSENSITIVE 2 promotes flowering by inhibiting OST1/ABI5-dependent FLOWERING LOCUS C transcription in Arabidopsis.

The plant hormone abscisic acid (ABA) is an important regulator of plant growth and development and plays a crucial role in both biotic and abiotic stress responses. ABA modulates flowering time, but the precise molecular mechanism remains poorly understood. Here we report that ABA INSENSITIVE 2 (ABI2) is the only phosphatase from the ABA-signaling core that positively regulates the transition to flowering in Arabidopsis. Loss-of-function abi2-2 mutant shows significantly delayed flowering both under long day and short day conditions. Expression of floral repressor genes such as FLOWERING LOCUS C (FLC) and CYCLING DOF FACTOR 1 (CDF1) was significantly up-regulated in abi2-2 plants while expression of the flowering promoting genes FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) was down-regulated. Through genetic interactions we further found that ost1-3 and abi5-1 mutations are epistatic to abi2-2, as both of them individually rescued the late flowering phenotype of abi2-2. Interestingly, phosphorylation and protein stability of ABA INSENSITIVE 5 (ABI5) were enhanced in abi2-2 plants suggesting that ABI2 dephosphorylates ABI5, thereby reducing protein stability and the capacity to induce FLC expression. Our findings uncovered the unexpected role of ABI2 in promoting flowering by inhibiting ABI5-mediated FLC expression in Arabidopsis.

Unlocking the plant ER stress code: IRE1-proteasome signaling cohort takes the lead.

In the intricate landscape of cellular function, proper protein folding is pivotal for cellular processes, particularly within the endoplasmic reticulum (ER). In a recent study, Ko et al. reveal a signaling role for inositol-requiring enzyme 1 (IRE1) in ER stress and identify PHOSPHATASE TYPE 2CA (PP2CA)-INTERACTING RING FINGER PROTEIN 1 (PIR1) as a crucial plant-specific regulator, balancing the unfolded protein response (UPR) and ubiquitin-proteasome system (UPS) by modulating ABI5 stability, unveiling intricate stress response connections.