The Transcription Factor INDUCER OF CBF EXPRESSION1 Interacts with ABSCISIC ACID INSENSITIVE5 and DELLA Proteins to Fine-Tune Abscisic Acid Signaling during Seed Germination in Arabidopsis.

ABSCISIC ACID INSENSITIVE5 (ABI5) is a crucial regulator of abscisic acid (ABA) signaling pathways involved in repressing seed germination and postgerminative growth in Arabidopsis (Arabidopsis thaliana). ABI5 is precisely modulated at the posttranslational level; however, the transcriptional regulatory mechanisms underlying ABI5 and its interacting transcription factors remain largely unknown. Here, we found that INDUCER OF CBF EXPRESSION1 (ICE1) physically associates with ABI5. ICE1 negatively regulates ABA responses during seed germination and directly suppresses ABA-responsive LATE EMBRYOGENESIS ABUNDANT6 (EM6) and EM1 expression. Genetic analysis demonstrated that the ABA-hypersensitive phenotype of the ice1 mutant requires ABI5. ICE1 interferes with the transcriptional activity of ABI5 to mediate downstream regulons. Importantly, ICE1 also interacts with DELLA proteins, which stimulate ABI5 during ABA signaling. Disruption of ICE1 partially restored the ABA-hyposensitive phenotype of the della mutant, gai-t6 rga-t2 rgl1-1 rgl2-1, indicating that ICE1 functions antagonistically with DELLA in ABA signaling. Consistently, DELLA proteins repress ICE1's transcriptional function and the antagonistic effect of ICE1 on ABI5. Collectively, our study demonstrates that ICE1 antagonizes ABI5 and DELLA activity to maintain the appropriate level of ABA signaling during seed germination, providing a mechanistic understanding of how ABA signaling is fine-tuned by a transcriptional complex involving ABI5 and its interacting partners.

Hu Y ，Han X ，Yang M ，Zhang M ，Pan J ，Yu D ... -  《-》

BRASSINOSTEROID INSENSITIVE2 interacts with ABSCISIC ACID INSENSITIVE5 to mediate the antagonism of brassinosteroids to abscisic acid during seed germination in Arabidopsis.

Seed germination and postgerminative growth are regulated by a delicate hormonal balance. Abscisic acid (ABA) represses Arabidopsis thaliana seed germination and postgerminative growth, while brassinosteroids (BRs) antagonize ABA-mediated inhibition and promote these processes. However, the molecular mechanism underlying BR-repressed ABA signaling remains largely unknown. Here, we show that the Glycogen Synthase Kinase 3-like kinase BRASSINOSTEROID INSENSITIVE2 (BIN2), a critical repressor of BR signaling, positively regulates ABA responses during seed germination and postgerminative growth. Mechanistic investigation revealed that BIN2 physically interacts with ABSCISIC ACID INSENSITIVE5 (ABI5), a bZIP transcription factor. Further genetic analysis demonstrated that the ABA-hypersensitive phenotype of BIN2-overexpressing plants requires ABI5. BIN2 was found to phosphorylate and stabilize ABI5 in the presence of ABA, while application of epibrassinolide (the active form of BRs) inhibited the regulation of ABI5 by BIN2. Consistently, the ABA-induced accumulation of ABI5 was affected in BIN2-related mutants. Moreover, mutations of the BIN2 phosphorylation sites on ABI5 made the mutant protein respond to ABA improperly. Additionally, the expression of several ABI5 regulons was positively modulated by BIN2. These results provide evidence that BIN2 phosphorylates and stabilizes ABI5 to mediate ABA response during seed germination, while BRs repress the BIN2-ABI5 cascade to antagonize ABA-mediated inhibition.

Hu Y ，Yu D 《-》

The gibberellic acid signaling repressor RGL2 inhibits Arabidopsis seed germination by stimulating abscisic acid synthesis and ABI5 activity.

Piskurewicz U ，Jikumaru Y ，Kinoshita N ，Nambara E ，Kamiya Y ，Lopez-Molina L ... -  《-》

SOS2-LIKE PROTEIN KINASE5, an SNF1-RELATED PROTEIN KINASE3-Type Protein Kinase, Is Important for Abscisic Acid Responses in Arabidopsis through Phosphorylation of ABSCISIC ACID-INSENSITIVE5.

Abscisic acid (ABA) plays an essential role in seed germination. In this study, we demonstrate that one SNF1-related protein kinase3-type protein kinase, SOS2-like protein kinase5 (PKS5), is involved in ABA signal transduction via the phosphorylation of an interacting protein, abscisic acid-insensitive5 (ABI5). We found that pks5-3 and pks5-4, two previously identified PKS5 superactive kinase mutants with point mutations in the PKS5 FISL/NAF (a conserved peptide that is necessary for interaction with SOS3 or SOS3-like calcium binding proteins) motif and the kinase domain, respectively, are hypersensitive to ABA during seed germination. PKS5 was found to interact with ABI5 in yeast (Saccharomyces cerevisiae), and this interaction was further confirmed in planta using bimolecular fluorescence complementation. Genetic studies revealed that ABI5 is epistatic to PKS5. PKS5 phosphorylates a serine (Ser) residue at position 42 in ABI5 and regulates ABA-responsive gene expression. This phosphorylation was induced by ABA in vivo and transactivated ABI5. Expression of ABI5, in which Ser-42 was mutated to alanine, could not fully rescue the ABA-insensitive phenotypes of the abi5-8 and pks5-4abi5-8 mutants. In contrast, mutating Ser-42 to aspartate rescued the ABA insensitivity of these mutants. These data demonstrate that PKS5-mediated phosphorylation of ABI5 at Ser-42 is critical for the ABA regulation of seed germination and gene expression in Arabidopsis (Arabidopsis thaliana).

Zhou X ，Hao H ，Zhang Y ，Bai Y ，Zhu W ，Qin Y ，Yuan F ，Zhao F ，Wang M ，Hu J ，Xu H ，Guo A ，Zhao H ，Zhao Y ，Cao C ，Yang Y ，Schumaker KS ，Guo Y ，Xie CG ... -  《-》

Arabidopsis VQ18 and VQ26 proteins interact with ABI5 transcription factor to negatively modulate ABA response during seed germination.

Seed germination and early seedling establishment, critical developmental stages in the life cycle of seed plants, are modulated by diverse endogenous hormones and the surrounding environment. Arabidopsis ABSCISIC ACID-INSENSITIVE5 (ABI5) is a central transcription factor of abscisic acid (ABA) signaling that represses those processes. ABI5 is precisely modulated at post-translational level; however, whether it interacts with other crucial transcriptional regulators remains to be investigated. In this study, VQ18 and VQ26, two members of the recently-identified VQ family, were found to interact with ABI5 in vitro and in vivo. Phenotypic analysis showed that VQ18 and VQ26 are responsive to ABA and negatively mediate ABA signaling redundantly during seed germination. Simultaneously decreasing VQ18 and VQ26 expression levels enhanced ABA signaling to suppress seed germination, whereas overexpressing these two VQ genes resulted in the germinated seeds being less ABA-sensitive. Consistently, the expression levels of several ABI5 targets were modulated by VQ18 and VQ26. The increased ABA signaling of plants in which VQ18 and VQ26 were simultaneously suppressed required ABI5. Additionally, VQ18 and VQ26 acted as negative interactors of the ABI5 transcription factor. Our study reveals a previously unidentified regulatory role of VQ proteins, which act antagonistically with ABI5 to maintain the appropriate ABA signaling level to fine-tune seed germination and early seedling establishment.

Pan J ，Wang H ，Hu Y ，Yu D ... -  《-》