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

Arabidopsis ABSCISIC ACID INSENSITIVE5 (ABI5) is a crucial regulator of abscisic acid (ABA) signaling pathways involved in repressing seed germination and postgerminative growth. ABI5 is precisely modulated at the posttranslational level; however, the transcriptional regulation mechanisms underlying ABI5 and its interacting transcription factors remain largely unknown. Here, INDUCER OF CBF EXPRESSION 1 (ICE1) was found to physically associate with ABI5. ICE1 negatively regulates ABA responses during seed germination and directly suppresses ABA-responsive LATE EMBRYOGENESIS ABUNDANT6 (EM6) and EM1. Genetic analysis demonstrated that the ABA-hypersensitive phenotype of the ice1 mutant requires ABI5. Mechanistic investigation revealed that ICE1 interferes with the transcriptional activity of ABI5 to mediate downstream regulons. Significantly, ICE1 also interacts with DELLA proteins, which stimulate ABI5 in ABA signaling. Disruption of ICE1 partially restores the ABA-hyposensitive phenotype of the gai-t6 rga-t2 rgl1-1 rgl2-1 mutant (della), showing that ICE1 functions antagonistically with DELLA in ABA signaling. Consistently, DELLA proteins repress ICE19s transcriptional function and the antagonistic effect of ICE1 on ABI5. Collectively, our study demonstrates that ICE1 antagonizes ABI5 and DELLA to maintain appropriate ABA signaling during seed germination and provides a mechanistic understanding of how ABA signaling is fine-tuned by the transcriptional complex involving ABI5 and its interacting partners.