AHG1-AFP interaction as a regulatory node in the ABA response during seed germination.

Seed dormancy and germination are tightly regulated by complex signaling networks that integrate internal and external cues, including the endogenous phytohormone abscisic acid (ABA). ABA HYPERSENSITIVE GERMINATION 1 (AHG1), a group A type 2C protein phosphatase (PP2C), is thought to modulate the activity of transcription factors such as ABA INSENSITIVE 5 (ABI5) in seeds and during germination. AHG1 is regulated by DELAY OF GERMINATION 1 (DOG1), a key regulator of seed dormancy, through physical interaction. We previously reported that AHG1 also interacts with ABI FIVE BINDING PROTEIN 2 (AFP2), a member of the AFP family; however, the molecular basis of AHG1-AFP coordination has remained unclear. In this study, we show that AHG1 interacts with all AFP family members and that AFP3 binds AHG1 and ABI5 through adjacent but distinct amino acid residues within its C-domain, allowing simultaneous association with both proteins. In addition, AHG1 modulates the phosphorylation status of AFP3 at Ser60 in a DOG1-dependent manner, suggesting that DOG1-AHG1 regulates AFP3 post-translationally. Transcriptomic analyses of AHG1- or AFP3-overexpressing lines revealed that these factors are associated with the regulation of a shared set of ABA-responsive genes, including AFPs, and that AFP3 overexpression is predominantly associated with altered expression of genes involved in transcriptional regulation. Large-scale protein interaction analyses showed that AFPs interact with multiple classes of transcription factors, suggesting their involvement in diverse regulatory pathways, including ABA signaling. Together, these findings demonstrate that DOG1 regulates ABI5 function and modulates ABA responses, at least in part, by controlling AHG1-mediated dephosphorylation of AFPs.