The RCC 1 family protein SAB 1 negatively regulates ABI 5 through multidimensional mechanisms during postgermination in Arabidopsis

Summary Abscisic acid‐insensitive 5 ( ABI 5) is an essential and conserved plant basic leucine zipper transcription factor whose level controls seed germination and postgerminative development. It has been demonstrated that activity of ABI 5 is transcriptionally and post‐translationally regulated. However, transcriptional regulation of ABI 5 is not fully understood. Here, we identified SAB 1 ( Sensitive to ABA 1 ) as a novel negative regulator of ABI 5 that simultaneously regulates its stability, promoter binding activity and histone methylation‐mediated gene silencing of ABI 5 . SAB 1 encodes a Regulator of Chromatin Condensation 1 ( RCC 1) family protein and is expressed in an opposite pattern to that of ABI 5 during early seedling growth in response to abscisic acid ( ABA ). SAB 1 mutation results in enhanced ABA sensitivity and acts upstream of ABI 5 . SAB 1 physically interacts with ABI 5 at phosphoamino acid Ser‐145, and reduces the phosphorylation of ABI 5 and the protein stability. SAB 1 reduces ABI 5 binding activity to its own promoter, leading to reduced transcriptional level of ABI 5 . SAB 1 inactivates ABI 5 transcription by increasing the level of histone H3K27me2 in the ABI 5 promoter. Our findings have identified SAB 1 as a crucial new component of ABA signaling which modulates early development of plant by precisely controlling ABI 5 activity through multiple mechanisms.