ABA signaling is involved in the regulation of BSK1 stability mediated by the UBP24-PUB25/26 module in Arabidopsis

<h2>Summary</h2> Abscisic acid (ABA) and brassinosteroid (BR) signaling coordinately regulate plant growth and stress adaptation. Here, we identify the BR pathway component BR signaling kinase (BSK1) to negatively regulate ABA signaling in Arabidopsis. The stability of BSK1 is dynamically controlled by E3 ubiquitin ligases plant U-box 25 (PUB25) and PUB26, which ubiquitinate BSK1 to promote its degradation via the 26S proteasome. This process is likely to be regulated by BR-insensitive 1-associated receptor kinase 1 (BAK1) phosphorylation. Conversely, the deubiquitination enzyme ubiquitin-specific protease 24 (UBP24) stabilizes BSK1 by removing ubiquitin chains. ABA-induced accumulation of PUB25/26 displaces UBP24 from BSK1, permitting BSK1 degradation while redirecting UBP24 to stabilize PUB25/26. This functional pair of E3 ligases (PUB25/26) and deubiquitinase (UBP24), centered on BSK1, plays a vital role in ABA signaling. Our study reveals an evolutionarily conserved paradigm in which the competition between E3 ligases and deubiquitinases spatially orchestrates hormone signaling plasticity.