Li et al. report that Arabidopsis BSK1 negatively regulates ABA signaling. The stability of BSK1 is dynamically controlled by PUB25 and PUB26, a process likely regulated by BAK1 phosphorylation, whereas UBP24 stabilizes BSK1 by removing ubiquitin chains. ABA shifts this balance toward degradation by inducing the accumulation of PUB25/26.