<scp>BSK</scp> family kinases are essential for brassinosteroid signaling and suppression of adventitious rooting by repressing the expression of <i>LBD16</i>

While BRASSINOSTEROID (BR) SIGNALING KINASEs (BSKs) are known to interact with the BR receptor BR-INSENSITIVE 1 (BRI1), the extent of their genetic redundancy and essential contribution to BR signaling remain unclear. This study aimed to systematically investigate the function of BSK family members and uncover novel developmental outputs controlled by BR signaling via BSKs mediated pathway in Arabidopsis. We generated an undecuple bsk knockout/knockdown mutant (bsk-u) in Arabidopsis. Comprehensive phenotypic observation, molecular analyses, and genetic complementation were performed to characterize the mutant. Subsequently, the role of the LATERAL ORGAN BOUNDARIES DOMAIN16 (LBD16) in mediating BR-suppressed adventitious root (AR) development was further examined using genetic and physiological approaches. The bsk-u mutant exhibited severe BR-insensitive phenotypes, including dwarfism, impaired BZR1 dephosphorylation, and attenuated transcriptional responses to BR. We identified LBD16 as a key BZR1 target repressed by BR signaling. BR application inhibited AR formation in wild-type explants, whereas BR-deficient mutants produced more ARs. Overexpression of LBD16 partially alleviated BR-mediated suppression of AR development. This study establishes that BSKs are indispensable for the BR signaling pathway in Arabidopsis. Furthermore, it reveals a novel role for BR in negatively regulating AR development through transcriptional repression of LBD16, expanding our understanding of BR-mediated developmental control.