Developmental Rewiring of a Brassinosteroid Signaling Module Controls Anther Patterning and Tapetum Specification in Arabidopsis

Male reproductive development in plants relies on the precise patterning of anther locules and the specification of the tapetum layer. While the BRASSINAZOLE RESISTANT (BZR) family transcription factors are essential for both processes, they function independently of the brassinosteroid (BR) receptors, leaving their upstream signaling mechanism unknown. Here, through systematic genetic analysis, we show that a core BR-signaling module, comprising BR-SIGNALING KINASEs (BSKs) and BRASSINOSTEROID-INSENSITIVE 2 (BIN2), is repurposed downstream of the tapetum regulator the receptor kinase EXCESS MICROSPOROCYTES1 (EMS1) and functions upstream of BZRs. Within this reconfigured pathway, EMS1 recruits BSKs, which in turn inactivate BIN2, thereby releasing BZRs in a conserved hierarchy cascade mirroring BR signaling to drive tapetum specification. Additionally, this BSKs-BIN2-BZRs signaling module also functions in anther locule formation. Conversely, BSU1-LIKE2/3 (BSL2/3), homologs of the positive BR regulator BSU1, act as repressors in this context. This pathway transcriptionally activates the cyclins such as CYCB1;1 to initiate tapetal cell division, while BSL2/3 ensure its timely termination by promoting CYCB1;1 protein degradation. Our work defines the signaling framework governing anther development and establishes a paradigm in which conserved signaling components are rewired to achieve developmental specificity.