A multifaceted kinase axis regulates plant organ abscission through conserved signaling mechanisms.

Plants have evolved mechanisms to abscise organs as they develop or when exposed to unfavorable conditions. 1 Uncontrolled abscission of petals, fruits, or leaves can impair agricultural productivity. 2,3,4,5 Despite its importance for abscission progression, our understanding of the IDA signaling pathway and its regulation remains incomplete. IDA is secreted to the apoplast, where it is perceived by the receptors HAESA (HAE) and HAESA-LIKE2 (HSL2) and somatic embryogenesis receptor kinase (SERK) co-receptors. 6,7,8,9 These plasma membrane receptors activate an intracellular cascade of mitogen-activated protein kinases (MAPKs) by an unknown mechanism. 10,11,12 Here, we characterize brassinosteroid signaling kinases (BSKs) as regulators of floral organ abscission in Arabidopsis. BSK1 localizes to the plasma membrane of abscission zone cells, where it interacts with HAESA receptors to regulate abscission. Furthermore, we demonstrate that YODA (YDA) has a leading role among other MAPKKKs in controlling abscission downstream of the HAESA/BSK complex. This kinase axis, comprising a leucine-rich repeat receptor kinase, a BSK, and an MAPKKK, is known to regulate stomatal patterning, early embryo development, and immunity. 10,13,14,15,16 How specific cellular responses are obtained despite signaling through common effectors is not well understood. We show that the identified abscission-promoting allele of BSK1 also enhances receptor signaling in other BSK-mediated pathways, suggesting conservation of signaling mechanisms. Furthermore, we provide genetic evidence supporting independence of BSK1 function from its kinase activity in several developmental processes. Together, our findings suggest that BSK1 facilitates signaling between plasma membrane receptor kinases and MAPKKKs via conserved mechanisms across multiple facets of plant development. [Display omitted] • A semidominant allele of BSK1 rescues floral abscission defects in ida mutants • BSK1 and BSK2 regulate floral abscission in abscission zone cells • BSK1 interacts with HAE and controls abscission independently of kinase activity • Multiple RLK-mediated pathways are activated by a hypermorphic allele of BSK1 Galindo-Trigo et al. show that BSK1 and BSK2 regulate floral organ abscission in Arabidopsis. BSK1 localizes to the plasma membrane and interacts with HAE/HSL2. A hypermorphic allele of BSK1 activates receptor kinase pathways that drive abscission and stomata development, among other processes, suggesting conservation of signaling mechanisms. [ABSTRACT FROM AUTHOR]