Divergent strigolactones or karrikins signaling is mediated by preferred AtD14/KAI2-MAX2-SMXL complex targeting...

Strigolactones (SLs) and karrikins (KARs) are structurally related signaling molecules that regulate different aspects of plant development. In Arabidopsis, both SL and KAR signaling require the F box protein MAX2 but are perceived by different receptors, AtD14 and KAI2, respectively. These signals target distinct SMAX1-LIKE (SMXL) repressors, the SMXL7 and SMAX1 clade, respectively, for degradation. However, the mechanism underlying selective SMXL degradation remains elusive. Here, we propose that selective SMXL degradation arises from RGKT motif-dependent ternary protein complexes involving MAX2 and the respective receptors. Domain-swapping analysis between SMAX1 and SMXL7 indicates that the D1M (D1+M) and D2 domains contribute to hormone responsiveness in Arabidopsis through targeted degradation. Biochemical and structural analyses highlight the D2 domain and its RGKT motif as central to ternary complex formation. Our findings provide insights into how a single E3 ubiquitin ligase recruits distinct substrate proteins via different signaling receptors to achieve specific signal transduction.