Novel mechanisms of strigolactone-induced DWARF14 degradation in Arabidopsis thaliana

In angiosperms, the strigolactone receptor is the alpha/beta hydrolase DWARF14 (D14) that, upon strigolactone binding, undergoes conformational changes, triggers strigolactone-dependent responses, and hydrolyses strigolactones. Strigolactone signalling involves the formation of a complex between strigolactone-bound D14, the E3-ubiquitin ligase SCFMAX2, and the transcriptional corepressors SMXL6/7/8, which become ubiquitinated and degraded by the proteasome. Strigolactone also destabilizes the D14 receptor. The current model proposes that D14 degradation occurs after ubiquitination of the SMXLs via SCFMAX2 and proteasomal degradation. Using fluorescence and luminescence assays on transgenic lines expressing D14 fused to GREEN FLUORESCENT PROTEIN or LUCIFERASE, we showed that strigolactone-induced D14 degradation may also occur independently of SCFMAX2 and/or SMXL6/7/8 through a proteasome-independent mechanism. Furthermore, strigolactone hydrolysis was not essential for triggering either D14 or SMXL7 degradation. The activity of mutant D14 proteins predicted to be non-functional for strigolactone signalling was also examined, and their capability to bind strigolactones in vitro was studied using differential scanning fluorimetry. Finally, we found that under certain conditions, the efficiency of D14 degradation was not aligned with that of SMXL7 degradation. These findings indicate a more complex regulatory mechanism governing D14 degradation than previously anticipated and provide novel insights into the dynamics of strigolactone signalling in Arabidopsis.