Plant-specific casein kinases phosphorylate and stabilize SMXL6/7/8 to suppress strigolactone signaling and promote shoot branching.

Strigolactones (SLs) significantly impact agricultural production because of their central role in regulating plant morphology. As core switch controllers of SL signaling, the transcriptional repressors SUPPRESSOR OF MAX2 1-LIKE (SMXL6/7/8) are ubiquitinated by F-box E3 ligase MORE AXILLARY GROWTH 2 (MAX2) for the 26S proteasome-mediated degradation through receptor D14. However, the post-translational modification and underlying regulatory mechanisms of SMXL6/7/8 proteins remain unknown. Here, we demonstrates that Arabidopsis seedlings deficient in or overexpressing a category of evolutionarily conserved, plant-specific protein kinase Arabidopsis EL1-like (AEL1-4) exhibit significantly reduced or increased branching, respectively. The results reveal that AEL interact with and directly phosphorylate SMXL6/7/8 proteins, inhibiting their interaction with MAX2 and thus suppressing their ubiquitination and degradation, thereby negatively interfering with SL-regulated branching. Notably, SL signaling downregulated the expression of AEL genes dependent on SMXL6/7/8 action and diminished AEL-SMXL protein interactions. This study reveals the importance of protein phosphorylation in regulating SL signaling and effects, highlighting the fine-tuning mechanism of SL signaling by the transition of SMXL6/7/8 between active and inactive forms through phosphorylation.