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

Strigolactones (SLs) significantly impact agricultural production due to their central role in regulating plant morphology. As switch controllers of SL signaling, the transcriptional repressors, suppressor of MAX2 1-like 6/7/8 (SMXL6/7/8), are ubiquitinated by the F-box E3 ligase, more axillary growth 2 (MAX2) for degradation through the 26S proteasome, which is mediated by the receptor DWARF14. However, post-translational modifications and regulatory mechanisms of SMXL6/7/8 proteins remain unknown. Here, we demonstrate that Arabidopsis seedlings deficient in or overexpressing an evolutionarily conserved, plant-specific protein kinase Arabidopsis EL1-like (AEL1-4) exhibit significantly reduced or enhanced branching, respectively. Biochemical assays reveal that AEL interacts with and phosphorylates SMXL6/7/8 proteins, inhibiting their interactions with MAX2 and suppressing their degradation, thereby negatively interfering with SL-regulated branching. Notably, SL signaling reduces the expression of AEL genes dependent on action of SMXL6/7/8 and diminishes AEL-SMXL protein interactions. In summary, this study reveals the importance of protein phosphorylation in regulating SL signaling and its effects, highlighting a fine-tuning mechanism of SL signaling through phosphorylation-mediated transition between active and inactive forms of SMXL6/7/8.