Antagonistic MADS-box Transcription Factors SEEDSTICK and SEPALLATA3 Form a Transcriptional Regulatory Network That Regulates Oil Accumulation

Transcriptional regulation is essential for balancing multiple metabolic pathways that influence oil accumulation in seeds. Thus far, the transcriptional regulatory mechanisms that govern seed oil accumulation remain largely unknown. Here, we identified the transcriptional regulatory network composed of MADS-box transcription factors SEEDSTICK (STK) and SEPALLATA3 (SEP3), which bridges several key genes for fatty acid (FA) biosynthesis and degradation to regulate oil accumulation in seeds. We found that STK, highly expressed in the developing embryo, positively regulates seed oil accumulation in Arabidopsis (Arabidopsis thaliana). Furthermore, we discovered that SEP3 physically interacts with STK in vivo and in vitro. Seed oil content is increased by the SEP3 mutation, while it is decreased by SEP3 overexpression. The chromatin immunoprecipitation, electrophoretic mobility shift assay, and transient dual-luciferase reporter assays showed that STK positively regulated seed oil accumulation by directly repressing the expression of MYB5, SEP3, and SEED FATTY ACID REDUCER 4 (SFAR4). Moreover, genetic and molecular analyses demonstrated that STK and SEP3 antagonistically regulated seed oil production and that SEP3 weakened the binding ability of STK to MYB5, SEP3, and SFAR4. Additionally, we demonstrated that TT8 and AAD3 were direct targets of MYB5 during seed oil accumulation in Arabidopsis. Together, our findings provide the transcriptional regulatory network antagonistically orchestrated by STK and SEP3, which fine-tunes oil accumulation in seeds.