In many higher plants, seed oil accumulation is precisely controlled by intricate multilevel regulatory networks, among which transcriptional regulation mainly influences oil biosynthesis. In Arabidopsis (Arabidopsis thaliana), the master positive transcription factors, WRINKLED1 (WRI1) and LEAFY COTYLEDON1-LIKE (L1L), are important for seed oil accumulation. We found that an R2R3-MYB transcription factor, MYB89, was expressed predominantly in developing seeds during maturation. Oil and major fatty acid biosynthesis in seeds was significantly promoted by myb89-1 mutation and MYB89 knockdown; thus, MYB89 was an important repressor during seed oil accumulation. RNA sequencing revealed remarkable up-regulation of numerous genes involved in seed oil accumulation in myb89 seeds at 12 d after pollination. Posttranslational activation of a MYB89-glucocorticoid receptor fusion protein and chromatin immunoprecipitation assays demonstrated that MYB89 inhibited seed oil accumulation by directly repressing WRI1 and five key genes and by indirectly suppressing L1L and 11 key genes involved in oil biosynthesis during seed maturation. These results help us to understand the novel function of MYB89 and provide new insights into the regulatory network of transcriptional factors controlling seed oil accumulation in Arabidopsis.
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