bZIP67 Regulates the Omega-3 Fatty Acid Content of Arabidopsis Seed Oil by Activating FATTY ACID DESATURASE3

Arabidopsis thaliana seed maturation is accompanied by the deposition of storage oil, rich in the essential ω-3 polyunsaturated fatty acid α-linolenic acid (ALA). The synthesis of ALA is highly responsive to the level of FATTY ACID DESATURASE3 ( FAD3 ) expression, which is strongly upregulated during embryogenesis. By screening mutants in LEAFY COTYLEDON1 (LEC1)–inducible transcription factors using fatty acid profiling, we identified two mutants ( lec1-like and bzip67 ) with a seed lipid phenotype. Both mutants share a substantial reduction in seed ALA content. Using a combination of in vivo and in vitro assays, we show that bZIP67 binds G-boxes in the FAD3 promoter and enhances FAD3 expression but that activation is conditional on bZIP67 association with LEC1-LIKE (L1L) and NUCLEAR FACTOR-YC2 (NF-YC2). Although FUSCA3 and ABSCISIC ACID INSENSITIVE3 are required for L1L and bZIP67 expression, neither protein is necessary for [bZIP67:L1L:NF-YC2] to activate FAD3 . We conclude that a transcriptional complex containing L1L, NF-YC2, and bZIP67 is induced by LEC1 during embryogenesis and specifies high levels of ALA production for storage oil by activating FAD3 expression.