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TRANSPARENT TESTA2(TT2)is a key regulator of proanthocyanidins(PAs)biosynthesis in the seed coat of Arabidopsis.We recently discovered that TT2 also has a function in the inhibition of fatty acid(FA)biosynthesis in seeds.However,the mechanism by which TT2 suppresses the accumulation of seed FA remains unclear.In a this study,we discovered that TT2 was expressed in embryos at an early developmental stage.TT2 directly bound to the regulatory region of FUSCA3(FUS3)and mediated the expression of numerous genes on the FA biosynthesis pathway,including BCCP2,CAC2,MOD1,and KASII,which encode the initial steps of FA chain formation; FAD2 and FAD3,which are responsible for FA desaturation; and FAE1,which catalyzes very long chain FA elongation.The loss of function of TT2 downregulated the expression of GLABRA2(GL2),but did not lead to a significant reduction in the mucilage attached to seed coats,which competes with FA for photosynthates.Although TT2 was expressed in both maternal seed coats and embryonic tissues,PAs were only found in wild-type seed coats,and not in embryonic tissues.The amount of PAs in the seed coat was negatively correlated with the amount of FA in the embryo.To the best of our knowledge,this study is the first to report on the function of TT2 in developing embryos.