Fig. 8.

Site-specific phosphorylation of TTG1 by SK11/SK12 regulates carbon flow in Arabidopsis seeds. During seed development, TT2 interacts with unphosphorylated TTG1 and facilitates recruitment of TTG1 to the GL2 locus, thus upregulating GL2 expression. This promotes the formation of seed mucilage and flavonoid pigments. In contrast, two GSK3-like kinases, SK11 and SK12, interact with TTG1 and specifically phosphorylate TTG1 at Ser 215. This abolishes TTG1 interaction with TT2, which compromises TTG1 binding to GL2, thus downregulating GL2 expression to promote biosynthesis of fatty acids in seeds. Therefore, site-specific phosphorylation of TTG1 by SK11/SK12 specifically modulates the direction of carbon flow to either formation of seed mucilage and flavonoid pigments or synthesis of fatty acids via regulating GL2 expression levels in Arabidopsis seeds