Circadian clock factors regulate seed oil accumulation by promoting fatty acid synthesis in Arabidopsis

The circadian clock regulates temporal metabolic activities, enabling organisms to adapt to cyclic environmental changes, but how it affects lipid metabolism in plants is poorly understood. Our previous finding showed that the central clock transcription factors LATE ELONGATED HYPOCOTYL (LHY) and CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) increased seed oil contents in Arabidopsis. Here we investigated the molecular and metabolic mechanism underlying the LHY and CCA1 regulated oil accumulation. Triacylglycerol (TAG) accumulation in Arabidopsis developing seeds was increased in LHY-overexpressing (LHY-OE) and decreased in lhycca1 plants compared to wild-type (WT). Metabolic tracking of lipids in developing seeds indicated that fatty acids (FAs) of major lipid precursors for TAG production increased more rapidly in LHY-OE and slowly in lhycca1 than in WT, suggesting that LHY enhanced FA synthesis. Transcript analysis revealed that the expression of genes involved in FA synthesis, including the one encoding β-ketoacyl-ACP synthase III (KASIII), was oppositely changed in developing seeds of LHY/CCA1-OEs and those of lhycca1. Chromatin immunoprecipitation, electrophoretic mobility shift, and transactivation assays indicated that LHY directly bound and activated the promoter of KASIII. Furthermore, phosphatidic acid, a metabolic precursor to TAG, inhibited LHY binding to KASIII promoter elements. Our data reveal a new regulatory mechanism by the core clock regulators for storage lipid production during plant seed development.