Transcriptional Regulation of Lipid Catabolism During Seedling Establishment

Lipid catabolism in germinating seeds provides energy and substrates for initial seedling growth, but how this process is regulated is not well understood. Here, we show that an AT-hook motif-containing nuclear localized (AHL) protein regulates lipid mobilization and fatty acid β-oxidation during seed germination and seedling establishment. AHL4 was identified to directly interact with the lipid mediator phosphatidic acid (PA). Knockout (KO) of  AHL4  enhanced, but overexpression (OE) of  AHL4  attenuated, triacylglycerol (TAG) degradation and seedling growth. Normal seedling growth of the OE lines was restored by sucrose supplementation to the growth medium.  AHL4 -OE seedlings displayed decreased expression of genes involved in TAG hydrolysis and fatty acid oxidation, whereas the opposite was observed in  AHL4 -KOs. These genes contained AHL4-binding  cis  elements, and AHL4 was shown to bind to the promoter regions of genes encoding the TAG lipases  SDP1  and  DALL5  and acyl-thioesterase  KAT5.  These AHL4–DNA interactions were suppressed by PA species that bound to AHL4. These results indicate that AHL4 suppresses lipid catabolism by repressing the expression of specific genes involved in TAG hydrolysis and fatty acid oxidation, and that PA relieves AHL4-mediated suppression and promotes TAG degradation. Thus, AHL4 and PA together regulate lipid degradation during seed germination and seedling establishment.