PHR1-like 7 and phosphatidic acid oppositely regulate TAG degradation and seed oil accumulation in Arabidopsis

Triacylglycerol (TAG) degradation plays an important role in seed oil accumulation, but how this catabolic process is transcriptionally regulated in developing seeds remains poorly understood. Here, we identified a myb-like HTH transcriptional regulator family protein, PHR1-like 7 (PHL7), that enhances TAG degradation and decreases seed oil accumulation in Arabidopsis thaliana. PHL7 knockout (KO) plants phl7 exhibited an approximately 10% increase in seed oil content, whereas its overexpression (OE) decreased oil content by 8-13%. Compared to those in wild-type (WT), PHL7-KO lines phl7 displayed a lower, but its OE a higher, expression of TAG degradation genes, including SUGAR-DEPENDENT 1 (SDP1). Chromatin immunoprecipitation, electrophoretic mobility shift assay, and transactivation analyses indicated that PHL7 bound to the SDP1 promoter and activated SDP1 expression. In addition, we found that PHL7 binds to phosphatidic acid (PA), a key intermediate in TAG biosynthesis, and PA suppresses the binding of PHL7 to the promoter of SDP1. The lysine residue at position 61 (K61) of PHL7 is involved in the PA binding and introducing the PHL7K61S mutation into phl7 failed to genetically complement the oil content phenotype of the mutant to that of wild-type seeds. These findings suggest that the transcription factor PHL7 promotes the expression of SDP1, whereas PA attenuates its action, and the lipid mediator-PHL7 interaction regulates TAG degradation and seed oil accumulation.