Phosphatidic acid (PA) functions as a lipid signaling molecule in plants. Physiological analysis showed that PA triggers early signal transduction events that lead to responses to abscisic acid (ABA) during seed germination. We measured PA production during seed germination and found increased PA levels during early germination. To investigate the role of PA during seed germination, we focused on the PA catabolic enzyme lipid phosphate phosphatase (LPP). LPP catalyzes the conversion of PA to diacylglycerol (DAG). There are 4 LPP genes in the Arabidopsis genome. Among them, AtLPP2 and AtLPP3 are expressed during seed germination. Two AtLPP2 T-DNA insertional mutants (lpp2-1 and lpp2-2) showed hypersensitivity to ABA and significant PA accumulation during germination. Furthermore, double-mutant analysis showed that ABA-insensitive 4 (ABI4) is epistatic to AtLPP2 but ABA-insensitive 3 (ABI3) is not. These results suggest that PA is involved in ABA signaling and that AtLPP2 functions as a negative regulator upstream of ABI4, which encodes an AP2-type transcription factor, in ABA signaling during germination.