Phospholipase D (PLD) and protein phosphatase 2C (PP2C) both play a role in mediating plant responses to abscisic acid (ABA). In this article, we show that PLD alpha 1 and its product, phosphatidic acid (PA), regulate a PP2C, ABI1, which is a negative regulator of ABA responses in Arabidopsis. Leaves from a T-DNA insertional mutant of PLD alpha 1 and PLD alpha 1-antisense plants lose more water than do wild-type plants. The stomatal closure of PLD alpha 1-null leaves is insensitive to ABA but is promoted by PA. ABA treatment promotes an increase in PA from phosphatidylcholine in wild type but not in PLD alpha 1-null cells. PLD alpha 1-derived PA binds to ABI1; the PA-ABI1 binding is demonstrated by coprecipitating PA with ABI1 from plant cells, measuring binding of PA from vesicles to ABI1, and assaying ABI1 bound to PA immobilized on a filter. Deletion and site-specific mutational analyses show that arginine 73 in ABI1 is essential for PA-ABI1 binding. PA binding decreases the phosphatase activity of ABI1. The lack of ABA-induced production of PA in PLD alpha 1-null cells results in a decrease in the association of ABI1 with the plasma membrane in response to ABA. These results indicate that PA produced by PLD alpha 1 inhibits the function of the negative regulator ABI1, thus promoting ABA signaling. The identification of ABI1 as a direct target of the lipid messenger PA provides a functional link between the two families of important signaling enzymes, PLD and PP2C.