Phospholipase Dα1-derived phosphatidic acid interacts with ABI1 phosphatase 2C and regulates abscisic acid signaling

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 PLDalpha1 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 PLDalpha1 and PLDalpha1-antisense plants lose more water than do wild-type plants. The stomatal closure of PLDalpha1-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 PLDalpha1-null cells. PLDalpha1-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 PLDalpha1-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 PLDalpha1 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.