Cooperative Function of PLDδ and PLDα1 in Abscisic Acid-Induced Stomatal Closure in Arabidopsis

Phospholipase D (PLD) is involved in responses to abiotic stress and abscisic acid (ABA) signaling. To investigate the roles of two Arabidopsis (Arabidopsis thaliana) PLDs, PLD alpha 1 and PLD delta, in ABA signaling in guard cells, we analyzed ABA responses in guard cells using Arabidopsis wild type, pld alpha 1 and pld delta single mutants, and a pld alpha 1 pld delta double mutant. ABA-induced stomatal closure was suppressed in the pld alpha 1 pld delta double mutant but not in the pld single mutants. The pld alpha 1 and pld delta mutations reduced ABA-induced phosphatidic acid production in epidermal tissues. Expression of either PLD alpha 1 or PLD delta complemented the double mutant stomatal phenotype. ABA-induced stomatal closure in both pld alpha 1 and pld delta single mutants was inhibited by a PLD inhibitor (1-butanol), suggesting that both PLD alpha 1 and PLD delta function in ABA-induced stomatal closure. During ABA-induced stomatal closure, wild-type guard cells accumulate reactive oxygen species and nitric oxide and undergo cytosolic alkalization, but these changes are reduced in guard cells of the pld alpha 1 pld delta double mutant. Inward-rectifying K+ channel currents of guard cells were inhibited by ABA in the wild type but not in the pld alpha 1 pld delta double mutant. ABA inhibited stomatal opening in the wild type and the pld delta mutant but not in the pld alpha 1 mutant. In wild-type rosette leaves, ABA significantly increased PLD delta transcript levels but did not change PLD alpha 1 transcript levels. Furthermore, the pld alpha 1 and pld delta mutations mitigated ABA inhibition of seed germination. These results suggest that PLD alpha 1 and PLD delta cooperate in ABA signaling in guard cells but that their functions do not completely overlap.