Connections between Sphingosine Kinase and Phospholipase D in the Abscisic Acid Signaling Pathway in Arabidopsis

Phosphatidic acid (PA) and phytosphingosine 1-phosphate (phyto-S1P) both are lipid messengers involved in plant response to abscisic acid (ABA). Our previous data indicate that PA binds to sphingosine kinase (SPHK) and increases its phyto-S1P-producing activity. To understand the cellular and physiological functions of the PA-SPHK interaction, we isolated Arabidopsis thaliana SPHK mutants sphk1-1 and sphk2-1 and characterized them, together with phospholipase D alpha 1 knockout, pld alpha 1, in plant response to ABA. Compared with wild-type (WT) plants, the SPHK mutants and pld alpha 1 all displayed decreased sensitivity to ABA-promoted stomatal closure. Phyto-S1P promoted stomatal closure in sphk1-1 and sphk2-1, but not in pld alpha 1, whereas PA promoted stomatal closure in sphk1-1, sphk2-1, and pld alpha 1. The ABA activation of PLD alpha 1 in leaves and protoplasts was attenuated in the SPHK mutants, and the ABA activation of SPHK was reduced in pld alpha 1. In response to ABA, the accumulation of long-chain base phosphates was decreased in pld alpha 1, whereas PA production was decreased in SPHK mutants, compared with WT. Collectively, these results indicate that SPHK and PLD alpha 1 act together in ABA response and that SPHK and phyto-S1P act upstream of PLD alpha 1 and PA in mediating the ABA response. PA is involved in the activation of SPHK, and activation of PLD alpha 1 requires SPHK activity. The data suggest that SPHK/phyto-S1P and PLD alpha 1A are co-dependent in amplification of response to ABA, mediating stomatal closure in Arabidopsis.