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, along with
phospholipase Dα1 knockout, pldα1, in plant
response to ABA. Compared to wild-type
(WT) plants, the SPHK mutants and pldα1 all
displayed decreased sensitivity to
ABA-promoted stomatal closure. Phyto-S1P
promoted stomatal closure in sphk1-1 and
sphk2-1, but not in pldα1, whereas PA
promoted stomatal closure in sphk1-1,
sphk2-1, and pldα1. The ABA activation of
PLDα1 in leaves and protoplasts was
attenuated in the SPHK mutants, and the
ABA activation of SPHK was reduced in
pldα1. In response to ABA, the accumulation
of long-chain base phosphate (LCBP) was
decreased in pldα1 whereas PA production
was decreased in SPHK mutants, compared
WT. Collectively, these results indicate that
SPHK and PLDα1 act together in ABA
response and that SPHK and phyto-S1P act
upstream of PLDα1 and PA in mediating the
ABA response. Whereas SPHKs are involved in the activation of PLDα1, the
activity of PLDα1 further enhances SPHK
activation via PA binding SPHKs. The data
suggest that SPHK/phyto-S1P and PLDa1/PA
are co-dependent in amplification of response
to ABA, mediating stomatal closure in
Arabidopsis.
