ABA Regulation of Plant Responses to Drought and Salt Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 317

1998 ; Lebaudy et al. 2010 ; Marten et al. 1999 ; Pilot et al. 2001 ; Xicluna et al.
2007 ). ABA up-regulated the expression of AKT2 and decreased the expression
of SKOR under drought and high salinity stresses (Deeken et al. 2002 ; Gaymard
et al. 1998 ; Lacombe et al. 2000 ; Marten et al. 1999 ). Cherel et al. ( 2002 ) pro-
vided evidence that AtPP2CA, a protein phosphatase involved in ABA signaling,
interacts with AKT2 and is considered a partner of AKT2. It was reported that
SRK2E/OST1/SnRK2.6, one of the subclass ʲ SnRK2 family members, global
regulators of ABA signaling, inhibits the activity of KAT1, an inward K + channel,
through phosphorylation of the threonine site at the C-terminal domain of KAT1.
This in turn enhances drought tolerance (Ichida et al. 1997 ; Sato et al. 2009 ;
Szyroki et al. 2001 ). GORK encodes a K + -release channel that is expressed mainly
in guard cells, roots, and vascular tissue and is up-regulated by ABA, drought, and
high salinity. It is responsible for the K + efflux from guard cells, a main mecha-
nism to reduce cell turgor and induce stomata closure. PP2Cs, core ABA-signaling
components, regulate the expression of GORK (Becker et al. 2003 ). The multiple
gene mutants of the K + uptake transporter gene, kup268 and kup68 gork, exhibit
decreased drought stress tolerance. Furthermore, SRK2E phosphorylates KUP6,
indicating that ion transport system directly regulated by ABA signaling plays
important roles in response to drought stress (Osakabe et al. 2013 a).
In conclusion, drought and high salinity stresses may lead to an imbalance in
ion homeostasis. Maintenance of ion homeostasis is vital for the adaptation and
survival of plants under stressful conditions. Ion transporters or channels are
mainly responsible for ion transport and flow to maintain ion homeostasis and
to avoid ion toxicity. ABA may participate by the interaction of ABA-signaling
components with ion transporters/channels. According to relatively recent reports,
ABA is primarily involved in the modulation of ion transporters/channels by
phosphorylation/dephosphorylation modifications in the cytosolic domains of the
ion transporters/channels through protein kinase SnRK2s, the ABA “core sign-
aling components” (Hubbard et al. 2010 ; Osakabe et al. 2013b ; Umezawa et al.
2010 ).
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