ABA Regulation of Plant Responses to Drought and Salt Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 316

NHX6, are also involved in cellular Na + /K + homeostasis as well as pH bal-
ance. The nhx5nhx6 double mutant exhibited hypersensitivity to high salinity
stress (Bassil et al. 2011 ). The homolog of SOS1 in the salinity-tolerant species
Chrysanthemum crassum (CcSOS1) was identified and found to share approxi-
mately 62 % amino acid sequence identity with the SOS1 in Arabidopsis . The
CcSOS1 transcript abundance is increased by drought stress. However, its tran-
script level is down-regulated in the leaves of C. crassum in the presence of ABA,
which indicates that ABA may participate in the expression of CcSOS1 (Song
et al. 2012 ). In addition to SOS3, the protein phosphatase 2C, ABI2, has been
recognized as another SOS2-interacting protein based on a yeast two-hybrid
screening. Their interaction depended on the conserved protein phosphatase inter-
action (PPI) motif that presented in SOS2 family proteins with 37 amino acid
residues. The abi2 - 1 mutant showed increased salt tolerance, decreased ABA sen-
sitivity, and a disrupted interaction between SOS2 and ABI2 (Ohta et al. 2003 ).
At least, 23 SOS2-like protein kinase (PKSs) and 8 SOS3-like binding proteins
(SCaBPs) exist in the Arabidopsis genome (Guo et al. 2001 ). Guo et al. reported
that pks3 and scabp5 mutants displayed an ABA hyperinsensitive phenotype as
well as an up-regulated expression of drought stress-responsive genes, such as
RD29A . Furthermore, the ScaBP5-PKS3 complex specifically responds to ABA-
induced Ca 2 + signaling. This complex together with ABI2 constitutes part of a
negative regulatory loop that controls ABA sensitivity (Guo et al. 2002 ). Another
SOS2-like protein kinase gene PKS18 was identified in Arabidopsis . Transgenic
plants expressing constitutively active forms of PKS18 (PKS18T/D) were
hypersensitive to ABA, and the PKS18T/D knockdown plant showed an ABA-
insensitive phenotype (Gong et al. 2002 ).
AtHKT1, a Na + transporter localized at the plasma membrane and expressed
especially in vascular tissues, is putatively responsible for the Na + efflux from
xylem vessels under high salinity (Kato et al. 2001 ; Rus et al. 2001 ). The hkt1 - 1
mutant was sensitive to salt stress because of the excessive accumulation of Na +
accumulated in the shoots (Berthomieu et al. 2003 ). Hence, AtHKT1 is involved
in Na + translocation in roots and shoots and in Na + accumulation in leaves (Horie
et al. 2007 ; Moller et al. 2009 ; Sunarpi et al. 2005 ). HKT transporter genes have
also been identified in crop plants, such as wheat and the wheat-relative Triticum
monococcum and have similar functions as those in Arabidopsis (Munns et al.
2012 ). Other types of HKT family members, such as TaHKT2;1 from Triticum
aestivum , TsHKT1;2 from Thellungiella salsuginea , and EcHKT1;2 from
Eucalyptus camaldulensis , also participate in K + transport to maintain K + /Na +
balance under salt stress (Ali et al. 2012 ; Ardie et al. 2009 ; Laurie et al. 2002 ; Liu
et al. 2001 ; Schachtman and Schroeder 1994 ). Shkolnik-Inbar et al. reported that
ABI4 reduced the expression of AtHKT1 to further affect salt tolerance. This indi-
cates that ABA-signaling components are involved in the regulation and expres-
sion of ion transporters (Shkolnik-Inbar et al. 2013 ).
Additionally, K + channels such as AKT2 and KAT2, both encoding K + inward
rectifying channels, and SKOR, a K + outward rectifying channel, are all expressed
in vascular tissue and play important roles in K + homeostasis (Gaymard et al.