ABA Regulation of Plant Responses to Drought and Salt Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 313

as severely impaired expression of ABA-responsive genes (Gonzalez-Guzman
et al. 2012 ). With the discovery of the ABA receptor, quinabactin, a sulfona-
mide ABA agonist, was found to form a hydrogen bond with the PYL2/PP2C
complex, which elicited ABA responses in vegetative tissues, such as promot-
ing stomata closure, reducing water loss, and enhancing drought tolerance in
Arabidopsis and soybean (Okamoto et al. 2013 ). Cao et al. ( 2013 ) recently iden-
tified a small-molecule ABA mimic (AM1) that activates multiple members of
ABA receptors in Arabidopsis, according to crystal structural evidence and the
layer of gene expression, to protect plants from drought stress and decreased
water loss. These reports provide new technologies for improving stress toler-
ance of plants mediated through enhanced ABA signaling by receptors or ABA
agonists/mimics.
Many of the genes encoding enzymes involved in ABA biosynthesis have been
identified, such as 9-cis-epoxycarotenoid dioxygenase (NCED), abscisic aldehyde
oxidase (AAO3), MoCo sulfurase (AtABA3), and zeaxanthin epoxidase (AtZEP),
which all show increased expression during plant dehydration (Iuchi et al. 2001 ;
Nambara and Marion-Poll 2005 ; Seo et al. 2000 ; Vogel et al. 2005 ; Xiong et al.
2001 ). Among them, NCED is the key enzyme responsible for ABA biosynthe-
sis; the expression of the NCED gene in either Arabidopsis or tomato increased
the level of ABA (Iuchi et al. 2001 ; Thompson et al. 2000b ). Furthermore, NCED
mRNA accumulates mainly in vascular tissues, consistent with ABA biosyn-
thetic sites. NCED is reportedly induced by drought stress in several plant spe-
cies, including Arabidopsis , tomato, maize, bean, and cow pea (Iuchi et al.
2000 ; Qin and Zeevaart 1999 ; Tan et al. 1997 ; Thompson et al. 2000a , b ). In
Arabidopsis , AtNCED3 is induced by drought and high salinity. The overexpres-
sion of AtNCED3 in transgenic plants enhanced dehydration stress tolerance,
whereas knockouts of AtNCED3 resulted in a dehydration-sensitive phenotype
(Iuchi et al. 2001 ). Transgenic tobacco plants overexpressing SgNCED1 originat-
ing from Stylosanthes guianensis exhibit increased accumulation of ABA in leaves
as well as enhanced stress resistance to drought and high salinity. The overexpres-
sion of LeNCED1 in tomato plants also resulted in ABA accumulation (Thompson
et al. 2007 ). Under drought stress, the induction of VuNCED1 in cowpeas pre-
cedes ABA accumulation. This suggests that the transcriptional regulation of the
VuNCED1 gene is responsible for the drought-induced ABA accumulation (Iuchi
et al. 2000 ). The regulation of endogenous ABA levels depends not only on the
rate of ABA synthesis but also on the rate of its catabolism. The CYP707A3 gene
encoding an ABA 8'-hydroxylase in Arabidopsis is highly induced by ABA and
by rehydration. It is the major enzyme responsible for catalyzing ABA degrada-
tion (Saito et al. 2004 ). The cyp707a3 mutant plants show reduced transpiration
rates, enhanced drought tolerance, higher ABA accumulation, and hypersensitiv-
ity to exogenous ABA compared with the wild type. By contrast, plant constitu-
tive expression of the CYP707A3 gene exhibited an opposite phenotype (Umezawa
et al. 2006 ). In light of the finding that NCED overexpression in plants also
hyperaccumulates phaseic acid, it is likely that ABA regulates the accumulation