Improvement of Stress Tolerance in Crops by Genetic Manipulation of ABA Metabolism, Signaling, and Regulation - Abscisic Acid: Metabolism, Transport and Signaling - page 439

regulation of ABA signaling, and downstream functional proteins. A comprehensive
review on this information will be useful to the communities for the future efforts in
developing stress-resistant crops by exploring the knowledge of ABA biology.
22.2 Enhancing Stress Resistance by Manipulating ABA
Metabolism
The endogenous ABA level is determined by ABA biosynthesis, catabolism, and
the release of ABA from ABA-glucose conjugates (Nambara and Marion-Poll
2005 ; Lee et al. 2006 ). Therefore, the identification of all of the components affect-
ing ABA metabolism is essential for a complete understanding of the action of the
hormone, which in turn helps in manipulating ABA levels for stress resistance.
The mutants that were unable to produce carotenoid precursors for endogenous
ABA synthesis exhibited pre-harvest seed sprouting and/or wilted leaves in maize,
rice, and tomato, and such mutants often showed drought-sensitive phenotypes
(Schwartz et al. 1997 ; Burbidge et al. 1999 ; Fang et al. 2008 ). Some genes control-
ling the early steps of ABA biosynthesis have been used in genetic engineering for
improving plant drought tolerance. For example, constitutive over-expression of a
bacterial ʲ -carotene hydroxygenase (BCH) gene crtZ in tobacco led to increased
zeaxanthin (an ABA precursor) synthesis and enhanced UV tolerance (Gotz et al.
2002 ), providing an association between the xanthophyll cycle and stress tolerance
in plants. Similarly, over-expression of a rice DSM2 gene which encodes BCH and
which converts ʲ -carotene to zeaxanthin resulted in significantly improved drought
resistance of the transgenic rice at both the seedling and reproductive stages (Du
et al. 2010 ). NCED (9-cis-epoxycarotenoid dioxygenase) is a drought-inducible
gene encoding a rate-limiting enzyme for ABA biosynthesis. Over-expression of
NCED could increase endogenous ABA levels, trigger stomatal closure, and lead
to improved drought tolerance in transgenic tomato (Thompson et al. 2000 ) and
tobacco (Qin and Zeevaart 2002 ). The over-expression of the NCED gene by a con-
stitutive promoter resulted in increased ABA accumulation and water-use efficiency
(WUE), but with apparently little impact on long-term biomass production. Positive
effects on drought stress under field conditions were also observed in transgenic
rice over-expressing AtNCED2 (Xiao et al. 2009 ). LOS5/ABA3 encodes a molyb-
denum cofactor (MoCo) sulfurase which functions in the final step of ABA biosyn-
thesis in Arabidopsis. When LOS5/ABA3 was over-expressed in rice, transgenic rice
showed improved spikelet fertility and grain yield under drought stress conditions
in the field (Xiao et al. 2009 ). LOS5 also had a positive effect on improving drought
tolerance when it was over-expressed in soybean, cotton, and maize, and the phe-
notype was associated with ABA accumulation and the expression of stress-related
genes via ABA signaling, which sequentially caused a set of stress-related physi-
ological and biochemical responses (Yue et al. 2012 ; Li et al. 2013 ; Lu et al. 2013 ).
Genetic association mapping identified allelic variation in ZmAO3 which controls