Improvement of Stress Tolerance in Crops by Genetic Manipulation of ABA Metabolism, Signaling, and Regulation - Abscisic Acid: Metabolism, Transport and Signaling - page 435

factors are known as abiotic stresses, and they affect plant growth and devel-
opment and in particular the productivity of field crops worldwide. Thus, the
production of increased quantities of food with limited land and adverse envi-
ronmental conditions are immense challenges to the sustainable development
of agriculture (Mittler 2006 ; Borlaug 2007 ). These goals might be attained by
conventional plant breeding approaches, and in fact, using the traditional meth-
ods of crossing and selecting progeny, breeders have produced new varieties with
improved stress resistance (Witcombe et al. 2008 ). Meanwhile, increasing evi-
dence suggests that modern plant biotechnologies have much greater potential
to make substantial stress resistance improvements in major crops (Zurbriggen
et al. 2010 ).
Abscisic acid (ABA) is recognized as a stress hormone that coordinates the
complex networks of stress responses. Under drought or salt stress conditions,
the endogenous plant ABA level can rise to about 40-fold, triggering the clo-
sure of stomata, and the accumulation of dehydrins and late embryogenesis
abundant proteins for osmotic adjustment (Verslues et al. 2006 ). Cold or heat
stress also increases endogenous ABA levels in plants but to a much lesser
extent. Many ABA-deficient rice mutants showed enhanced water loss from
stomata under drought stress conditions and displayed drought-sensitive pheno-
types (Du et al. 2013 ). Due to the physiological importance of ABA in drought
responses, the genetic basis of drought-induced ABA accumulation was investi-
gated in rice (Quarrie et al. 1997 ) and maize (Tuberosa et al. 1998 ; Landi et al.
2005 ).
ABA signaling also plays critical roles in stress response pathways by pro-
ducing various yet specific outputs such as stomatal closure, changes in the root
system, and an increase in the osmotic adjustment ability. Recently, one of the
breakthroughs in ABA biology was the discovery of a core ABA signaling cascade
consisting of the ABA receptor PYR/PYL/RCAR, negative regulator PP2C protein
phosphatases, and positive regulator SnRK2 protein kinases which can activate
transcription factors to regulate the expression of downstream genes (Described
in the previous chapters). Numerous genes involved in ABA signaling and regu-
lation have been tested by transgenic approaches for their effects in influencing
stress tolerance through proper regulation of ABA responsiveness, water loss, and
stress-related gene expression (Hu and Xiong 2014 ; Nakashima and Yamaguchi-
Shinozaki 2013 ), some of which may be promising for improving crop perfor-
mance under stress conditions.
As a “stress hormone,” ABA has been well studied from production to signal-
ing pathways, especially as a key regulator under drought stress conditions. The
above chapters on ABA focused on metabolism, transportation, signaling and regu-
lation, and responses to particular stresses, mainly in the model plant Arabidopsis.
This chapter focuses on the recent studies of ABA-related genes that have been
well characterized in physiological, molecular, or biochemical processes involved
in abiotic stress tolerance. For ease of reference, the ABA-related genes that have
been tested in crops for improving stress resistance are listed in Table 22.1 , and
these genes are described in different categories: ABA metabolism, ABA signaling,