Improvement of Stress Tolerance in Crops by Genetic Manipulation of ABA Metabolism, Signaling, and Regulation - Abscisic Acid: Metabolism, Transport and Signaling - page 447

higher levels of ABA accumulation in leaves and sap, but no negative effects on
seed dormancy. However, these plants exhibited a set of severe abnormal symp-
toms resulting from the consequences of long-term and high levels of ABA
accumulation (Tung et al. 2008 ). Therefore, the moderate and timely increase in
ABA biosynthesis may be critical for genetic improvement of stress resistance in
crops. In addition, stress-inducible promoters may be useful for engineering of
ABA-related genes. For example, transgenic rice over-expressing the trehalose-
6-phosphate synthase/phosphatase gene by a drought-inducible promoter showed
increased amounts of trehalose and drought tolerance without obvious negative
effects on plant growth or grain yield (Garg et al. 2002 ). In the future, the applica-
tion of specific genes on stress resistance needs to be optimized by using stress-
inducible and/or tissue-specific promoters.
Even though many ABA-related genes have proven to be effective in enhancing
stress resistance by transgenic approaches, the effects of these genes were mostly
shown based on greenhouse experiments with plants grown in small pots or only
at the seedling stages. Due to the complex nature of stress in the field, those genes
proven to be effective in the greenhouse have to be further evaluated in the field
before being adopted in breeding programs.
The last challenge is how to tackle the different abiotic stresses that simultane-
ously occur under natural field conditions. In the future, it is essential to examine
the effects of ABA-related genes for stress resistance under multiple stress com-
bination conditions. Meanwhile, the nature of different crops in stress adaptation
may further complicate the stress resistance effect for the same gene(s). Therefore,
intensive knowledge on ABA biology obtained from model plants needs to be inte-
grated into specific crops according to their inherent nature of stress response and
adaptation to specific stresses.
Acknowledgments The work from the authors' laboratory cited in this review was supported
by grants from the National Program of China for Transgenic Research (2011ZX08009-003-002,
2011ZX08001-003), the National Program for Basic Research of China (2012CB114305), and
the National Program on High Technology Development (2012AA10A303).
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