Improvement of Stress Tolerance in Crops by Genetic Manipulation of ABA Metabolism, Signaling, and Regulation - Abscisic Acid: Metabolism, Transport and Signaling - page 440

the biosynthesis of ABA, and the variation was associated significantly with the
endogenous ABA levels in the silks of water-stressed plants (Setter et al. 2011 ).
Fleury et al. ( 2010 ) found that the “drought tolerant” wheat genotype RAC875 pro-
duced fewer tillers, showed moderate osmotic adjustment, increased ABA content,
and reduced stomatal conductance, providing a link between drought resistance and
allelic variation of ABA biosynthesis in natural crop varieties.
ABA biosynthesis can also be affected by changes in the levels of other
endogenous phytohormones. For example, TLD1 is a rice GH3 - 13 gene which
encodes indole-3-acetic acid (IAA)-amido synthetase, and it is dramatically
induced by drought stress. A gain-of-function mutant, tld1 - D , showed increased
ABA biosynthesis and expression of late embryogenesis abundant (LEA) pro-
tein genes and enhanced drought tolerance (Zhang et al. 2009 ). However, over-
expressing OsGH3 - 2 , which also encodes an IAA-amido synthetase, led to
significantly reduced ABA levels but increased resistance to cold stress (Du et al.
2012 ).
22.3 Improving Stress Tolerance by Manipulating ABA
Signaling
A core ABA signaling model has been proposed which includes the binding of
ABA to the receptors PYR/PYL/RCAR which inhibit the type 2C protein phos-
phatases (PP2C) of group A, resulting in the activation of SNF1-related type 2
protein kinases (SnRK2s), which subsequently activate various ion channels and
ABA-dependent gene expression by phosphorylating bZIP transcription fac-
tors (Ma et al. 2009b ; Raghavendra et al. 2010 ). The group A PP2C are negative
regulators of ABA signaling. The elimination of group A PP2C is sufficient to
ensure moss survival to full desiccation, implying that group A PP2Cs emerged
in land plants to confer desiccation tolerance, possibly facilitating the propaga-
tion of plants on land (Komatsu et al. 2013 ). The maize ZmSAPK8/ZmOST1
sequence is highly homologous to that of the AtOST1 and is required for drought
and salt tolerance responses, and this kinase can phosphorylate ZmSNAC1
together with ZmOST1 to function as a positive regulator of water deficit sign-
aling in guard cells (Vilela et al. 2013 ). However, whether manipulation of
ZmOST1 can enhance drought tolerance in maize remains to be investigated.
The SnRK2 subfamily consists of ten members that were designated as SAPK1
to SAPK10 (osmotic stress/ABA-activated protein kinases) in rice. All of the
SAPKs were shown to be activated by hyperosmotic stress (Kobayashi et al.
2004 ). The SAPK8, SAPK9, and SAPK10 kinases were also activated by ABA
and were able to phosphorylate the rice bZIP transcription factor TRAB1, sug-
gesting that the regulation of the bZIP transcription factors by SnRK2 kinases is
conserved among plant species (Kobayashi et al. 2005 ). In vitro phosphorylation
assays demonstrated that the rice OsbZIP46 protein could be phosphorylated by