Improvement of Stress Tolerance in Crops by Genetic Manipulation of ABA Metabolism, Signaling, and Regulation - Abscisic Acid: Metabolism, Transport and Signaling - page 445

and drought tolerance (Zhang et al. 2007 ). Ectopic expression of SDIR1 in rice
and tobacco plants also led to improved drought tolerance (Zhang et al. 2008b ).
Transgenic rice over-expressing an ortholog of SDIR1 and OsSDIR1 showed
strong drought tolerance compared to control plants (Gao et al. 2011b ). Drought-
induced expression of ZmRFP1 , a putative ortholog of SDIR1 in maize, was
impaired in the ABA-deficient mutant viviparous14 , indicating that ZmRFP1
responds to drought stress in an ABA-dependent way (Xia et al. 2012 ). OsDSG1,
which is most similar to Arabidopsis AIP2, has E3 ubiquitin ligase activity and can
target ABI3, and the osdsg1 mutant plants that exhibited significantly increased
expression of ABA signaling and responsive genes were more tolerant to salt and
drought stresses (Park et al. 2010 ). In addition, transgenic tobacco constitutively
over-expressing OsBIRF1 , which encodes a rice RING-H2 finger protein, exhib-
ited reduced ABA sensitivity and increased drought tolerance (Liu et al. 2008 ).
Small interfering RNAs (siRNAs) may also be involved in the regulation of
ABA-related stress responses. In rice, siR441 and siR446 are positive regulators
of ABA signaling and tolerance to abiotic stress, possibly by regulating MAIF1
expression (Yan et al. 2011a ). The expression of MAIF1 , encoding an F-box
domain protein mainly localized in the plasma membrane and nucleus, is induced
rapidly and strongly by ABA and abiotic stresses, and over-expression of MAIF1
reduced ABA sensitivity and abiotic stress tolerance in rice (Yan et al. 2011b ).
Mitogen-activated protein kinase (MAPK) cascades play an important role in
mediating ABA and stress responses in eukaryotic organisms. In rice, basal-level
OsMAPK5 can be activated by ABA very quickly before the accumulation of its
mRNA and protein, and OsMAPK5 -over-expression rice exhibited increased
OsMAPK5 kinase activity and increased tolerance to drought, salt, and cold
stresses (Xiong and Yang 2003 ).
22.6 Improving Stress Resistance by Manipulating Other
ABA-Related Functional Genes
The output of ABA signaling and/or its regulation includes expression changes
or the activation of numerous downstream functional proteins which translate
the original signal input into diverse physiological responses to the stresses (see
reviews by Nambara and Marion-Poll 2005 ; Boursiac et al. 2013 ; Fukao and
Xiong 2013 ; Hu and Xiong 2014 ). Here, we only introduce some of the ABA-
related downstream genes which were seldom reviewed previously.
ABA accumulation in the roots of stressed plants can stimulate root growth
and increase root hydraulic conductivity (Zhu et al. 2005 ; Lian et al. 2006 ). In this
process, aquaporins, or water channel proteins which are members of major intrin-
sic proteins (MIPs) and translocate water across cell membranes, have been dem-
onstrated for their roles in stress-induced physiological processes (Ishibashi et al.
2000 ; Javot and Maurel 2002 ). Plasma membrane intrinsic proteins (PIPs) have
also been investigated for their induction under water deficit conditions (Malz and