ABA Regulation of Plant Responses to Drought and Salt Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 312

(Hu et al. 2006 , 2008 ). In addition, the yield of transgenic rice plants expressing
SNAC1 was not penalized under favorable growing conditions (Hu et al. 2006 ),
and under severe drought stress conditions, these plants still showed stronger
drought tolerance compared with control plants. OsNAC6 , another NAC transcrip-
tion factor, is induced not only by drought, high salinity, and ABA but also by
blast disease, jasmonic acid, and wounding. Therefore, OsNAC6 overexpression
in transgenic rice confers enhanced stress tolerance to drought and high salinity
as well as a strong tolerance to blast disease. The NAC transcription factor gene,
ONAC010, regulated senescence and improved grain protein and ion content in
wheat (Uauy et al. 2006 ). These results indicate that compared with Arabidopsis ,
NAC transcription factors in crops not only function in response to drought and
salt stresses but are also involved in functions such as seed quality and disease
resistance.
In conclusion, in the regulatory circuits responding to drought and salt stresses,
many common mechanisms are shared by Arabidopsis and crops. However, it is
essential to attend to their differences when effectively improving a correspond-
ing trait in agriculturally important crops using gene transfer technology from
Arabidopsis . The transcription factors, as central regulators of gene expression,
will likely become preferential targets for genetic engineering.
16.4 Enhanced ABA Perception, Biosynthesis,
and Transportation Improve Plant Dehydration
Stress Tolerance
The discovery of the novel ABA-soluble receptor pyrabactin resistance1
(PYR)/PYR1-like (PYL)/regulatory components of ABA receptors (RCAR) by
two independent research groups is known as the crucial breakthrough in recent
ABA signaling studies. ABA binds these receptors and inhibits the activity of
protein phosphatases 2C (PP2Cs), including ABI1, ABI2, and HAB1, abro-
gating the inhibition of subclass III sucrose non-fermenting-1 (SNF1)-related
protein kinase 2 (SnRK2s), switching on the expression of downstream stress-
responsive genes. PYR/PYL/RCAR together with ABA, PP2Cs, and SnRK2s
constitute the core ABA signaling components that play essential roles in plants
for responding to cellular dehydration. The overexpression of PYR/PYL/PCAR
proteins, such as PYL5/RCAR3, PYL8/RCAR3, and RCAR1/PYL9, in trans-
genic Arabidopsis enhanced the response to ABA and afforded higher resist-
ance to drought by inhibiting the activity of downstream PP2Cs (Ma et al. 2009 ;
Park et al. 2009 ; Saavedra et al. 2010 ; Santiago et al. 2009 ). By contrast, the
pyr1 pyl1 pyl2 pyl4 quadruple mutant exhibited less sensitivity to ABA-induced
stomatal closure and ABA-inhibited stomatal opening (Nishimura et al. 2010 ).
Similarly, the pyr1 pyl1 pyl2 pyl4 pyl5 pyl8 sextuple mutant was insensitive to
ABA and showed increased stomatal aperture and leaf transpiration, as well