Agriculture Reference
In-Depth Information
Sauter
1999
). ABA application can increase hydraulic conductance, thus promot-
ing water movement from roots to leaves (Hose et al.
2000
). The early induction
of
ZmPIP2
-
4
may also be mediated by ABA, implying that ABA may transiently
induce the water conductivity of roots and the water permeability of cortex cells in
maize (Zhu et al.
2005
). The expression of the water channel protein gene
RWC3
was increased in upland rice at the early stages of treatment with 20 % polyethylene
glycol (PEG) 6000, whereas no significant expression change was detected in low-
land rice, indicating that
RWC3
probably plays a role in drought avoidance in rice
(Lian et al.
2004
). Treatment with ABA also enhanced the expression of several PIP
genes in upland rice (Lian et al.
2006
), and transgenic Arabidopsis over-expressing
OsPIP2
-
2
showed enhanced tolerance to salt and drought stresses (Guo et al.
2006
).
The functions of some protein-modification enzymes are also involved in
ABA responses. Squalene synthase (SQS) is a farnesyl-diphosphate farnesyl-
transferase which catalyses the first reaction of the branch of the isoprenoid meta-
bolic pathway committed specifically to sterol biosynthesis (Tansey and Shechter
2001
). It has been demonstrated that RNAi-mediated disruption of a rice SQS
gene resulted in reduced sensitivity to ABA and improved drought tolerance at
both the vegetative and reproductive stages (Manavalan et al.
2012
). Transgenic
canola over-expressing a farnesyltransferase gene
ERA1
driven by a drought and
ABA-inducible
rd29A
promoter showed increased ABA sensitivity, a significant
decrease in stomatal conductance, and increased drought tolerance at mid-flower-
ing under field conditions (Wang et al.
2005
).
22.7 Conclusions and Perspectives
It is apparent that ABA, as an important signal molecule, can mediate signal trans-
duction involved in the responses to multiple abiotic stresses. The core ABA sig-
nal perception components which involve PYR/PYL/RCAR, PPC2, and SnRK2
in Arabidopsis are especially important in understanding the molecular basis of a
regulatory network controlling ABA-dependent stress responses (Ma et al.
2009b
;
Nishimura et al.
2009
). A recent study showed that a large number of ABA bio-
synthesis and signaling components are conserved in plants (Hauser et al.
2011
).
These findings will help us to discover similar ABA-regulated pathways and net-
works in crops and to discover potential candidate genes for genetic improvement
of stress tolerance in crops. However, when compared to Arabidopsis, the amount
of data on ABA-regulated genes and their functions in crop species is scattered.
Further intensive studies on ABA biology in economically important crops need to
be carried out to gain better insights into the mechanisms of stress responses.
It should also be noted that there are negative side effects of ABA on normal
growth and yield potential. For example, over-expression of
NCED
by a consti-
tutive promoter in tomato had negative effects (increased seed dormancy), while
transgenic tomato over-expressing
LeNCED1
by a Rubisco rbcS3C promoter,
which is light-responsive and expressed in a circadian manner, showed much
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