Agriculture Reference
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and/or long-distant transport of ABA between cells in the regulation of physiologi-
cal responses is largely unknown. ABA has long been considered as a mobile sig-
nal that is produced in roots upon soil drying and is transported to shoots (leaves)
to regulate water loss by reducing stomatal conductance (Davies and Zhang
1991
;
Schachtman and Goodger
2008
). In contrast, root-derived ABA is not always
required for stomatal closure because leaves can potentially synthesize ABA
and induce stomatal closure in response to water deficit (Ikegami et al.
2009
;
Holbrook et al.
2002
; Christmann et al.
2007
). Also, ABA moves from leaves to
leaves, leaves to roots, vegetative organs to seeds, and maternal tissues to zygotic
tissues (embryo and/or endosperm) in seeds (Everatbourbouloux
1982
; Zeevaart
and Boyer
1984
; Rodriguez-Gamir et al.
2011
; Kudoyarova et al.
2011
; Ernst
et al.
2010
; Goodger and Schachtman
2010
; Ikegami et al.
2009
; Frey et al.
2004
;
Karssen et al.
1983
; Kanno et al.
2010
).
To understand the physiological responses mediated through ABA transport, it
is necessary to define the site of ABA biosynthesis and the sites of ABA action.
Earlier studies deduced the site of ABA biosynthesis and subsequent movement
by measuring endogenous concentrations of ABA from different parts of a plant
as a function of time. For example, application of water stress to a limited part of
the root system without changing the leaf water status resulted in reduced stomatal
conductance, and this phenomenon was associated with an increase in xylem sap
ABA levels (Davies and Zhang
1991
). This finding suggests that the ABA syn-
thesized in roots moved to the leaves (guard cells) through the xylem; however,
measurements of ABA concentrations from plant tissues at a specific time point do
not simply reveal the sites of ABA biosynthesis and movement, especially if ABA
synthesis takes place ubiquitously within plants and if the direction of ABA move-
ment is nonpolar or multi-directional.
Another strategy to identify the site for ABA biosynthesis is to assume that
ABA is synthesized where ABA biosynthetic enzymes are present. Almost all
of the genes and enzymes involved in ABA biosynthesis have been identified
(Nambara and Marion-Poll
2005
; Seo and Koshiba
2011
), thereby facilitating our
investigation of the site of ABA biosynthesis. Immunohistochemical staining was
used to determine the localization of three ABA biosynthesis enzymes, AtNCED3,
AtABA2, and AAO3, in Arabidopsis by using antibodies raised against the respec-
tive proteins (Koiwai et al.
2004
; Endo et al.
2008
). AtABA2 and AAO3 were con-
sistently present in leaf vascular tissues, irrespective of water stress conditions. In
contrast, AtNCED3 was detected in the same region as AtABA2 and AAO3 only
after, but not before, the stress treatment. These observations indicate that ABA
biosynthesis is activated in vascular tissues in response to water stress. This, in
turn, indicates that ABA synthesized in vascular tissues has to be translocated to
the guard cells to close stomata in response to water stress; however, we cannot
exclude the possibility that ABA is synthesized not only in vascular tissues but
also other tissues. It is still possible that ABA biosynthetic enzymes existing at
undetectable levels could contribute to physiological responses to some extent.
Transient expression of two ABA biosynthesis genes,
AtNCED3
and
AAO
3, in
the guard cells of broad beans (
Vicia faba
) after particle bombardment induced
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