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vascular cells under non-stressed conditions, because under non-stressed condi-
tions both mesophyll and vascular cells contain much ABA. A reasonable explana-
tion may be that drought stress not only results in an increase in the ABA level in
mesophyll and vascular cells, but also increases the release of ABA from the mes-
ophyll and vascular cells. Therefore, the ABA in guard cells themselves may well
have a function in stomatal movement, and this possibility merits further investiga-
tion. There is strong evidence that guard cells contain a large amount of ABA in
guard cells. For example, in
Valerianella locusta
, the ABA content in guard cells
was estimated to be 600 fg per guard cell, corresponding to an even concentration
of 0.65 mM in the cells (Behl and Hartung
1986
), and in isolated protoplasts from
Vicia faba,
it was estimated to be 24-150 fg per guard cell protoplast (Lahr and
Raschke
1988
). More importantly, ABA accumulation in guard cells appears to
increase in response to drought stress. For example, Cornish and Zeevaart (
1986
)
found that the guard cells in
V. faba L
. plants contained 18 times more ABA when
isolated from stressed leaves than when isolated from turgid leaves. Given that
some ABA receptors were localized within cells, a great increase in the ABA con-
tent within the guard cells is likely to contribute to stomatal closure.
4.3.2 ABA Transport and Distribution in Relation
to Seed and Fruit Development
ABA plays important roles in seed development and germination. During seed
development, ABA levels changed to different degrees in different tissues. In
Arabidopsis, ABA levels in whole siliques slowly increased during late develop-
ment, whereas they concomitantly sharply decreased in the seeds and seed enve-
lopes (i.e., the pedicles, receptacles, valves, replums, septa, and funiculi) during
the middle to final stages of development (Kanno et al.
2010
), implying that
ABA may be transported among different tissues. Given that the seed coat origi-
nates from the mother plant, whereas the endosperm and embryo originate from
zygotes, ABA is expected to be synthesized only in the zygotic tissues of the
seeds obtained by crossing an ABA-deficient female and a wild-type male. By
quantifying the ABA in the F1 and F2 populations derived from crosses between
the wild type and an ABA-deficient mutant aba2-2, Kanno et al. (
2010
) demon-
strated that in the absence of zygotic ABA, ABA synthesized in maternal tissues
could be translocated into the embryos to induce seed dormancy. Grafting analy-
ses using wild-type tobacco (
Nicotiana plumbaginifolia
) and its corresponding
ABA-deficient mutants revealed that ABA synthesized in vegetative tissues could
be transported to the seeds to promote seed development and growth (Frey et al.
2004
). Generating crosses or grafts between the wild type and an ABA-deficient
mutant is a good approach for investigating ABA transport between tissues.
However, the observed ABA translocation from wild-type tissues to those of the
ABA-deficient tissues may not reflect the cases in non-crossed or non-grafted
plants, because it has been proposed that ABA may be transported within seed
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