ABA Transport and Distribution in Relation to Its Function in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 63

the vascular parenchyma cells associated with sieve elements and xylem vessels,
and in the cytoplasm and nucleus of cells in the axillary bud tip and procambial
strands. Immunogold analysis of tomato showed that free ABA mainly localized in
the apoplast, cytoplasmic vesicles, and amyloplasts, around starch grains, of root
cap cells, and in the columella and meristematic cells, mainly at the junction area
with root cap cells and in the wall (Bertrand et al. 1992 ). Water stress induced
a significant increase in ABA labeling in the chloroplast, nucleus, and cell walls,
with the increase being greatest in the cell wall. Based on this observation, the
authors' proposed that the increase in apoplastic ABA in lavender exposed to water
stress was not attributable to ABA release from the chloroplast.
The information provided by immunolocalization studies of ABA at vari-
ous levels largely agrees with the data of ABA quantification in specific tissues
or cells, and the spatial localization of enzymes involved in ABA biosynthesis,
such as NCED, ABA2, and AAO3. All of these studies suggest that ABA is par-
ticularly concentrated in buds, root tips, vascular tissues, and stomata. However,
at the cellular and subcellular levels, there is some disagreement between the
data. As described above, both histochemical staining of promoter::GUS activity
and immunofluorescence localization demonstrated that AAO3, the last enzyme
in the ABA biosynthesis pathway, is mainly present in phloem companion cells
and xylem parenchyma cells of Arabidopsis (Koiwai et al. 2004 ). In contrast,
immunolocalization of ABA in the vascular tissues of Chenopodiu plants did not
reveal any ABA in the parenchyma cells or the cambial zone (Sotta et al. 1985 ).
This may be due to the different plant materials used; however, the ABA distri-
bution pattern is thought to be basically the same among common plant species.
Furthermore, it should be noted that immunolocalization analysis does not exactly
reflect the distribution of ABA in plants. Because ABA is a small molecule, ABA
will only be detected by immunolocalization when it is coupled to a cellular pro-
tein, which implies that the distribution of cellular proteins is an important factor
affecting ABA localization. Given that the protein content in companion cells is
not less than in other cells, the reduction or lack of ABA in the companion cells
of vascular tissues implies that the ABA synthesized in companion cells is trans-
ported through vascular strands to various target tissues and cells.
4.3 Intercellular Transport in Relation to ABA Function
4.3.1 ABA Transport and Distribution in Relation to
Stomatal Movement
ABA is a major regulator of stomatal movement and is generally thought to be
a critical mediator of water deficit-induced stomatal closure. Although the signal
transduction pathways underlying ABA-regulated stomatal movement have been
extensively studied, there is still much debate about the origin of the ABA sig-
nal. As described above, in leaves, ABA is mainly localized to chloroplasts and
vascular tissues; therefore, for ABA to mediate stomatal closure, ABA must be