ABA Transport and Distribution in Relation to Its Function in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 70

ABA compartmentalization and redistribution. Nevertheless, the existence of other
mechanisms involved in ABA transport and localization in relation to the synergis-
tic action of ABA transporters and pH needs to be investigated.
4.6 Concluding Remarks
As for any signaling molecule, the sites of ABA metabolism must be separated
from its sites of perception, and the localization and transport of ABA are impor-
tant factors in ABA signaling. ABA is a weak acid that is ubiquitous in plant tis-
sues. Both the ion-trap theory of pH and experimental evidence suggest that ABA
is compartmentalized in plant cells, rather than being evenly distributed. On one
hand, this compartmentalization ensures that ABA does not interfere with other
physiological actions in plant tissues or cells containing a high level of ABA
under normal conditions. On the other hand, the redistribution of ABA follow-
ing changes in pH in cellular compartments under specific conditions ensures that
ABA may possibly function even if its content remains unchanged. Besides com-
partmentalization and redistribution, intercellular or long-distance transport is an
important basis for ABA signaling in plants. Particularly, the expressional pattern
of genes encoding the key enzymes in ABA biosynthesis was found to be highly
similar to that of ABA transporters, and moreover, loss or gain of the ABA trans-
porters was found to alter the response sensitivity to ABA, strongly demonstrating
the importance of ABA transport in ABA functioning. The root-to-shoot signaling
mediated by the long-distance transport of ABA is a good example of the criti-
cal roles of ABA transport in ABA functioning. Although the recent identification
of some ABA transporters has revealed the molecular basis of ABA transport, lit-
tle is known about the mechanisms that control ABA transport and localization
in relation to ABA accumulation at action sites. A major reason for this is the
technical difficulty involved in localizing ABA. The commonly adopted immu-
nolocalization method is not actually an ideal indicator of ABA localization and
distribution, because it depends on the conjugation of ABA to cellular proteins.
Immunolocalization in combination with ultracryotomy may be a good technical
system for this purpose; however, this technique needs to be further developed.
The development of new technologies to observe ABA localization as well as
thorough investigations of ABA transporters is required to fully understand ABA
transport and localization in relation to ABA functioning.
References
Astle MC, Rubery PH. A study of abscisic acid uptake by apical and proximal root segments of
Phaseolus coccineus L. Planta. 1980;150:312-20.
Astle MC, Rubery PH. Carriers for abscisic acid and indole-3-acetic acid in primary roots: their
regional localization and thermodynamic driving forces. Planta. 1983;157:53-63.