ABA Transmembrane Transport and Transporters - Abscisic Acid: Metabolism, Transport and Signaling - page 54

translocated to the ER to be inactivated. ABA may also be inactivated by gluco-
sylation; glucosylated ABA (ABA-glucosyl ester; ABA-GE) is hydrolyzed to form
bioactive free ABA (Nambara and Marion-Poll 2005 ). The ʲ -glucosidase encoded
by BG1 in Arabidopsis hydrolyzes ABA-GE (Lee et al. 2006 ). Since BG1 is local-
ized in the ER, ABA generated by BG1 in the ER has to be translocated to the
cytosol to induce physiological responses. Also ABA or ABA-GE has to be trans-
located to the ER before or after glucosylation. Several glucosyltransferases in the
UGT family that have activities to synthesize ABA-GE have been identified (Priest
et al. 2006 ; Xu et al. 2002 ); however, the physiological functions of these glyco-
syltransferases are still not well established. ABA-GE has been detected in the
apoplast, suggesting that ABA-GE moves between cells (Wilkinson and Davies
2002 ; Hartung et al. 2002 ).
Figure 3.1 illustrates a model for the possible transport or movement of
ABA. Note that this is a simplified model in which ABA transport into ABA-
synthesizing cells and ABA biosynthesis in the receiving cells are not taken into
account. It is speculated that the transmembrane transport of ABA, ABA precur-
sors, and ABA metabolites regulate physiological responses in a highly complex
manner. As described above, three ABA transporters have been identified; how-
ever, relatively mild phenotypes of the mutants defective in the possible ABA
transporters AtABCG25, AtABCG40 and AtNPF4.6 compared to typical ABA-
deficient mutants such as Arabidospsi aba1 , aba2, and aba3 , indicate that the
transport mechanisms are highly redundant. Therefore, further identification of
ABA transporters will be required for a total understanding of ABA transport.
Also, to identify how these transporters regulate ABA transport within plants,
development of techniques to visualize local concentrations of ABA will be
required.
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