ABA Conjugates and Their Physiological Roles in Plant Cells - Abscisic Acid: Metabolism, Transport and Signaling - page 82

5.6 Perspective and Concluding Remarks
The physiological role of ABA-GE has long been controversial. Now, it is clear
that ABA-GE, one type of ABA conjugate, is crucial in ABA-related physiol-
ogy by functioning as a stored form of ABA because ABA-GE can be rapidly
converted to active ABA through a one-step hydrolysis by AtBG1 and AtBG2.
However, there are many unanswered questions regarding the physiological mean-
ing of ABA. First of all, the identity of the ABA-GE transporter localized at the
ER and possibly plasma membrane remains elusive, not to mention their action
mechanisms. In addition, how much ABA-GE is stored in different subcellular
organelles and how much of the active ABA pool is contributed by the ABA-GE
pathways remains unclear. Furthermore, how are these different pathways coor-
dinated to maintain the active ABA pool at the cellular and whole plant lev-
els? Therefore, future work should be directed to investigate: (i) the identity of
ABA-GE transporters at various subcellular organellar membranes and their
mechanisms of action; (ii) how these transporters are coordinated for homeosta-
sis of ABA levels at the cellular level; (iii) what is(are) the signaling pathway(s)
involved in the coordination of these multiple biosynthetic and catabolic pathways
involving multiple organelles under normal growth and abiotic stress conditions
at the cell level; (iv) how the long-distance transport, if there is any, contributes to
the homeostasis of ABA at the whole plant level.
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