How Can We Interpret the Large Number and Diversity of ABA Transporters?

Abstract

Abscisic acid (ABA) is generally known as the plant stress hormone. Functioning in a wide range of environmental responses, ABA plays a major role in drought tolerance. In addition to inducing stomatal closure during drought stress, ABA promotes suberization of the exodermis and endodermis, which reduces water loss from the root. Furthermore, ABA increases freezing tolerance and has a complex, but not completely understood, role in plant–pathogen interactions. ABA also functions in plant development; for example, ABA is a central player in maintaining seed dormancy. Whereas the enzymatic steps of ABA biosynthesis have been known for some time, our knowledge of ABA receptors and transporters is quite recent. This is due, at least partially, to redundancy among members of both the ABA receptor and transporter families. Many transporters from different transporter families cooperate to transport ABA. The weak but distinct phenotypes described for the different loss-of-function mutants indicate that each of these transporters plays a specific role and, at least under a given condition or in a specific tissue, they are not completely redundant. However, for each function described so far, delivery of ABA at the target site requires the activity of several different ABA transporters. This strategy may ensure that ABA is transported to the correct target even if one of the transporters is nonfunctional or that plants can transport ABA under a given condition via several routes.