ABA Transmembrane Transport and Transporters - Abscisic Acid: Metabolism, Transport and Signaling - page 49

stomatal closure (Melhorn et al. 2008 ). Similarly, expression of the functional
AtABA3 gene in the Arabidopsis aba3 mutant background under the control of a
guard cell-specific promoter complemented the defective stomatal closure at least
in part (Bauer et al. 2013 ). These studies suggest that guard cells can potentially
synthesize ABA and induce stomatal closure. Also, expression of some ABA-
inducible genes are up-regulated in vascular tissues, whereas others are activated
in non-vascular cells (Endo et al. 2008 ), indicating that ABA acts at the site of its
synthesis and also acts in surrounding and/or distant cells. Therefore, the contribu-
tion of ABA derived from a different source to a particular physiological response
at a particular site needs to be considered very carefully.
3.3 Transmembrane ABA Transporters
Several auxin transporters have been identified by characterizing mutants isolated
based on their defects in organ development or based on their altered responses
to exogenously applied auxins or environmental stimuli (Benjamins and Scheres
2008 ; Petrasek and Friml 2009 ). Such mutants contributed significantly toward
understanding the physiological processes mediated by auxin transport. In con-
trast, forward genetics approaches based on physiological or hormone responses
have not succeeded in isolating mutants defective in the transport of other hor-
mones including ABA. These findings suggest that hormone transport systems are
highly redundant or that hormone transport is not required for physiological func-
tions. The latter hypothesis seems unlikely since two different types of ABA trans-
porters, the ATP-binding cassette (ABC) type and the NRT1/PTR FAMILY (NPF)
member proteins, were identified recently in Arabidopsis (Kuromori et al. 2010 ;
Kang et al. 2010 ; Kanno et al. 2012 ). These studies demonstrated the importance
of ABA transmembrane transport in regulating physiological responses.
3.3.1 ABCG
Two independent studies identified ABC proteins belonging to subfamily G in
Arabidopsis, namely ABCG25 and ABCG40, as an ABA exporter and ABA
importer, respectively. ABC proteins are defined as proteins that contain an ATP-
binding cassette (Rea 2007 ; Verrier et al. 2008 ; Kang et al. 2011 ). A full-size ABC
transporter is composed of two ATP-binding cassettes and two transmembrane
domains, whereas a half-size ABC transporter contains one ATP-binding cassette
and two transmembrane domains. Half-size ABC proteins are proposed to function
as a homo- and/or a heterodimer. ABC proteins exist in prokaryotes and eukary-
otes, including bacteria, fungi, yeast, insects, animals, and plants. ABC proteins
mediate the export or import of various compounds by using the energy generated
by ATP hydrolysis. There are about 130 ABC genes in Arabidopsis. ABC proteins