ABA Transmembrane Transport and Transporters - Abscisic Acid: Metabolism, Transport and Signaling - page 50

from eukaryotes are grouped into 8 subfamilies (A-H), but plants do not have the
H subfamily. Instead, plants contain prokaryote types (subfamily I).
An Arabidopsis mutant defective in AtABCG25 , encoding a half-size ABC
transporter, was isolated from a population of Ac/Ds insertion lines based on its
altered response to exogenously applied ABA during seed germination and/or
early seedling growth (Kuromori et al. 2010 ). The abcg25 mutant was more sensi-
tive to exogenously applied ABA than wild type, suggesting that the function of
AtABCG25 is related to ABA transport. Using isolated membrane vesicles from
Sf9 insect cells, the AtABCG25 protein expressed with the baculovirus expres-
sion system was shown to have sufficient activity to transport ABA (Kuromori
et al. 2010 ). The K m value of AtABCG25 for ABA [racemic mixture of ( + ) and
( − ) enantiomers] is 260 nM, and the ABA transport activity of AtABCG25 was
inhibited by ( + )-ABA but not by ( − )-ABA, gibberellin (GA), or indole-3-acetic
acid (IAA). These results suggest that AtABCG25 has a relatively high affin-
ity and specificity for the naturally occurring ( + ) enantiomer of ABA. Promoter
activity of AtABCG25 was induced by ABA in vascular tissues, suggesting that
AtABCG25 regulates ABA export from ABA-synthesizing cells in response to
elevated endogenous ABA concentrations. Transgenic plants overexpressing
AtABCG25 had higher leaf surface temperatures with reduced water loss com-
pared to wild type. This result suggests that elevated apoplastic ABA levels caused
by constitutive ABA export activities of AtABCG25 resulted in an increased
amount of ABA to be transported from vascular tissues to the guard cells.
However, the guard cells of overexpressors should have more ABA exporting
activity that could inhibit stomatal closure even if increased levels of ABA were
present in the apoplast. The use of tissue/cell-specific promoters to locally regulate
ABA transport activities should help elucidating the roles of the ABA transporter
at specific sites.
AtABCG40 (PDR12), a full-size ABC transporter, was originally identified
based on its involvement in lead (PbII) resistance (Kang et al. 2010 ). Expression
of AtABCG40 was induced by PbII treatment. Mutants defective in AtABCG40
were sensitive whereas AtABCG40 overexpressors were resistant to PbII. Some
members of the full-size ABCG subfamily have been implicated the transport
of terpenoids and are also involved in responses to pathogens, salinity, cold, and
heavy metal stresses (Lee et al. 2005 ; Moons 2008 ; Stein et al. 2006 ; Campbell
et al. 2003 ; Rea 2007 ). These observations motivated Kang et al. ( 2010 ) to exam-
ine the involvement of this family in ABA transport, because ABA is a terpe-
noid (sesquiterpene) and is involved in responses to biotic and abiotic stresses.
Knockout mutants of 13 out of 15 Arabidopsis ABCG/PDR family members were
investigated for their phenotypes. One mutant, abcg40 , was found to be less sensi-
tive to exogenously applied ABA than wild type in terms of inhibiting seed ger-
mination and inducing stomatal closure. These results indicate that AtABCG40 is
involved in ABA transport. Yeast and tobacco BY2 cells expressing AtABCG40
incorporated more ABA than the corresponding control cells. The K m value
of AtABCG40 for ABA is 1 ʼ M when determined in the tobacco BY2 system.
Also, isolated mesophyll cells from atabcg40 were less active in taking up ABA