ABA Regulation of Plant Responses to Drought and Salt Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 314

of phaseic acid by activating its catabolism (Qin and Zeevaart 2002 ). Therefore,
the levels of ABA are tightly modulated by the balance between ABA biosynthesis
and catabolism under stressful conditions, and it is feasible to regulate the level of
endogenous ABA, which can change the drought tolerance of plants.
ABA-induced stomata closure to reduce transpiration and prevent water loss is
crucial for plants under osmotic stresses, such as those induced by drought and
high salinity. Many signaling components participate in this reaction, including
Ca 2 + , nitric oxide, reactive oxygen species, protein kinases, protein phosphatases,
G protein, G protein-coupled receptors, and phosphatidic acid. The effect of ABA
on stomatal closure is to reduce turgor and volume in guard cells, which is medi-
ated by cation and anion effluxes and leads to the depolarization of guard cells
(Roychoudhury and Paul 2012 ). ABA is known to function in stomata consist-
ing of two guard cells, but is mainly biosynthesized in vascular tissues (Cheng
et al. 2002 ; Koiwai et al. 2004 ; Okamoto et al. 2009 ). This indicates that ABA
transport is crucial for proper ABA function and thus for correct physiologi-
cal responses. AtABCG40 , an ATP-binding cassette (ABC) transporter gene, was
identified in Arabidopsis as an ABA importer in plant cells (Kang et al. 2010 ).
Yeast or BY2 cells expressing AtABCG40 show an increase in ABA uptake. By
contrast, protoplasts of atabcg40 mutants show a decrease in ABA uptake. In the
atabcg40 mutant, ABA-induced stomata closure is slower than that in the wild
type and this directly results in reduced drought tolerance. The expression of the
ABA-responsive genes is accordingly delayed in response to exogenous ABA.
AtABCG22 has been identified as a member of AtABCG family gene encoding a
half-size ABC transporter in Arabidopsis and is responsible mainly for stomatal
regulation. The atabcg22 mutants were sensitive to drought stress, with lower leaf
temperatures and increased water loss compared with the wild type (Kuromori
et al. 2011 ). Another ABA importer, the ABA-importing transporter1 ( AIT1) ini-
tially named NRT1.2/NPF4.6 , has also been characterized (Huang et al. 1999 ;
Kanno et al. 2012 ). AIT1 is mainly expressed in vascular tissues, where ABA is
biosynthesized, and acts as an ABA importer responsible for ABA transport from
its site of synthesis to its site of function, where it facilitates and regulates sto-
mata closure responding to drought stress. The ait1 mutants showed decreased
sensitivity to exogenously applied ABA, whereas plants overexpressing AIT1
were more sensitive to ABA (Kanno et al. 2012 ). In addition, AtABCG25, another
ABC transporter, is responsible for exporting ABA from inside to outside of cells
in vascular tissues, playing an important role in stomatal closure. Thus, the over-
expression of AtABCG25 causing an ABA-insensitive phenotype as well as an
increase in leaf surface temperature may be a result of stomatal closure (Kuromori
et al. 2010 ).
In conclusion, newly discovered ABA receptors and genes involved in the regu-
lation of the levels of ABA, as well as ABA transporters responsible for translocat-
ing ABA from sites of biosynthesis to functional sites have greatly enriched our
knowledge regarding plant responses to drought and salt stresses and have broad-
ened our gene resources and generated new technologies in genetic engineering to
improve stress tolerance in plants.