ABA Transmembrane Transport and Transporters - Abscisic Acid: Metabolism, Transport and Signaling - page 47

et al. 2004 ; Holbrook et al. 2002 ; Everatbourbouloux 1982 ; Zeevaart and Boyer
1984 ). ABA is synthesized inside cells. Thus, transmembrane transport of ABA
from the inside to the outside of ABA-synthesizing cells is the first step required for
movement unless ABA moves through plasmodesmata. ABA has been detected in
xylem sap (Tardieu et al. 2010 ; Hartung et al. 2002 ), indicating that ABA is exported
to the outside of cells after biosynthesis. Once exported from the site of biosyn-
thesis, ABA can be delivered to neighboring or distant cells and can induce physi-
ological responses. Sites for ABA recognition by receptors that occur both inside
and on the surface (plasma membrane) of cells have been proposed (Allan et al.
1994 ; Anderson et al. 1994 ; Gilroy and Jones 1994 ; Schwartz et al. 1994 ; Yamazaki
et al. 2003 ). Studies in Arabidopsis demonstrated that at least two different types
of proteins, the plasma membrane-localized GPCR-type G proteins and the soluble
START proteins (PYR/PYL/RCAR), function as ABA receptors (Ma et al. 2009 ;
Pandey et al. 2009 ; Park et al. 2009 ). The soluble receptor PYR/PYL/RCAR family
plays a central role in most of the physiological responses mediated by ABA since
multiple knockout mutants in this family display severe ABA-insensitive phenotypes
(Gonzalez-Guzman et al. 2012 ; Park et al. 2009 ). Therefore, ABA present in the
apoplast needs to be taken into cells again across a biological membrane to induce
physiological responses through the PYR/PYL/RCAR receptors.
ABA is a weak acid with a p K a of 4.7 and is ionized at its carboxyl group to form
ABA − depending on the pH. Compared to ionized ABA − , non-ionized ABA (proto-
nated ABA; ABA-H) can pass relatively easily through biological membrane by pas-
sive diffusion because of its hydrophobic nature. Therefore, pH conditions can affect
the distribution of ABA between the inside and the outside of cells. The last step of
ABA biosynthesis takes place in the cytosol where the pH is estimated to be 7.2-7.4.
Thus, most of ABA in the cytosol exists as ABA − that requires specific transporters
for extracellular ABA export. Once exported, ABA can move through the apoplastic
space unless it is taken into cells (Kramer 2006 ). The pH in the apoplast (estimated
to be 5.0-6.0) is relatively lower than that in the cytosol; however, the pH of the
apoplast is still higher than the p K a of ABA, suggesting that active transport mech-
anisms are also required for efficient uptake of apoplastic ABA into cells. Several
studies have indicated the presence of saturable components (carriers or transport-
ers) that mediate cellular ABA uptake (Bianco-Colomas et al. 1991 ; Perras et al.
1994 ; Windsor et al. 1994 ; Windsor et al. 1992 ). On the other hand, apoplastic pH
increases up to approximately pH 7.0 during water stress, enabling a relatively large
proportion of ABA-H to move long distances through the apoplast (Wilkinson 1999 ;
Wilkinson and Davies 2002 ; Hartung et al. 2002 ; Sauter et al. 2001 ).
3.2 The Sites of ABA Biosynthesis and Action
As described above, transmembrane ABA transport could be associated with
local and/or long-distance cell-to-cell movement of ABA. Despite many inves-
tigations, the significance of transmembrane transport of ABA and, hence, local