ABA Transmembrane Transport and Transporters - Abscisic Acid: Metabolism, Transport and Signaling - page 53

Synthesizing cells

Chl

zeaxanthin

xanthoxin

ER

′

xanthoxin 8 -OH ABA

ABA-GE

ABA ABA

PYR/PYL/RCAR

ABA-GE

ABCG25

GPCR ABA

ABA ABA-GE

ABA

GPCR

ABCG40 NPF4 6

ABA-GE ER

ABA

ABA

PYR/PYL/RCAR

′

8 -OH ABA

Receiving cells

Fig. 3.1 A simplified model for the transport of ABA. For simplification, the transport of ABA,
ABA precursors and metabolites into ABA-synthesizing cells ( upper ), and ABA biosynthesis in
the receiving cells ( lower ) are not considered. Simple arrows and bold outlined arrows indicate
metabolism (biosynthesis and catabolism) of ABA and transmembrane transport of ABA, respec-
tively. The early steps of ABA biosynthesis take place in chloroplasts ( Chl ). Xanthoxin, a C 15
product, is produced by the reaction mediated by 9- cis epoxycarotenoid dioxygenase ( NCED )
in chloroplasts. Specific transporters are required for the transmembrane transport of xanthoxin
from the inside to the outside of chloroplasts. The final reactions of ABA biosynthesis pro-
ceed in the cytosol. ABA is recognized by the soluble PYR/PYL/RCAR receptors and induces
physiological responses. ABA catabolism mediated by CYP707A to produce 8′-hydroxyl ABA
( 8 -OH ABA ) takes place in the endoplasmic reticulum ( ER ), indicating that transmembrane

transport of ABA into the ER is required. It is possible that ABA is inactivated also by glucosida-
tion to produce an ABA-glucosyl ester ( ABA-GE ). Because the ABA-GE hydrolyzing enzyme
BG1 is localized in the ER, ABA, or ABA-GE has to be transported into the ER. ABA synthe-
sized from ABA-GE could induce physiological responses via PYR/PYL/RCAR. ABA in the
cytosol is exported extracellularly by ABCG25 and moves through the apoplastic space. ABA in
the apoplast could induce physiological responses via plasma membrane-localized GPCR recep-
tors. After local or long-distance transport through the apoplast, ABA is imported into cells by
ABCG40 and/or NPF4.6 and induces physiological responses via PYR/PYL/RCAR. It is also
possible that ABA-GE is exported from cells, moves through the apoplast, is taken again into
cells, and induces physiological responses

′