ABA Metabolism and Signaling in Fleshy Fruits - Abscisic Acid: Metabolism, Transport and Signaling - page 271

Fig. 14.1 A model for ABA metabolism and signaling in fleshy fruits. In response to develop-
mental and environmental cues, fleshy fruit can synthesize ABA mainly by NCEDs and BGs
while catabolize ABA by CYP707As and GTs. ABA levels in fleshy fruits are determined by
the balance between ABA biosynthesis and catabolism. One-step dissociation of ABA-EG by
BGs may rapidly accumulate ABA. The accumulated ABA in fleshy fruit can be perceived by
its receptors, at least including PYR and ABAR. In turn, the initial signals may be relayed by
the way of reversible protein phosphorylation, which involves ABI1, SnRK2, ACPK1, MKK1,
and MPK1. Subsequently, the cascade signaling of phosphorylation can activate downstream
transcript factors and corresponding cis -elements, including bZIP5, MYb4, ABI3, ABI4, ABI5,
ABRE, then promote the expression of the ripening-related genes: softening-related genes
include PG , PL , PME , TBG , XET , Cels , and Exp ; sugar metabolism-related genes include SUT1 ,
HTs , ADH2 , and H

+

- ATPase , and coloring-related genes include CHS , CHI , F3H , DFR , ANS ,
and UFGT . Finally these expressed genes accelerate fleshy fruit ripening, to a large extent, the
processes of which could be cooperated by the interaction of ABA with sugar and ethylene.
Abbreviations: NCDEs, 9-cis-epoxycarotenoid dioxygenases; CYP707As, P450 monooxyge-
nases; GTs, glucosyltransferases; BGs, beta-glucosidases; ABA-GE, ABA-glucosylester; PYR,
pyrabactin resistance; ABAR, ABA receptor; SnRK2: sucrose non-fermenting 1-related protein
kinase 2; ACPK1, ABA-stimulated calcium-dependent protein kinase1; MKK1, mitogen-acti-
vated protein kinase kinase 1; MPK1, mitogen-activated protein kinase 1; bZIP5, basic leucine-
zipper 5; MYB4, dehydration responsive element-related transcription factor 4; ABI3-5, abscisic
acid insensitive 3-5; ABRE, ABA response element; PG , polygalacturonase gene; PL , pectate
lyase gene; PME , pectin methyl esterase gene; TBG , ʲ -galactosidase gene; XET , xyloglucan
endotransglycosylase gene; Cels , endo-1,4- ʲ -cellulose gene; Exp , expansin gene; SUT1 , sucrose
transporter 1 gene; HTs , monosaccharide transporter genes; ADH2 , alcohol dehydrogenase
2 gene; H

+

- ATPase , proton-pump ATPase gene; CHS , chalcone synthase gene; CHI , chalcone
isomerase gene; F3H , flavanone 3-hydroxylase gene; DFR , dihydroflavonol 4-reductase gene;
ANS , anthocyanidin synthase gene; UFGT , UDP-glucose:flavonoid 3-O-glucosyltransferase gene

The initiation of ABA action is triggered by the perception by its receptors, includ-
ing PYRs and ABAR, causing signaling to be relayed by reversible protein phospho-
rylation of protein kinases and phosphatases, including ABI1, SnRK2, ACPK1, SK1,