ABA Metabolism and Signaling in Fleshy Fruits - Abscisic Acid: Metabolism, Transport and Signaling - page 270

Taken together, much progress has been made toward understanding ABA
signaling, including the identification of ABA receptors, protein kinases, protein
phosphatases, transcription factors, cis -elements, and target genes in fleshy fruit.
These include the identification of core signaling components controlling the key
links to fleshy fruit development, including strawberry 'ABA-FaPYR1-FaPP2C-
FaSnRK2', grape 'ABA-VvACPK1-H + -ATPase' and 'ABA-VvSK1-VvHTs',
apple 'ABA-MdMKK1-MdMPK1-ABI5', and persimmon 'ABA- DkbZIP5-
DkMyb4-PA'. To date, the target genes include those involved in fruit soften-
ing ( PG , PL , PME , TBG , XET , Cels , and Exp )-, sugar metabolism ( VvHTs ,
MiADH2 )-, pigmentation ( PA , CHS , CHI , F3H , DFR , ANS , and UFGT ), and rip-
ening ( FaASR , DlGH3.2 )-related genes.
14.5 Understanding of the Mechanisms for ABA
Regulation of Fleshy Fruit Ripening
There is considerable interest at present in the relationship between ABA, ethyl-
ene and sugar content in ripening fleshy fruits. Two previous reports suggested that
ABA facilitates the initiation and progression of ethylene-mediated ripening events,
possibly by enhancing the sensitivity to ethylene (Jiang et al. 2000 ) or ethylene
levels (Riov et al. 1990 ). In grape berries, sugar and ABA signaling orthologs are
activated at the onset of the ripening, including the putative sucrose sensor SUT2,
core G-protein signaling components, GPA1 and RGS1, hexose kinases (Hxk),
PP2C protein phosphatases, Snf1-related kinases (SnRK), the sugar-related WRKY,
ABA-related homeodomain-leucine zipper, as well as homeobox (HB) transcrip-
tions factors, ABRE-binding factor (ABF), and AP2 transcription factors (Gambetta
et al. 2010 ). Conserved changes in the dynamics of metabolic processes during
fruit development and ripening across species were found (Klie et al. 2014 ). During
tomato fruit growth, ABA stimulates cell enlargement by suppressing ethylene syn-
thesis (Nitsch et al. 2012 ; Sun et al. 2012b ) and during the ripening, a significant
reduction in ABA levels is thought to promote coloring and firmness, resulting in the
carbon that normally channels to ABA biosynthesis and catabolism being targeted to
compounds upstream of ABA biosynthesis, including lycopene, carotene, and pectin
(Sun et al. 2012a , b ). Interestingly, in citrus fruit, ABA my induce its own biosynthe-
sis at the transcriptional level and the feedback regulation of ABA has been shown
to lead to decreases in carotenoid content in citrus juice sacs in vitro (Zhang et al.
2012 ). In strawberry fruit, sucrose may serve as a signalling molecule to promote
mRNA expression levels of FaNCED1 , as well as playing an important role in ABA
accumulation and fruit ripening (Jia et al. 2011 , 2013 ).
Fleshy fruit can synthesize ABA in response to developmental and environmental
cues and the accumulation of ABA in fleshy fruit is controlled by four main enzymes
classes that are fundamentally important in ABA biosynthesis and catabolism:
NCEDs, CYP707As, GTs, and BGs. In the ripening of fleshy fruit, the interaction
between ABA, ethylene and sugars occurs at physiological and molecular levels.