ABA Metabolism and Signaling in Fleshy Fruits - Abscisic Acid: Metabolism, Transport and Signaling - page 269

membranes and chloroplasts/plastids. The fact that ACPK1 is expressed in a devel-
opmental stage-dependent manner, and that it positively regulates the plasma
membrane H + -ATPase in vitro, together with the observation that ABA stimu-
lates ACPK1 in a dose-dependent manner, suggest that ACPK1 may be involved
in the ABA-signalling pathway during berry development (Yu et al. 2006 ). Similar
studies found that an apple MAPK signalling cascade, MdMKK1-MdMPK1, is
involved in ABA signalling, and that MdMPK1 phosphorylates the ABI5 protein
through a unique residue, Ser314, making ABI5 a potential direct downstream
component of MAPK in ABA signalling (Wang et al. 2010 ). In grape berries, a
glycogen synthase kinase 3 protein kinase, VvSK1, is strongly expressed when
the berries accumulate glucose, fructose, and ABA, and overexpression of VvSK1
results in an upregulation of the transcripts of four monosaccharide transporters
( VvHT3 , VvHT4 , VvHT5 , and VvHT6 ), an up to 5-fold increase in the rate of glu-
cose uptake, and a doubling in the amount of glucose and sucrose accumulation,.
This indicates that VvSK1 controls sugar uptake and accumulation by a sugar/
ABA-inducible protein kinase (Lecourieux et al. 2010 ). More importantly, the core
phosphorylation signaling of PP2C-SnRK2 has been established in response to
ABA during strawberry fruit ripening (Jia et al. 2013 ).
14.4.2.2 Transcription Factor, Cis -Elements and Target Genes
Many transcription factors, cis -elements and target genes related to ABA signal-
ing have been identified in developing fleshy fruit. For example, in persimmon
fruit, DkbZIP5 was shown to recognize ABA-responsive elements in the pro-
moter region of DkMyb4 and to act as a direct regulator of DkMyb4 in an ABA-
dependent manner. It is reported to suggest that ABA signals may be involved
in proanthocyanidin (PA) biosynthesis via DkMyb4 activation by DkbZIP5
(Akagi et al. 2012 ). In tomato fruit, the higher hexose and acid concentration in
SlAREB1 -overexpressing lines can be correlated with an increased expression of
genes encoding a vacuolar invertase and a sucrose synthase, suggesting that an
AREB-mediated ABA signal affects the metabolism of acid and sugar in tomato
fruit development (Bastas et al. 2011 ). ABA can also affect cell wall catabolism
during tomato fruit ripening via down-regulation of the expression of major cata-
bolic genes ( SlPG , SlPME , SlTBG , SlXET , SlCels , and SlExp ; Sun et al. 2012b ),
but an ABA biosynthesis inhibitor can increase transcript levels corresponding to
enzyme activities associated with other aspects of fruit ripening, as is the case with
an alcohol dehydrogenase from mango (Singh et al. 2010 ) and an indole-3-acetic
acid-amido synthetase, DlGH3.2, in longan (Kuang et al. 2011 ). It has been pro-
posed that the expression of FaASR , a member of the ABA-, stress- and ripening-
induced (ASR) set of genes might partially contribute to the acceleration of the
fruit ripening (Chen et al. 2011 ). Alterations in strawberry FaABI1 expression
were reported to regulate the transcripts of a set of both ABA-responsive and rip-
ening-related genes, including ABI3 , ABI4 , ABI5 , SnRK2 , ABRE1 , CHS , PG1 , PL ,
CHI , F3H , DFR , ANS , and UFGT (Jia et al. 2013 ).