ABA Metabolism and Signaling in Fleshy Fruits - Abscisic Acid: Metabolism, Transport and Signaling - page 266

and softening is inhibited, indicating a key role of ʲ -glucosidases in regulation of
ripening (Li et al. 2013 ). Interestingly, similar results were generated through stud-
ies of tomato fruit, where ABA levels are regulated by SlNCED1 and SlCYP707A1
(Nitsch et al. 2009 ; Zhang et al. 2009 ). It was observed that suppressing the
expression of the SlNCED1 using RNAi lead to a 20-50 % decrease in both ABA
accumulation and SlNCED1 transcript compared with control fruit, and prolonged
fruit shelf life by up to 3 weeks (Sun et al. 2012b ).
In addition to these important breakthroughs made through observations of straw-
berry and tomato fruits (Li et al. 2011 , 2013 ; Jia et al. 2011 ; Sun et al. 2012b ), simi-
lar studies have been reported of ABA biosynthesis and catabolism in other fruits.
Water deficit results in a nearly two-fold increase in ABA concentration in berries
of a red-wine grape Cabernet Sauvignon, the same stress induces a decrease in ABA
levels in the Chardonnay white-wine grape at veraison and shortly thereafter (Deluc
et al. 2009 ). The higher transcript levels of VvNCED1 and VvBG1 together with
lower transcript abundance of VvCYP1 and VvGT contribute to a continuous accu-
mulation of ABA, especially VvBG1 transcripts increased more rapidly than that of
VvNCED1 during berry red-coloring. An incubation test in vitro indicated that the
Escherichia coli -expressed VvBG1 protein had high enzymatic activity, validating
that ʲ -glucosidase (VvBG1) has high enzymatic activity and might play a role in
berry ripening (Sun et al. 2014 ). In avocado fruits, PaNCED1 and PaNCED3 , but
not PaNCED2 , were strongly induced as the fruit ripened, and a correlation with
water stress was also established, since PaNCED1 was induced under these condi-
tions. Furthermore, recombinant PaNCED1 and PaNCED3, but not PaNCED2 could
cleave 9- cis -xanthophylls into xanthoxin and C (25)-apocarotenoids in vitro, indicat-
ing that ABA biosynthesis in avocado is regulated at the level of carotenoid cleav-
age (Chernys and Zeevaart 2000 ). In citrus fruit, the accumulation of both ABA and
ABA-conjugates occurs during fruit maturation (Harris et al. 1986 ). In citrus peels,
high levels of ABA are detected and CsNCED1 is likely to play a primary role in
ABA biosynthesis (Rodrigo et al. 2014 ). Moreover, in the flavedo and juice sacs,
expression of CitNCED2 and/or CitNCED3 increases coincident with a substan-
tial accumulation of ABA (Kato et al. 2006 ). In watermelon fruit, the expression of
ClBG1 , ClNCED4 and ClCYP707A1 has been shown to increase rapidly along with
ripening, and reaches the highest levels at the stage of harvest. This trend was show
to be consistent with ABA accumulation, which was also shown to be modulated
by the described dynamic balance between biosynthesis and catabolic processes via
these genes, indicating the importance of glucosidase genes in the ripening process
(Li et al. 2012 ). Similarly, it was shown that the endogenous ABA content of sweet
cherry fruit is regulated by PacNCED1 , PacCYP707A1 and PacCYP707A3 tran-
scripts during maturation (Ren et al. 2010 ), and in apple fruits, the major portion
of the ABA has been shown to pool is conjugated to ʲ -D-glucopyranosyl abscisate
(ABA-GE) which leads to markedly lower ABA levels (Rock and Zeevaart 1990 ).
The peach PpNCED1 gene has been shown to initiate ABA biosynthesis at the onset
of fruit ripening (Zhang et al. 2009 ). These reports again reinforce the idea that ABA
accumulation is involved in the regulation of ripeness and senescence in a broad
range of fleshy fruits.