Agriculture Reference
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type frutis. In addition, the results of targeted studies with transgenic tomato sug-
gest that ABA regulates the degree of pigmentation, carotenoid composition, and
fruit firmness during ripening (Sun et al.
2012a
,
b
). Furthermore, the ABA-deficient
notabilis
/
lacca
(
not
/
lc
) tomato double mutant has been used to demonstrate that
ABA stimulates cell enlargement and increases fruit size (Nitsch et al.
2012
).
In addition, a recent report revealed that ABA may also be involved in suberi-
zation-based wound healing processes in tomato fruit stem scar tissue (Leide et al.
2011
). Other studies have shown that ABA is involved in the regulation of avocado
fruit growth (Cowan et al.
1997
) and peach fruit sugar accumulation (Kobashi
et al.
2001
), suggesting that ABA is likely a common signaling molecule that mod-
ulates many processes in both climacteric and non-climacteric fleshy fruits.
14.3 ABA Metabolism in Fleshy Fruits
In land plants, ABA levels in a specific tissue are determined by the balance
between ABA biosynthesis, catabolism and conjugation. Biosynthesis results
from the oxidative cleavage of carotenoids, where the activity of the enzyme
9-
cis
-epoxycarotenoid dioxygenase (NCED) represents a major rate limiting step.
Conversely, ABA catabolism results mainly from 8′-hydroxylation reactions cata-
lyzed by 8′-hydroxylase enzymes, which are encoded by a small family of P450
monooxygenase CYP707A genes (Zeevaart et al.
1989
; Nambara and Marion-Poll
2005
). In addition, ABA conjugation can result from the action of ABA-induced
glucosyltransferases (GTs), which convert free ABA to an ABA-glucosylester
(ABA-GE; Xu et al.
2002
). Earlier reports suggested that ABA-GE represents an
inactive end product of ABA catabolism (Lehmann and Schutte
1984
; Zeevaart
1999
). However, this idea is called into question as a result of the identification
of Arabidopsis beta-glucosidase1 (AtBG1), an enzyme that catalyzes the conver-
sion of ABA-GE back into the pool of biologically active ABA, allowing a rapid
change in ABA levels (Lee et al.
2006
). A contemporary model might therefore
involve the control of plant cellular ABA levels by NCEDs and CYPs in a syn-
thesis/degradation pathway or/and by GTs and BGs in a conjugation/dissociation
pathway. Potentially, the one-step pathway catalyzed by GTs or BGs allows rapid
dynamic changes in ABA levels to meet developmental and adaptive requirements.
This foundation of knowledge resulting from studies of
A. thaliana
and model
crop species has been valuable in elucidating ABA biosynthesis and catabolism
in fleshy fruits and in identifying conserved mechanisms of ABA metabolism (Li
et al.
2011
,
2013
; Jia et al.
2011
; Sun et al.
2012b
). For example, down regula-
tion of
FaNCED1
expression in strawberry using virus induced gene silencing
(VIGS) resulted in a significant decrease in ABA levels, as well as uncolored
fruits, demonstrating that
FaNCED1
is a key gene for ABA biosynthesis in fruits
and plays an important role in fruit ripening (Jia et al.
2011
). Moreover, Li et al.
(
2013
) reported the importance of BG enzymes in strawberry, showing that
FaBG3
-RNAi-treated fruit has reduced ABA levels and fruit color development
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