ABA Metabolism and Signaling in Fleshy Fruits - Abscisic Acid: Metabolism, Transport and Signaling - page 267

In all, the detection of key ABA biosynthesis and catabolism genes in fleshy
fruit tissues suggests that although ABA could be transported from the leaves
to the fruits via the phloem (Shiozaki et al. 1999 ), it can also be synthesized in
fleshy fruits in situ. ABA levels in fleshy fruits are regulated mainly by NCED1
and CYP707A1, the accumulation of ABA-GE catalyzed by GT enzymes, contrib-
utes to a rapid response by one-step dissociation of ABA-EG catalyzed by BG in
response to developmental and stress cues. During later stages of fruit develop-
ment, beta-glucosidases (BGs) appear to play a more important role in ripening.
14.4 ABA Signaling in Fleshy Fruits
The presence of both transmembrane and cytosolic ABA receptors in plants was
suggested by early studies using experimental model plants (Hornberg and Weiler
1984 ; Allan et al. 1994 ; Schwarz and Schroeder 1998 ), such as the identification of
multiple receptors, secondary messengers, protein kinases and phospholipases, tran-
scription factors, cis -elements, and target genes in A. thaliana . Known ABA recep-
tors include the plasma membrane localized GTG1/GTG2 (Pandey et al. 2009 ), a
class of cytosolic PYR/PYL/RCAR (Park et al. 2009 ; Ma et al. 2009 ) proteins and a
plastid/chloroplast magnesium-chelatase H subunit ABAR/CHLH (Shen et al. 2006 ;
Wu et al. 2009 ; Shang et al. 2010 ). Signalling components include G proteins, phos-
pholipases, and various protein kinases, such as receptor-like kinases, SNF1-related
protein kinases (SnRKs), calcium dependent protein kinases (CDPKs), calcineurin
B-like protein kinases (CIPKs), and mitogen-activated protein kinases (MAPKs).
Protein phosphatases of type-2C/A protein phosphatase (PP2C/A), various classes
of transcription factors, including MYBs/MYCs, B3 domain transcription factors,
APETALA2 domain transcription factors, bZIP domain transcription factor, and
WRKY transcription factors, are also among important regulators of ABA levels
(Nambara and Marion-Poll 2005 ; Hirayama and Shinozaki 2007 ; Verslues and Zhu
2007 ; Wang and Zhang 2008 , and Cutler et al. 2010 ). To date, two core ABA signal-
ling pathways in A. thaliana have been proposed: the 'ABA-PYR/PYL/RCAR-PP2C-
SnRK2' pathway (Fujii et al. 2009 ), and the'ABA-ABAR-WRKY40-ABI5'pathway
(Shang et al. 2010 ). At the mechanistic level, a detailed gate latch-lock mechanism
has been identified, in which ABA promotes the interaction of PYR1 and PP2C,
resulting in PP2C inhibition and SnRK2 activation. This transduces ABA signals
through phosphorylation of downstream factors such as AREB/ABF, ion channels,
and NADPH oxidases (Park et al. 2009 ; Ma et al. 2009 ; Fujii et al. 2009 ).
14.4.1 ABA Perception in Flesh Fruits
In addition to pioneering studies in model experimental plants, there has also been
much interest in understanding the molecular basis of ABA detection by specific
receptors and subsequent downstream signalling in fleshy fruits. In studies of