ABA Metabolism and Signaling in Fleshy Fruits - Abscisic Acid: Metabolism, Transport and Signaling - page 268

grape berry microsomes, a high proportion of specific ABA receptors (which had
a measured K d value of 17.5-50 nM) were shown to be localized mainly in the
endomembranes and not in the plasma membrane or in the cytoplasm (Zhang et al.
1999 ). In contrast, ABA binding activity was scarcely detectable in microsomes
derived from the flesh of developing apple fruit, but high ABA binding activity
was detected in the cytosolic fraction with both high affinity (K d 2.3 nM) and low
affinity sites (K d 58.8 nM; Zhang et al. 2001 ). Progress has also been made in
understanding ABA perception in strawberry. Using a newly established tobacco
rattle virus (TRV)-induced gene silencing technique in strawberry fruit, down-reg-
ulation of the expression levels of FaPYR1 or FaCHLH/ABAR , homologous to the
A. thaliana ABA receptor genes, was shown to inhibit ripening (Jia et al. 2011 ;
Chai et al. 2011 ). On the basis of these results, a model was proposed for ABA
perception and signalling transduction during non-climacteric fruit ripening (Li
et al. 2011 ). More recently, the Type 2C protein phosphatase FaABI1 was dem-
onstrated to be a negative regulator of strawberry fruit ripening (Jia et al. 2013 )
and it was suggested that the 'ABA-FaPYR1-FaPP2C-FaSnRK2' signalling path-
way, involving the positive regulatory effect of FaPYR1 in combination with the
negative regulatory effect of FaABI1, represents a core mechanism by which ABA
regulates strawberry fruit ripening (Jia et al. 2013 ).
Recently, the ABA perception mechanisms and core signalling systems in other
fruit tree species have also been studied. The V. vinifera proteins VvRCAR6 and
VvRCAR5 may be the major receptors involved in ABA perception and signalling
in grape, mainly through VvPP2C4 (Boneh et al. 2012 ), while VvPYL1 may be an
ABA receptor that modulates ABA signalling by inhibiting type PP2C activity (Li
et al. 2012 ). In tomato, SlPYL1 and SlPYL2 are expressed at high levels through-
out fruit development and ripening, and SlPP2C1 and SlPP2C5 are both strongly
expressed during the breaker stage, while SlSnRK2.2 , SlSnRK2.3 , SlSnRK2.4 , and
SlSnRK2C are highly expressed at all maturity stages (Sun et al. 2011 ).
14.4.2 Downstream Components of ABA Signaling
in Fleshy Fruits
14.4.2.1 Protein Kinases and Phosphatases
Plant protein kinases and phosphatases include CDPKs, SNF1-related kinases
(SnRKs), MAPKs, a receptor-type kinase (RPK1), and protein phosphatase PP2Cs
(Hirayama and Shinozaki 2007 ). As described above, reversible protein phos-
phorylation has been demonstrated to play key roles in ABA signal transduction
through the protein kinase SnRK2 and the phosphataseABI1 (Fujii et al. 2009 ).
In grape berries, earlier studies revealed high activities of both the calcium-
dependent protein kinase (CDPK) and the mitogen-activated protein kinase
(MAPK) in the lag phase of fruit growth, prior to the ripening stage (Shen et al.
2004 ). The subsequent experiments lead to the identification and purification of
a 58-kD ABA-stimulated CDPK, ACPK1, which localizes to both the plasma