Aroma Volatiles - Fruit Ripening: Physiology, Signalling and Genomics - page 70

acetate (Defi lippi et al. , 2009a). Despite
strong sequence identity, SAAT and VAAT
(wild strawberry) proteins differ in their
substrate preference (Beekwilder et al. ,
2004). Site-directed mutagenesis has
demonstrated that a threonine residue
plays a crucial role in AAT enzyme activity
(El-Sharkawy et al. , 2005). AAT enzyme
activity and gene expression increase
during fruit ripening (Defi lippi et al. ,
2009a).
The breakdown of fatty acids through
oxidation at the E -carbon and the sub-
sequent removal of two carbon units was
fi rst discovered in 1904, and the detailed
mechanisms of E -oxidation have been
reviewed (Baker et al. , 2006). Analysis of
mutants has revealed essential roles for
E -oxidation in plant development and in
response to stresses (Goepfert and Poirier,
2007). Involvement of E -oxidation in the
biosynthesis of volatile aroma lactones has
been suggested (Schwab et al. , 2008).
Aliphatic long-chain acyl-CoA is fi rst
converted to 2- trans -enoyl-CoA by acyl-
CoA oxidase (ACX) and fi nally yields an
acyl-CoA molecule. During the E -oxidation
of fatty acids, the breakdown of acetyl-CoA
can be stopped between E -oxidation cycles
or inside the reaction sequence as a result
of many factors, resulting in the liberation
of volatile lactones (Husan, 2010).
Although lactones play an important
sensory role in fruit aroma quality, there is
a lack of information on the char-
acterization of both the enzymes and the
genes associated with their biosynthesis.
ACX is the fi rst enzyme involved in fatty
acid E -oxidation and is regarded as a key
step controlling fl ux through the pathway
(Arent et al. , 2008). ACX is widely involved
in embryo development, seed germination,
seedling establishment, natural senescence
and the biosynthesis of jasmonic acid in
response to stresses (Baker et al. , 2006; Yang
and Ohlrogge, 2009). Because model plants
such as Arabidopsis , rice and tomato lack
lactone compounds, an association of ACX
with fruity-note lactone formation is
unclear. In peach fruit, there are at least
four ACX gene members, of which PpACX1
has a distinct expression profi le. Transcript

levels of PpACX1 increase with accumu-
lated lactones during peach fruit ripening,
and a positive correlation between long-
chain ACX activity and lactones has been
reported (Xi et al. , 2012).

5.3.3 Amino acid pathway

Branched-chain volatile compounds, in-
cluding alcohols, aldehydes, esters, lac-
tones, acids and sulfur-containing aroma
compounds, are important for fruit aroma.
The important amino acids responsible for
the biosynthesis of aromatic volatile com-
pounds are leucine, isoleucine, valine,
alanine, phenylalanine, tyrosine and trypto-
phan (Defi lippi et al. , 2009a; Tzin and
Galili, 2010). Volatile compounds derived
from leucine, such as 3-methyl butanoic
acid and 2-methyl butanoic acid, and
from phenylalanine, such as benzaldehyde,
phenylacetaldehyde, benzyl alcohol,
2-phenylethanol, eugenol and chavicol,
have been identifi ed from peach, grape,
tomato and strawberry fruit (Schwab et al. ,
2008; Eduardo et al. , 2010). Branched-chain
esters, including 2-methylpropyl acetate,
2-methylbutyl acetate, 3-methylbutyl
acetate and 3-methylbutyl butanoate, were
the most abundant volatiles in ripe banana
fruit (Yang et al. , 2011). The ester
2-methylbutyl acetate has a strong apple
scent and is associated with apple fruit,
and 2-methyl butanoate determines the
characteristic aroma of prickly pear
(reviewed by Schwab et al. , 2008).
The biosynthesis pathways of aromatic
amino acids and their regulation have been
explored extensively in bacteria because of
their utility in the food and drug industry.
In plants, the aromatic amino acids are
synthesized through the shikimate path-
way, with chorismate serving as a major
intermediate branch point metabolite (Fig.
5.2). The progress in terms of the char-
acterization of enzymes and genes and the
regulation of aromatic amino acid bio-
synthesis has recently been reviewed by
Tzin and Galili (2010), whose evidence is
mainly based on results from the model
plant Arabidopsis .