Aroma Volatiles - Fruit Ripening: Physiology, Signalling and Genomics - page 71

Shikimate pathway

Chorismate

Chorismate mutase
(CM)

Anthranilate synthase
(AS α , AS β )

Anthranilate

Prephenate

Prephenate
dehydrogenase
(PDH)

Prephenate
dehydratase
(PDT)

Prephenate
aminotransferase
(PAT)

Phosphoribosylanthranilate

p -Hydroxyphenylpyruvate Arogenate Phenylpyruvate

Phosphoribosylanthranilate
isomerase (PAI)

Aromatic amino acid
aminotransferase
(AAAAT)

Arogenate
dehydrogenase
(TyrA)

Arogenate
dehydratase
(ADT)

Aromatic amino acid
aminotransferase
(AAAAT)

1-( o -carboxyphenylamino)-1-
deoxyribulose-5-phosphate
Indole-3-clycerol phosphate
synthase (IGPS)
Indole-3-glycerol phosphate

Tyrosine Phenylalanine

Tryptophan synthase α -subunit

(TS α )

Indole

Tryptophan synthase β -subunit

(TS β )

Tryptophan

Fig. 5.2. Biosynthesis pathways of the aromatic amino acids in plants . From Tzin and Galili (2010).

Although amino acids are known as
important precursors in the generation of
aroma volatiles, the initial steps in the
catabolism of amino acids into volatiles
remains unclear. The results from tomato
fruit showed that conversion of
L -phenylalanine into aroma volatiles is
initially catalysed by decarboxylation
followed by deamination (Tieman et al. ,
2006). Three gene family members en-
coding aromatic L -amino acid decarbo-
xylase (AADC) were cloned from tomato
fruit, in which overexpression of
LeAADC1A and LeAADC2 resulted in
tenfold increases in the emission of aroma
volatiles derived from phenethylamine,
whilst antisense reduction of LeAAD2
produced signifi cantly lower levels (Tieman
et al. , 2006). In contrast, studies from
melon fruit showed that transamination of
amino acids through aromatic amino acid
transaminase (ArAT) or branched-chain
amino acid transaminase (BCAT) was the
initial step involved in the formation of
branched-chain volatiles (Gonda et al. ,
2010). Functional expression of CmArAT1
and CmBCAT1 in Escherichia coli had
aromatic and branched-chain amino acid
activities and converted amino acids into
corresponding aroma volatiles. Accumu-
lation of aroma volatiles is concomitant

with increased expression of CmArAT1 and
CmBCAT1 during melon fruit ripening. In
addition, ripe melon fruit of climacteric
aromatic cultivars showed high expression
of CmArAT1 and CmBCAT1 during
ripening in contrast to non-climacteric non-
aromatic fruit (Gonda et al. , 2010). In
banana fruit, the production of branched-
chain esters such as 3-methylbutyl acetate,
3-methylbutyl butanoate and 3-methylbutyl
2-methylbutanoate was signifi cantly in-
creased by ethylene treatment during
postharvest ripening (Yang et al. , 2011).
The expression of banana BanBCAT was
induced during fruit ripening and showed
signifi cantly higher levels in ethylene-
treated fruit than in the controls (Yang et
al. , 2011).

5.4 Regulation of Fruit Aroma Volatiles

5.4.1 Cultivation practices

The formation of aroma volatiles is a
dynamic process, and changes in the
volatile profi le are both qualitative and
quantitative during fruit growth and
ripening. Grassy-note aroma volatiles such
as C6 aldehydes and alcohols showed a
decreasing trend during peach and