Aroma Volatiles - Fruit Ripening: Physiology, Signalling and Genomics - page 67

volatile compounds (Wang et al. , 2009).
Differences in the aroma volatiles of fruit
cultivars have also been reported in plum
(Chai et al. , 2012), mango (Pino et al. ,
2005) and kiwifruit (Garcia et al. , 2012).

phases are involved in the basic pathway
of volatile terpenoid biosynthesis: (i)
formation of the C5 units; (ii) condensation
of C5 units into C10, C15 or C20 prenyl
diphosphates; and (iii) conversion of the
resulting prenyl diphosphates to end
products (reviewed by Dudareva et al. ,
2004). In plants, the fi ve-carbon com-
pound isopentenyl pyrophosphate (IPP)
and its isomer dimethylallyl pyrophos-
phate (DMAPP) are produced through
either the plastidic methylerythritol phos-
phate pathways or cytosolic mevalonate.
The methylerythritol phosphate pathway is
responsible for the formation of hemiter-
pene, monoterpene and diterpene by
providing IPP and DMAPP. In the cytosol,
one DMAPP and two IPP molecules are
condensed, resulting in farnesyl pyro-
phosphate (FPP), which is further con-
verted into sesquiterpene. Recently, a novel
pathway for sesquiterpene biosynthesis
from Z , Z -FPP in the wild tomato has been
discovered, suggesting that sesquiterpene
could be generated using IPP and DMAPP
from the plastidic 1-deoxy- D -xylulose
5-phosphate (DXP) pathway (Sallaud et al. ,
2009).
Terpene synthases (TPSs) catalyse
the formation of hemi-, mono-, sesqui-
and diterpenes from DMAPP, geranyl

5.3 Pathways for the Biosynthesis of
Aroma Volatiles

The volatile compounds that are re-
sponsible for the aroma quality of fruit are
usually small molecules with low boiling
points and high vapour pressures at
ambient temperature. These compounds
contain different chemical functional
groups such as aldehydes, alcohols, esters,
lactones, alkenes and ethers, which are
derived from many different biosynthesis
pathways. Generally, fatty acids, carbo-
hydrates and amino acids are the major
natural carbon pools for the biosynthesis of
aroma volatiles (Fig. 5.1).

5.3.1 Carbohydrate pathway

Terpenoids consist of hemiterpenes (C5),
monoterpenes (C10), sesquiterpenes (C15)
and diterpenes (C20) and are the largest
class of plant secondary metabolites. Three

Isothiocyanates
Thiocyanates
Nitriles

R

Disulfides
Trisulfides
Thiosulfinates

R

C n H m C x H y

R

C n H m

Methylbranched:
alcohols
aldehydes
acids
esters

R S R'

Aliphatic:
alcohols
aldehydes
methylketones
acids
lactones
esters

R'

S-containing:
thioester
thioether
thiols

Aldehydes
Ketones

R''

Hydrolysis

Aromatic:
alcohol
aldehyde
acids
ester

Glucosinolates

Hydrolysis

H
N

R R'

Degradation

Cyanogenic glycosides

Amines:
indol

Oxidation
lox pathway

R Terpenoids:

mono-
sesqui-
apocarotenals

S -alk(en)yl cysteine sulphoxides

Phenylpropanes
Benzenoids

Degradation

Terpenoid
pathway

Biosynthesis

O R

Furanones

Degradation

Fatty acids

Pyrones

R'

O

Carbohydrates

CO 2 Amino acids

Fig. 5.1. Biosynthesis pathways of volatile compounds in plants. From Schwab et al. (2008).