2.1.3 Aromatic Compounds
Aromatic compounds are those which contain at least one conjugated planar ring.
Like terpenoids and fatty acid derivatives, they are very common in flowers across
plant families [32, 47] and are involved in pollinator attraction [56] and leaf defense
[13]. Aromatic VOCs are common in ripe fruit [15, 38–40, 51] and constitute
a significant portion of the scent emitted by wild fruits in Uganda and Madagascar
[15]. The vast majority of volatile plant aromatic compounds are synthesized by
a complex biosynthetic process whose precursors are aromatic amino acids synthesized via the shikimate pathway [33, 57, 58]. VOC synthesis is the result of
deamination of the amino acid L-phenylalanine and reduction to C9 compounds
[33, 49, 57]. One volatile product of this process is trans-cinnamic acid, which was
identified in several Malagasy fruit species in its methyl ester form [51] (Fig. 1, 6).
Further reduction of trans-cinnamic acid by removal of a C2 unit is the basis for
synthesis of many other aromatic VOCs [49]. Other common aromatic compounds
include methyl- and ethyl-salicylate, both esterized forms of the phytopheromone
salicylic acid [59, 60]. While of lesser importance compared to other plant VOCs
like terpenoids, aromatic compounds also play a role in both active and passive leaf
defense [13, 58] and possibly play a role in fruit defense too [16].
2.1.4 Nitrogen- and Sulfur-Containing Compounds
While terpenoids, fatty acid derivatives, and aromatic compounds dominate the fruit
scent profiles of most cultivated and wild species [14, 15, 51], fruits of some species
emit less common compounds. Several wild fruits in Madagascar have been found to
contain nitrogen- and sulfur-containing compounds [15, 51], although even in these
species the relative contribution of these compounds to the scent profile was minor.
However, compounds of these classes dominate the scent profiles of at least one wild
species, the (in)famous durian (Durio sp.). Durian fruits, which are also cultivated in
Southeast Asia, are known for their strong and distinct scent, although some cultivars
are almost odorless [61]. While there has been some debate over which specific VOCs
are responsible for the foul scent, it is known that Durian scent contains, in addition to
more typical compounds like alcohols and esters, primarily sulfur-containing compounds [61–64]. Notably, as opposed to some past claims, the sulfur-containing
compounds are synthesized by the fruit itself and not by bacteria inhabiting the flesh
[65]. Interestingly, nitrogen- and sulfur-containing compounds are a product of protein
metabolism [47]. This led to the speculation that their presence and amount in fruit
scent could serve as an honest signal of protein content [15].
2.2
Patterns of Scent Emission
Although based on few cultivated model systems, it appears that at least in some
species fruit VOCs are synthesized by specialized cells situated on the fruit’s skin
[66]. It is generally assumed that VOC release is predominantly passive through
diffusion and that therefore it can only be regulated by up- or downregulating VOC
synthesis [67]. Indeed, VOC synthesis is strongly regulated by the presence and
17 Fruit Scent: Biochemistry, Ecological Function, and Evolution
407
Aromatic compounds are those which contain at least one conjugated planar ring.
Like terpenoids and fatty acid derivatives, they are very common in flowers across
plant families [32, 47] and are involved in pollinator attraction [56] and leaf defense
[13]. Aromatic VOCs are common in ripe fruit [15, 38–40, 51] and constitute
a significant portion of the scent emitted by wild fruits in Uganda and Madagascar
[15]. The vast majority of volatile plant aromatic compounds are synthesized by
a complex biosynthetic process whose precursors are aromatic amino acids synthesized via the shikimate pathway [33, 57, 58]. VOC synthesis is the result of
deamination of the amino acid L-phenylalanine and reduction to C9 compounds
[33, 49, 57]. One volatile product of this process is trans-cinnamic acid, which was
identified in several Malagasy fruit species in its methyl ester form [51] (Fig. 1, 6).
Further reduction of trans-cinnamic acid by removal of a C2 unit is the basis for
synthesis of many other aromatic VOCs [49]. Other common aromatic compounds
include methyl- and ethyl-salicylate, both esterized forms of the phytopheromone
salicylic acid [59, 60]. While of lesser importance compared to other plant VOCs
like terpenoids, aromatic compounds also play a role in both active and passive leaf
defense [13, 58] and possibly play a role in fruit defense too [16].
2.1.4 Nitrogen- and Sulfur-Containing Compounds
While terpenoids, fatty acid derivatives, and aromatic compounds dominate the fruit
scent profiles of most cultivated and wild species [14, 15, 51], fruits of some species
emit less common compounds. Several wild fruits in Madagascar have been found to
contain nitrogen- and sulfur-containing compounds [15, 51], although even in these
species the relative contribution of these compounds to the scent profile was minor.
However, compounds of these classes dominate the scent profiles of at least one wild
species, the (in)famous durian (Durio sp.). Durian fruits, which are also cultivated in
Southeast Asia, are known for their strong and distinct scent, although some cultivars
are almost odorless [61]. While there has been some debate over which specific VOCs
are responsible for the foul scent, it is known that Durian scent contains, in addition to
more typical compounds like alcohols and esters, primarily sulfur-containing compounds [61–64]. Notably, as opposed to some past claims, the sulfur-containing
compounds are synthesized by the fruit itself and not by bacteria inhabiting the flesh
[65]. Interestingly, nitrogen- and sulfur-containing compounds are a product of protein
metabolism [47]. This led to the speculation that their presence and amount in fruit
scent could serve as an honest signal of protein content [15].
2.2
Patterns of Scent Emission
Although based on few cultivated model systems, it appears that at least in some
species fruit VOCs are synthesized by specialized cells situated on the fruit’s skin
[66]. It is generally assumed that VOC release is predominantly passive through
diffusion and that therefore it can only be regulated by up- or downregulating VOC
synthesis [67]. Indeed, VOC synthesis is strongly regulated by the presence and
17 Fruit Scent: Biochemistry, Ecological Function, and Evolution
407