important seed dispersal vectors, its evolution, and other functions it may fulfill.
We end with recommendation for future studies, in the hope that the next decade
will be at least as fruitful as the previous one.
Keywords
Co-evolution · Constraints · Frugivory · Mutualism · Odor · Olfaction · Seed
dispersal · Sense of smell
1
Introduction
Like all plant tissues, fruits are packed with secondary metabolites [1]. This diversity
of secondary metabolites has been suggested to fulfill a plethora of ecological
functions, from attraction of seed-dispersing frugivores (fruit-eating animals),
through regulation of their interaction with the seeds, to repellance of fruit antagonists [2, 3]. In contrast, it has been suggested that fruit secondary metabolites are
primarily defensive and in most cases similar to those present in nonreproductive
tissue or unripe fruits, thus making them an extension of the plant’s general line of
defense [4, 5]. The assertion that fruit secondary metabolites are primarily the result
of “leakage” from nonreproductive tissue has since been refuted [6, 7]. Nonetheless,
among the myriad possible functions of secondary metabolites, defense against
microbial, insect, and vertebrate antagonists has been in the focus of most studies
[6–11]. This is not surprising given that in other plant tissues, the most explored and
probably also prominent function of plant secondary metabolites is defense [12].
The prominence of defense in the discussion regarding fruit secondary metabolites can probably be attributed to the fact that most studies have focused on the
large, nonvolatile, secondary metabolites. Indeed, many large, often water-soluble,
secondary metabolites such as cyanogenic glucosides, glucosides, and polyphenols
are defensive [12, 13]. But much less attention has been given to volatile secondary
compounds (or volatile organic compounds: henceforth VOCs) – lighter and often
more hydrophobic compounds that constitute what we would colloquially recognize
as “scent” or “odor” [14, 15]. VOCs are ubiquitous in fruits and some are likely to
play a role in fruit defense [16, 17]. Yet as recognized by the much more developed
study of commercial fruit production, they are responsible for the aroma of fruits and
their attractivity to human consumers [14, 18, 19]. Thus, another reason explaining
the focus on the defensive rather than attractive function of fruit secondary metabolites is likely to have originated from the fact that VOCs of wild fruits have until
recently rarely been studied [14, 15].
Over the past decade, the interest in wild fruit VOCs has increased substantially,
in particular in their role as an attractant of animal seed dispersers [20]. This has been
the result of a growing understanding that despite various constraints [21, 22], many
fruit traits are likely to have evolved in response to animal seed disperser preferences
[23–26]. Along with the proliferation of methods that allow analysis of scent [27,
28], the decades-old hypothesis that ripe fruit scent – i.e., VOC profile – is
an evolved trait whose function is to attract seed dispersers [29] began receiving
404
O. Nevo and M. Ayasse
We end with recommendation for future studies, in the hope that the next decade
will be at least as fruitful as the previous one.
Keywords
Co-evolution · Constraints · Frugivory · Mutualism · Odor · Olfaction · Seed
dispersal · Sense of smell
1
Introduction
Like all plant tissues, fruits are packed with secondary metabolites [1]. This diversity
of secondary metabolites has been suggested to fulfill a plethora of ecological
functions, from attraction of seed-dispersing frugivores (fruit-eating animals),
through regulation of their interaction with the seeds, to repellance of fruit antagonists [2, 3]. In contrast, it has been suggested that fruit secondary metabolites are
primarily defensive and in most cases similar to those present in nonreproductive
tissue or unripe fruits, thus making them an extension of the plant’s general line of
defense [4, 5]. The assertion that fruit secondary metabolites are primarily the result
of “leakage” from nonreproductive tissue has since been refuted [6, 7]. Nonetheless,
among the myriad possible functions of secondary metabolites, defense against
microbial, insect, and vertebrate antagonists has been in the focus of most studies
[6–11]. This is not surprising given that in other plant tissues, the most explored and
probably also prominent function of plant secondary metabolites is defense [12].
The prominence of defense in the discussion regarding fruit secondary metabolites can probably be attributed to the fact that most studies have focused on the
large, nonvolatile, secondary metabolites. Indeed, many large, often water-soluble,
secondary metabolites such as cyanogenic glucosides, glucosides, and polyphenols
are defensive [12, 13]. But much less attention has been given to volatile secondary
compounds (or volatile organic compounds: henceforth VOCs) – lighter and often
more hydrophobic compounds that constitute what we would colloquially recognize
as “scent” or “odor” [14, 15]. VOCs are ubiquitous in fruits and some are likely to
play a role in fruit defense [16, 17]. Yet as recognized by the much more developed
study of commercial fruit production, they are responsible for the aroma of fruits and
their attractivity to human consumers [14, 18, 19]. Thus, another reason explaining
the focus on the defensive rather than attractive function of fruit secondary metabolites is likely to have originated from the fact that VOCs of wild fruits have until
recently rarely been studied [14, 15].
Over the past decade, the interest in wild fruit VOCs has increased substantially,
in particular in their role as an attractant of animal seed dispersers [20]. This has been
the result of a growing understanding that despite various constraints [21, 22], many
fruit traits are likely to have evolved in response to animal seed disperser preferences
[23–26]. Along with the proliferation of methods that allow analysis of scent [27,
28], the decades-old hypothesis that ripe fruit scent – i.e., VOC profile – is
an evolved trait whose function is to attract seed dispersers [29] began receiving
404
O. Nevo and M. Ayasse