11
1
taste receptors in the mouth (Lee et al. 2009, 2015). These receptors belong to the same
family of seven-transmembrane gustatory receptors that allow animals to detect different
kinds of taste such as bitter, salty, sweet, sour or umami, but they are very specific for the
compounds that should be avoided. Thereby, the animals can detect the presence of a
toxin early on and avoid continued feeding.
In a more indirect manner, plants have also developed “the enemy of my enemy is my
friend” strategies to fight pests and herbivores. It is quite common that plants release volatiles that attract parasitic or predatory enemies of an attacker and help them to identify
invested plants (Takabayashi and Dicke 1996). Various species have been shown to attract
predatory mites when invested by plant-feeding mites or predatory insects when attacked
by larvae. The Southern catalpa tree, for instance, increases the sugar content of its extrafloral nectar to attract ants when attacked by caterpillars of the moth Ceratomia catalpae
(Ness 2003).
1.3.4 Plant-to-Plant Communication
It has also been suggested that plants use volatile secondary metabolites to communicate
with each other in response to abiotic and biotic stress (Karban et al. 2014). These volatiles
are often collectively labelled as green leaf volatiles (Dudareva et al. 2013). Brought forward in the early 1980s, the idea of plant-to-plant communication was initially discredited
by many scientists and has only been studied thoroughly since the 1990s (Farmer and
Ryan 1990). Volatile-mediated communication has now been shown for over 30 different
species including trees, shrubs and herbaceous plants, albeit mostly as a response to herbivore attack and under laboratory conditions. By contrast, evidence for the use of volatiles to communicate abiotic stress is rather scarce. Moreover, it should be noted that there
is still an ongoing debate, whether plants use these volatiles to explicitly communicate
with other plants. Alternatively, or additionally, they could be used to warn potential
predators of their defensive status or to quickly distribute information across distances
within the context of a single plant, e.g. within the crown of a tree. In this case, other plants
can simply “listen” into the communication as long as the compounds are not yet dispersed too much.
1.3.5 Constitutive versus Induced Expression
As mentioned above, the production of secondary metabolites is quite costly for a plant.
Resources allocated to their synthesis cannot be used for plant growth or production of
seeds. Careful consideration has to be given, whether a compound is synthesized at all
times and in all parts of the plant. Therefore, the presence of many secondary metabolites
is restricted to tissues that are potential targets of an attack, i.e. only in roots or leaves or
floral nectaries. Also, attractants or defence compounds might only be present at a certain
time or developmental stage. Depending on the specific environment and grade of occurrence of specific stresses on one hand and the cost of synthesis and storage of a secondary
metabolite on the other hand, the plant has to “decide” on a constitutive or induced expression of a specific substance.
If a stress or attack occurs frequently or the speed of synthesis is too slow to be effective
before existential damage is done, constitutive expression might be favourable despite the
1.3 · Overview of the Function of Secondary Metabolites in Plants (Details in Part IV)
1
taste receptors in the mouth (Lee et al. 2009, 2015). These receptors belong to the same
family of seven-transmembrane gustatory receptors that allow animals to detect different
kinds of taste such as bitter, salty, sweet, sour or umami, but they are very specific for the
compounds that should be avoided. Thereby, the animals can detect the presence of a
toxin early on and avoid continued feeding.
In a more indirect manner, plants have also developed “the enemy of my enemy is my
friend” strategies to fight pests and herbivores. It is quite common that plants release volatiles that attract parasitic or predatory enemies of an attacker and help them to identify
invested plants (Takabayashi and Dicke 1996). Various species have been shown to attract
predatory mites when invested by plant-feeding mites or predatory insects when attacked
by larvae. The Southern catalpa tree, for instance, increases the sugar content of its extrafloral nectar to attract ants when attacked by caterpillars of the moth Ceratomia catalpae
(Ness 2003).
1.3.4 Plant-to-Plant Communication
It has also been suggested that plants use volatile secondary metabolites to communicate
with each other in response to abiotic and biotic stress (Karban et al. 2014). These volatiles
are often collectively labelled as green leaf volatiles (Dudareva et al. 2013). Brought forward in the early 1980s, the idea of plant-to-plant communication was initially discredited
by many scientists and has only been studied thoroughly since the 1990s (Farmer and
Ryan 1990). Volatile-mediated communication has now been shown for over 30 different
species including trees, shrubs and herbaceous plants, albeit mostly as a response to herbivore attack and under laboratory conditions. By contrast, evidence for the use of volatiles to communicate abiotic stress is rather scarce. Moreover, it should be noted that there
is still an ongoing debate, whether plants use these volatiles to explicitly communicate
with other plants. Alternatively, or additionally, they could be used to warn potential
predators of their defensive status or to quickly distribute information across distances
within the context of a single plant, e.g. within the crown of a tree. In this case, other plants
can simply “listen” into the communication as long as the compounds are not yet dispersed too much.
1.3.5 Constitutive versus Induced Expression
As mentioned above, the production of secondary metabolites is quite costly for a plant.
Resources allocated to their synthesis cannot be used for plant growth or production of
seeds. Careful consideration has to be given, whether a compound is synthesized at all
times and in all parts of the plant. Therefore, the presence of many secondary metabolites
is restricted to tissues that are potential targets of an attack, i.e. only in roots or leaves or
floral nectaries. Also, attractants or defence compounds might only be present at a certain
time or developmental stage. Depending on the specific environment and grade of occurrence of specific stresses on one hand and the cost of synthesis and storage of a secondary
metabolite on the other hand, the plant has to “decide” on a constitutive or induced expression of a specific substance.
If a stress or attack occurs frequently or the speed of synthesis is too slow to be effective
before existential damage is done, constitutive expression might be favourable despite the
1.3 · Overview of the Function of Secondary Metabolites in Plants (Details in Part IV)