12
1
costs. Typical components of the constitutive defence system are, for example,
antimicrobial- acting phenols, lignins and tannins that are commonly found in cell walls
and vacuoles (Rehman et al. 2012). Also, cyanogenic glycosides and other glycosides such
as saponins are often expressed in a constitutive manner. In line with the complexity of
environmental challenges, plants often contain mixtures of different defence metabolites.
Thereby, the effectiveness of defence compounds can be increased, and resistance to a
specific metabolite is less likely to occur (Wittstock and Gershenzon 2002). With constitutively expressed defence compounds, the economy of resources advocates a preferential
allocation to those tissues or parts of a plant that are more prone to herbivory or pathogen
attack.
Alternatively, a specific compound can be synthesized only when required, i.e. a
defence compound that is produced after an initial attack by a herbivore or a pathogen. In
these cases, induced synthesis often has a spatial as well as temporal component. Indeed,
combinations of these strategies can be found with a low level of constitutive expression
aided by an increased production upon stress or attack. This interplay of constitutive and
induced expression is, for example, well studied in tobacco. While tobacco plants always
contain a basal level of nicotine – it is indeed the most abundant alkaloid in tobacco
leaves – it was shown that biotic attack leads to an increased synthesis (Baldwin et al.
1997) and thereby enhanced protection of the plant.
1.3.6 Counterstrategies of the Attacker and Use of Plant-Derived
Secondary Metabolites by Other Animals
Co-evolution of plants and animals has not only produced systems of plant protection.
Herbivores have often developed means to overcome toxicity of defence compounds,
thereby enabling them to feed on plants that would otherwise be toxic (Foley and Moore
2005). This is less important for specific rare toxins but essential for secondary metabolites
that are more common. Even with low toxicity, these might affect an animal when digested
in higher amounts or if they accumulate in the body over time. Several different systems
have evolved in animals to allow the consumption of toxic plant material (Heckel 2014).
Mutations in the cellular target of a toxin that renders it insensitive are the most efficient
way to protect an animal from the toxic effects. Also, if active transport is required, uptake
of toxic compounds can be eliminated or reduced by changes in the affinity of transporters. Once internalized, metabolic degradation or alteration of the toxin can counter toxicity. The large superfamily of cytochrome P450 enzymes plays an important role in this
system. Cytochrome P450 proteins are found abundantly in organs such as the liver and
kidney, where they oxidize or hydrolyse toxic compounds to yield a product that can subsequently be conjugated with hydrophilic molecules such as glucuronic acid. This detoxifies the compounds and/or aids in their excretion. Indeed, glycosylation of toxins is a
similar strategy that plants use to avoid auto-toxicity (see below). These detoxification
systems are costly and often limited to smaller amounts of toxins. Thus, while allowing the
herbivore to feed on a specific plant, they still protect the plant from overgrazing. Also,
fungi have been shown to metabolize defensive plant saponins by the use of secreted
enzymes (Morrissey et al. 2000).
Moreover, once animals have adapted these protective systems, they often use
plant- derived metabolites for their own defence against predators or as precursors for
pheromonic substances (Wittstock and Gershenzon 2002). For example, monarch butChapter 1 · Plant Secondary Metabolites and Their General Function in Plants
1
costs. Typical components of the constitutive defence system are, for example,
antimicrobial- acting phenols, lignins and tannins that are commonly found in cell walls
and vacuoles (Rehman et al. 2012). Also, cyanogenic glycosides and other glycosides such
as saponins are often expressed in a constitutive manner. In line with the complexity of
environmental challenges, plants often contain mixtures of different defence metabolites.
Thereby, the effectiveness of defence compounds can be increased, and resistance to a
specific metabolite is less likely to occur (Wittstock and Gershenzon 2002). With constitutively expressed defence compounds, the economy of resources advocates a preferential
allocation to those tissues or parts of a plant that are more prone to herbivory or pathogen
attack.
Alternatively, a specific compound can be synthesized only when required, i.e. a
defence compound that is produced after an initial attack by a herbivore or a pathogen. In
these cases, induced synthesis often has a spatial as well as temporal component. Indeed,
combinations of these strategies can be found with a low level of constitutive expression
aided by an increased production upon stress or attack. This interplay of constitutive and
induced expression is, for example, well studied in tobacco. While tobacco plants always
contain a basal level of nicotine – it is indeed the most abundant alkaloid in tobacco
leaves – it was shown that biotic attack leads to an increased synthesis (Baldwin et al.
1997) and thereby enhanced protection of the plant.
1.3.6 Counterstrategies of the Attacker and Use of Plant-Derived
Secondary Metabolites by Other Animals
Co-evolution of plants and animals has not only produced systems of plant protection.
Herbivores have often developed means to overcome toxicity of defence compounds,
thereby enabling them to feed on plants that would otherwise be toxic (Foley and Moore
2005). This is less important for specific rare toxins but essential for secondary metabolites
that are more common. Even with low toxicity, these might affect an animal when digested
in higher amounts or if they accumulate in the body over time. Several different systems
have evolved in animals to allow the consumption of toxic plant material (Heckel 2014).
Mutations in the cellular target of a toxin that renders it insensitive are the most efficient
way to protect an animal from the toxic effects. Also, if active transport is required, uptake
of toxic compounds can be eliminated or reduced by changes in the affinity of transporters. Once internalized, metabolic degradation or alteration of the toxin can counter toxicity. The large superfamily of cytochrome P450 enzymes plays an important role in this
system. Cytochrome P450 proteins are found abundantly in organs such as the liver and
kidney, where they oxidize or hydrolyse toxic compounds to yield a product that can subsequently be conjugated with hydrophilic molecules such as glucuronic acid. This detoxifies the compounds and/or aids in their excretion. Indeed, glycosylation of toxins is a
similar strategy that plants use to avoid auto-toxicity (see below). These detoxification
systems are costly and often limited to smaller amounts of toxins. Thus, while allowing the
herbivore to feed on a specific plant, they still protect the plant from overgrazing. Also,
fungi have been shown to metabolize defensive plant saponins by the use of secreted
enzymes (Morrissey et al. 2000).
Moreover, once animals have adapted these protective systems, they often use
plant- derived metabolites for their own defence against predators or as precursors for
pheromonic substances (Wittstock and Gershenzon 2002). For example, monarch butChapter 1 · Plant Secondary Metabolites and Their General Function in Plants