10
1
animals. At the same time, unripe fruits often contain bitter or even toxic components to
further deter animals from premature feeding. In rarer cases, plants want to deter
unwanted feeders. Consumption of fruits by these animals does not enable successful distribution of seeds, e.g. because they are damaged during digestion. Capsaicin is a wellstudied example for the deterrence of unwanted feeders and is discussed in detail in
7 Chap. 3.
Humans have become an important factor in seed dispersal that differs significantly
from those of animals. One important aspect is the high mobility of humans due to technological advances. Many crops but also ornamental plants and trees are nowadays grown
in areas very far away from their native range. Initially humans brought these plants with
them along trade routes or when they migrated to other countries or even continents.
Nowadays, global markets are satisfied by growing important and high-prized plants
wherever possible.
1.3.3 Defence Against Herbivores, Pathogens and Other Pests
Defence against biotic attack is maybe the most important and definitely the best studied
role of secondary metabolites. Phytopathogenic viruses, bacteria and fungi infect susceptible or wounded plants, aphids and other insects feed on their sap and herbivores, ranging from small caterpillars to full-size cattle, graze on plants as their food source. Therefore,
plants have evolved a variety of chemical defence systems to battle against pathogens and
herbivoral predators. Many secondary metabolites have antimicrobial properties.
Pathogen- induced defence metabolites are called phytoalexins and include compounds
such as isoflavonoids, terpenes, alkaloids and polyacetylenes (Ahuja et al. 2012). Bitterness
and toxicity of secondary metabolites are a common way for plants to avoid being eaten or
to at least reduce the extent of feeding to a manageable level. Accordingly, animals will
either shun a toxic plant completely or will limit its intake.
In order for secondary metabolites to act as effective deterrents for herbivores, they
must be recognized by the predator in a way to ensure the avoidance reaction. Bittertasting compounds are directly sensed by taste receptors in the mouth. Toxins, however,
act on cellular targets in muscles, organs or brains only after they have been internalized.
To efficiently deter herbivores before too much damage is done to the plant tissue, a toxin
either has to kill very quickly or its presence needs to be perceived by the animal. This
might involve learning, in those cases where the toxin is not lethal but the effect is strong
enough to make feeding on the plant undesirable. While not very well studied, it is known
from Drosophila that metabolites such as caffeine and strychnine are recognized by specific
Box 1.2 Did Humans Save the Avocado from Extinction?
During the Cenozoic era, when large mammals and other megafauna roamed the earth, many
plants also developed large seeds and fruits. However, these large seed dispersers disappeared
around 13000 years ago, and yet, fruits such as the avocado retained their large size. While surviving for a long time in small ranges probably aided by the occasional scattering of seeds via
unusual disperser, e.g. jaguars, the plants have celebrated their comeback since about 4000–2800
B.C, when its cultivation by humans in Mesoamerica begun. Also other plants that lost their large
seed dispersers have faced slow extinction by dwindling numbers but are now rescued by human
intervention (Janzen and Martin 1982).
Chapter 1 · Plant Secondary Metabolites and Their General Function in Plants
1
animals. At the same time, unripe fruits often contain bitter or even toxic components to
further deter animals from premature feeding. In rarer cases, plants want to deter
unwanted feeders. Consumption of fruits by these animals does not enable successful distribution of seeds, e.g. because they are damaged during digestion. Capsaicin is a wellstudied example for the deterrence of unwanted feeders and is discussed in detail in
7 Chap. 3.
Humans have become an important factor in seed dispersal that differs significantly
from those of animals. One important aspect is the high mobility of humans due to technological advances. Many crops but also ornamental plants and trees are nowadays grown
in areas very far away from their native range. Initially humans brought these plants with
them along trade routes or when they migrated to other countries or even continents.
Nowadays, global markets are satisfied by growing important and high-prized plants
wherever possible.
1.3.3 Defence Against Herbivores, Pathogens and Other Pests
Defence against biotic attack is maybe the most important and definitely the best studied
role of secondary metabolites. Phytopathogenic viruses, bacteria and fungi infect susceptible or wounded plants, aphids and other insects feed on their sap and herbivores, ranging from small caterpillars to full-size cattle, graze on plants as their food source. Therefore,
plants have evolved a variety of chemical defence systems to battle against pathogens and
herbivoral predators. Many secondary metabolites have antimicrobial properties.
Pathogen- induced defence metabolites are called phytoalexins and include compounds
such as isoflavonoids, terpenes, alkaloids and polyacetylenes (Ahuja et al. 2012). Bitterness
and toxicity of secondary metabolites are a common way for plants to avoid being eaten or
to at least reduce the extent of feeding to a manageable level. Accordingly, animals will
either shun a toxic plant completely or will limit its intake.
In order for secondary metabolites to act as effective deterrents for herbivores, they
must be recognized by the predator in a way to ensure the avoidance reaction. Bittertasting compounds are directly sensed by taste receptors in the mouth. Toxins, however,
act on cellular targets in muscles, organs or brains only after they have been internalized.
To efficiently deter herbivores before too much damage is done to the plant tissue, a toxin
either has to kill very quickly or its presence needs to be perceived by the animal. This
might involve learning, in those cases where the toxin is not lethal but the effect is strong
enough to make feeding on the plant undesirable. While not very well studied, it is known
from Drosophila that metabolites such as caffeine and strychnine are recognized by specific
Box 1.2 Did Humans Save the Avocado from Extinction?
During the Cenozoic era, when large mammals and other megafauna roamed the earth, many
plants also developed large seeds and fruits. However, these large seed dispersers disappeared
around 13000 years ago, and yet, fruits such as the avocado retained their large size. While surviving for a long time in small ranges probably aided by the occasional scattering of seeds via
unusual disperser, e.g. jaguars, the plants have celebrated their comeback since about 4000–2800
B.C, when its cultivation by humans in Mesoamerica begun. Also other plants that lost their large
seed dispersers have faced slow extinction by dwindling numbers but are now rescued by human
intervention (Janzen and Martin 1982).
Chapter 1 · Plant Secondary Metabolites and Their General Function in Plants