8
1
1.3.1 Adaptation to a Life on Land
A wide range of environmental stresses are potentially harmful to plants, and specific
protective compounds were developed during evolution. Secondary metabolites such as
flavonoids can already be found in algae, but chemical and genomic analyses suggest that
the expansion in specialized secondary metabolites started about 500 million years ago
with the evolution of land plants (Weng et al. 2012). The transition from a life in the ocean
to a conquest of earth’s surface was an enormous step that brought with it many new challenges including exposure to UV radiation, lack of structural support, drought stress and
attack by newly evolving herbivores and pathogens. Not surprisingly, this step is also
marked by a rise of new metabolic pathways to produce compounds helpful in addressing
these new threats. These include the formation of phenylpropanoids for UV protection in
the early land plants (Lowry et al. 1980), followed by the development of lignins (phenolic
compounds) in the vascular plants, which provide the necessary solid scaffold that allows
these plants to grow to heights not reached in any older lineages such as the bryophytes.
Lignin also formed a material well suited to protect the plant surface from damage by
wind or herbivores (Bateman et al. 1998), and both substance classes can already be found
in mosses. Cyanogenic glycosides are found from the pteridophytes onwards (Buchanan
et al. 2015), and evolutionarily their occurrence coincides closely with the emergence of
insects. Alkaloids emerged at about the same time but all in all secondary metabolites are
most abundant in flowering plants.
1.3.2 Attraction (or Deterrent) of Pollinators and Seed Dispersers
Animals play an important role for pollination and seed dispersal. Many plants can propagate by self-pollination or rely on wind or other forms of abiotic pollination. However,
flowering plants commonly depend on the work of animals to carry pollen from one
flower to another for cross-pollination. This allows a wider spreading of their pollen and
ensures the new combination of genes that is the hallmark of sexual reproduction. Indeed,
it is believed that the success of flowering plants after their first appearance about 200 million years ago is partially due to their efficient system of fertilization. While entomophily
(pollination by insects) is probably the most common form of biotic pollination, it can
also involve other animals such as birds and bats. During feeding, these animals come in
contact with the sexual organs of the plants, thereby transferring pollen from the stamen
of one plant to the stigma of another plant. An important feature for the attraction of pollinators are the often elaborate and colourful flowers of angiosperms. However, in addition to pigmentation, also flavours and volatile scents provide a means to attract insects or
other animals for fertilization. Last but not the least, a reward system in the form of nectar
ensures the ongoing cooperation between the plants and their pollinators.
While the attraction of pollinators is desired by the plant, the nutrient-rich nectar is
also a lure for unwanted predators. Nectar robbers partake in a meal without successful
pollination. They might reach the nectar via holes that they bite into the base of a flower,
or they use the floral opening but without contacting the anthers and stigma. Floral
larceny is quite common, but surprisingly very little studied (Irwin et al. 2004). Loss of
nectar without pollination can severely affect plant fitness, and therefore plants want to
discourage nectar robbers. However, in contrast to the deterrence of herbivores
Chapter 1 · Plant Secondary Metabolites and Their General Function in Plants
1
1.3.1 Adaptation to a Life on Land
A wide range of environmental stresses are potentially harmful to plants, and specific
protective compounds were developed during evolution. Secondary metabolites such as
flavonoids can already be found in algae, but chemical and genomic analyses suggest that
the expansion in specialized secondary metabolites started about 500 million years ago
with the evolution of land plants (Weng et al. 2012). The transition from a life in the ocean
to a conquest of earth’s surface was an enormous step that brought with it many new challenges including exposure to UV radiation, lack of structural support, drought stress and
attack by newly evolving herbivores and pathogens. Not surprisingly, this step is also
marked by a rise of new metabolic pathways to produce compounds helpful in addressing
these new threats. These include the formation of phenylpropanoids for UV protection in
the early land plants (Lowry et al. 1980), followed by the development of lignins (phenolic
compounds) in the vascular plants, which provide the necessary solid scaffold that allows
these plants to grow to heights not reached in any older lineages such as the bryophytes.
Lignin also formed a material well suited to protect the plant surface from damage by
wind or herbivores (Bateman et al. 1998), and both substance classes can already be found
in mosses. Cyanogenic glycosides are found from the pteridophytes onwards (Buchanan
et al. 2015), and evolutionarily their occurrence coincides closely with the emergence of
insects. Alkaloids emerged at about the same time but all in all secondary metabolites are
most abundant in flowering plants.
1.3.2 Attraction (or Deterrent) of Pollinators and Seed Dispersers
Animals play an important role for pollination and seed dispersal. Many plants can propagate by self-pollination or rely on wind or other forms of abiotic pollination. However,
flowering plants commonly depend on the work of animals to carry pollen from one
flower to another for cross-pollination. This allows a wider spreading of their pollen and
ensures the new combination of genes that is the hallmark of sexual reproduction. Indeed,
it is believed that the success of flowering plants after their first appearance about 200 million years ago is partially due to their efficient system of fertilization. While entomophily
(pollination by insects) is probably the most common form of biotic pollination, it can
also involve other animals such as birds and bats. During feeding, these animals come in
contact with the sexual organs of the plants, thereby transferring pollen from the stamen
of one plant to the stigma of another plant. An important feature for the attraction of pollinators are the often elaborate and colourful flowers of angiosperms. However, in addition to pigmentation, also flavours and volatile scents provide a means to attract insects or
other animals for fertilization. Last but not the least, a reward system in the form of nectar
ensures the ongoing cooperation between the plants and their pollinators.
While the attraction of pollinators is desired by the plant, the nutrient-rich nectar is
also a lure for unwanted predators. Nectar robbers partake in a meal without successful
pollination. They might reach the nectar via holes that they bite into the base of a flower,
or they use the floral opening but without contacting the anthers and stigma. Floral
larceny is quite common, but surprisingly very little studied (Irwin et al. 2004). Loss of
nectar without pollination can severely affect plant fitness, and therefore plants want to
discourage nectar robbers. However, in contrast to the deterrence of herbivores
Chapter 1 · Plant Secondary Metabolites and Their General Function in Plants