Sometimes, the same or other PSM are more directed against microorganisms
and help to ward off bacteria, fungi and viruses, which are abundant in the environment. Antimicrobial PSM interfere with biomembranes of microbes (saponins,
mono- and sesquiterpenes), of proteins (polyphenols), protein biosynthesis, and
DNA replication and transcription (many alkaloids). In addition to antimicrobial
PSM and antimicrobial peptides (AMPs), plants can block vessels by storing callose
or can inhibit microbial infection by secreting chitinase, glucanase, and peroxidase
[5, 11, 13–15].
Plants compete with other plants for light, water, and nutrients. In some instances,
for example, in desert plants, a strong competition between plants of the same or
different species can be observed, in a way that individual plants are surrounded by
an empty space. It could be demonstrated that plants excrete PSM from their
rhizosphere or from leaves, which can inhibit the germination or development of
other competing plants. This phenomenon has been termed “allelopathy” (Fig. 2)
[2, 5, 7, 19].
1.3
Functions of Plant Secondary Metabolites as Signal
Compounds
It appears to be contradictory that plants also use PSM as signal compounds to
attract pollinating insects, fruit-dispersing animals, or symbiotic root bacteria (Fig. 2).
Mostly, we see the same PSM, which are employed in defense against herbivores. How
to explain this contradiction? Flowering plants (angiosperms) produce conspicuous
flowers to attract pollinators. Attraction is achieved via colors (mostly anthocyanins,
some flavonoids, and carotenoids) which insect can perceive in normal but also UV
light. In addition, many plants employ aromatic PSM (mostly terpenoids) as an
additional olfactory attractants. Some plants produce foully smelling PSM, such as
amines to attract flies and beetles [2, 5, 6, 19–21]. These strategies help to attract
pollinators to the vicinity of a flower. But the pollinator should not feed on the flower
itself, but instead is rewarded by nectar. Nectar is usually rich in sucrose or glucose and
may contain some lipids and amino acids. In some plants, the nectar also sequesters
PSM [22–24]. Thus, the PSM in flowers function as deterrent at low distance. In
addition to insects (honey bees, solitary bees, bumblebees, pollen wasps, ants, flies,
bee flies, hover flies, mosquitos, butterflies, moths, flower beetles), some plants (often
with red flowers) employ birds (sugar birds, sun birds, humming birds, bats) as
pollinators [2].
In contrast to plants which use animals (entomophilous and zoophilous species),
the majority of gymnosperms and many angiosperms (taxa within Poales, such as
grasses, sedges, and rushes; Fagaceae, Betalaceae, Junglandaceae, Vitaceae) use
wind-pollination (anemophily). Anemophilous plants do not produce showy
flowers, lack nectar but produce large amounts of pollen grains. As discussed later,
wind-pollinated plants are often without strong poisons, such as alkaloids.
Some plants, which produce fruits, are interested that their seeds are dispersed
away from the producing plants [3–5]. To achieve this purpose, plants produce fruits,
8 Evolution of the Angiosperms and Co-evolution of Secondary Metabolites. . .
155
and help to ward off bacteria, fungi and viruses, which are abundant in the environment. Antimicrobial PSM interfere with biomembranes of microbes (saponins,
mono- and sesquiterpenes), of proteins (polyphenols), protein biosynthesis, and
DNA replication and transcription (many alkaloids). In addition to antimicrobial
PSM and antimicrobial peptides (AMPs), plants can block vessels by storing callose
or can inhibit microbial infection by secreting chitinase, glucanase, and peroxidase
[5, 11, 13–15].
Plants compete with other plants for light, water, and nutrients. In some instances,
for example, in desert plants, a strong competition between plants of the same or
different species can be observed, in a way that individual plants are surrounded by
an empty space. It could be demonstrated that plants excrete PSM from their
rhizosphere or from leaves, which can inhibit the germination or development of
other competing plants. This phenomenon has been termed “allelopathy” (Fig. 2)
[2, 5, 7, 19].
1.3
Functions of Plant Secondary Metabolites as Signal
Compounds
It appears to be contradictory that plants also use PSM as signal compounds to
attract pollinating insects, fruit-dispersing animals, or symbiotic root bacteria (Fig. 2).
Mostly, we see the same PSM, which are employed in defense against herbivores. How
to explain this contradiction? Flowering plants (angiosperms) produce conspicuous
flowers to attract pollinators. Attraction is achieved via colors (mostly anthocyanins,
some flavonoids, and carotenoids) which insect can perceive in normal but also UV
light. In addition, many plants employ aromatic PSM (mostly terpenoids) as an
additional olfactory attractants. Some plants produce foully smelling PSM, such as
amines to attract flies and beetles [2, 5, 6, 19–21]. These strategies help to attract
pollinators to the vicinity of a flower. But the pollinator should not feed on the flower
itself, but instead is rewarded by nectar. Nectar is usually rich in sucrose or glucose and
may contain some lipids and amino acids. In some plants, the nectar also sequesters
PSM [22–24]. Thus, the PSM in flowers function as deterrent at low distance. In
addition to insects (honey bees, solitary bees, bumblebees, pollen wasps, ants, flies,
bee flies, hover flies, mosquitos, butterflies, moths, flower beetles), some plants (often
with red flowers) employ birds (sugar birds, sun birds, humming birds, bats) as
pollinators [2].
In contrast to plants which use animals (entomophilous and zoophilous species),
the majority of gymnosperms and many angiosperms (taxa within Poales, such as
grasses, sedges, and rushes; Fagaceae, Betalaceae, Junglandaceae, Vitaceae) use
wind-pollination (anemophily). Anemophilous plants do not produce showy
flowers, lack nectar but produce large amounts of pollen grains. As discussed later,
wind-pollinated plants are often without strong poisons, such as alkaloids.
Some plants, which produce fruits, are interested that their seeds are dispersed
away from the producing plants [3–5]. To achieve this purpose, plants produce fruits,
8 Evolution of the Angiosperms and Co-evolution of Secondary Metabolites. . .
155