Among angiosperms, some orders have more families producing alkaloids, than
others (Fig. 4): Alkaloid-rich are Nympheales, Piperales, Magnoliales, Laurales,
Liliales, Ranunculales, Buxales, Zygophyllales, Malpighiales, Fabales, Sapindales,
Solanales, Gentianales, and Boraginales [3–5, 46, 47, 59].
Some types of alkaloids occur in closely related taxa (Table 2), which was the
base for chemotaxonomy some time ago [59–63]. But a closer look shows
that this feature is not consistent. For example, quinolizidine alkaloids occur
predominantly in the tribe Genisteae of Fabaceae. But alkaloids with
identical structures have also been detected in unrelated orders/families, such as
Chenopodiaceae or Ranunculaceae. Or pyrrolizidine alkaloids occur in
Boraginaceae, the tribe Crotalarieae of Fabaceae, and in the tribe Senecioneae of
Asteraceae, which are unrelated families. Similar examples can be found for tropane,
indole, and isoquinoline alkaloids (Table 2) [6–9, 15]. How to explain the irregular
occurrences? It could be argued that the occurrence of a PSM in unrelated families is
due to convergent evolution. Although convergence cannot be ruled out in all
instances, genetic data favor a different hypothesis. When genomes of different
plants and microorganisms were analyzed for the presence/absence of key
enzymes of PSM biosynthesis, it turned out that the key enzymes are widely
distributed in the plant kingdom, irrespective of whether a particular species
produces a particular PSM or not. We have therefore postulated that the key genes
of PSM biosynthesis are present in most if not in all plants and that the PSM profile
observed in a particular plant is a consequence of differential gene expression.
Furthermore, the origin of the key genes may be found in microorganisms, which
also are active producers of secondary metabolites. These microbial genes probably
entered plant genomes by horizontal gene transfer. According to the endosymbiont
theory, plants have mitochondria, which derived from alpha Protobacteria and
chloroplasts, which came from Cyanobacteria. The originally microbial genomes
were incorporated into plant genomes, which thus obtained many genes which
could be used later for the biosynthesis of PSM [7–9, 59, 64].
3
Evolution of Angiosperms, Pollinating Insects, and
Defense via Alkaloids
First land plants occurred around 420 million years ago in the Silurian and became
more common in the Devonian 416–359 million years ago. Fossils from this period
mainly show plants which produced spores and resembled ferns (Psilophytatae).
Later in the Devonian, tree-like lycopods, ferns, and horsetails followed which
formed dense forests in the Devonian and Carbon (a source of our coal today).
Starting in the Carboniferous, first gymnosperms occurred with cycads and gingko.
Gymnosperms were abundant in Perm and the following Mesozoicum. About 140
million years ago in the Cretaceous, the largest group of plants, the angiosperms
evolved and became the dominant flora, especially in the Palaeogene. Roots of most
angiosperm orders go back to 100–120 million years ago. However, radiation and
168
M. Wink
others (Fig. 4): Alkaloid-rich are Nympheales, Piperales, Magnoliales, Laurales,
Liliales, Ranunculales, Buxales, Zygophyllales, Malpighiales, Fabales, Sapindales,
Solanales, Gentianales, and Boraginales [3–5, 46, 47, 59].
Some types of alkaloids occur in closely related taxa (Table 2), which was the
base for chemotaxonomy some time ago [59–63]. But a closer look shows
that this feature is not consistent. For example, quinolizidine alkaloids occur
predominantly in the tribe Genisteae of Fabaceae. But alkaloids with
identical structures have also been detected in unrelated orders/families, such as
Chenopodiaceae or Ranunculaceae. Or pyrrolizidine alkaloids occur in
Boraginaceae, the tribe Crotalarieae of Fabaceae, and in the tribe Senecioneae of
Asteraceae, which are unrelated families. Similar examples can be found for tropane,
indole, and isoquinoline alkaloids (Table 2) [6–9, 15]. How to explain the irregular
occurrences? It could be argued that the occurrence of a PSM in unrelated families is
due to convergent evolution. Although convergence cannot be ruled out in all
instances, genetic data favor a different hypothesis. When genomes of different
plants and microorganisms were analyzed for the presence/absence of key
enzymes of PSM biosynthesis, it turned out that the key enzymes are widely
distributed in the plant kingdom, irrespective of whether a particular species
produces a particular PSM or not. We have therefore postulated that the key genes
of PSM biosynthesis are present in most if not in all plants and that the PSM profile
observed in a particular plant is a consequence of differential gene expression.
Furthermore, the origin of the key genes may be found in microorganisms, which
also are active producers of secondary metabolites. These microbial genes probably
entered plant genomes by horizontal gene transfer. According to the endosymbiont
theory, plants have mitochondria, which derived from alpha Protobacteria and
chloroplasts, which came from Cyanobacteria. The originally microbial genomes
were incorporated into plant genomes, which thus obtained many genes which
could be used later for the biosynthesis of PSM [7–9, 59, 64].
3
Evolution of Angiosperms, Pollinating Insects, and
Defense via Alkaloids
First land plants occurred around 420 million years ago in the Silurian and became
more common in the Devonian 416–359 million years ago. Fossils from this period
mainly show plants which produced spores and resembled ferns (Psilophytatae).
Later in the Devonian, tree-like lycopods, ferns, and horsetails followed which
formed dense forests in the Devonian and Carbon (a source of our coal today).
Starting in the Carboniferous, first gymnosperms occurred with cycads and gingko.
Gymnosperms were abundant in Perm and the following Mesozoicum. About 140
million years ago in the Cretaceous, the largest group of plants, the angiosperms
evolved and became the dominant flora, especially in the Palaeogene. Roots of most
angiosperm orders go back to 100–120 million years ago. However, radiation and
168
M. Wink