144
9
What You Will Learn in This Chapter
Plants usually contain many different secondary metabolites, but some species contain very
specific groups of secondary metabolites. The amount of the compounds and the kind of
compounds vary between different cells, tissues and developmental stages and can be
influenced by external stressors. This means that the enzymatic pathways have to be tightly
controlled. To synthetize secondary metabolites, plants use products from the primary
metabolism as building blocks. The metabolites are further fused and modified by different
processes to lead to the variations observed in nature.
9.1 Role of Secondary Metabolites for Plant Development
Secondary metabolites are in principle characterized as such because their presence is not
essential for a plant’s life. Rather they help plants to optimize growth and to adapt to the
changing environmental conditions. This explains also the huge variation of secondary
metabolites between different organs and under different conditions. Furthermore, the
ability to synthetize specific secondary metabolites has been selected for during evolution
when such compounds are important for specific needs. Therefore, some compounds are
very specific for a narrow group of species. The adaptation of plants to life on land during
evolution (terrestrialization) was accompanied by a large increase in the production of
secondary (or specialized) metabolites, as protection against UV light and loss of water
became important, as well as the attraction of pollinators.
Nevertheless, some derivatives of these so-called secondary pathways are essential for
development as they constitute hormones (such as abscisic acid, gibberellin, cytokinin,
brassinosteroid, strigolactone) or are necessary for photosynthesis, like chlorophyll (which
contains a phytoene-derived tail), ubiqionone, plastoquinone, tocopherol and carotenoids
(all derivatives from the isoprene pathway). Additionally, these compounds are required
for the mechanical stability of plants, for instance, lignin (a phenylpropanoid). Other
components, such as suberin (a phenylpropanoid), are important to avoid water loss
through cell walls. Therefore, the discrimination between pathways that are essential or
not essential for plants’ life is blurry. In plants, many of the secondary metabolic pathways
are located in plastids. This provides these organelles with an important function for adaptation and defence in addition to their essential role for photosynthesis and fatty acid
biosynthesis.
Metabolic pathways for secondary metabolites are usually no constitutively expressed,
as this would be a waste of energy if the compounds were not needed for survival.
Furthermore, compounds can be very selectively produced in specific tissues and/or
organs (flowers, green tissues, seeds or roots) according to their function. For example,
secondary metabolites to attract pollinators will be produced in floral tissues, whereas
compounds important for defence mechanisms are often accumulating either in root or in
leaf tissues, depending on the predator that is targeted. In several cases, it has also been
shown that compounds can be produced in tissues that are distant from the place of
release, which indicates the involvement of a transport mechanism. Especially for toxic
compounds, the expression of the genes coding for the necessary enzymes can be limited
to individual tissues or cells, such as to the glandular trichomes and to cells close to the
phloem or to the pericycle of the root (. Fig. 9.1).
Equally important to the spacial regulation of metabolic pathways is the regulation in
time, as some components are needed at different stages of development. One example is
the expression of genes for such pathways in developing fruits, where either the unripe
Chapter 9 · Secondary Metabolites in Plants: General Introduction
9
What You Will Learn in This Chapter
Plants usually contain many different secondary metabolites, but some species contain very
specific groups of secondary metabolites. The amount of the compounds and the kind of
compounds vary between different cells, tissues and developmental stages and can be
influenced by external stressors. This means that the enzymatic pathways have to be tightly
controlled. To synthetize secondary metabolites, plants use products from the primary
metabolism as building blocks. The metabolites are further fused and modified by different
processes to lead to the variations observed in nature.
9.1 Role of Secondary Metabolites for Plant Development
Secondary metabolites are in principle characterized as such because their presence is not
essential for a plant’s life. Rather they help plants to optimize growth and to adapt to the
changing environmental conditions. This explains also the huge variation of secondary
metabolites between different organs and under different conditions. Furthermore, the
ability to synthetize specific secondary metabolites has been selected for during evolution
when such compounds are important for specific needs. Therefore, some compounds are
very specific for a narrow group of species. The adaptation of plants to life on land during
evolution (terrestrialization) was accompanied by a large increase in the production of
secondary (or specialized) metabolites, as protection against UV light and loss of water
became important, as well as the attraction of pollinators.
Nevertheless, some derivatives of these so-called secondary pathways are essential for
development as they constitute hormones (such as abscisic acid, gibberellin, cytokinin,
brassinosteroid, strigolactone) or are necessary for photosynthesis, like chlorophyll (which
contains a phytoene-derived tail), ubiqionone, plastoquinone, tocopherol and carotenoids
(all derivatives from the isoprene pathway). Additionally, these compounds are required
for the mechanical stability of plants, for instance, lignin (a phenylpropanoid). Other
components, such as suberin (a phenylpropanoid), are important to avoid water loss
through cell walls. Therefore, the discrimination between pathways that are essential or
not essential for plants’ life is blurry. In plants, many of the secondary metabolic pathways
are located in plastids. This provides these organelles with an important function for adaptation and defence in addition to their essential role for photosynthesis and fatty acid
biosynthesis.
Metabolic pathways for secondary metabolites are usually no constitutively expressed,
as this would be a waste of energy if the compounds were not needed for survival.
Furthermore, compounds can be very selectively produced in specific tissues and/or
organs (flowers, green tissues, seeds or roots) according to their function. For example,
secondary metabolites to attract pollinators will be produced in floral tissues, whereas
compounds important for defence mechanisms are often accumulating either in root or in
leaf tissues, depending on the predator that is targeted. In several cases, it has also been
shown that compounds can be produced in tissues that are distant from the place of
release, which indicates the involvement of a transport mechanism. Especially for toxic
compounds, the expression of the genes coding for the necessary enzymes can be limited
to individual tissues or cells, such as to the glandular trichomes and to cells close to the
phloem or to the pericycle of the root (. Fig. 9.1).
Equally important to the spacial regulation of metabolic pathways is the regulation in
time, as some components are needed at different stages of development. One example is
the expression of genes for such pathways in developing fruits, where either the unripe
Chapter 9 · Secondary Metabolites in Plants: General Introduction