4
1
What You Will Learn in This Chapter
Primary metabolites are compounds that are associated with essential cellular functions.
Therefore, they are very much ubiquitously found in all plants. By contrast, secondary
metabolites have much more specific functions. They are often species specific and can be
dispensable under many conditions. Nevertheless, the basis of most secondary metabolites
are by-products or intermediates of primary metabolism. Secondary metabolites do not
generally increase a plant fitness, but in the natural environment, they might be essential
for survival and reproduction. They are thus mostly made under controlled conditions for a
specific purpose such as defence against pathogens and herbivores, improved tolerance to
abiotic stresses, attraction of insects and animals for fertilization and/or seed dispersal or
repellence of unwanted feeders.
1.1 General Classification of Secondary Metabolites
(Details in Part IV)
The term metabolite comes from the Greek word “μεταβολιτησ” – meaning the “changed” –
and comprises intermediates (and end products) of enzymatic reactions that occur as part
of biological pathways in living organisms. Metabolites are thus not a defined class of molecules, but the term encompasses compounds of very different chemical forms. There is not
even a universally valid definition of this term, and not every substrate and product of an
enzymatic reaction is considered a metabolite. The usage is normally restricted to lowmolecular-weight compounds and mostly excludes biopolymers such as proteins, polynucleotides and polysaccharides or non-polymeric larger compounds such as lipids. Also, it is
often used in a manner restricting it to compounds that have a defined biological role,
thereby excluding intermediates of pathways that do not have a function on their own.
More important in the context of this book is the distinction between primary and secondary metabolites. Primary metabolites are compounds that are associated with essential
cellular functions, e.g. components of growth and energy metabolism such as basic sugars,
amino acids, nucleosides and small organic acids. Functionally, they are directly involved in
normal growth, development and reproduction and as such are usually indispensable for
the viability of an organism under any condition. Due to the evolutionary conservation of
many of these basal cellular functions, primary metabolites mostly show a very broad distribution over a wide range of organisms, i.e. they are considered to be ubiquitous.
Initially, secondary metabolites were defined as “such compounds, which are formed
by metabolism but which are no longer used for the formation of new cells” (Sachs 1873;
as translated by Hartmann (1996)). This definition was based on the fact that the effect of
certain secondary metabolites as human drugs was already known, however, the significance of these compounds for the plants was not yet understood. Only a few years later, a
protective function of secondary metabolites for the plant was suggested for the first time
(Errera and Durand 1886; Stahl 1888; Kerner von Marilaun 1879). The concept of chemical defence as part of animal-plant interactions was put forward, and it was suggested that
phytochemicals play a role in animal attraction and deterrence (Stahl 1888). It should take
another 60 years before the inherent biological role of secondary metabolites came back
into the focus of plant science (Fraenkel 1959; Hartmann 2008), but the field of plant
secondary metabolites has been gaining momentum ever since.
By now it is generally accepted that secondary metabolites are made for a specific
intrinsic purpose even if their precise role is only known for a fraction of compounds.
Chapter 1 · Plant Secondary Metabolites and Their General Function in Plants
1
What You Will Learn in This Chapter
Primary metabolites are compounds that are associated with essential cellular functions.
Therefore, they are very much ubiquitously found in all plants. By contrast, secondary
metabolites have much more specific functions. They are often species specific and can be
dispensable under many conditions. Nevertheless, the basis of most secondary metabolites
are by-products or intermediates of primary metabolism. Secondary metabolites do not
generally increase a plant fitness, but in the natural environment, they might be essential
for survival and reproduction. They are thus mostly made under controlled conditions for a
specific purpose such as defence against pathogens and herbivores, improved tolerance to
abiotic stresses, attraction of insects and animals for fertilization and/or seed dispersal or
repellence of unwanted feeders.
1.1 General Classification of Secondary Metabolites
(Details in Part IV)
The term metabolite comes from the Greek word “μεταβολιτησ” – meaning the “changed” –
and comprises intermediates (and end products) of enzymatic reactions that occur as part
of biological pathways in living organisms. Metabolites are thus not a defined class of molecules, but the term encompasses compounds of very different chemical forms. There is not
even a universally valid definition of this term, and not every substrate and product of an
enzymatic reaction is considered a metabolite. The usage is normally restricted to lowmolecular-weight compounds and mostly excludes biopolymers such as proteins, polynucleotides and polysaccharides or non-polymeric larger compounds such as lipids. Also, it is
often used in a manner restricting it to compounds that have a defined biological role,
thereby excluding intermediates of pathways that do not have a function on their own.
More important in the context of this book is the distinction between primary and secondary metabolites. Primary metabolites are compounds that are associated with essential
cellular functions, e.g. components of growth and energy metabolism such as basic sugars,
amino acids, nucleosides and small organic acids. Functionally, they are directly involved in
normal growth, development and reproduction and as such are usually indispensable for
the viability of an organism under any condition. Due to the evolutionary conservation of
many of these basal cellular functions, primary metabolites mostly show a very broad distribution over a wide range of organisms, i.e. they are considered to be ubiquitous.
Initially, secondary metabolites were defined as “such compounds, which are formed
by metabolism but which are no longer used for the formation of new cells” (Sachs 1873;
as translated by Hartmann (1996)). This definition was based on the fact that the effect of
certain secondary metabolites as human drugs was already known, however, the significance of these compounds for the plants was not yet understood. Only a few years later, a
protective function of secondary metabolites for the plant was suggested for the first time
(Errera and Durand 1886; Stahl 1888; Kerner von Marilaun 1879). The concept of chemical defence as part of animal-plant interactions was put forward, and it was suggested that
phytochemicals play a role in animal attraction and deterrence (Stahl 1888). It should take
another 60 years before the inherent biological role of secondary metabolites came back
into the focus of plant science (Fraenkel 1959; Hartmann 2008), but the field of plant
secondary metabolites has been gaining momentum ever since.
By now it is generally accepted that secondary metabolites are made for a specific
intrinsic purpose even if their precise role is only known for a fraction of compounds.
Chapter 1 · Plant Secondary Metabolites and Their General Function in Plants