5
1
Also, some derivatives of pathways involved in secondary metabolite production are
important for plant development as they constitute hormones (ABA, gibberellin, cytokinin, brassinosteroids, strigolactones) or are necessary for photosynthesis, such as the phytoene tail of chlorophyll, ubiquinone, plastoquinone, tocopherol and carotenoids. At this
point, the distinction between primary and secondary metabolism becomes blurred.
Nevertheless, secondary metabolites are often found to be species specific, and even in
those plants where they are very abundant, they might be dispensable under many growth
conditions. Therefore, secondary metabolites do not generally increase a plant’s fitness,
but in the natural plant environment, they might still be essential for survival and reproduction. Consequently, the content of secondary metabolites varies enormously between
different species.
1.2 Overview of Classes of Secondary Metabolites
(Details in Part IV)
By-products or intermediates of primary metabolism form the basis of most secondary
metabolites (Hartmann 1996). Products of the carbon and nitrogen metabolism form the
basic structures of the three major classes, the terpenoids (isoprenoids), alkaloids and
phenylpropanoids, but also the polyketides, quinones and cyanogenic glycosides
(. Fig. 1.1). The alkylamides are derivatives of fatty acids, while the glucosinolates derive
from sulphur metabolism.
The number of primary metabolites in plants is probably less than 10,000 (Pichersky
and Lewinsohn 2011). The total content of plant secondary metabolites has been estimated to be more than 200,000 (Dixon and Strack 2003; Yonekura-Sakakibara and Saito
2009), but it should be kept in mind that (i) the metabolic content of very few plants has
ever been studied systematically and (ii) this estimate includes transitory intermediates of
metabolic pathways that might not have specific functions, not even as branch points for
deviating pathways. However, the number of secondary metabolites vastly exceeds the
number of primary ones. Based on a small number of principal molecular scaffolds, plants
produce a wide variety of secondary metabolites often with very different biological functions. These variants are due to different sets of enzymes changing the substrate or product
range within certain metabolic pathways (Schwab 2003). In parts, this genetic variability
resulting in an immense variety of secondary metabolites might be an advantage in dealing with a changing and/or demanding environment. Moreover, the non-essentiality of
secondary metabolites for basic cellular functions might have allowed a less stringent
selection process on enzymes/genes and resulted in the formation of novel components
whose potential could then be exploited by the plant.
The evolution of different synthesis pathways and their specific enzymes was driven by
gene duplications and adaptation to the requirements of a specific environment.
Furthermore, there was not only gain but also secondary loss of the ability to form certain
compounds. Analyses of biosynthesis genes and metabolites have indicated that there are
many examples of convergent evolution, meaning that different plants have either independently evolved the ability to make similar or equal compounds (see caffeine) or to
make structurally completely different compounds that nevertheless fulfil the same biological function (Pichersky and Lewinsohn 2011).
The presence and distribution of specific secondary metabolites within the plant kingdom, therefore, is neither ubiquitous nor does it follow a clear phylogenetic pattern (Wink
1.2 · Overview of Classes of Secondary Metabolites (Details in Part IV)
1
Also, some derivatives of pathways involved in secondary metabolite production are
important for plant development as they constitute hormones (ABA, gibberellin, cytokinin, brassinosteroids, strigolactones) or are necessary for photosynthesis, such as the phytoene tail of chlorophyll, ubiquinone, plastoquinone, tocopherol and carotenoids. At this
point, the distinction between primary and secondary metabolism becomes blurred.
Nevertheless, secondary metabolites are often found to be species specific, and even in
those plants where they are very abundant, they might be dispensable under many growth
conditions. Therefore, secondary metabolites do not generally increase a plant’s fitness,
but in the natural plant environment, they might still be essential for survival and reproduction. Consequently, the content of secondary metabolites varies enormously between
different species.
1.2 Overview of Classes of Secondary Metabolites
(Details in Part IV)
By-products or intermediates of primary metabolism form the basis of most secondary
metabolites (Hartmann 1996). Products of the carbon and nitrogen metabolism form the
basic structures of the three major classes, the terpenoids (isoprenoids), alkaloids and
phenylpropanoids, but also the polyketides, quinones and cyanogenic glycosides
(. Fig. 1.1). The alkylamides are derivatives of fatty acids, while the glucosinolates derive
from sulphur metabolism.
The number of primary metabolites in plants is probably less than 10,000 (Pichersky
and Lewinsohn 2011). The total content of plant secondary metabolites has been estimated to be more than 200,000 (Dixon and Strack 2003; Yonekura-Sakakibara and Saito
2009), but it should be kept in mind that (i) the metabolic content of very few plants has
ever been studied systematically and (ii) this estimate includes transitory intermediates of
metabolic pathways that might not have specific functions, not even as branch points for
deviating pathways. However, the number of secondary metabolites vastly exceeds the
number of primary ones. Based on a small number of principal molecular scaffolds, plants
produce a wide variety of secondary metabolites often with very different biological functions. These variants are due to different sets of enzymes changing the substrate or product
range within certain metabolic pathways (Schwab 2003). In parts, this genetic variability
resulting in an immense variety of secondary metabolites might be an advantage in dealing with a changing and/or demanding environment. Moreover, the non-essentiality of
secondary metabolites for basic cellular functions might have allowed a less stringent
selection process on enzymes/genes and resulted in the formation of novel components
whose potential could then be exploited by the plant.
The evolution of different synthesis pathways and their specific enzymes was driven by
gene duplications and adaptation to the requirements of a specific environment.
Furthermore, there was not only gain but also secondary loss of the ability to form certain
compounds. Analyses of biosynthesis genes and metabolites have indicated that there are
many examples of convergent evolution, meaning that different plants have either independently evolved the ability to make similar or equal compounds (see caffeine) or to
make structurally completely different compounds that nevertheless fulfil the same biological function (Pichersky and Lewinsohn 2011).
The presence and distribution of specific secondary metabolites within the plant kingdom, therefore, is neither ubiquitous nor does it follow a clear phylogenetic pattern (Wink
1.2 · Overview of Classes of Secondary Metabolites (Details in Part IV)