146
9
tems and growth. This is important, because both processes compete for the same
resources (growth-defence trade-off), and thus investing into secondary metabolism
always means that the allocated energy and resources (carbon and nitrogen) might be
lacking for other reactions (Huot et al. 2014; Havko et al. 2016). In many cases, a close
co-evolution of plants with pollinators or predators has been observed.
The active form of jasmonate, JA-Ile, is perceived in the cell by the SCF COI1 complex.
COI1 is an E3 ubiquitin ligase which, upon binding of JA-Ile (usually when the levels are
elevated), ubiquitinylates the JASMONATE ZIM DOMAIN (JAZ) repressors thereby targeting them for degradation by the 26S proteasome. This releases transcription factors,
such as MYC2, which then regulate JA-dependent genes together with other transcription
factors. In this way the transcription of many genes responsible for the biosynthesis of
secondary metabolites is controlled (Patra et al. 2013; Gimenez-Ibanez et al. 2015;
Chezem and Clay 2016; Zhou and Memelink 2016). JA has been shown to induce the
biosynthesis of terpenes such as sesquiterpenes, phenylpropanoids and alkaloids.
Examples are the biosyntheses of nicotine in tobacco (Dewey and Xie 2013) and terpenoid indole alkaloids in Catharanthus roseus (Zhang et al. 2018). Additionally, JA has
been shown to increase trichrome density on newly formed leaves of Arabidopsis and
tomato (Traw and Bergelson 2003; Boughton et al. 2005; Qi et al. 2011). This shows that
not only the production of enzymes of the biosynthetic pathways but also the availability
of storage places is regulated.
In addition to JA, other hormones are also involved in the regulation of secondary
metabolites. These include especially cytokinins and ethylene. Cytokinin, for example,
acts on downstream enzymatic steps of the indole alkaloid biosynthesis pathway (Papon
et al. 2005). Ethylene can induce the formation of traumatic resin ducts in conifers
(Hudgins and Franceschi 2004).
The roles of light in modulating secondary metabolites can be manifold; one of them
is the entrainment of the circadian clock. The ability to anticipate the different times of the
day provides a tool to produce secondary metabolites at times when they are needed and
thereby to reduce the costs of overproduction. Protective compounds against UV light are
only needed during daytime, and defence mechanisms are expressed according to the
lifestyle of predators (Kim et al. 2011). Cabbage loopers (Trichoplusia ni), for example,
display rhythmic feeding behaviour, and plants that are in the same entrainment are less
attacked by this herbivore compared to out-of-phase plants (Goodspeed et al. 2012). Even
fruits and vegetables that have already been harvested are still reacting to the daily cycles
of light and darkness and accumulate secondary metabolites accordingly (Liu et al. 2015).
Diurnal emission patterns of volatiles to attract specific pollinators have also been
observed (Fenske and Imaizumi 2016).
In recent years not only many transcription factors responsible for the developmental
and spatiotemporal modulation of gene expression have been identified (Yamada and
Sato 2013; Kant et al. 2015), but also additional regulatory mechanisms such as regulation
via miRNAs have been reported. miRNAs, for example, target genes encoding for enzymes
important for benzylisoquinoline alkaloid biosynthesis (Boke et al. 2015). In addition to
the transcriptional modulation of biosynthetic genes, post-translational regulatory mechanisms have been described. These include regulation of enzyme stability and modification by phosphatases/kinases. Often key enzymes of a pathway are regulated in this way,
for instance, the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase that
catalyses crucial regulatory steps of the mevalonic acid pathway (Leivar et al. 2011; Doblas
et al. 2013) and the phenylalanine ammonia-lyase leading to the phenolpropanoids
(Zhang and Liu 2015). Feedback mechanisms that limit the production of secondary
Chapter 9 · Secondary Metabolites in Plants: General Introduction
9
tems and growth. This is important, because both processes compete for the same
resources (growth-defence trade-off), and thus investing into secondary metabolism
always means that the allocated energy and resources (carbon and nitrogen) might be
lacking for other reactions (Huot et al. 2014; Havko et al. 2016). In many cases, a close
co-evolution of plants with pollinators or predators has been observed.
The active form of jasmonate, JA-Ile, is perceived in the cell by the SCF COI1 complex.
COI1 is an E3 ubiquitin ligase which, upon binding of JA-Ile (usually when the levels are
elevated), ubiquitinylates the JASMONATE ZIM DOMAIN (JAZ) repressors thereby targeting them for degradation by the 26S proteasome. This releases transcription factors,
such as MYC2, which then regulate JA-dependent genes together with other transcription
factors. In this way the transcription of many genes responsible for the biosynthesis of
secondary metabolites is controlled (Patra et al. 2013; Gimenez-Ibanez et al. 2015;
Chezem and Clay 2016; Zhou and Memelink 2016). JA has been shown to induce the
biosynthesis of terpenes such as sesquiterpenes, phenylpropanoids and alkaloids.
Examples are the biosyntheses of nicotine in tobacco (Dewey and Xie 2013) and terpenoid indole alkaloids in Catharanthus roseus (Zhang et al. 2018). Additionally, JA has
been shown to increase trichrome density on newly formed leaves of Arabidopsis and
tomato (Traw and Bergelson 2003; Boughton et al. 2005; Qi et al. 2011). This shows that
not only the production of enzymes of the biosynthetic pathways but also the availability
of storage places is regulated.
In addition to JA, other hormones are also involved in the regulation of secondary
metabolites. These include especially cytokinins and ethylene. Cytokinin, for example,
acts on downstream enzymatic steps of the indole alkaloid biosynthesis pathway (Papon
et al. 2005). Ethylene can induce the formation of traumatic resin ducts in conifers
(Hudgins and Franceschi 2004).
The roles of light in modulating secondary metabolites can be manifold; one of them
is the entrainment of the circadian clock. The ability to anticipate the different times of the
day provides a tool to produce secondary metabolites at times when they are needed and
thereby to reduce the costs of overproduction. Protective compounds against UV light are
only needed during daytime, and defence mechanisms are expressed according to the
lifestyle of predators (Kim et al. 2011). Cabbage loopers (Trichoplusia ni), for example,
display rhythmic feeding behaviour, and plants that are in the same entrainment are less
attacked by this herbivore compared to out-of-phase plants (Goodspeed et al. 2012). Even
fruits and vegetables that have already been harvested are still reacting to the daily cycles
of light and darkness and accumulate secondary metabolites accordingly (Liu et al. 2015).
Diurnal emission patterns of volatiles to attract specific pollinators have also been
observed (Fenske and Imaizumi 2016).
In recent years not only many transcription factors responsible for the developmental
and spatiotemporal modulation of gene expression have been identified (Yamada and
Sato 2013; Kant et al. 2015), but also additional regulatory mechanisms such as regulation
via miRNAs have been reported. miRNAs, for example, target genes encoding for enzymes
important for benzylisoquinoline alkaloid biosynthesis (Boke et al. 2015). In addition to
the transcriptional modulation of biosynthetic genes, post-translational regulatory mechanisms have been described. These include regulation of enzyme stability and modification by phosphatases/kinases. Often key enzymes of a pathway are regulated in this way,
for instance, the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase that
catalyses crucial regulatory steps of the mevalonic acid pathway (Leivar et al. 2011; Doblas
et al. 2013) and the phenylalanine ammonia-lyase leading to the phenolpropanoids
(Zhang and Liu 2015). Feedback mechanisms that limit the production of secondary
Chapter 9 · Secondary Metabolites in Plants: General Introduction