13
1
terflies have become insensitive to cardenolides and accumulate these compounds to
become unpalatable for their predators (Holzinger et al. 1992). While this strategy
requires the direct ingestion of the metabolite by feeding on the plant, some species
such as Utetheisa ornatrix ingest the toxin during their larva state and can pass it on
through metamorphosis to the adult moth state and even further to the eggs (Eisner
and Eisner 1991).
Animals also use toxic plant compounds in more subtle ways. Blue tits have been
shown to line their nest with parts from aromatic plants such as lavender, curry or mint to
protect their offspring from parasites (Petit et al. 2002). Similarly, the leaves of tobacco are
used by birds to repel parasites. It was even suggested that city birds use nicotine- containing
stubs from smoked cigarette for the same purpose (Suarez-Rodriguez et al. 2013).
1.3.7 Avoidance of Auto-Toxicity and Premature Toxin Release
One severe problem that plants encounter when synthesizing secondary metabolites as
defence compounds is auto-toxicity in those cases where these compounds are not
only toxic to the attacker but also to the plant itself. A good example is hydrogen cyanide, which is commonly found as a defence compound. Hydrogen cyanide inhibits
the mitochondrial cytochrome c oxidase and thereby aerobic respiration. This affects
plants as much as animals. Other secondary metabolites interfere with conserved processes in the cell cycle. In all these cases, it has to be ensured that during synthesis and
storage, toxic compounds do not come into contact with potential targets within the
plant cell. Furthermore, because they are quite costly, unnecessary release of defence
metabolites should be avoided. Plants have developed several mechanisms to address
these issues.
1. Cellular compartmentalization of biosynthetic pathways: Many biosynthetic pathways for secondary metabolites are separated into different cells or compartments
(for more details see 7 Chap. 4). Non-toxic initial and intermediate compounds
are made in one type of cell, while toxic intermediates and final synthesis steps are
restricted to cells/compartments, where a compound toxicity does not affect the
plant. This way the plant can ensure that active compounds and their toxic intermediates are prevented from coming into contact with cellular targets in sensitive cells.
However, it should be noted that compartmentalization of biosynthetic pathways
has also been shown for compounds that, to our knowledge, are not phytotoxic.
Box 1.3 Defensive Halitosis
Nicotine is a potent alkaloid found in nightshade plants such as tobacco, which makes these
plants or certain parts of the plants poisonous to muscle-moving pests, livestock and humans.
The tobacco hornworm, Manduca sexta, is a moth whose larvae often feed on the leaves of
tobacco or tomato. While the exact mechanism of detoxification is still debated, it appears that
nicotine is metabolized via the enzyme cytochrome P450 6B46. However, part of the resulting
substance is transported to the haemolymph, reconverted into nicotine and released into the air
through spiracles. This mechanism is called defensive halitosis and protects the hornworm from
predators such as spiders (Kumar et al. 2014).
1.3 · Overview of the Function of Secondary Metabolites in Plants (Details in Part IV)
1
terflies have become insensitive to cardenolides and accumulate these compounds to
become unpalatable for their predators (Holzinger et al. 1992). While this strategy
requires the direct ingestion of the metabolite by feeding on the plant, some species
such as Utetheisa ornatrix ingest the toxin during their larva state and can pass it on
through metamorphosis to the adult moth state and even further to the eggs (Eisner
and Eisner 1991).
Animals also use toxic plant compounds in more subtle ways. Blue tits have been
shown to line their nest with parts from aromatic plants such as lavender, curry or mint to
protect their offspring from parasites (Petit et al. 2002). Similarly, the leaves of tobacco are
used by birds to repel parasites. It was even suggested that city birds use nicotine- containing
stubs from smoked cigarette for the same purpose (Suarez-Rodriguez et al. 2013).
1.3.7 Avoidance of Auto-Toxicity and Premature Toxin Release
One severe problem that plants encounter when synthesizing secondary metabolites as
defence compounds is auto-toxicity in those cases where these compounds are not
only toxic to the attacker but also to the plant itself. A good example is hydrogen cyanide, which is commonly found as a defence compound. Hydrogen cyanide inhibits
the mitochondrial cytochrome c oxidase and thereby aerobic respiration. This affects
plants as much as animals. Other secondary metabolites interfere with conserved processes in the cell cycle. In all these cases, it has to be ensured that during synthesis and
storage, toxic compounds do not come into contact with potential targets within the
plant cell. Furthermore, because they are quite costly, unnecessary release of defence
metabolites should be avoided. Plants have developed several mechanisms to address
these issues.
1. Cellular compartmentalization of biosynthetic pathways: Many biosynthetic pathways for secondary metabolites are separated into different cells or compartments
(for more details see 7 Chap. 4). Non-toxic initial and intermediate compounds
are made in one type of cell, while toxic intermediates and final synthesis steps are
restricted to cells/compartments, where a compound toxicity does not affect the
plant. This way the plant can ensure that active compounds and their toxic intermediates are prevented from coming into contact with cellular targets in sensitive cells.
However, it should be noted that compartmentalization of biosynthetic pathways
has also been shown for compounds that, to our knowledge, are not phytotoxic.
Box 1.3 Defensive Halitosis
Nicotine is a potent alkaloid found in nightshade plants such as tobacco, which makes these
plants or certain parts of the plants poisonous to muscle-moving pests, livestock and humans.
The tobacco hornworm, Manduca sexta, is a moth whose larvae often feed on the leaves of
tobacco or tomato. While the exact mechanism of detoxification is still debated, it appears that
nicotine is metabolized via the enzyme cytochrome P450 6B46. However, part of the resulting
substance is transported to the haemolymph, reconverted into nicotine and released into the air
through spiracles. This mechanism is called defensive halitosis and protects the hornworm from
predators such as spiders (Kumar et al. 2014).
1.3 · Overview of the Function of Secondary Metabolites in Plants (Details in Part IV)