9
1
(see below), defences against nectar robbers should not affect mutualistic pollinators.
This can be achieved, when a toxin is targeted towards specific species. For example, the
floral nectar of the northern catalpa tree (Catalpa speciose) contains iridoid glycosides
that make it toxic or unpalatable for nectar thieves such as ants, but it remains edible to
legitimate pollinators such as bumble bees and moths (Stephenson 1982). Instead of
lacing the nectar, plants also can contain toxins in the petal tissue that affect nectar robbers when they bite through the flower base. It was suggested that this is one role of
nicotine, which is found enriched in the basal parts of the corolla (Euler and Baldwin
1996).
Indeed, plant floral nectaries are a nice example for the combined function of primary
and secondary metabolites (Heil 2011). The major purpose of nectar is the attraction of
pollinators. To this end, its major constituents are primary sugars and free amino acids,
providing nutritious carbons and a potential nitrogen source. Attraction of pollinators is
aided by volatile organic compounds emitted by the flower. Also, nectar was shown to
contain antimicrobial proteins (nectarines) for protection against microbial infections.
This is helpful since the opening of the floral nectaries at the base of a flower provides an
easy entrance point for microbes. In case of the phytopathogen fire blight, this was shown
to be the major entrance point of infection (Buban et al. 2003). Secondary metabolites in
the nectar enhance the antimicrobial properties, while their presence in the nectar or the
corolla tissue deters nectar robbers.
In a similar fashion, many plants rely on animals for the dispersion of their seeds.
Since they are sessile organisms, plants had to develop systems to spread out their progeny
over a larger area than their direct surrounding. Seeds of plants such as dandelion, maples
or sycamore have parachute- or winglike structures that allow for easy dispersal by wind.
Alternatively, seeds can be covered by a nutrient-rich tissue, i.e. fruit, that is eaten by animals. While insects play an important role for pollination, birds, bats and mammals are
the most important seed dispersers. They carry the seeds with them until they have moved
through the digestive tract. This ensures a wider distribution of the seed and the provision
of a fertile environment for their growth after they are discarded with the excrements of
the animal. Similar to flowers, also fruits attract animals by colour, flavour and volatile
scents. Since it is important that fruits are not eaten before the seeds are mature, colour
change from green to red or black is often used to advertise the ripening process to
Box 1.1 Toxic Honey
For humans, the presence of toxic secondary metabolites in nectar can be problematic. They can
be transferred into the honey by bees either feeding directly on nectar or on honeydew exudates
from sap-sucking insects. Honey poisoning was reported already more than 2000 years ago in
Europe and Asia (Gunduz et al. 2008). Basis for the toxic property in this case are grayanotoxins
that can be found in Rhododendron and other species of the Ericaceae. While rarely lethal they
can cause the so-called mad honey disease when grayanotoxin-laced honey is consumed. Various
other toxins have been detected in honey including strychnine. Tutin was found in toxic honey
after the European honey bee was introduced to New Zealand and started obtaining nectar from
the Tutu plant. While honey poisoning mostly occurs accidentally, mad honey has been deliberately harvested in areas such as the Black Sea for a long time, due to its halogenic properties and
the belief that it acts as an aphrodisiac.
1.3 · Overview of the Function of Secondary Metabolites in Plants (Details in Part IV)
1
(see below), defences against nectar robbers should not affect mutualistic pollinators.
This can be achieved, when a toxin is targeted towards specific species. For example, the
floral nectar of the northern catalpa tree (Catalpa speciose) contains iridoid glycosides
that make it toxic or unpalatable for nectar thieves such as ants, but it remains edible to
legitimate pollinators such as bumble bees and moths (Stephenson 1982). Instead of
lacing the nectar, plants also can contain toxins in the petal tissue that affect nectar robbers when they bite through the flower base. It was suggested that this is one role of
nicotine, which is found enriched in the basal parts of the corolla (Euler and Baldwin
1996).
Indeed, plant floral nectaries are a nice example for the combined function of primary
and secondary metabolites (Heil 2011). The major purpose of nectar is the attraction of
pollinators. To this end, its major constituents are primary sugars and free amino acids,
providing nutritious carbons and a potential nitrogen source. Attraction of pollinators is
aided by volatile organic compounds emitted by the flower. Also, nectar was shown to
contain antimicrobial proteins (nectarines) for protection against microbial infections.
This is helpful since the opening of the floral nectaries at the base of a flower provides an
easy entrance point for microbes. In case of the phytopathogen fire blight, this was shown
to be the major entrance point of infection (Buban et al. 2003). Secondary metabolites in
the nectar enhance the antimicrobial properties, while their presence in the nectar or the
corolla tissue deters nectar robbers.
In a similar fashion, many plants rely on animals for the dispersion of their seeds.
Since they are sessile organisms, plants had to develop systems to spread out their progeny
over a larger area than their direct surrounding. Seeds of plants such as dandelion, maples
or sycamore have parachute- or winglike structures that allow for easy dispersal by wind.
Alternatively, seeds can be covered by a nutrient-rich tissue, i.e. fruit, that is eaten by animals. While insects play an important role for pollination, birds, bats and mammals are
the most important seed dispersers. They carry the seeds with them until they have moved
through the digestive tract. This ensures a wider distribution of the seed and the provision
of a fertile environment for their growth after they are discarded with the excrements of
the animal. Similar to flowers, also fruits attract animals by colour, flavour and volatile
scents. Since it is important that fruits are not eaten before the seeds are mature, colour
change from green to red or black is often used to advertise the ripening process to
Box 1.1 Toxic Honey
For humans, the presence of toxic secondary metabolites in nectar can be problematic. They can
be transferred into the honey by bees either feeding directly on nectar or on honeydew exudates
from sap-sucking insects. Honey poisoning was reported already more than 2000 years ago in
Europe and Asia (Gunduz et al. 2008). Basis for the toxic property in this case are grayanotoxins
that can be found in Rhododendron and other species of the Ericaceae. While rarely lethal they
can cause the so-called mad honey disease when grayanotoxin-laced honey is consumed. Various
other toxins have been detected in honey including strychnine. Tutin was found in toxic honey
after the European honey bee was introduced to New Zealand and started obtaining nectar from
the Tutu plant. While honey poisoning mostly occurs accidentally, mad honey has been deliberately harvested in areas such as the Black Sea for a long time, due to its halogenic properties and
the belief that it acts as an aphrodisiac.
1.3 · Overview of the Function of Secondary Metabolites in Plants (Details in Part IV)