14
1
2. Storage in protected/safe compartments: Plants have developed several structures
that allow for the safe storage of toxic compounds. Common examples for such
structures are resin ducts, laticifers, internal glands or glandular trichomes. Often,
the same structures allow the easy release of toxic or antimicrobial compounds
when the tissue is ruptured upon herbivore feeding or mechanical tissue damage
(Wittstock and Gershenzon 2002). While resin ducts are hollow spaces that are
filled with resin from surrounding secretory cells, laticifers are elongated living
cells that contain secondary metabolites but also defence proteins such as proteinases and chitinases. Internal glands can occur as isolated idioblast or small cell
groups, which might contain a central cavity. Glandular or stinging trichomes are
external structures covering the surface of leaves or stems. In glandular hairs the
secretory substance accumulates in a thin-walled storage cavity above the secretory cells that easily ruptures upon pressure. Stinging trichomes such as the ones
covering the common nettle (Urtica dioica) are built in such a fashion that the tip
breaks easily when touched. The specific build of the calcified cell creates a needlelike end that injects the irritating content into the predator. Plants also sequester
hydrophilic phytotoxins in their vacuoles, which are a “safe” compartment within
the cellular context.
3. Storage of inactive precursors: Another way to avoid auto-toxicity is the storage
of defence compounds as inactive precursors. Many secondary metabolites are
found as glycosides, molecules in which sugar (often D-glucose) is bound to a
functional group (aglycone), which can be activated by enzymatic removal of
the sugar moiety. To prevent early release, enzymes and glycosides are separated
into different cells or different compartments (. Fig. 1.3). Disruption of the tissue/cell brings enzyme and glycoside in contact with each other, resulting in the
release of the active defence compound. Good examples are the glucosinolates
(thioglucosides derived from glucose) found in pungent plants of Brassicales such
as mustard, cabbage or horseradish. Isothiocyanate is released from glucosinolates
upon cleavage by a family of enzymes called myrosinases. These enzymes are
stored in the vacuole or cytosol of idioblastic so-called “myrosin” cells that are
scattered within the plant tissue surrounded by cells harbouring the glucosinolates
(Koroleva et al. 2000). Another example are cyanogenic glycosides such as amygdalin found in pits of plants from the rose family, e.g. almonds, cherries, apples or
plums (Moller 2010). Cyanogenic glycosides are stored in the vacuole of the same
cells that contain the activating glycosidases. Damage of the cell results in mixing
of the vacuolar content with the cytoplasma and thus the release of toxic hydrogen
cyanide by enzymatic removal of the sugar. Even after take-up of undamaged tissue, the hydrogen cyanide is released by glycosidases in the gut of animals during
the digestion process. Sorghum plants have been shown to store the cyanogenic
glycoside dhurrin, which makes them resistant to pests such as rootworms
(Diabrotica spp.). However, the diurnal turnover of dhurrin implies that it might
rather function as a source of nitrogen and glucose with its defensive properties as
a useful by-product (Adewusi 1990), a role that has been generally suggested for
cyanogenic glycosides (Moller 2010).
Chapter 1 · Plant Secondary Metabolites and Their General Function in Plants
1
2. Storage in protected/safe compartments: Plants have developed several structures
that allow for the safe storage of toxic compounds. Common examples for such
structures are resin ducts, laticifers, internal glands or glandular trichomes. Often,
the same structures allow the easy release of toxic or antimicrobial compounds
when the tissue is ruptured upon herbivore feeding or mechanical tissue damage
(Wittstock and Gershenzon 2002). While resin ducts are hollow spaces that are
filled with resin from surrounding secretory cells, laticifers are elongated living
cells that contain secondary metabolites but also defence proteins such as proteinases and chitinases. Internal glands can occur as isolated idioblast or small cell
groups, which might contain a central cavity. Glandular or stinging trichomes are
external structures covering the surface of leaves or stems. In glandular hairs the
secretory substance accumulates in a thin-walled storage cavity above the secretory cells that easily ruptures upon pressure. Stinging trichomes such as the ones
covering the common nettle (Urtica dioica) are built in such a fashion that the tip
breaks easily when touched. The specific build of the calcified cell creates a needlelike end that injects the irritating content into the predator. Plants also sequester
hydrophilic phytotoxins in their vacuoles, which are a “safe” compartment within
the cellular context.
3. Storage of inactive precursors: Another way to avoid auto-toxicity is the storage
of defence compounds as inactive precursors. Many secondary metabolites are
found as glycosides, molecules in which sugar (often D-glucose) is bound to a
functional group (aglycone), which can be activated by enzymatic removal of
the sugar moiety. To prevent early release, enzymes and glycosides are separated
into different cells or different compartments (. Fig. 1.3). Disruption of the tissue/cell brings enzyme and glycoside in contact with each other, resulting in the
release of the active defence compound. Good examples are the glucosinolates
(thioglucosides derived from glucose) found in pungent plants of Brassicales such
as mustard, cabbage or horseradish. Isothiocyanate is released from glucosinolates
upon cleavage by a family of enzymes called myrosinases. These enzymes are
stored in the vacuole or cytosol of idioblastic so-called “myrosin” cells that are
scattered within the plant tissue surrounded by cells harbouring the glucosinolates
(Koroleva et al. 2000). Another example are cyanogenic glycosides such as amygdalin found in pits of plants from the rose family, e.g. almonds, cherries, apples or
plums (Moller 2010). Cyanogenic glycosides are stored in the vacuole of the same
cells that contain the activating glycosidases. Damage of the cell results in mixing
of the vacuolar content with the cytoplasma and thus the release of toxic hydrogen
cyanide by enzymatic removal of the sugar. Even after take-up of undamaged tissue, the hydrogen cyanide is released by glycosidases in the gut of animals during
the digestion process. Sorghum plants have been shown to store the cyanogenic
glycoside dhurrin, which makes them resistant to pests such as rootworms
(Diabrotica spp.). However, the diurnal turnover of dhurrin implies that it might
rather function as a source of nitrogen and glucose with its defensive properties as
a useful by-product (Adewusi 1990), a role that has been generally suggested for
cyanogenic glycosides (Moller 2010).
Chapter 1 · Plant Secondary Metabolites and Their General Function in Plants