a single species has the genetic potential to produce about the equal number of the
SMs to that earlier claimed for the entire genus [65].
The in silico analyses of genomic sequences of a wide range of Fusarium species
revealed surprisingly high level of differences in the distribution of secondary
metabolite biosynthetic genes and, therefore, differences in the genetic potential
of individual species to produce SMs [52, 66, 69]. Namely, the PKS gene PGL1,
which is necessary for the production of a blackish perithecial pigment and a family
of reddish mycelial pigments (fusarubins), was occurring in all Fusaria examined in
multiple studies [52, 64, 70, 71]. Moreover, there are some reports indicating that
the SM’s biosynthetic gene clusters are well-conserved among organisms. From the
evolutionary point of view, their maintenance could only be beneficial for the fungus
if the final product would confer any advance to the producing organism, even if the
effect of their action is subtle or not directly obvious [59, 60]. This statement applies
for mycotoxins, like the narrowly distributed fumonisin and gibberellin gene clusters
that are exhibited in only some species of the F. fujikuroi and F. oxysporum species
complexes [72, 73]. Additionally, the fusarin biosynthetic genes, which are extensively spread in Fusarium, are occurring in all F. oxysporum isolates that have been
analyzed [52].
5.1.1 Trichothecenes
Trichothecenes are the major group of mycotoxins produced by various Fusarium
plant pathogens [61, 64, 74]. Due to their toxicity and economic significance,
trichothecenes are among the best characterized mycotoxins. Structurally, they are
sesquiterpenoid compounds with a tricyclic 12,13-epoxytrichothec-9-ene ring that
can be chemically substituted at several positions, which result in multiple derivatives [75, 76].
There are over 200 trichothecene derivatives which can be grouped into four
main groups: types A, B, C, and D. Type A trichothecenes characterized by
hydroxyl, or ester substitution at C-8, contain diacetoxyscirpenol (DAS), T-2
toxin, HT-2 toxin, and neosolaniol, and T-2 toxin is the most toxic trichothecene
in animals. Recently, a new chemotype has been discovered among type A trichothecenes and designed NX-2. Surprisingly, it can be produced by F. graminearum,
which is a typical type B trichothecene producer [77, 78]. The most important
producers of type A trichothecenes are F. sporotrichioides, F. langsethiae, F. poae,
F. sambucinum, F. armeniacum, and F. venenatum. They may develop on variety of
cereal grains especially in cold climate regions or during storage conditions [76, 79].
Type B trichothecenes contain a C-8 keto group and are produced by various
Fusarium species, particularly from the Fusarium graminearum species complex:
F. graminearum sensu lato, F. culmorum, F. pseudograminearum, and F. cerealis.
The most common type B trichothecenes are deoxynivalenol (DON), nivalenol
(NIV), and the DON-acetylated derivatives AcDONs. Type C trichothecenes are
a minor group of toxins produces by several other genera of fungi, and type D
includes compounds produced by Stachybotrys species that are considered as important indoor mold hazards [17, 74, 80].
10 Fusarium Secondary Metabolism Biosynthetic Pathways: So Close but So. . .
219
SMs to that earlier claimed for the entire genus [65].
The in silico analyses of genomic sequences of a wide range of Fusarium species
revealed surprisingly high level of differences in the distribution of secondary
metabolite biosynthetic genes and, therefore, differences in the genetic potential
of individual species to produce SMs [52, 66, 69]. Namely, the PKS gene PGL1,
which is necessary for the production of a blackish perithecial pigment and a family
of reddish mycelial pigments (fusarubins), was occurring in all Fusaria examined in
multiple studies [52, 64, 70, 71]. Moreover, there are some reports indicating that
the SM’s biosynthetic gene clusters are well-conserved among organisms. From the
evolutionary point of view, their maintenance could only be beneficial for the fungus
if the final product would confer any advance to the producing organism, even if the
effect of their action is subtle or not directly obvious [59, 60]. This statement applies
for mycotoxins, like the narrowly distributed fumonisin and gibberellin gene clusters
that are exhibited in only some species of the F. fujikuroi and F. oxysporum species
complexes [72, 73]. Additionally, the fusarin biosynthetic genes, which are extensively spread in Fusarium, are occurring in all F. oxysporum isolates that have been
analyzed [52].
5.1.1 Trichothecenes
Trichothecenes are the major group of mycotoxins produced by various Fusarium
plant pathogens [61, 64, 74]. Due to their toxicity and economic significance,
trichothecenes are among the best characterized mycotoxins. Structurally, they are
sesquiterpenoid compounds with a tricyclic 12,13-epoxytrichothec-9-ene ring that
can be chemically substituted at several positions, which result in multiple derivatives [75, 76].
There are over 200 trichothecene derivatives which can be grouped into four
main groups: types A, B, C, and D. Type A trichothecenes characterized by
hydroxyl, or ester substitution at C-8, contain diacetoxyscirpenol (DAS), T-2
toxin, HT-2 toxin, and neosolaniol, and T-2 toxin is the most toxic trichothecene
in animals. Recently, a new chemotype has been discovered among type A trichothecenes and designed NX-2. Surprisingly, it can be produced by F. graminearum,
which is a typical type B trichothecene producer [77, 78]. The most important
producers of type A trichothecenes are F. sporotrichioides, F. langsethiae, F. poae,
F. sambucinum, F. armeniacum, and F. venenatum. They may develop on variety of
cereal grains especially in cold climate regions or during storage conditions [76, 79].
Type B trichothecenes contain a C-8 keto group and are produced by various
Fusarium species, particularly from the Fusarium graminearum species complex:
F. graminearum sensu lato, F. culmorum, F. pseudograminearum, and F. cerealis.
The most common type B trichothecenes are deoxynivalenol (DON), nivalenol
(NIV), and the DON-acetylated derivatives AcDONs. Type C trichothecenes are
a minor group of toxins produces by several other genera of fungi, and type D
includes compounds produced by Stachybotrys species that are considered as important indoor mold hazards [17, 74, 80].
10 Fusarium Secondary Metabolism Biosynthetic Pathways: So Close but So. . .
219