of PM ones. Therefore, each of the clusters should be carefully and individually
checked for its usefulness in the species studied.
5.1
Mycotoxins
Mycotoxins are SMs produced by vast majority of filamentous fungi, mostly under
favorable environmental conditions. Fusarium species have the genetic potential
to produce hundreds of structurally diverse SMs, most of which have poorly
understood or completely unknown ecological functions [24, 55–57]. These
substances are usually produced in complex biochemical processes, including polyketide, terpenoid, and amino acid metabolic pathways, and can be accumulated
in crop plants. Thereby, they pose a health risk to human and livestock [4, 58–60].
Many known mycotoxins are the virulence factors related to plant disease development [61], or they might play a role in improvement of the survival of the spores and,
consequently, influence the development of the producing organism by enhancing
the fitness of a given community/species [59].
Throughout the past two decades, numerous studies have been made to better
understand the molecular mechanisms of mycotoxin biosynthesis and the direct
and indirect regulatory agents and patterns controlling these processes. Mycotoxin
biosynthetic pathways involve several coordinately regulated and functionally
related genes physically grouped into clusters that can be co-expressed under
specified conditions. Generally, these genes can be identified through the presence
of four classes of enzymes: terpene cyclases (TCs), dimethylallyltryptophan synthases (DMATSs), polyketide synthases (PKSs), and nonribosomal peptide synthetases (NRPSs) [55, 62, 63], which catalyze the condensation or rearrangement of
simple molecules to form more complex structures. Typically, the clusters contain
also the core genes responsible for structural modifications of the initial metabolite,
transporters for metabolite transport, and transcription factors for coordinated transcriptional regulation of genes in the cluster. These chemical products undergo
multiple enzymatic modifications to form biologically active SMs and are transported to their site of activity [64, 65].
The release of the full genomic sequences of F. fujikuroi [62] and closely related
Fusarium species, such as F. verticillioides [66], F. mangiferae, and F. proliferatum
[63], revealed that the species have the genetic capacity of producing even more SMs
than previously thought. Before the publication of the first Fusarium genomic
sequence, the members of the entire genus were believed to produce about 40
structurally distinct families of SMs, while some groups, for instance, fumonisins
and trichothecenes, contain tens of different analogs [67, 68]. Regardless of this
metabolic diversity within the genus, single species and isolates were reported to
produce a relatively low number of metabolites, e.g., there is some evidence showing
that F. graminearum produced eight secondary metabolite families, such as
aurofusarin, butenolide, fusarins, trichothecenes, culmorin, cyclonerodiol,
chlamydosporol, and zearalenones. However, the study of the F. graminearum
genome sequence identified 16 PKSs, 19 NRPSs, and 8 TSs, which suggests that
218
Ł. Stępień et al.
checked for its usefulness in the species studied.
5.1
Mycotoxins
Mycotoxins are SMs produced by vast majority of filamentous fungi, mostly under
favorable environmental conditions. Fusarium species have the genetic potential
to produce hundreds of structurally diverse SMs, most of which have poorly
understood or completely unknown ecological functions [24, 55–57]. These
substances are usually produced in complex biochemical processes, including polyketide, terpenoid, and amino acid metabolic pathways, and can be accumulated
in crop plants. Thereby, they pose a health risk to human and livestock [4, 58–60].
Many known mycotoxins are the virulence factors related to plant disease development [61], or they might play a role in improvement of the survival of the spores and,
consequently, influence the development of the producing organism by enhancing
the fitness of a given community/species [59].
Throughout the past two decades, numerous studies have been made to better
understand the molecular mechanisms of mycotoxin biosynthesis and the direct
and indirect regulatory agents and patterns controlling these processes. Mycotoxin
biosynthetic pathways involve several coordinately regulated and functionally
related genes physically grouped into clusters that can be co-expressed under
specified conditions. Generally, these genes can be identified through the presence
of four classes of enzymes: terpene cyclases (TCs), dimethylallyltryptophan synthases (DMATSs), polyketide synthases (PKSs), and nonribosomal peptide synthetases (NRPSs) [55, 62, 63], which catalyze the condensation or rearrangement of
simple molecules to form more complex structures. Typically, the clusters contain
also the core genes responsible for structural modifications of the initial metabolite,
transporters for metabolite transport, and transcription factors for coordinated transcriptional regulation of genes in the cluster. These chemical products undergo
multiple enzymatic modifications to form biologically active SMs and are transported to their site of activity [64, 65].
The release of the full genomic sequences of F. fujikuroi [62] and closely related
Fusarium species, such as F. verticillioides [66], F. mangiferae, and F. proliferatum
[63], revealed that the species have the genetic capacity of producing even more SMs
than previously thought. Before the publication of the first Fusarium genomic
sequence, the members of the entire genus were believed to produce about 40
structurally distinct families of SMs, while some groups, for instance, fumonisins
and trichothecenes, contain tens of different analogs [67, 68]. Regardless of this
metabolic diversity within the genus, single species and isolates were reported to
produce a relatively low number of metabolites, e.g., there is some evidence showing
that F. graminearum produced eight secondary metabolite families, such as
aurofusarin, butenolide, fusarins, trichothecenes, culmorin, cyclonerodiol,
chlamydosporol, and zearalenones. However, the study of the F. graminearum
genome sequence identified 16 PKSs, 19 NRPSs, and 8 TSs, which suggests that
218
Ł. Stępień et al.