1
Introduction
Secondary metabolites (SMs) are universal messengers between plants and pathogens, of which the most widespread are filamentous fungi. SMs are responsible for
pathogen recognition by the plant host and for pathogen actions during host infection. They belong to multiple classes concerning their chemical structures and
influence diverse biochemical processes exhibiting signaling, toxic, eliciting, priming, growth-promoting, or defense response-inducing actions [1]. The ability to
produce the SMs is often governed by the presence and activity of the specific
gene clusters present in fungal genomes, which usually contain several enzymeencoding genes devoted exclusively to biosynthesize specific group of compounds
[2, 3]. The distribution of these gene clusters among fungal taxa generally resembles
their phylogenetic relationships but sometimes may serve as the evidence of the past
horizontal gene transfer events, since the same biosynthetic pathways may be found
in species that not share close relationship [4, 5]. Moreover, it seems that SM
biosynthetic gene clusters not only undergo common regulation and expression
patterns but also share evolutionary fate, which often depends strongly on the
genomic context and differs from main primary metabolism (PM) regions. Many
SMs are universal for diverse fungal microorganisms, and their actions are similar
for various plant-pathogen systems; nevertheless, significant level of specificity may
be observed in comparative metabolomic analyses of pathogenic fungi.
Fusarium genus consists of a large number of diverse species of different
lifestyles. Many of them are opportunistic pathogens infecting multiple plant species
in a range of climatic zones (e.g., F. graminearum species complex (FGSC), F.
fujikuroi species complex (FFSC), F. equiseti species complex (FESC), F.
avenaceum, and F. culmorum), and some are more typical soil-borne pathogens
and are more likely isolated from the rhizosphere of plants (mainly F. oxysporum
species complex (FOSC) and F. solani species complex (FSSC)).
Fusaria are extremely variable in terms of genetics, biology, and ecology; thus,
they produce also very diverse repertoire of SMs. This divergence relates partially to
the ecological niches occupied by individual species but also seems to play some, yet
unknown role in the organism ecological flexibility. On the other hand, closely
related species may vary in biosynthetic potentials. Fumonisins may serve as an
example. F. verticillioides and F. proliferatum are the main producers of fumonisins,
both capable of infecting maize as the typical host. Yet, the sequence divergence of
the FUM biosynthetic cluster responsible for fumonisin biosynthetic ability reaches
20% when those two species are compared [2, 6–8]; in F. oxysporum, FUM cluster
has been found and characterized for just one strain O-1890 [9], and another maize
pathogens from the FFSC – F. subglutinans and F. temperatum – are essentially
fumonisin nonproducers [10, 11].
Genes inside the clusters responsible for the SMs’ biosynthesis remain under
strong selection pressure, exerted by ecological factors (environment, competitive
organisms, host availability, and resistance) which keep the structures and functions
of encoded enzymes relatively stable. Still, even intraspecific polymorphism can be
observed for some of the pathways, like FUM cluster divergence in populations of F.
proliferatum [12, 13]. Similar examples of discrepancies in phylogenetic
10 Fusarium Secondary Metabolism Biosynthetic Pathways: So Close but So. . .
213
Introduction
Secondary metabolites (SMs) are universal messengers between plants and pathogens, of which the most widespread are filamentous fungi. SMs are responsible for
pathogen recognition by the plant host and for pathogen actions during host infection. They belong to multiple classes concerning their chemical structures and
influence diverse biochemical processes exhibiting signaling, toxic, eliciting, priming, growth-promoting, or defense response-inducing actions [1]. The ability to
produce the SMs is often governed by the presence and activity of the specific
gene clusters present in fungal genomes, which usually contain several enzymeencoding genes devoted exclusively to biosynthesize specific group of compounds
[2, 3]. The distribution of these gene clusters among fungal taxa generally resembles
their phylogenetic relationships but sometimes may serve as the evidence of the past
horizontal gene transfer events, since the same biosynthetic pathways may be found
in species that not share close relationship [4, 5]. Moreover, it seems that SM
biosynthetic gene clusters not only undergo common regulation and expression
patterns but also share evolutionary fate, which often depends strongly on the
genomic context and differs from main primary metabolism (PM) regions. Many
SMs are universal for diverse fungal microorganisms, and their actions are similar
for various plant-pathogen systems; nevertheless, significant level of specificity may
be observed in comparative metabolomic analyses of pathogenic fungi.
Fusarium genus consists of a large number of diverse species of different
lifestyles. Many of them are opportunistic pathogens infecting multiple plant species
in a range of climatic zones (e.g., F. graminearum species complex (FGSC), F.
fujikuroi species complex (FFSC), F. equiseti species complex (FESC), F.
avenaceum, and F. culmorum), and some are more typical soil-borne pathogens
and are more likely isolated from the rhizosphere of plants (mainly F. oxysporum
species complex (FOSC) and F. solani species complex (FSSC)).
Fusaria are extremely variable in terms of genetics, biology, and ecology; thus,
they produce also very diverse repertoire of SMs. This divergence relates partially to
the ecological niches occupied by individual species but also seems to play some, yet
unknown role in the organism ecological flexibility. On the other hand, closely
related species may vary in biosynthetic potentials. Fumonisins may serve as an
example. F. verticillioides and F. proliferatum are the main producers of fumonisins,
both capable of infecting maize as the typical host. Yet, the sequence divergence of
the FUM biosynthetic cluster responsible for fumonisin biosynthetic ability reaches
20% when those two species are compared [2, 6–8]; in F. oxysporum, FUM cluster
has been found and characterized for just one strain O-1890 [9], and another maize
pathogens from the FFSC – F. subglutinans and F. temperatum – are essentially
fumonisin nonproducers [10, 11].
Genes inside the clusters responsible for the SMs’ biosynthesis remain under
strong selection pressure, exerted by ecological factors (environment, competitive
organisms, host availability, and resistance) which keep the structures and functions
of encoded enzymes relatively stable. Still, even intraspecific polymorphism can be
observed for some of the pathways, like FUM cluster divergence in populations of F.
proliferatum [12, 13]. Similar examples of discrepancies in phylogenetic
10 Fusarium Secondary Metabolism Biosynthetic Pathways: So Close but So. . .
213