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8.1 Sirodesmin PL, an Epipolythiodioxopiperazine Toxin

Sirodesmin PL is a non-host specific toxin produced by the ascomyceteLeptosphaeria

maculans, which causes blackleg (phoma stem canker) of oilseed rape (Brassica

napus), the most damaging disease of this crop worldwide (Fitt et al. 2006). As well

as causing chlorotic (yellow) lesions on leaves, sirodesmin PL has antibacterial and

antiviral properties (Rouxel et al. 1988). Sirodesmin PL is a member of the epi-

polythiodioxopiperazine (ETP) class of fungal secondary metabolites, which are

characterised by a sulphur-bridged dioxopiperazine ring synthesised from two

amino acids (Fig. 8.1a, b). The disulphide bridge confers toxicity by enabling ETPs

to cross-link proteins via cysteine residues, and to generate reactive oxygen species

through redox cycling.

At least 14 different ETPs are known, and all are produced by ascomycetes

(for review see Gardiner et al. 2005). The best-characterised ETP, gliotoxin

(Fig. 8.1c), is produced by the opportunistic human pathogenAspergillus fumigatus,

as well asA. terreus,A. flavus,A .niger, Penicillium terlikowskii and Trichoderma

virens, a fungus that controls root diseases of plants. Other ETPs with roles in

animal diseases include sporidesmins, produced by Pithomyces chartarum which

infects grasses and causes facial eczema and liver diseases of grazing animals.

Chaetomin and chaetocin are produced by Chaetomium globosum, a systemic

pathogen of immunocompromised humans. The cytotoxicity of some ETPs has

made them attractive as potential anticancer agents and accordingly interest in

these molecules is increasing.

B.J. Howlett (*), E.M. Fox, A.J. Cozijnsen, A.P. Wouw, and C.E. Elliott

School of Botany, The University of Melbourne, Vic 3010, Australia

e-mail: bhowlett@unimelb.edu.au

Chapter 8

The Secondary Metabolite Toxin,

Sirodesmin PL, and Its Role in Virulence

of the Blackleg Fungus

Barbara J. Howlett, Ellen M. Fox, Anton J. Cozijnsen,

Angela P. Van de Wouw, and Candace E. Elliott

R.N. Strange and M.L. Gullino (eds.), The Role of Plant Pathology in Food Safety

and Food Security, Plant Pathology in the 21st Century 3,

DOI 10.1007/978-1-4020-8932-9_8, Springer Science+Business Media B.V. 2010

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90 B.J. Howlett et al.

8.2 Biosynthesis of Sirodesmin PL

Sirodesmin PL is derived from serine and tyrosine. Labelling experiments and analysis

of putative intermediates have been used to deduce the biosynthetic pathway (Ferezou

et al. 1980a, b; BuLock and Clough 1992). A prenyl transferase is predicted to

catalyse the addition of a dimethylallyl group to either the dipeptide cyclo-l-tyrosyl-

l-serine or free tyrosine, before condensation with serine, to produce the intermediate

cyclic dipeptide, phomamide. The genes responsible for the biosynthesis of sirodesmin

PL inL. maculans were identified 4 years ago by an approach that took advantage

of the clustering of genes encoding biosynthetic enzymes for secondary metabolites

in fungal genomes (Gardiner et al. 2004). A homologue of prenyl transferase

(a dimethyl tryptophan synthetase) from the endophyteNeotyphodium coenphialum

was identified from an L. maculans Expressed Sequence Tag (EST) library. This

EST was used to probe anL. maculans cosmid DNA library and a cosmid (35 kb)

containing the prenyl transferase was sequenced. Regions flanking this prenyl

transferase gene (named sirD) contained a cluster of 18 genes which, based on best

matches to genes in databases, could be assigned roles in sirodesmin PL biosynthesis.

These included a two module non-ribosomal peptide synthetase (sirP), acetyl trans-

ferase (sirH), methyl transferases (sirMand sirN), an ATP-binding cassette (ABC) type

transporter (sirA), responsible for toxin efflux, and a member of the zinc binuclear

cluster (Zn(II)2Cys6) family (sirZ) (Fig. 8.2).Of the nine cluster genes tested, all were co-regulated and timing of expression

was consistent with the production of sirodesmin PL in culture, suggesting the gene

cluster is involved in sirodesmin PL biosynthesis. Furthermore, the disruption of the

peptide synthetase, sirP, resulted in a mutant isolate unable to produce sirodesmin

PL, confirming that this gene is essential for the synthesis of this toxin, and that the

biosynthetic gene cluster was correctly identified (Fox and Howlett 2008a).

8.3 Regulation of Sirodesmin PL Biosynthesis

Members of the zinc binuclear cluster (Zn(II)2Cys

6) family of transcription

factors are unique to fungi (Todd and Andrianopoulos 1997). Genes in this family

control production of other secondary metabolites such as aflatoxins, therefore,

Fig. 8.1 Structures of epithiodioxopiperazines. (a) Core epithiodioxopiperazine (ETP) moiety;

(b) sirodesmin PL; (c) gliotoxin

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918 The Secondary Metabolite Toxin

SirZ was an obvious candidate for the regulation of sirodesmin PL. RNAi-

induced silencing of this gene led to minimal production of sirodesmin PL and

very low expression of several of the biosynthetic genes (Fox et al. 2008).

Binding sites for Zn(II)2Cys6 transcription factors consist of conserved terminaltrinucleotides, usually in a symmetrical configuration, spaced by an internal

variable sequence of defined length (Todd and Andrianopoulos 1997). Such

sequences are present in the promoters of most of the sirodesmin PL biosyn-

thetic genes, and it is likely that these are binding sites for the pathway specific

transcription factor, sirZ (Fox et al. 2008).

In order to identify regulators of sirodesmin PL biosynthesis upstream ofsirZ,

a library ofL. maculans insertional mutants has been screened for lack of

sirodesmin PL production, using an assay that relies on the antibacterial properties of

sirodesmin (E.M. Fox and B.J. Howlett, unpublished, 2009). The insertionalmutants were created via Agrobacterium tumefaciens-mediated transformation

whereby T-DNA is inserted randomly in the genome (Elliott and Howlett 2006).

Ten-day-old colonies of individualL. maculans mutants growing on an agar plate

were assayed for loss of the ability to produce a ring of clearing when overlaid

with molten agar containing Bacillus subtilis. Five sirodesmin-deficient mutants

have been isolated; four of which have insertions in genes with best matches to

transcription factors, whilst the other is a hypothetical gene (E.M. Fox and B.J.

Howlett, unpublished, 2009). One of the transcription factors was cpcA, a general

amino acid transcriptional regulator in fungi including A. fumigatus (Krappmann

et al. 2004). When amino acids are unavailable, fungi activate a complex regula-

tory network that allows the coordinated expression of a whole suite of genes

required for amino acid biosynthesis. The central control element of this network

is CpcA. The role ofcpcA in the regulation of sirodesmin PL biosynthesis is currently

being assessed.

Fig. 8.2 TheLeptosphaeria maculans sirodesmin PL (a) andAspergillus fumigatus gliotoxin (b)

biosynthetic gene clusters. Common ETP moiety genes (white texton black background) include

those with best matches to non-ribosomal peptide synthetase (P), thioredoxin reductase (T), methyl

transferases (MandN), glutathione-S-transferase (G) and cytochrome P450 mono-oxygenase (C),

amino cyclopropane carboxylate synthase (ACCS) (I), dipeptidase (J), as well as a transcriptional

regulator (Z) and a transporter (A). Other genes (black text on white background) do not have

obvious homologues in the other cluster and are thought to be involved in modification of the side

chains of the core ETP moiety. These encode cytochrome P450 mono-oxygenases (F,B andE), a

prenyl transferase (D), an acetyl transferase (H), epimerases (Q, SandR), an oxidoreductase (O)and a hypothetical protein (K) (Gardiner and Howlett 2005). Genes shaded in grey encode proteins

with best matches to proteins with no potential roles in ETP biosynthesis. Theforward slash marks

represent a 17 kb region of repetitive DNA (reproduced from Fox and Howlett (2008a), with

permission from Elsevier)

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938 The Secondary Metabolite Toxin

non-ribosomal peptide synthetase gene (sirP). When the sirP mutant was inoculated

onto cotyledons ofB. napus, it caused similar-sized lesions as the wild type isolate,

indicating that sirodesmin PL was not a virulence factor at this stage of infection.

Subsequently, the mutant caused fewer stem lesions and was half as effective as the

wild type in colonising stems, as shown by quantitative PCR analyses (Elliottet al. 2007) (Fig. 8.3). Thus sirodesmin PL contributes to virulence inB. napus stems.

The expression of two cluster genes, the peptide synthetase, sirP and an ABC trans-

porter, sirA, was also studied during infection. Fungal isolates containing fusions

of the green fluorescent protein gene (GFP) with the promoters of these genes

fluoresced 10 days post-inoculation (dpi). This expression pattern was consistent

with the distribution of sirodesmin PL in both cotyledons and stems, as revealed by

mass spectrometry experiments (Elliott et al. 2007).

It is intriguing to speculate as to why sirodesmin PL contributes to colonisation

of stems but not to generation of the necrotic symptoms of lesions in the cotyledons.Sirodesmin PL may suppress plant defences during the biotrophic growth down the

stem, or it might play a role in competition between L. maculans and other micro-

organisms in planta, or even on stubble during the saprophytic stage of its life cycle.

Germination of several fungal species including Fusarium graminaerum and

L. biglobosa brassicae was inhibited in the presence of a 13 day old colony of the

wild type sirodesmin PL-producing isolate when grown in vitro, illustrating the anti-

fungal activity of this molecule (Elliott et al. 2007).

The role that secondary metabolites play in the biology of fungi is elusive. In many

cases fungi producing toxins do not rely on growth on a host to complete their lifecycle. The most likely advantage of secondary metabolites to organisms that produce

such molecules is that they enhance survival. Many such organisms live saprophyti-

cally in the soil where they are exposed to a harsh environment with a plethora of

competing organisms. Fungal virulence has been proposed to have evolved to protect

fungi in such an environment against amoebae, nematodes or other invertebrates that

Fig. 8.3 Quantification of fungal biomass in infected stems ofBrassica napus cv. Monty.

Cotyledons and first leaves ofB. napus cv. Monty were inoculated with wild type (wt) or the

sirodesmin-deficient mutant, sirP and were analysed at 33 dpi. Genomic DNA was used as a

template for quantitative PCR. The relative amount of fungal biomass was calculated as the ratio

of the amplification of a fragment of fungal actin to that of plant actin. Each barrepresents the

average of three replicate PCRs on three independent sets of infected stems. The asteriskindicates

that fungal biomass of wild type is significantly different from that of the mutant

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94 B.J. Howlett et al.

can feed on fungi (Mylonakis et al. 2007); Fox and Howlett (2008b). Secondary

metabolite toxins could play a role in such behaviour. The recent availability of

defined mutants in the biosynthesis of secondary metabolites will enable this hypoth-

esis to be tested for some molecules and their producing-organisms.

Acknowledgements We thank the Australian Research Council and the Grains Research and

Development Corporation for funding our research.

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