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Abstract
SummaryThe economically important necrotrophic fungal pathogen, Pyrenophora tritici‐repentis (Ptr), causes tan spot of wheat, a disease typified by foliar necrosis and chlorosis. The culture filtrate of an Australian Ptr isolate, M4, possesses phytotoxic activity and plant bioassay guided discovery led to the purification of necrosis inducing toxins called triticone A and B. High‐resolution LC–MS/MS analysis of the culture filtrate identified an additional 37 triticone‐like compounds. The biosynthetic gene cluster responsible for triticone production (the Ttc cluster) was identified and deletion of TtcA, a hybrid polyketide synthase (PKS)‐nonribosomal peptide synthase (NRPS), abolished production of all triticones. The pathogenicity of mutant (ttcA) strains was not visibly affected in our assays. We hypothesize that triticones possess general antimicrobial activity important for competition in multi‐microbial environments.
Highlights
At least 15 000 specialized or secondary metabolites have been identified in ascomycete fungi and many have been shown to have important roles in signalling, defence or as toxins (Losada et al, 2009; Spiteller, 2015; Macheleidt et al, 2016; Wakefield et al, 2017; Künzler, 2018; Chapman and Hall, 2019)
Emodin, catenarin, helminthosporin, islandicin and their hydroxylated forms have been detected in culture filtrates, infected leaves and grains of wheat (Kachlicki and Wakulinski, 2002; Wakulinski et al, 2003)
Infiltration of the culture filtrate of Pyrenophora tritici-repentis (Ptr) M4, a ToxA and ToxC producing Ptr isolate, into leaves of the ToxA insensitive wheat cultivar Eagle Rock produced a beige ovoid region of necrosis which did not extend to the entire infiltration zone (Fig. 1A)
Summary
At least 15 000 specialized or secondary metabolites have been identified in ascomycete fungi and many have been shown to have important roles in signalling, defence or as toxins (Losada et al, 2009; Spiteller, 2015; Macheleidt et al, 2016; Wakefield et al, 2017; Künzler, 2018; Chapman and Hall, 2019). The biosynthesis of these specialized metabolites is generally facilitated by any of four core biosynthetic enzymes; polyketide synthase (PKS), nonribosomal peptide synthase (NRPS), terpene synthase (TPS) or dimethylallyl tryptophan synthase (DMATS) (Keller et al, 2005) or are ribosomally synthesized and post-translationally modified (Hetrick and van der Donk, 2017). Catenarin is thought to be the cause of red smudge, a red discolouration of grains predominantly found on infected durum wheat (Bouras and Strelkov, 2008)
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