Isolation of Solanapyrones A, B and C from Culture Filture and Spore Germination Fluids of Ascochyta rabiei and Aspects of Phytotoxin Action

Abstract

Nine isolates of the fungus Ascochyta rabiei have been assayed for their ability to produce solanapyrone toxins. All isolates formed solanapyrone A, B and C which were secreted into the culture medium. Pronounced production of the toxins only occurred after onset of sporulation. The identification of the fungal products was achieved by cochromatography (TLC, HPLC), 1H‐NMR (solanapyrone A and B) and mass spectrometry (solanapyrone B). Work with A. rabiei isolate X showed that cultivation in chickpea seed extract medium in a surface culture provided best conditions for maximal toxin production. The accumulation of solanapyrones over the growth cycle was monitored. Germinating spores produced solanapyrones C and B whereas solanapyrone A was formed from the 6th day of the culture period on. Application of a mixture of solanapyrones A, B and C to leaflets of intact plants from an A. rabiei resistant cultivar (ILC 3279) and a susceptible cultivar (ILC 1929) led to characteristic changes in leaf morphology which had earlier been obsevad in susceptible plants following infection with spores of A. rabiei. Attempts to demonstrate the occurrence of toxins in the infected leaf were unsuccessful. Application of solanapyrones to solanapyrones to chickpea cell suspension cultures (derived from both cultivars) led to pronounced losses in viability and to plasmolysis of cells.

Comparison of the epidemiology of ascochyta blights on grain legumes

“…Therefore, fungi responsible for ascochyta blights may be considered as hemibiotrophs characterised by an initial biotrophic phase that is followed by a necrotrophic phase. However, phytotoxins characteristic for necrotrophic pathogens have been isolated from the germination fluid of A. rabiei spores and were suggested to be of importance in early ascochyta blight development on chickpea (Hö hl et al 1991). Histological studies for the first critical 48-72 h have not been published for this pathogen to determine whether host invasion follows a biotrophic or necrotrophic strategy.…”

“…Among toxins, enzymes and suppressors, toxins have received most attention to date, whereas less is known about enzymes and suppressors. All three types of compounds are produced by A. rabiei which has been studied most extensively (reviewed by Welle 1992 andJayakakumar et al 2005), and the toxins solanopyrones A, B and C have been isolated from culture filtrates and spore germination fluids (Hö hl et al 1991;Kaur 1995). The role of the phytotoxin ascochitine has been comprehensively studied on faba bean infected with A. fabae (Beed et al 1994), and on pea infected with A. pisi (Marcinkowska et al 1991).…”

A Novel Type Pathway-Specific Regulator and Dynamic Genome Environments of a Solanapyrone Biosynthesis Gene Cluster in the Fungus Ascochyta rabiei

“…Synteny analyses of the A. rabiei draft genome data and data on the genomes of other fungal species within the order Pleosporales, available on the JGI website (64), showed that a large portion of the contig harboring the solanapyrone gene cluster is syntenic (mesosyntenic sensu stricto) to the largest scaffold of related species (the 3.38-Mb scaffold of Leptosphaeria maculans and the 4.03-Mb scaffold of Setosphaeria turcica), indicating that the solanapyrone gene cluster resides in one of the core chromosomes of A. rabiei. Accordingly, despite the high degree of karyotype variation observed in the A. rabiei strains (65), all strains examined so far are known to produce solanapyrones (26,27,66).…”

Towards identifying pathogenic determinants of the chickpea pathogen Ascochyta rabiei

“…Among the ascochyta pathogens, A. rabiei on chickpea is probably the most intensively studied pathosystem in terms of biochemical interactions between the host and the pathogen. Ascochyta rabiei, causal agent of chickpea ascochyta blight, produces toxin solanopyrones through the polyketide synthesis pathway (Alam et al 1989;Hohl et al 1991), and hydrolytic or cell-wall degrading enzymes (Tenhaken and Barz 1991;Tenhaken et al 1997). Several lines of evidence show the roles of the phytotoxins in causing blight (Chen and Strange 1991;Kaur 1995).…”

“…Ascochyta rabiei produces melanin through the 1,8-dihydroxynaphthalene pathway via polyketide synthesis (Chen et al 2004b). Thus, polyketide synthases could potentially be pathogenicity factors in A. rabiei through their involvement in melanin biosynthesis or in the synthesis of phytotoxin solanapyrones (Hohl et al 1991).…”