Genomic analysis of secondary metabolism in Ramularia collo-cygni, causative agent of Ramularia leaf spot disease of barley
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Date
30/11/2017Author
Dussart, François Mathieu Didier
Metadata
Abstract
In the past two decades a new barley threat has emerged as the disease
Ramularia leaf spot (RLS) became more prevalent in temperate regions worldwide.
This disease, first identified in the late 19th century, is caused by the filamentous
fungus Ramularia collo-cygni (Rcc) and can cause substantial yield losses as well as
reduce grain quality. RLS typically occurs late in the growing season and
characteristic disease symptoms are usually seen after the crop has flowered.
Expression of RLS lesions is thought to be associated with the action of fungal
secondary metabolism products. The one group of secondary metabolites (SMs)
characterised to date from Rcc, the anthraquinone toxins rubellins, are known to
cause necrosis to plant tissues in a non-host specific manner. Therefore, it appears
that fungal secondary metabolism might be a key component in understanding the
interaction between Rcc and its host.
In this study, more than 23 core genes involved in the biosynthesis of SMs
belonging to the polyketide and non-ribosomal peptide pathways were identified in
the genome of Rcc. Putative clusters containing genes with a predicted function
relating to secondary metabolism were identified by in silico genome walking in the
genetic loci adjacent to Rcc SM core genes. Two gene clusters containing no SM
core gene were also identified. Five of the putative SM clusters exhibited similarity
to the known fungal SM biochemical pathways involved in gliotoxin,
monodictyphenone, ferricrocin, betaenone and chaetoglobosins biosynthesis. Several
gene clusters exhibited similarity to SM clusters from fungal species where the SM
pathway is uncharacterised.
Changes in transcript abundance of selected SM core genes during RLS
development in artificially inoculated barley seedlings were tested. Transcript levels
were found to be the highest at an early stage of disease development, typically
during the asymptomatic and early lesions formation stages and declined over time,
suggesting that the associated SMs in Rcc, may not necessarily be involved in
symptoms appearance. The in planta mode of action of the non-host specific photoactivated toxin
rubellin D was studied in the model plant Arabidopsis thaliana. Rubellin-induced
cell death appeared phenotypically reminiscent of programmed cell death (PCD).
Full expression of rubellin D-induced cell death required the host salicylic acid (SA)
pathway and the host proteasome supporting the PCD response to this fungal SM.
However, a clear correlation between toxin sensitivity and disease susceptibility
could not be found, suggesting a potential alternative role for rubellin in disease
symptom development.