Abstract
Corynespora cassiicola is an Ascomycetes fungus with a broad host range and diverse life styles. Mostly known as a necrotrophic plant pathogen, it has also been associated with rare cases of human infection. In the rubber tree, this fungus causes the Corynespora leaf fall (CLF) disease, which increasingly affects natural rubber production in Asia and Africa. It has also been found as an endophyte in South American rubber plantations where no CLF outbreak has yet occurred. The C. cassiicola species is genetically highly diverse, but no clear relationship has been evidenced between phylogenetic lineage and pathogenicity. Cassiicolin, a small glycosylated secreted protein effector, is thought to be involved in the necrotrophic interaction with the rubber tree but some virulent C. cassiicola isolates do not have a cassiicolin gene. This study set out to identify other putative effectors involved in CLF. The genome of a highly virulent C. cassiicola isolate from the rubber tree (CCP) was sequenced and assembled. In silico prediction revealed 2870 putative effectors, comprising CAZymes, lipases, peptidases, secreted proteins and enzymes associated with secondary metabolism. Comparison with the genomes of 44 other fungal species, focusing on effector content, revealed a striking proximity with phylogenetically unrelated species (Colletotrichum acutatum, Colletotrichum gloesporioides, Fusarium oxysporum, nectria hematococca, and Botrosphaeria dothidea) sharing life style plasticity and broad host range. Candidate effectors involved in the compatible interaction with the rubber tree were identified by transcriptomic analysis. Differentially expressed genes included 92 putative effectors, among which cassiicolin and two other secreted singleton proteins. Finally, the genomes of 35 C. cassiicola isolates representing the genetic diversity of the species were sequenced and assembled, and putative effectors identified. At the intraspecific level, effector-based classification was found to be highly consistent with the phylogenomic trees. Identification of lineage-specific effectors is a key step toward understanding C. cassiicola virulence and host specialization mechanisms.
Highlights
CCP was selected as the reference among a collection of Cucumis sativus (Cs). cassiicola isolates from various hosts and of diverse geographical origins, for its high virulence in the rubber tree (Breton et al, 2000; Déon et al, 2012a,b, 2014)
This study provides the genomic description of a plant pathogenic C. cassiicola isolate and the first transcriptomic analysis for this species
By combining in silico mining of all putative effectors and experimental identification of the fungal genes differentially expressed during the compatible interaction with the rubber tree, we were able to identify pertinent candidate effectors potentially involved in Corynespora leaf fall (CLF) disease, in addition to the already characterized effector cassiicolin
Summary
A. Curtis) is an Ascomycetes fungus responsible for diseases in a wide range of plants (Farr and Rossman, 2016), mainly in tropical and subtropical areas or greenhouses. Curtis) is an Ascomycetes fungus responsible for diseases in a wide range of plants (Farr and Rossman, 2016), mainly in tropical and subtropical areas or greenhouses It has been found in nematodes (Carris et al, 1986), sponges (Zhao et al, 2015) and in rare cases of human infections (Mahgoub, 1969; Huang et al, 2010; Yamada et al, 2013; Yan et al, 2016). C. cassiicola causes the Corynespora leaf fall (CLF) disease, characterized by necrotic lesions on the leaves and massive defoliation in susceptible cultivars. CLF has gradually spread to most rubber producing areas in Asia and Africa, impairing natural rubber production yields (Kuruvilla Jacob, 2006)
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