Mining the effector repertoire of the biotrophic fungal pathogen Ustilago hordei during host and non‐host infection

Abstract

The success of plant-pathogenic fungi mostly relies on their arsenal of virulence factors which are expressed and delivered into the host tissue during colonization. The biotrophic fungal pathogen Ustilago hordei causes covered smut disease on both barley and oat. In this study, we combined cytological, genomics and molecular biological methods to achieve a better understanding of the molecular interactions in the U. hordei-barley pathosystem. Microscopic analysis revealed that U. hordei densely colonizes barley leaves on penetration, in particular the vascular system. Transcriptome analysis of U. hordei at different stages of host infection revealed differential expression of the transcript levels of 273 effector gene candidates. Furthermore, U. hordei transcriptionally activates core effector genes which may suppress even non-host early defence responses. Based on expression profiles and novelty of sequences, knockout studies of 14 effector candidates were performed in U. hordei, which resulted in the identification of four virulence factors required for host colonization. Yeast two-hybrid screening identified potential barley targets for two of the effectors. Overall, this study provides a first systematic analysis of the effector repertoire of U. hordei and identifies four effectors (Uvi1-Uvi4) as virulence factors for the infection of barley.

UhAVR1, an HR-triggering avirulence effector ofUstilago hordei, is secreted via the ER-Golgi pathway to the cytosol of barley coleoptile cells and contributes to virulence early in infection

“…Transcriptome analysis of leaves of susceptible barley (cv. Golden Promise) infected by Uh showed that 65% of all in planta induced Uh effector genes are stage-specific including UhAvr1 [18], similar to our study for UhAvr1 in cvs. Odessa and Hannchen.…”

“…Similarly, deletion of UhAvr1 and its paralogous gene (UHOR_10021) did not compromise virulence of Uh during a compatible interaction as measured in the number of diseased plants at 2 to 3 months [16], suggesting the involvement of additional effectors. Although the UhAvr1 paralogous gene has not been functionally characterized, recent Uh-barley leaf transcriptomic data revealed up-regulation of UHOR_10021 from 3 dpi [18]. This time point (72 hpi) coincides with our observed recovery of fungal growth emanating from UhAvr1m teliospores.…”

“…A gene family formed by UhAvr1 and its paralogous gene (UHOR_10021) encompasses five genes in Ustilago maydis (Um), three in Sporisorium reilianum, six in Sporisorium scitamineum and only one in the related epiphytic fungus Moesziomyces albugensi [16,17]. Recently, four effectors (Uvi1-Uvi4) contributing to virulence in Uh have been identified by gene deletion assays [18].…”

Transcriptome Analysis Identifies Plasmodiophora brassicae Secondary Infection Effector Candidates

“…), or the smut pathogen Ustilago hordei (Ökmen et al. ) with hundreds of predicted secreted proteins, 32 SSPbPs is a small number, but there are some points that should be considered for P. brassicae : (i) This pathogen is an obligate parasite only able to survive in the plant cell or as a dormant resting spore in the soil, (ii) the life cycle is less complex than any currently known biotrophic fungal plant pathogen, and (iii) the RNA‐Seq data were generated during P. brassicae secondary infection in Arabidopsis.…”

UhAVR1, an HR-Triggering Avirulence Effector of Ustilago hordei, Is Secreted via the ER–Golgi Pathway, Localizes to the Cytosol of Barley Cells during in Planta-Expression, and Contributes to Virulence Early in Infection

“…A gene family formed by UhAvr1 and its paralogous gene (UHOR_10021) encompasses five genes in Ustilago maydis ( Um ), three in Sporisorium reilianum , six in Sporisorium scitamineum and only one in the related epiphytic fungus Moesziomyces albugensi [ 16 , 17 ]. Recently, four effectors ( Uvi1-Uvi4 ) contributing to virulence in U. hordei have been identified by gene deletion assays [ 18 ].…”