Sporisorium reilianum possesses a pool of effector proteins that modulate virulence on maize

Abstract

The biotrophic maize head smut fungus Sporisorium reilianum is a close relative of the tumour-inducing maize smut fungus Ustilago maydis with a distinct disease aetiology. Maize infection with S. reilianum occurs at the seedling stage, but spores first form in inflorescences after a long endophytic growth phase. To identify S. reilianum-specific virulence effectors, we defined two gene sets by genome comparison with U. maydis and with the barley smut fungus Ustilago hordei. We tested virulence function by individual and cluster deletion analysis of 66 genes and by using a sensitive assay for virulence evaluation that considers both disease incidence (number of plants with a particular symptom) and disease severity (number and strength of symptoms displayed on any individual plant). Multiple deletion strains of S. reilianum lacking genes of either of the two sets (sr10057, sr10059, sr10079, sr10703, sr11815, sr14797 and clusters uni5-1, uni6-1, A1A2, A1, A2) were affected in virulence on the maize cultivar 'Gaspe Flint', but each of the individual gene deletions had only a modest impact on virulence. This indicates that the virulence of S. reilianum is determined by a complex repertoire of different effectors which each contribute incrementally to the aggressiveness of the pathogen.

Effectors with Different Gears: Divergence of Ustilago maydis Effector Genes Is Associated with Their Temporal Expression Pattern during Plant Infection

“…The rapid disease development of U. maydis makes it a model for smut fungi, and many virulence effectors have been functionally characterized [ 3 ]. In contrast, the SUPPRESSOR OF APICAL DOMINANCE1 (SAD1) is hitherto the only functionally characterized effector in S. reilianum [ 19 ], and several virulence factors have been identified [ 20 ]. Secreted proteins alter their expression in a wave like fashion during the course of infection [ 16 ].…”

Cross-species analysis between the maize smut fungiUstilago maydisandSporisorium reilianumhighlights the role of transcriptional plasticity of effector orthologs for virulence and disease

“…reilianum is largely unknown. Deletion of sr10075 alone did not seem affect virulence, while a knockout sr10079 reduced S. reilianum virulence (Ghareeb et al, 2019). The orthologs of UMAG_05318…”

Genetic Manipulation of the Brassicaceae Smut Fungus Thecaphora thlaspeos

“…We have generated a hygromycin resistance cassette, where expression of the hygromycin-phospho-transferase, hpt, is controlled by the T. thlaspeos hsp70 promoter sequence, similar to the cassettes used in U. maydis [ 3 ]. Interestingly, promoter sequences seem to be exchangeable between smut fungi, since the T. thlaspeos promoters were active in U. maydis and several groups have successfully used U. maydis constructs [ 23 , 45 ]. This now enables us to generate reporter strains for a broad range of scientific questions.…”