Mycovirus induced hypervirulence of Leptosphaeria biglobosa enhances systemic acquired resistance to Leptosphaeria maculans in Brassica napus

Phoma stem canker is one of the most important diseases of winter oil seed rape (Brassica napus) world-wide and is caused by a complex that comprises at least two species: Leptosphaeria maculans and Leptosphaeria biglobosa. Screening a panel of field Leptosphaeria isolates from B. napus for the presence of mycoviruses revealed the presence of a novel double stranded (ds) RNA virus in L. biglobosa and no viruses in L. maculans. The virus forms isometric particles ca. 40–45 nm in diameter and has four genomic segments, each possessing a single open reading frame flanked by untranslated regions. Phylogenetic analysis revealed modest similarities to known and suspected members of the family Quadriviridaeand therefore the virus was nominated Leptosphaeria biglobosa quadrivirus-1. Following eradication of the mycovirus, virus-infected and virus-free isogenic lines of L. biglobosa were created. A direct comparison of the growth and virulence of these isogenic lines illustrated that virus infection caused hypervirulence and resulted in induced systemic resistance towards L. maculans in B. napus following lower leaf pre-inoculation with the virus-infected isolate. Analysis of the plant transcriptome suggests that the presence of the virus leads to subtle alterations in metabolism and plant defences. For instance, transcripts involved in carbohydrate and amino acid metabolism are enriched in plants treated with the virus-infected isolate, while pathogenesis-related proteins, chitinases and WRKY transcription factors are differentially expressed. These results illustrate the potential for deliberate inoculation of plants with hypervirulent L. biglobosa to decrease the severity of phoma stem canker later in the growing season.