BackgroundRetrotransposons are genetic elements inducing mutations in all domains of life. Despite their detrimental effect, retrotransposons become temporarily active during epigenetic reprogramming and cellular stress response, which may accelerate host genome evolution. In fungal pathogens, a positive role has been attributed to retrotransposons when shaping genome architecture and expression of genes encoding pathogenicity factors; thus, retrotransposons are known to influence pathogenicity.ResultsWe here uncovered a hitherto unknown role of fungal retrotransposons as being pathogenicity factors, themselves. Studying the aggressive fungal plant pathogenBotrytis cinerea, that is known to deliver some long-terminal repeat (LTR) deriving regulatory trans-species small RNAs (BcsRNAs) into plant cells to suppress host gene expression for infection we found that naturally occurring, less aggressiveB. cinereastrains possess considerably lower copy numbers of LTR retrotransposons and had lost retrotransposonBcsRNA production. By a transgenic proof-of-concept approach, we reconstituted retrotransposon expression in aBcsRNA-lackingB. cinereastrain, which resulted in enhanced aggressiveness in a retrotransposon andBcsRNA expression-dependent manner. Moreover, retrotransposon expression inB. cinerealed to suppression of plant defence-related genes during infection.ConclusionsWe propose that retrotransposons are pathogenicity factors that manipulate host plant gene expression by encoding trans-speciesBcsRNAs. Taken together, the novelty that retrotransposons are pathogenicity factors will have general impact on studies of host-microbe interactions and pathology.
