Soybean seedlings as a new model to study the virulence factors of Fusarium graminearum.

Recent evidences have shown that soybean is a new host of F. graminearum mostly at the early phase of seedlings development. Some F. graminearum mutants previously characterized for virulence in wheat spike infection were tested by using a rapid virulence assay in order to establish their behavior in soybean. The disrupted mutants used were affected in DON (Δtri5), lipase (ΔFgl1) or in two MAPK signaling pathways, regulating cell wall degrading enzymes production (Δgpmk1) and osmotic balance (ΔFgOS-2), respectively. The ability of these mutants to colonize soybean mirrors their behavior in wheat. In fact, the mutants that show reduced virulence (Δtri5, ΔFgl1 and ΔFgOS-2) or non pathogenicity (Δgpmk1) on wheat spike are, correspondently, less virulent or non-pathogenic in soybean seedlings. However, a different ranking of symptom severity occurs in the two hosts: in wheat the Δtri5, ΔFgl1 and ΔFgOS-2 mutants give similar reduction of symptom severity in comparison to the WT while in soybean the ΔFgOS-2 mutant was less virulent than Δtri5 and ΔFgl1. The anatomy of the infected organs may partially explain these differences. In wheat the rachis node represents an histological barrier which stops the Δtri5 and ΔFgl1 mutants since DON and lipase are necessary to overcome this barrier. In soybean the absence of discontinuity between root and hypocotyl tissues allows the progress of infection of the Δtri5 and ΔFgl1 mutants, thus the absence of DON and lipase seem only to slow down seedling colonization. This soybean infection assay could be effectively used to rapidly screen F. graminearum mutants for their virulence, possibly anticipating results of wheat infection.