δ-Aminolaevulinic acid synthesis is required for virulence of the wheat pathogen Stagonospora nodorum

Abstract

δ-Aminolaevulinic acid (ALA) is synthesized in fungi by ALA synthase, a key enzyme in the synthesis of haem. The requirement for ALA synthase inStagonospora nodorumto cause disease in wheat was investigated. The single gene encoding ALA synthase (Als1) was cloned and characterized. Expression analysis determined thatAls1transcription was up-regulated during germination and also towards the latter stages of the infection. TheAls1gene was further characterized by homologous gene replacement. The inactivation ofAls1resulted in strains producing severely stunted germ tubes leading quickly to death. The strains could be recovered by supplementation with 33 μM ALA. Pathogenicity assays revealed theals1strains were essentially non-pathogenic, inferring a key role for the synthesis of ALA duringin plantagrowth. Supplementing the strains with ALA restored growthin vitroand also pathogenicity for up to 5 days after inoculation. Further examination by inoculating theals1strains onto wounded leaves found that pathogenicity was only partially restored, suggesting that host-derivedin plantalevels of ALA are not sufficient to support growth. This study has identified a key role for fungal ALA synthesis during infection and revealed its potential as an antifungal target.