δ-Aminolevulinic 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 in Stagonospora nodorum to cause disease in wheat was investigated. The single gene encoding ALA synthase (Als1) was cloned and characterized. Expression analysis determined that Als1 transcription was up-regulated during germination and also towards the latter stages of the infection. The Als1 gene was further characterized by homologous gene replacement. The inactivation of Als1 resulted 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 the als1 strains were essentially non-pathogenic, inferring a key role for the synthesis of ALA during in planta growth. Supplementing the strains with ALA restored growth in vitro and also pathogenicity for up to 5 days after inoculation. Further examination by inoculating the als1 strains onto wounded leaves found that pathogenicity was only partially restored, suggesting that host-derived in planta levels 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.