Suppressive effect of crotonylidene diurea degradation intermediates on root-hair infection by Plasmodiophora brassicae, the causal agent of clubroot disease

Abstract

The application of crotonylidene diurea (2-oxo-4-methyl-6-ureido-hexahydropyrimidine, CDU) is known to suppress or alleviate soilborne crop diseases, an effect most likely due to alterations in the soil microbial flora. In this study, we show that two CDU degradation intermediates, 2-oxo-4-methyl-6-oxy-hexahydropyrimidine (OMHP) and the (R)-isomer of 3-aminobutyric acid (β-aminobutyric acid, BABA), suppressed root-hair infection in Chinese cabbage seedlings caused by Plasmodiophora brassicae, the causal agent of clubroot disease in cruciferous crops. In line with previous studies, we show that BABA possibly induced systemic resistance to infection that is not specific to clubroot disease. OMHP additionally suppressed infection by more virulent P. brassicae strains, unlike the case for BABA, suggesting a different mode of action. Assuming that OMHP induced root resistance to infection, we expected such resistance to disappear very rapidly after OMHP removal. On the other hand, while OMHP did not inhibit the germination of primary zoospores from resting spores of P. brassicae, it suppressed root-hair infection even under these conditions. These results suggest that the mechanism by which OMHP suppress infection either (1) triggers resistance in the roots but with a very short residual effect, or (2) negatively affects the primary zoospores.