Tracing seed to seedling transmission of the wheat blast pathogen Magnaporthe oryzae pathotype Triticum

Wheat blast caused by Magnaporthe oryzae pathotype Triticum (MoT), initially restricted to South America, is a global threat for wheat after spreading to Asia in 2016 by the introduction of contaminated seeds, raising the question about transmission of the pathogen from seeds to seedlings, a process so far not well understood. We thereforestudied the relationship between seed infection and disease symptoms on seedlingsand adult plants. To accomplish this objective, we inoculated spikes of wheat cv.Apogee with a transgenic isolate (MoT-DsRed,with the addition of being resistant to hygromycin). We identified MoT-DsRed in experiments using hygromycin resistance for selection or by observation of DsRed fluorescence. The seeds from infected plants looked either apparently healthy or shrivelled. To evaluate the transmission, two experimental designs were chosen (blotter test and greenhouse) and MoT-DsRed was recovered from both. This revealed that MoT is able to colonize wheat seedlings from infected seeds under the ground. The favourable conditions of temperature and humidity allowed a high recovery rate of MoT from wheat shoots when grown in artificialmedia. Around 42 days after germination of infected seeds, MoT-DsRedcould not be reisolated, indicating that fungal progression, at this time point, did not proceedsystemically/endophytically. We hypothesize that spike infection might occur via spore dispersal from infected leaves rather than within the plant. Because MoT-DsRedwas not only successfully reisolated from seed coats and germinating seeds with symptoms, but also from apparently healthy seeds, urgent attention is needed tominimize the risks of inadvertent dispersal of inoculum.