Tracing seed to seedling transmission of the wheat blast pathogenMagnaporthe oryzaepathotypeTriticum

Abstract

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 therefore studied the relationship between seed infection and disease symptoms on seedlings and 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 artificial media. Around 42 days after germination of infected seeds, MoT‐DsRed could not be reisolated, indicating that fungal progression, at this time point, did not proceed systemically/endophytically. We hypothesize that spike infection might occur via spore dispersal from infected leaves rather than within the plant. Because MoT‐DsRed was not only successfully reisolated from seed coats and germinating seeds with symptoms, but also from apparently healthy seeds, urgent attention is needed to minimize the risks of inadvertent dispersal of inoculum.

Role of seed infection for the near and far distance dissemination of wheat blast caused by Magnaporthe oryzae pathotype Triticum

“…The disease incidence on seedlings of Sumai 3 grown from infected seeds ranged up to 64%, causing necrosis and death of seedlings. In contrast, Martinez et al (2021) did not detect blast symptoms 41 days after sowing infected wheat seeds. Cruz et al (2015) stated that old basal leaves are the initial inoculum source of wheat blast, and sampling at anthesis from the lower three senescent leaves resulted in recovery of MoT conidia.…”

“…Neither the systemic growth of MoT from seed to seed nor the potential spread of MoT by seeds at the local, regional or long distance scale have been so far explored in-depth. In a previous study MoT could not be detected by reisolation from infected wheat plants (susceptible cultivar Apogee) later than 42 days after sowing ( Martinez et al, 2021 ). In addition, till today, the role of cultivar resistance in the transmission and dissemination of MoT and its localization in seeds is not known.…”

“…Data from previous studies indicate that MoT infected seeds may cause more than 20% disease incidence in the offspring ( Goulart et al, 1990 ; Goulart and Paiva, 2000 ). However, 42 days after sowing, MoT could not be isolated from wheat plants grown from infected seeds ( Martinez et al, 2021 ). Taking previous reports together, the mode of disease transfer by seeds infected with MoT is not clear.…”

“…Proper localization of MoT in the seeds, however, is crucial to understand the systemic transmission of MoT from seeds to plants or next-generation seeds. Previous studies investigating transmission of blast pathogens ( Faivre-Rampant et al, 2013 ; Martinez et al, 2021 ) used visual identification followed by plating and fluorescence microscopy. Until today, a comprehensive study addressing the potential of vertical seed-to-seed transmission of MoT in wheat and the localization of MoT in seeds is lacking.…”

Taxonomic and pathogenic diversity of the blast pathogen populations infecting wheat and grasses in Minas Gerais

“…Another cultural practice is to avoid crop rotation with barley, oats, and rye, or, if feasible, the elimination of signal grass, goosegrass, and ryegrass via chemical control (Ceresini et al 2019, Singh et al 2021. As the blast pathogen can be seed-transmitted, using certified seeds is an important measure to avoid pathogen introduction into disease-free areas (Gomes et al 2018;Martinez et al 2021).…”