Wheat blast, caused by Magnaporthe oryzae pathotype Triticum (MoT) is a global threat for wheat. The first report of the disease was in Brazil, but wheat blast has spread to other South American countries, and there are currently reports of its presence in Bangladesh and Africa, indicating that it could have a worldwide distribution. Initially restricted to South America, the disease spread to Asia in 2016 by the introduction of contaminated seeds, raising the question: how is MoT transmitted from seeds to seedlings?
This pathogen attacks the spikes and can cause yield losses of 5-51%, but can destroy the whole crop under the right environmental conditions. Total or partial bleaching of spikes is the most significant symptom of wheat blast and could lead to grain infertility and/or failure of grain filling. At its worst, the disease can partially or completely damage the spike, resulting in no grain production. The fungus can also attack the leaves: the symptoms appear as diamond-shaped water-soaked lesions that gradually turn necrotic with the progression of time.
This fungus, during the period that the wheat is not being cultivated, jumps to secondary hosts, which are the common weeds that grow near to cultivated fields. It is known that short-distance dispersal is through conidia, but over long distances it is propagated by seeds, which do not always present the symptoms described. Therefore, seeds that are apparently healthy can harbor the pathogen and spread disease far from its origin. This risks epidemics in areas where the conditions for its establishment and development are favorable.
We studied the relationship between seed infection and disease symptoms on seedlings and adult plants. To accomplish this objective, we inoculated spikes of wheat (to a variety named Apogee) with transgenic fungal isolates. Two isolates of Wheat Blast fungus were used (PY15W and PY34W) and transformed using Agrobacterium tumefaciens– to introduce a gene encoding a red fluorescent protein (DsRed). These transgenic isolates were identified in experiments using hygromycin (antibiotic) resistance for selection, or by observation of DsRed fluorescence. By tracking their presence, from seeds underground to seedlings, we demonstrated that wheat blast can spread from seed to seedling.
Using these transgenic fungi, labelled with antibiotics and fluorescence, we were able to track the infection of wheat plants from seed to spike (seed head) production 42 days after sowing. Two experimental designs were chosen: Blotter test (26 ± 2 °C and a 16 hr light period for 21 days) and greenhouse (pots with standard soil at 65% humidity, 22 ± 2 °C, and a 16 hr light period for 42 days). For the evaluation of recovery, randomized plant parts were plated on CM agar plates supplemented antibiotic and incubated at 22 ± 2 °C for 7 days. Wheat blast colonies recovered from each specimen were then further evaluated by fluorescence microscopy. Colonies fulfilling this criterion were classified as successful re-isolation and numbers were used to calculate percentage of fungal recovery and transmission rate. The transgenic isolates were recovered from both experiments. This revealed that this pathogen grows endophytically through the plant, from seed to seedling.
The favorable conditions of temperature (22°C) and humidity (65%) allowed a high recovery rate of Wheat Blast from wheat shoots when grown in artificial media. Around 42 days after germination of infected seeds, red fluorescent transgenic Wheat Blast isolates could not be re-isolated, indicating that fungal progression, at this time point, did not proceed systemically/endophytically in the plant. So, we hypothesize that wheat spike infection might occur via spore dispersal from infected leaves to healthy leaves rather than inside the plant.
Our experiments revealed that this pathogen is able to colonize wheat seedlings, systemically (through the plant), from infected seeds underground. Many of the shrivelled and/or discoloured seeds grew into seedlings with high levels of Wheat Blast fungus. Some infected seeds which looked healthy also germinated into plants showing high levels of infection. This proves that seeds which appear healthy can spread disease. These findings highlight the urgent attention needed to minimize the risks of accidental spread of this dangerous fungal disease of wheat.
Sergio I. Martinez, Alex Wegner, Stefan Bohnert, Ulrich Schaffrath and Analía Perelló published this study in Plant Pathology:
TITLE IMAGE:
Partial bleaching of spikes with mycelium caused by Magnaporthe oryzae triticum pathotype.
alt=”Two wheat ears against a black background. The wheat plants are a dark green colour, but one of the wheat ears is significantly bleached from the second. This variety has long awms. From the third row of spikelets to the top of the ear, all of the spikes are white or yellow and look completely dry.”
All images used with permission of the authors.
