By Nathan Kleczewski
Extension Plant Pathologist
Toxins are one means that some fungal pathogens can circumvent plant defenses and cause disease. For example, the ToxA toxin, produced by the tan spot pathogen (Pyrenophora triciti-repentis) is responsible for disease in many susceptible wheat varieties. We know that bacteria may share virulence factors such as toxins. By this I mean that two unrelated species may share genetic information such as toxin genes. By doing this, both species may become virulent on the same host, expanding the host range for a given pathogen or allowing one pathogen to overcome host resistance. This has never been observed in fungal pathogen systems.
In a recent paper in the science journal Nature, researchers examined the newly sequenced genome of Stagonospora nodorum, the causal agent of glume blotch in wheat. Surprisingly, they found the gene responsible for the production of ToxA in the genome of S. nodorum! The researchers then looked at several hundred isolates of S. nodorum from different locations in the world and found that 183 (25%) contained the Tox A gene. The ToxA gene was not found in other closely related pathogens. When the gene was inserted into avirulent (non-disease causing) S. nodorum it became virulent (disease causing).
What does this mean? The researchers note that tan spot was not a disease of concern until 1941. In fact, before that time, P. triciti-repentis was considered a saprophyte (living on decaying tissue) and not a pathogen. Glume blotch, on the other hand, has been a pathogen for much longer. The thought is that sometime around 1941, the ToxA gene moved from S. nodorum into P. tricitici-repentis, resulting in the development of what we now know as tan spot in wheat. This finding provides insight as to how new diseases can evolve in our field crops. It also serves to highlight the complexity of plant/pathogen systems and the need for continued research in disease identification, management, and control.