The susceptible wheat leaf at left was exposed to a purified form of ToxA, while the leaf at right was exposed to the fungus Stagonospora nodorum, which makes the toxin.

Genetic Gumshoes Trace Fungus' Turn to Serious Crop-Crime

By Jan Suszkiw
August 7, 2006

Pyrenophora tritici-repentis, which causes tan spot of wheat, wasn't always the worldwide disease threat it is today. Before 1941, its damage was considered minor—that is, until it acquired another fungus' toxin-producing gene.

According to Agricultural Research Service (ARS) plant pathologist Tim Friesen, the exchange illustrates "horizontal gene transfer," a phenomenon that’s been shown to occur in bacteria, but less convincingly so in fungi. Friesen reports the discovery in the journal Nature Genetics along with other scientists from the ARS Red River Valley Agricultural Research Center, Fargo, N.D.; North Dakota State University-Fargo; the Australian Centre for Necrotrophic Fungal Pathogens (ACNFP) at Murdoch University in Perth; and the Institute of Integrative Biology (IISB) in Zurich, Switzerland.

Around 65 years ago, they suggest, Pyrenophora's threadlike growths (mycelia) intertwined with those of a more-virulent fungus, Stagonospora nodorum, perhaps while both occupied the same wheat crop. A connective tube formed, and in the ensuing exchange, Pyrenophora acquired Stagonospora's protein-toxin gene, ToxA.

In 1942, a new mystery disease was reported on U.S. wheat: Pyrenophora’s virulent new form. It spread worldwide, today inflicting major yield losses. How Pyrenophora obtained ToxA has eluded scientists, though--until now.

In 2004, Friesen and Fargo colleagues discovered that a protein toxin produced by Stagonospora interacts with Tsn1, a wheat gene that also confers sensitivity to the toxin produced by Pyrenophora. Then, in 2005, ACNFP collaborator Richard Oliver observed an almost identical ToxA present in Stagonospora. Suspecting a connection, the ARS-ACNFP scientists disabled Stagonospora’s ToxA gene, creating a less-virulent pathogen on susceptible wheat.

In Zurich, IISB scientists screened an international collection of the fungi and found ToxA in 80 percent of the Pyrenophora specimens, and in 20 percent of Stagonospora. Genetic differences for ToxA among the Stagonospora specimens indicate that Stagonospora has been producing the toxin far longer than Pyrenophora.

According to Friesen, the discovery shows that more-virulent plant pathogens can arise from horizontal gene transfer. However, this is a very rare event. This transfer may have occurred once, even though both pathogens have grown on millions of acres of wheat for many years. This work also increases the significance of Tsn1, which is targeted by two different wheat pathogens.

ARS is the U.S. Department of Agriculture's chief scientific research agency.