Genetic Gumshoes Trace Fungus' Turn to SeriousCrop-Crime

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

According to Agricultural Research Service (ARS) plant pathologistTimFriesen, the exchange illustrates "horizontal gene transfer," aphenomenon that’s been shown to occur in bacteria, but less convincinglyso in fungi. Friesen reports the discovery in the journal Nature Genetics along withother scientists from the ARSRedRiver Valley Agricultural Research Center, Fargo, N.D.;North Dakota State University-Fargo; theAustralian Centre for Necrotrophic Fungal Pathogens (ACNFP) atMurdoch University in Perth; and theInstitute 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, Stagonosporanodorum, perhaps while both occupied the same wheat crop. A connective tubeformed, and in the ensuing exchange, Pyrenophora acquiredStagonospora'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, todayinflicting major yield losses. How Pyrenophora obtained ToxA haseluded scientists, though--until now.

In 2004, Friesen and Fargo colleagues discovered that a protein toxinproduced by Stagonospora interacts with Tsn1, a wheat gene thatalso confers sensitivity to the toxin produced by Pyrenophora. Then, in2005, ACNFP collaborator Richard Oliver observed an almost identicalToxA present in Stagonospora. Suspecting a connection, the ARS-ACNFPscientists disabled Stagonospora’s ToxA gene, creating aless-virulent pathogen on susceptible wheat.

In Zurich, IISB scientists screened an international collection of the fungiand found ToxA in 80 percent of the Pyrenophora specimens, and in20 percent of Stagonospora. Genetic differences for ToxA amongthe Stagonospora specimens indicate that Stagonospora has beenproducing the toxin far longer than Pyrenophora.

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

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