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
By Jan Suszkiw
August 7, 2006
which causes tan spot of wheat, wasn't always the worldwide disease threat it
is today. Before 1941, its damage was considered minorthat is, until it
acquired another fungus' toxin-producing gene.
According to Agricultural Research Service (ARS) plant pathologist
Friesen, the exchange illustrates "horizontal gene transfer," a
phenomenon thats 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
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:
Pyrenophoras 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 Stagonosporas 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
ARS is the U.S. Department of
Agriculture's chief scientific research agency.