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Fungi's genetic sabotage in wheat discovered
Using molecular techniques, Agricultural Research Service (ARS) and collaborative scientists have shown how the subversion of a single gene in wheat by two fungal foes triggers a kind of cellular suicide in the grain crop's leaves.
Fortunately, the team has also developed DNA molecular markers that can be used to rapidly screen commercial cultivars for the gene, Tsn1, so it can be eliminated by selective breeding. This, in turn, would deprive the fungi of their primary means of killing off leaf tissue to feed and grow, explains Justin Faris, a plant geneticist with the ARS Cereal Crops Research Unit in Fargo, North Dakota.
The fungi, Pyrenophora tritici-repentis (also known as tan spot) and Stagonospora nodorum (leaf blotch), are often partners in crime, occurring in the same crop fields and producing the same toxin, ToxA. This toxin induces a Tsn1-controlled response in wheat called programmed cell death (PCD).
Normally, PCD protects plants by confining invading pathogens in dead cells. But the strategy doesn't work against the ToxA fungi because they're necrotrophs (pathogens that feed on dead tissue).
To better understand this genetic trickery, Faris led a team of scientists from seven different research organizations in isolating, sequencing, and cloning the DNA sequence for Tsn1 from cultivated wheat and its wild relatives. Based on their analysis, the researchers concluded that modern-day wheat inherited Tsn1 from goatgrass. They figure this happened after a goatgrass gene for the enzyme protein kinase fused with another gene, NB-LRR, which probably conferred resistance to biotrophs (pathogens that feed on living tissue).
Interestingly, Tsn1 is controlled by wheat's circadian clock and only initiates PCD in response to ToxA during daylight hours. At night, Tsn1 shuts down and ignores ToxA, suggesting the toxin may indirectly interfere with the plant's photosynthesis.
Diverse Wheat Tapped For Antifungal Genes
Scientists are looking to China, Korea, and Japan for new resistance.
Asian wheat may offer novel genes for shoring up the defenses of U.S. wheat varieties against Fusarium graminearum fungi that cause Fusarium head blight (FHB) disease.
According to Agricultural Research Service (ARS) plant molecular biologist Guihua Bai, the FHB resistance found in today's U.S. wheat varieties is primarily based on the Chinese wheat variety Sumai 3 and a few other sources. But there's concern that FHB-causing species of F. graminearum will overcome these resistant sources.
In susceptible varieties, the fungus infects the wheat heads, causing kernels to shrivel up and turn chalky white. The fungus can also produce mycotoxins that reduce the kernels' value and quality, according to Bai, who works at the ARS Hard Winter Wheat Genetics Research Unit in Manhattan, Kansas.
In collaboration with Kansas State University scientists, Bai has sought new sources of FHB resistance from exotic wheat lines collected from China, Korea, and Japan. These lines include landrace populations, which are domesticated plants that have changed very little since the advent of modern plant breeding.
Of 87 total Asian landrace accessions tested in greenhouse trials, 26 showed high levels of FHB resistance, Bai reports. Grain evaluations also revealed that 15 of them had exceptionally low levels of the mycotoxin deoxynivalenol, which is produced during disease development and can diminish the value of affected kernels as food or feed.
Six of the accessions possessed genes for different forms of FHB resistance known as types I, II, and III. Significantly, some of the genes appear unrelated to Sumai 3, suggesting the Asian landraces could broaden the genetic pool of resistance now available for use in breeding U.S. wheat varieties. This, in turn, could help the avoidance of repeat disasters such as the FHB epidemic that swept through the Great Plains from 1998 to 2000. That epidemic cost America's wheat industry $2.7 billion in losses.