The fungus Fusarium graminearum makes toxins that harm wheat kernels.
WANG ET AL./SCIENCE
Wheat scab hits farmers with a double punch. The fungal disease, also known as Fusarium head blight, shrivels grain and can significantly dent harvests of wheat and barley. Worse, the toxins released by the fungus Fusarium graminearum, a growing problem in the breadbaskets of Europe, North America, and China, remain in grain intended for food. Above legal limits, they can harm people and animals. Grain from infected plants must be discarded in many countries, although some allow blending with uninfected grain.
Fungicides are no panacea, in part because the pathogen infects during wet weather, when the chemicals wash away. But researchers have now found a protective gene in a wild relative called wheatgrass. Called Fhb7, it encodes a toxin-destroying enzyme, the team reports online today in Science. “The gene could have a very large impact in breeding for Fusarium resistance in wheat,” says James Anderson, a wheat breeder at the University of Minnesota, Twin Cities.
The gene originated in a benign fungus that lives inside wild grasses, then somehow slipped into the wheatgrass genome. Such symbiotic fungi can help their plant hosts defend against a destructive invader. The study authors suggest fungal DNA could yield other potential resistance genes for plants.
The best resistance so far to F. graminearum comes from an heirloom variety of Chinese wheat. Breeders have for decades transferred a chromosome segment from it containing a resistance gene, dubbed Fhb1, into other cereals. But the gene’s identity and mechanism remain disputed and the segment provides only modest protection. About 2 decades ago, geneticist Lingrang Kong, then a postdoc at Purdue University, and colleagues found another source of wheat scab resistance farther afield, in the wheatgrass Thinopyrum elongatum.
Over the years, they narrowed down the gene’s chromosomal location, and in the new research, Kong, now at Shandong Agricultural University, and many colleagues finished the job. They sequenced the wheatgrass—yielding genetic markers—then made multiple crosses of the plant to home in on candidate genes. By silencing the genes individually, they found one, Fhb7, required for resistance. Then, the team showed it codes for an enzyme, glutathione S-transferase, and demonstrated that it degrades several fungal toxins, called trichothecenes, that cause wheat scab symptoms.
“This is a great paper, describing an enormous amount of work,” says molecular biologist Gerhard Adam of the University of Natural Resources and Life Sciences, Vienna. The gene, Adam adds, is likely to be broadly effective against many other trichothecenes.
The team had assumed Fhb7 arose in plants, but they found a 97% match with a gene in Epichloë aotearoae, a fungus that protect its grass hosts from pathogens. Bacteria often transfer DNA to plant genomes, but there aren’t many examples of fungi doing so. “I thought it was an artifact” initially, says co-author Hongwei Wang of Shandong Agricultural University.
Encouragingly, field tests showed that adding Fhb7 to wheat had no adverse impacts on grain yield. Kong says the team hopes to release a commercial variety within 1 year, although it’s unclear how resistance provided by Fhb7 compares to that offered by Fhb1. A bigger question is whether the new gene will increase the resistance of strains with Fhb1. So far, Kong says, they found “only a little improvement.”
But the hope is to identify other, similar genes and “stack” them to toughen up wheat and barley, Anderson says. “There’s going to be a rush now to find other genes in the genome of wheat and related species that may do the same thing.”