Diverse plant sources tapped for disease resistance in wheat
By Carolyn King
By Carolyn King
Breeding programs may narrow genetic diversity by selecting the few plants with the best traits out of thousands of plants. But wider genetic diversity is needed for the crop to adapt to changing threats, like a new virulent strain of a pathogen. So wheat researchers are working to add vital new sources of diversity drawn from some exotic locations. “Genetic diversity is a determinant of genetic viability and vulnerability. When you have more genetic diversity, you have a better chance of adapting to different stresses in the changing environment,” explains Dr. Yong-Bi Fu at Plant Gene Resources of Canada (PGRC) in Saskatoon, Saskatchewan.
PGRC, which is part of Agriculture and Agri-Food Canada (AAFC), houses the national collection of plant germplasm – seeds and other plant tissues from which new plants can be grown.
Fu uses advanced molecular tools to assess the level of genetic diversity in Canadian crop breeding programs and in PGRC’s collections. As part of this effort, Fu and Dr. Daryl Somers, now with the Vineland Research and Innovation Centre in Vineland, Ontario, examined 75 Canadian hard red spring wheat cultivars released from 1845 to 2004. “We found that overall the genetic diversity has been reduced by about 20 percent. And the loss of diversity is not just in one particular spot on the wheat genome; the whole genome is losing genetic viability,” says Fu.
This result has important implications. He says, “One implication is that we have an increased risk of vulnerability to stresses like disease and drought. Another is that it shows the importance of germplasm conservation, to provide a source of diversity for crop breeders.
Germplasm conservation is like insurance; you never know when another epidemic, drought or some other catastrophe might be coming.”
The study gives wheat breeders feedback on exactly how much diversity has been maintained or lost, and reminds them of the importance of minimizing such reductions in their own breeding programs.
According to Fu, wheat breeding programs in other countries also show diversity reductions. Programs in some European countries have gone through decades with drops in diversity followed by periods with increasing diversity, depending on the breeding strategies used.
Examples of strategies that tend to cause larger reductions in diversity include: over-reliance on a few elite lines as parents; and intense selection pressure, for instance when breeders develop varieties to meet a very strict, narrow set of market requirements.
An important strategy to increase diversity is to bring new germplasm into a breeding program. Wheat researchers draw from a wide range of sources, such as other crop species, wild plants and landraces (very old, locally adapted lines, to find new genes for disease resistance. Fu notes, “If our breeders hadn’t been bringing in new material, the diversity reduction would have been much higher, and perhaps up to 50 percent.”
However, bringing in new genes from exotic sources is very challenging. It can be hard to make a cross between an elite wheat line and a wild species, and the cross results in offspring with a tangle of wanted and unwanted characteristics. So it requires backcrossing for many generations to get back to that elite quality. The development of molecular markers can be crucial in these efforts. Researchers use these markers to screen progeny to see if a specific gene or set of genes they want to transfer is actually present.
Fortunately, wheat researchers are meeting this challenge, producing wheat lines with resistance against two of the most significant disease threats to wheat: Fusarium head blight (FHB) and a virulent strain of stem rust.
FHB resistance genes from Asian landraces
Dr. Guihua Bai, a plant molecular geneticist with the Agricultural Research Service of the United States Department of Agriculture, is leading work on FHB resistance.” FHB reduces the yield and grade of cereal crops, and produce toxins that limit the use of the infected grain for food and feed.
Bai and colleagues at Kansas State University-Manhattan are screening diverse wheat lines, including landraces from China, Japan and Korea, to find new sources of FHB resistance. The researchers chose those Asian landraces because much of that region, especially southern China, has had high disease pressure from Fusarium for many years, so the surviving lines likely have some FHB resistance.
Additional sources of FHB resistance are needed for two key reasons. “Right now, the major source of resistance is a QTL, called Fhb1, from a Chinese wheat called Sumai 3. (A QTL – quantitative trait locus – is a chromosomal segment containing the gene for a trait.) That Sumai 3 QTL has been widely deployed worldwide and it has had great success, but it doesn’t completely give a desirable level of resistance. So we need additional QTLs from other sources to enhance the level of resistance,” explains Bai. “The other reason is, if we only use a single source of resistance, then one day the pathogen could evolve to overcome that resistance and we wouldn’t have any other alternative.”
Out of the first several hundred Asian lines screened, the researchers have found six very highly resistant lines, plus other lines with moderate to high resistance. All of the highly resistant lines have Fhb1 along with two or three other QTLs that contribute some resistance.
Bai is also leading in a major marker-assisted backcross program to transfer Fhb1 to popular US wheat varieties. He has taken a soft winter wheat variety called Clark that is susceptible to FHB and has created lines with high FHB resistance and yields equal to Clark’s. This is a very valuable advance because it is much easier for breeders to use one of these FHB-resistant US lines as parent material, instead of a Chinese line, which can carry a lot of unwanted traits.
Wide crosses for disease resistance in Ontario wheats
Another interesting effort involves crossing Ontario wheats with other plant species, including wild species, to introduce new genes for resistance to FHB, stem rust and leaf rust. This project is funded by the Grain Farmers of Ontario, AAFC and the Developing Innovative Agri-Products program. “The reason we go into crosses with wild species is to enrich the primary gene pool of wheat. There’s probably not enough variability in that gene pool for some traits, so we go to a secondary gene pool, which are the immediate wild relatives of wheat, and the tertiary gene pool, which includes some of the grasses like tall wheatgrass,” explains Dr. George Fedak, a wheat geneticist with AAFC’s Eastern Cereal and Oilseed Research Centre in Ottawa.
Fedak’s research team is finding new sources of FHB resistance in various wild species and is making good progress transferring these genes to wheat. He says, “For example, so far we’ve transferred resistance from Triticum monococcum, an early form of cultivated wheat, Aegilops speltoides, a wild cousin of wheat, Triticum timopheevii, a wheat species found in some parts of the Middle East, Aegilops cylindrical, jointed goatgrass, and Triticum miguschovae, a synthetic wheat developed from two wild species.”
The researchers have released the germplasm for some of these FHB-resistant lines, and for others, they are in the process of mapping the locations of the genes along the chromosomes and developing markers before releasing the germplasm.
Even quackgrass turns out to be a source of FHB resistance. Dr. Pierre Hucl at the University of Saskatchewan made the difficult cross between quackgrass and wheat, and gave Fedak a bulk population from the seventh generation after the initial cross. Fedak’s team has screened all samples, found some lines with resistance to both Fusarium and stem rust, and is now mapping those genes.
They are also in the process of transferring FHB resistance from other crop species, like rye landraces from Brazil and an unusual triticale sample they found with FHB resistance.
Another major aspect of Fedak’s project involves resistance to Ug99, a virulent race of stem rust. This race was first detected in Uganda in 1999. Since then, the pathogen has mutated twice to overcome another two resistance genes. Ug99 has spread to many other countries and has the potential to reach the Americas. Fedak’s team is monitoring samples of the pathogen from Africa to watch for further mutations. All work with this pathogen is conducted under very strict containment protocols so it cannot escape into the environment.
Canada is fortunate to have two wheat varieties that already have resistance to Ug99, so Fedak’s team is moving those genes into other wheat varieties. As well, the researchers are putting various combinations of five known Ug99 resistance genes into Ontario wheats that have some FHB resistance. Developing varieties that carry at least two Ug99 resistance genes is very important because that means the pathogen cannot overcome the variety’s resistance in a single mutation step.
Fedak notes, “Four of those five Ug99 resistance genes that we’re using were actually produced many years ago by Peter Dyck and Eric Kerber at Agriculture Canada’s Cereal Research Centre (CRC) in Winnipeg, Manitoba. They found the genes in wild relatives of wheat and produced these stocks. Other scientists at CRC such as Colin Hiebert are developing molecular markers for these genes. So the genes were sitting on the shelf, and now we’re using them to meet the threat of Ug99. “The pathogen is going to continue mutating and could eventually overcome these genes. So we have to continue going into wild species and finding new genes for resistance and have them ready and waiting.”
Fedak’s team is at work on that aspect, too.
The researchers are screening offspring from crosses between wheat and various wild relatives of wheat and other wild grasses, and believe they have found some new resistance genes. As well, the University of Minnesota sent Fedak 20 lines of rye and 20 lines of triticale that all appear to have different new genes for Ug99 resistance. Fedak’s team has started the first steps in bringing these new genes into Ontario wheat lines.
Fedak emphasizes, “This type of work should be ongoing, looking for more resistance genes in exotic sources, bringing them into a good wheat background, and having them ready as backup if and when the existing resistance genes break down.”