Durum breeding for FHB resistance
By Carolyn King
By Carolyn King
Fusarium head blight (FHB) is a devastating fungal disease that likes warm, moist conditions. With the soggy weather in many parts of the Prairies in 2010, observed FHB levels were higher than in recent years, and severe cases of the disease occurred even in some areas not usually strongly affected by FHB, such as west-central Saskatchewan. This underlines the importance of long-term efforts to develop FHB-tolerant varieties for western Canada’s two durum wheat breeding programs.
FHB is a difficult-to-control disease that reduces the yield and grade of cereal crops, and can produce toxins that limit the use of the infected grain for food and feed. Durum wheat tends to be more susceptible to the disease than bread wheats. As a result, FHB has seriously affected durum production in western Canada. “For durum wheat in most of Manitoba and southeast Saskatchewan, FHB has been a major issue, and it has shifted durum production from these regions to the western prairies since an outbreak in the early 1990s. There is also an issue under irrigation in Alberta and Saskatchewan,” says Dr. Asheesh (Danny) Singh, who leads the durum breeding program at the Semiarid Prairie Agricultural Research Centre of Agriculture and Agri-Food Canada (AAFC) in Swift Current.
The other program is at the Crop Development Centre (CDC) at the University of Saskatchewan, and is led by Dr. Curtis Pozniak.
These two programs are incrementally improving FHB tolerance in durum varieties by using advanced techniques and collaborative approaches. “The major challenge in developing durum cultivars with improved FHB tolerance has been lack of genetic resistance in durum wheat,” says Singh. He explains that durum wheat has 28 chromosomes (called “tetraploid,” with ‘A’ and ‘B’ genomes), while bread wheat has 42 chromosomes (called “hexaploid,” with ‘A,’ ‘B’ and ‘D’ genomes). Breeders of bread wheat varieties have been able to make use of several sources of FHB resistance found in hexaploid wheats from other countries to improve Canadian bread wheats. “But worldwide, durum breeders have experienced frustration in trying to utilize this resistance from hexaploid bread wheats in tetraploid durum wheats.”
Singh is working on overcoming this hurdle as part of his program. A challenge for both durum and bread wheat breeders is that FHB-resistance involves not just one but several genes. As well, some sources of FHB resistance are in wheats with very different characteristics from the high-quality wheats grown in Western Canada. So
crossbreeding can result in “linkage drag,” where the cross produces plants with a tangle of desired and unwanted traits.
Both Singh and Pozniak are seeking new sources of FHB resistance in durum wheat lines from Canada and other countries, and in wild cousins of durum. Since there is no known source of immunity to FHB, Singh believes durum breeders will need to combine several sources of moderate resistance in a cultivar to develop FHB resistance that is a step above current levels. While improving FHB resistance, durum breeders have to simultaneously select for production, quality and pest resistance traits for registration in the Canada Western Amber Durum (CWAD) class, and to ensure crop success.
The screening of breeding material to determine its level of FHB resistance also poses challenges. Singh says, “There is a lot of environmental variation and genotype/environment interaction in the expression of Fusarium resistance, so it requires good nurseries and repeated evaluation in those nurseries.”
Fortunately, various public and private agencies along with the Western Grains Research Foundation are funding FHB screening at outdoor and indoor nurseries for use by western Canadian breeders. Singh says, “Having better and dedicated resources of field and lab screening capabilities has definitely helped the ability to select for FHB resistance. It also helps in getting more consistent expression for better reliability of data.”
Along with new resistance sources and improved screening, both Singh and Pozniak are using advanced technologies, such as doubled haploids and molecular markers, to more quickly develop lines with better FHB tolerance.
“The doubled haploid technique helps create an inbred line that is breeding true more quickly than such a breeding line could be created through conventional breeding methods. Breeders may save up to two years in developing a cultivar using a doubled haploid technique versus the conventional technique,” explains Singh. Dr. Ron Knox’s lab at AAFC in Swift Current, is a leader in developing doubled haploid plants, and generated doubled haploid plants for Singh’s program.
A molecular marker is a short DNA sequence associated with a particular trait. Molecular markers allow screening for FHB in a laboratory, reliably and without a need to grow the plants. However, Singh points out that to identify and develop a useful marker is a very time-consuming and labour-intensive process with lots of detailed disease evaluations indoors and in disease nurseries. He says, “Both the AAFC and CDC breeding programs are using targeted regions of the DNA and we are able to reduce linkage drag, which is critical to using resistance from exotic sources to build resistance in Canadian-adapted durum genotypes. We are using publicly available molecular markers, and we’re also developing new markers and validating them.”
Another important element for success in both programs is collaboration with different researchers with different areas of expertise. For example, Singh’s and Pozniak’s programs have undertaken collaborative efforts with colleagues from AAFC, universities and overseas.
New improved durum varieties
This multi-pronged approach is paying off with incremental gains in FHB tolerance. Singh says, “Since 2008, our program has registered four cultivars: Brigade, Eurostar, Enterprise and DT801. We just received support for the registration of DT801 in 2010, and we haven’t named it yet. All four are improvements over existing durum cultivars for FHB tolerance. And the Crop Development Centre registered CDC Verona in 2008, and that also is an improvement over currently existing cultivars.
“Brigade, Eurostar, and CDC Verona certified seed should be available for 2011 planting. Enterprise should be available for 2012 planting. And DT801 should be available for 2013 planting.”
He adds, “DT801 is an example of the usefulness of doubled haploid technology, and DT801 is one of the first durum wheat doubled haploid cultivars developed in North America.”
Singh emphasizes that developing FHB-resistant durum varieties is a long-term effort “that requires a sustained, substantial and focused approach.” He believes it will take targeted funding, collaborations and patience. “And with advances in understanding of the resistance genes, new sources of resistance and molecular tools, it should lead to further progress.”
|A faster path to FHB resistance?|
|In his durum breeding program, Dr. Asheesh (Danny) Singh collaborates with many researchers so that development of improved varieties can be tackled from different angles. One of those collaborations is with researchers at Agriculture and Agri-Food Canada’s Cereal Research Centre in Winnipeg, Manitoba, including Dr. Steve Haber, a cereal virologist, and Dr. Jeannie Gilbert, a plant pathologist. Working with advanced lines from Singh’s program, Haber and Gilbert are using a novel approach to try to find a faster route to fusarium head blight (FHB)-resistant durum.
Their approach has developed out of basic scientific research, which discovered that subjecting a higher plant to repeated systemic stress, such as severe cold, severe heat or systemic virus infection, results in “increased genomic flexibility,” explains Haber. In effect, the stress causes the plant to produce very diverse offspring with various combinations of traits, including the expected traits seen in the progenitor plant and often some de novo traits, traits that are not seen in the original starting material. For instance an FHB-susceptible progenitor might produce not only the expected FHB-susceptible offspring but also some offspring that expressed better FHB resistance. Some of these de novo traits can be inherited and expressed in subsequent generations.
Haber worked with Dr. Dallas Seifers of Kansas State University to turn this basic discovery into a practical protocol that allows the researchers to take well adapted wheat lines, unleash a greater range of trait expression hidden in those lines, and look for desirable de novo traits that are heritable. “Instead of bringing in genetic information from another source by making a cross and then looking at the range of traits expressed in that progeny, we take a defined line, subject it to usually several generations of systemic stress, note the particularly strikingly variant individuals, use those to start new sublines and subject them to the challenge of interest, such as fusarium head blight,” explains Haber.
He has found that systemic infection with wheat streak mosaic virus, as well as a wheat host’s response to inoculation with this virus, appear to be the most effective ways to generate the additional variation provoked by stress.
Haber notes, “It’s a particularly appropriate approach to take in efforts to develop durum wheat that is more resistant to fusarium head blight because there are very few resources of resistance available for the conventional breeding approach.”
As well, the protocol is relatively simple and inexpensive. He says, “This approach can be started with very few resources; it doesn’t require large amounts of material or very sophisticated equipment.” And, as Haber and Seifers have demonstrated with de novo resistance to wheat streak mosaic virus and leaf rust, once the de novo traits are established, they can be exploited just like conventional genetic traits, so breeders can use the improved sublines to cross, backcross and select traits in the conventional manner.
Gilbert and Haber have been using this approach to develop FHB-resistant lines in various types of wheat. For example, they have had good success already with McKenzie, a hard red spring wheat. He says, “The FHB-resistant lines we derived from McKenzie have now had extensive trials, and the Fusarium resistance does indeed appear to be stable, uniform and robust. We are gaining additional confidence that the best results from this approach do stand up to continued testing and will continue to express resistance in the hands of breeders, agronomists and ultimately producers.”
And they are making good progress with Singh’s durum lines. Haber says, “We have identified several populations of interesting individuals in each generation of varieties or elite material, we have tested, advanced those as the founders for the next generation, and repeated the process. By the time we got to the third cycle, we were ready to test them in the field in a Fusarium nursery, which we did last year. We identified several sublines that appeared to be performing much better than the original starting lines, advanced those sublines through two further generations indoors during the winter. This summer we ran the sixth cycle, and we have some promising lines. It will require several more cycles to be confident that these really are more resistant and that the resistance is stable and uniform.”
Although it is too early to say for sure whether Gilbert and Haber will successfully generate FHB-resistant durum sublines, Haber says, “By fall 2012, I would have completed two more years of field trials, and the plants would have gone through between five and six more cycles. If I saw continuing progress towards better resistance, stability and uniformity within the lines, then I would say yes, at that point it would be ready for a breeder to start subjecting it to the agronomic and quality trials that they have to run as part of their programs.”