Performance Story: Accelerating the introgression of enhanced stem solidness from tall wheatgrass to spring wheat
Wheat stem sawfly (WSS) (Cephus cinctus Norton) is a major pest facing wheat producers in western Canada. This insect is responsible for economic losses estimated at US$350- 400 million annually in North America (Beres, et al. 2017). The main strategy to minimize the impact of this pest is the development of solid-stemmed wheat cultivars because it prevents the larvae from growing inside the stem. Perennial wheatgrass (Thinopyrum ponticum syn.Agropyron elongatum), or tall wheat grass (TWG) is a distant wild relative of wheat with significant levels of stem pith. Because TWG has adapted to local WSS infestations over many decades, it represents a suitable source of stem solidness and resistance to WSS. However, due to the substantial efforts required to introduce solidness into wheat lines, and to achieve a desired agronomic performance; only two wheat cultivars have been reported to include a solid stem phenotype derived from TWG sources. Hence, the main aim of this project was to identify alternative sources of stem solidness and transfer the trait to elite wheat lines. To do so, we implemented traditional breeding approaches coupled with modern DNA analysis.
Because TWG and modern wheat varieties belong to separate genera (i.e., distantly related), our initial crosses were “intergeneric”. First, these intergeneric crosses between the line Crocus (spring wheat) and TWG (Orbit) were developed at the Crop Development Centre (CDC) in Saskatoon, Saskatchewan. These lines were advanced to near homozygosity by self-crossing until the F7-F8 generations. Then, 21 lines with significant level of stem solidness were sown in trial plots, including three solid or partially solid-stemmed wheat cultivars as controls (AAC Concord, Lillian, and Unity) (2018 growing season). Our results indicated that 17 out of 21 lines were significantly more solid than the control cultivars (Lillian and Unity), and similar to the cultivar AAC Concord (a fully solid stem top-to-bottom cultivar). The intergeneric lines with significant pith expression were shorter than the checks, with a tendency to late heading and maturity. A majority of our intergeneric lines (13/21) showed similar yields (x̄=4315 kg/ha) when compared with Lillian and Unity (x̄ =4234 kg/ha). Based on these results, the best ten lines were selected for agronomical characterization over the next three growing seasons.
Similarly to the data from 2018, the analysis over three growing seasons (2019-2021) indicated that 100% of the intergeneric lines had significantly higher levels of pith expression (solid score >4.3, on a scale 1-5) compared with the cultivars Lillian and Unity (3.5 and 3.2, respectively). Also, 90% of the intergeneric lines were statistically similar or superior to AAC Concord (solid score=4.4). Although stem solidness had a slight negative association with grain yield (r=-0.27), 50% of the intergeneric lines showed similar yields (x̄= 3973kg/ha) to the commercial cultivars Lillian and Unity (x̄=4162 kg/ha). Furthermore, all the lines were shorter and more tolerant to lodging when compared to the control cultivars, due to the solidness and thickness of the basal internodes. These properties have shown to improve the plant architecture through an erect growth habit which translates in positive effects yield, grain quality, drying cost and harvest (Marone, et al. 2020). Although we successfully introduced stem solidness, not all the intergeneric lines exhibited an adequate combination of milling and physical dough attributes present in CWRS varieties (presumably due to linkage drag of undesirable traits present at the TWG donor parental)
To overcome this bottleneck, the researchers performed a backcross (BC) breeding approach to transfer the solidness from our intergeneric lines into several elite CWRS cultivars with hollow and semisolid stem. This method is widely used to select against unfavorable genes/traits present in the donor parent. Briefly, the frequency of the donor-parent genome is reduced on each of the advanced backcross lines while, at the same time, the frequency of desirable alleles from the recurrent cultivars is improved (Todorovska, et al. 2013). The researchers used two outstanding solid intergeneric lines as donor parents to transfer the solidness into CDC cultivars (CDC Hughes, CDC Landmark, CDC Adamant) and hollow-stem cultivars (AAC Alida, ACC Goodwin, AAC Starbuck). After four BC cycles, we developed more than 500 lines that combined high levels of solidness with the benefits of the CWRS genetic background. These lines were advanced to the F6 generation to stabilize pith expression. The researchers successfully transferred the genetic component responsible for solidness in TWG into wheat lines, we performed further genetics analysis to unravel the gene(s) associated with this trait. They identified potential genomic regions associated with stem solidness located on chromosomes 1B, 3B and 3D, with a consistent pattern of over/under expression between hollow and solid-stemmed genotypes. The researchers propose that further analyses (i.e., data mining, functional tests) should be performed to determine the molecular implications and gene function of these potential candidates.
Overall, the team generated promising solid-stemmed wheat lines (more than 500), based on a novel genetic source of solidness from intergeneric origin (Crocus/TGW). As TWG has adapted through coevolution to the most important WSS biotype in Canada, we expect that our lines have a greater protection against local WSS infestations. To confirm the competitiveness in agronomic performance of our lines when compared to top wheat cultivars, the researchers will perform an extensive agronomic characterization under field conditions (project Field evaluation of next-generation solid-stemmed CWRS heat”, ADF20210681, January 2022). These results will allow the selection of wheat lines with the potential to be released as new CWRS cultivars