- Wheat (Triticum aestivum L.) is one of the most important crops in the world supplying about 18.8 percent of the world's caloric energy supply and 20 percent of the world's protein. In the Pacific Northwest (PNW), over one million hectares of wheat are grown every year. Wheat production is typically grown in a wheat-fallow rotation with no-till to prevent soil erosion. However, the adoption of no-till eliminates tillage as a form of weed control, making herbicides the primary tool for weed management. Grassy weeds such as jointed goatgrass (Aegilops cylindrica Host) and cheatgrass (Bromus tectorum L.) are wheat's greatest competitors and can reduce yield and quality through competition for water, nutrients, and sunlight. Herbicide resistance (HR) in wheat is necessary due to the similar life cycle and physiology between wheat and the grassy weeds if herbicides are to be applied. Clearfield® wheat, which is resistant to the herbicide imidazolinone, an acetolactate synthase (ALS) inhibitor, is grown in the PNW. The issue with Clearfield® wheat is that imidazolinone can have a soil residual of up to eighteen months which limits growers to only fallow or another Clearfield® crop after a Clearfield® wheat harvest.
A new source of HR, CoAXium® wheat, confers resistance to quizalofop-p-ethyl herbicide, an acetyl Co-A carboxylase (ACCase) inhibitor, which does not have a long soil residual. CoAXium® wheat was developed at Colorado State University (CSU) in 2015 through induced chemical mutagenesis which caused a mutation at the ACC-1 gene. In order to make the CoAXium® technology available to growers in the PNW, the resistance trait will need to be introgressed from Colorado HR hard red winter (HRW) cultivars into PNW soft white winter (SWW) cultivars. Conventional breeding takes about 10-12 years to develop a new wheat cultivar from first cross to release. This has caused a search for techniques that decrease the time it takes to create a new cultivar by decreasing the number of generations needed for trait introgression or decreasing the generation time for each breeding cycle.
To quickly develop PNW CoAXium® wheat cultivars and further understand this source of resistance, a seed-based assay, plant level dose response, and a novel speed breeding method were studied. The objectives of this research were to: 1) Develop a seed or seedling-based screening method to facilitate introgression of the herbicide genes into soft white winter wheat adapted to the PNW; 2) Perform a dose response study to evaluate the level of tolerance of plants carrying two, three, and four copies of the resistance allele; 3) Determine the significance of the number of alleles and significance of genome location of the resistance trait for level of herbicide tolerance; 4) Compare a speed breeding technique using soft white spring (SWS) wheat in the first few backcrosses to a conventional backcross breeding scheme for introgression of the HR trait into PNW adapted germplasm to determine if the time to develop a PNW adapted HR cultivars can be shortened.
Two seed assay methods, seed soak (SS) and substrate imbibition (SI) were compared to distinguish between resistant CoAXium® genotypes and susceptible wheat. The HR donor lines from CSU were either homozygous resistant on the A and D (A/D) genome or the B and D genome (B/D) of wheat. The seeds tested in the seed assay were A/D, B/D, or a susceptible cultivar. The SS method at the 5 µM concentration reliably distinguished trait and non-trait seedlings, and was therefore established as the protocol to be used for trait introgression by the Oregon State University (OSU) wheat breeding program and Seed Lab. The SI method was inconclusive since, even at the 20 µM concentration, 36% of susceptible seedlings would have been categorized as trait. It was observed that seedlings carrying the B/D form of resistance had a significantly lower tolerance to quizalofop than the A/D genotype at the 5 µM concentration. This trend was also observed at the plant level in the dose response study.
For the dose response, the dry biomass weight, survival rate, and image analysis were used as methods to study whether there was an additive effect of the ACC-1 mutation, if the ACC-1 mutation between wheat's three genomes (A, B, and D) conferred different levels of tolerance, whether the B/D genotype had high enough tolerance for commercial production, and to test the usefulness of the image analysis. An additive effect with the resistance alleles was observed, as well as varying levels of tolerance depending on which genome the resistance alleles were present. Resistance alleles on the B genome conferred the lowest tolerance and resistance alleles on the D genome conferred the highest tolerance. It is not advised to develop CoAXium® cultivars that carry the B/D resistance combination, and instead to focus on A/D resistance or even A/B/D. The image analysis parameters of quantifying the area of green healthy tissue and the percentage of green healthy tissue in a photo were not as predictive as the dry biomass and survival rate data.
To study whether the time to introgress the HR trait from a Colorado HR HRW lines to PNW SWW wheat cultivars, a speed breeding scheme was designed. In the spring wheat breeding scheme (SBS), a PNW SWS cultivar was crossed with the HR Colorado HRW lines, followed by two backcrosses to the SWS before being crossed with SWW. This was compared to a conventional winter wheat breeding scheme (WBS). Since spring wheat does not have a vernalization requirement, approximately six weeks per generation could be saved compared to the WBS. The SBS did not save as much time as anticipated, however other benefits were gained. Of the nine SWW cultivars that were initially crossed with the HR HRW in the conventional breeding scheme, only crosses with two SWW cultivars were successful due to hybrid necrosis with the other seven F1 populations. However, the breeding scheme with the SWS did not cause hybrid necrosis when the SWS lines introgressed with the HR trait was crossed back to SWW breeding lines and cultivars. This allowed for introgression of the HR trait into the SWW lines that previously died from hybrid necrosis with the Colorado lines. In addition, two more crosses to a PNW cultivar were performed in the SBS breeding scheme than in the conventional breeding scheme during the two-year study. This increases the chances for the transfer of disease resistance genes for PNW diseases such as stripe rust (Puccinia striiformis) and for favorable phenotypes of other quantitative traits, such as straw strength, into the HR SWW breeding lines which may be observed during the first field trials in 2021. Future research includes the continuation of the speed breeding study through the end of the field trials to observe any of the potential benefits from the speed breeding scheme, the quizalofop tolerance levels of the HR breeding lines in the field, and looking into the mechanisms of why certain allele locations confer a higher tolerance. The addition of a second form of HR wheat provides improved weed control in no-till fields, in addition to the possibility of crop and herbicide rotation to delay the evolution of HR weeds. The findings presented in this thesis delivers insight and resources on this new source of HR for optimal PNW HR wheat cultivar development.