Genetic regulation of cold tolerance in winter wheat (Triticum aestivum L.)

Andrew McCutchen Horgan

Winter wheat (Triticum aestivum L.) grown in the United States Pacific Northwest must be able to emerge from deep planting (as deep as 20cm) as well as have the ability to survive prolonged exposure to freezing temperatures. Decades of research has interpreted much of the genetic complexity of freezing tolerance and seedling emergence but breeding for these phenotypes has proven to be considerably difficult. This research identifies novel genomic regions associated with improved freezing tolerance phenotypes, and describes the three-way interaction of varietal emergence capabilities, rht semi-dwarfing alleles, and GA seed application of deepplanted wheat. A genome-wide association study of 464 diverse winter wheat varieties identified four significant marker/trait associations for freezing tolerance. Freezing tolerance loci were identified on chromosomes 5A, 1A, 2A, and 7B. The markers on chromosomes 5A and 7B were located near previously identified freezing tolerance loci, while the 1A and 2A markers were located in regions that are currently not identified as major freezing tolerance contributors. Furthermore, the pyramiding of these loci showed significant additive increases (p< 0.05) in freezing tolerance abilities. Identification of these freezing tolerance loci could be beneficial in advancing breeding attempts to increase freezing tolerance phenotypes among global wheat germplasm. Comparison of an rht-isoline group along with a panel of eleven Pacific Northwest wheat cultivars confirmed that coleoptile and first leaf lengths are significantly affected by the presence of rhtsemi-dwarfing alleles. When treated with GA, only individuals either lacking dwarfing genes or containing the Rht8 gene displayed a significant elongation response in the coleoptile and subcrown internode tissue. However, Mela CL+, a semi-dwarf variety, displayed significant coleoptile responses to GA treatment despite the presence of the GA-insensitive Rht-B1b allele. Mela CL+ also showed the longest first leaf length of all eleven cultivars, and the longest sub-crown internode length of all semi-dwarf cultivars. This study has identified a pathway in Mela CL+ that maintains adult dwarfism via the Rht-b1b allele without suffering the negative consequences of excessive reduced seedling elongation often experienced in semi-dwarfed wheat. Locating and utilizing this pathway could greatly increase emergence capabilities of semi-dwarfed wheat varieties in dryland farming regions.

Genetic regulation of cold tolerance in winter wheat (Triticum aestivum L.)

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