Dissecting phenotypic inheritance of very-long-chain fatty acid inhibitor resistance in waterhemp (Amaranthus tuberculatus) using a novel greenhouse assay: 'PRIM'
Weedy plant species, especially within the genus Amaranthus, capable of surviving the selective phytotoxic effects of an EPA-approved chemical at its most vulnerable growth stage, represent a clear and present danger to the United States and perhaps the world. Such a threat, if left unchecked, may very well thwart future generations’ ability to feed the world's growing population. Herbicides applied preemergence (PRE) are on the frontlines in the arms race of herbicide formulation development and rotating sites-of-action (SoA) to ameliorate herbicide resistance in weedy plants. A pseudo-POST germination method is needed to accurately phenotype known sensitive and resistant populations through a direct chemical × seedling interaction method capable of delivering low herbicide concentrations. Non-target site resistance (NTSR) to Group 15 herbicides represents an important research topic for the future of weed science. The genetic basis for Group 15 resistance is still unknown, with several genetic and environmental (G × E) factors likely involved. Environmental, ecological, and genetic factors are compounded with additional variability in the dioecious species, waterhemp [Amaranthus tuberculatus (Moq.) Sauer]. The use of a single (one male × one female) cross provides a more controlled plant breeding environment, which is important when breeding a dioecious weed species whereby maternal and paternally derived resistant lines of waterhemp were independently produced. This crossing strategy used over generations of selection yielded a pseudo-homogenous waterhemp population ideal for the crossing experiments with a sensitive waterhemp population used in this study. Additionally, phenotypic analysis of resistant × sensitive progeny revealed the probability of resistance to help determine effective weed management strategies for NTSR waterhemp populations. Chapter 1 of this thesis covers a literature review of the Group 15 herbicides, S-metolachlor, and pyroxasulfone. Their developmental history, mode of action, selectivity in weedy plants, edaphic impacts, and agronomic importance in row crop cultivation are discussed. Herbicide resistance in waterhemp, including its agronomic importance and biological implications, is also included. Chapter 2 of this thesis presents a novel greenhouse assay designed to identify and quantify resistance to soil-applied herbicides. Results from Chapter 2 demonstrate the use of vermiculite, soaked with herbicide solution, as a plausible alternative to using soil when phenotyping unknown weedy plants for resistance to soil-applied herbicides. Chapter 3 of this thesis focuses on the genetics and inheritance of S-metolachlor resistance using a three-generation full-sibling inbred population of waterhemp selected with field rates of S-metolachlor under greenhouse conditions. Results from dose-response studies, probability of resistance calculations, and phenotypic segregation ratios indicate that S-metolachlor resistance in waterhemp is a qualitative trait with one major gene inherited paternally, although results from reciprocal crosses suggest the possible involvement of a second gene. The dioecious nature of waterhemp allows for such resistance traits capable of being inherited from a pollen donor or acceptor. By restricting the flow of pollen from one male to one female waterhemp, loci conferring resistance can be ‘stacked’ into one population and compared to the original field population.
S-metolachlor, pyroxasulfone, soilless, vermiculite, preemergence, PRE, resistance, waterhemp
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign