Evaluation of Pollen-Mediated Gene Flow and Seed-Associated Microbial Communities in Northern Wild Rice (Zizania palustris L.)

Abstract

As the official state grain of Minnesota, northern wild rice (NWR; Zizania palustris L.) represents an economically important specialty crop with a rich cultural heritage. Native to the Great Lakes region of North America, domestication of the plant began in the 1950s and today, the vast majority of cultivated NWR is produced in Minnesota and California (Zepp, Harwood, & Somwaru, 1996). Like maize, NWR is an open-pollinated crop, making pollen difficult to control. NWR flower morphology promotes out-crossing, thus necessitating an understanding of pollen travel. By understanding pollen travel, the breeding program is ultimately able to understand spatial limitations while developing inbred lines and maintaining varietal purity. Here, the characterization of a recessive white male floret (WMF) population in contrast to the dominant, purple male floret (PMF) color of cultivated NWR along with estimates of pollen-mediated gene flow in a cultivated paddy setting is presented. Studies revealed that the primary amount of pollen-mediated gene flow occurred within the first 7 m from the PMF donor source with no gene flow detected past 63 m. This study demonstrates that the WMF trait is an excellent candidate for use in pollen-mediated gene flow studies in NWR. As a wild crop relative of Oryza sativa, the Zinaniiae subtribe within Oryzaea has seed that is desiccation-sensitive, which is unique among grain crops. Additionally, seed must be stored submerged at ~3 °C for a period of three to four months to help break dormancy, after which time seed still may not germinate. Commercial NWR seed germination rates range from 15% to 95%, with only approximately 60% of germinated seeds going on to establish plants. In research settings, the wet storage environment is optimal for microbial growth, but the effect microbial growth on NWR germination/dormancy is currently unknown. While several fungal diseases have been noted to plague plants of cultivated NWR, diagnoses of stored seed diseases are not widely known in the research community. In this study, we isolated and sequenced microbes found in 27 NWR seed stocks collected over a five year period from three locations and four genotypes (three released NWR varieties and one elite breeding line). Results revealed that microbial communities were heavily dependent on seed viability and began shifting after one year of NWR seed storage. We also evaluated the efficacy of four antimicrobial treatments for the reduction of microbial growth in hydrated NWR seed storage. These treatments did not reduce or drastically change microbial growth or seed viability. Overall, this study introduces common microbial constituents found in the seed storage of an aquatic, cold-adapted, recalcitrant species and provides a foundation for future studies to evaluate the effect of microbial communities on NWR seed viability during hydrated storage. Ultimately, these results suggest that the likelihood of pollen-mediated gene flow between cultivated NWR and natural stands remains low, reducing the perceived threat to growers, conservationists, or concerned communities. Additionally, antimicrobial seed treatments are not an effective mechanism of controlling microbial growth in NWR seed storage, but the identification of the core NWR microbiome will be useful for researchers going forward.