Document Type

Dissertation - Open Access

Award Date

2022

Degree Name

Doctor of Philosophy (PhD)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Melanie Caffe

Abstract

Oat is an important cereal crop grown worldwide. Oats have the potential to contribute to human health due to their unique nutritional attributes. Developing oat cultivars with efficient root systems able to extract heterogeneously distributed soil resources can help maintain yield under drought conditions and in nutrient poor soil. Various root traits determine the soil volume that is explored by the root system for resource acquisition. Knowledge about the genetic control of oat root traits and response to biotic and abiotic environmental factors is lacking. Identifying quantitative trait loci associated with root traits and understanding the response of roots to abiotic and biotic environmental factors such as drought and endophytic bacteria may enable plant breeders to develop oat cultivars with efficient roots that can maintain yield under unstable climates. To understand the genetic basis of various root traits in oats and how the oat root and shoot development is impacted by drought and by plant growth-promoting endophytic bacteria, we conducted three different experiments. First, we studied the response of oat root and shoot development to endophytic bacterial inoculation by conducting a root vigor assay and a greenhouse experiment. Several endophytic bacteria significantly increased the root length, root area and root volume for one of the two oat cultivars evaluated in the root xiii vigor assay. The greenhouse study revealed that the response of oat cultivars to endophytic bacterial inoculation varied depending on the growth parameters evaluated, the nitrogen fertilization level, the oat genotype, and their interactions. Thus, identifying a specific strain of bacteria for overall growth promotion in oats might be difficult. To gain a better understanding of the extent of phenotypic differences in roots among oat genotypes and how those variations are controlled genetically, a genome-wide association study of root system architectural traits was conducted. Root traits were phenotyped at the seedling stage using a germination paper-based growth platform and a high-throughput image analysis system. Significant variability in root traits among the 285 genotypes evaluated was observed and broad-sense heritability ranged from 0.17 to 0.59 depending on the trait. We identified 82 significant marker-trait associations using a mixed linear model approach. Markers significantly associated with root traits explained from 7.6 to 19.9 % of the phenotypic variation. We identified multiple candidate genes located close to the significant markers that are known to have a role in root development. Finally, we evaluated the morphological and physiological responses of root and shoot development of ten oat genotypes under drought stress. After withholding watering for two weeks on 21 days old seedlings, we measured chlorophyll content, relative water content, stomatal conductance, stomata number, shoot dry weight, root dry weight, root length, root area, and root volume. Seed yield per plant was also collected by continuing the drying and rewatering cycle until physiological maturity. All traits measured were significantly impacted by the water regime. Oat cultivar Hayden showed the smallest reduction in vigor assay. The greenhouse study revealed that the response of oat cultivars to endophytic bacterial inoculation varied depending on the growth parameters evaluated, the nitrogen fertilization level, the oat genotype, and their interactions. Thus, identifying a specific strain of bacteria for overall growth promotion in oats might be difficult. To gain a better understanding of the extent of phenotypic differences in roots among oat genotypes and how those variations are controlled genetically, a genome-wide association study of root system architectural traits was conducted. Root traits were phenotyped at the seedling stage using a germination paper-based growth platform and a high-throughput image analysis system. Significant variability in root traits among the 285 genotypes evaluated was observed and broad-sense heritability ranged from 0.17 to 0.59 depending on the trait. We identified 82 significant marker-trait associations using a mixed linear model approach. Markers significantly associated with root traits explained from 7.6 to 19.9 % of the phenotypic variation. We identified multiple candidate genes located close to the significant markers that are known to have a role in root development. Finally, we evaluated the morphological and physiological responses of root and shoot development of ten oat genotypes under drought stress. After withholding watering for two weeks on 21 days old seedlings, we measured chlorophyll content, relative water content, stomatal conductance, stomata number, shoot dry weight, root dry weight, root length, root area, and root volume. Seed yield per plant was also collected by continuing the drying and rewatering cycle until physiological maturity. All traits measured were significantly impacted by the water regime. Oat cultivar Hayden showed the smallest reduction in yield in response to drought treatment. Hayden also showed a smaller reduction in relative water content, chlorophyll content, and a strong reduction in stomata number. Results indicated that the larger root system may not necessarily provide a yield advantage under drought conditions in oats. The importance of root mass distribution into lower and upper soil layers should be investigated to improve our understanding of mechanisms involved in coping with drought.

Number of Pages

191

Publisher

South Dakota State University

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Rights Statement

In Copyright