| dc.description.abstract | As the human population expands, there comes a pressing need to increase crop output to meet the rising demand. Plants are subject to constant stressors that influence their successful germination and production capacity. These include abiotic factors such as changes in pH, salinity, drought, and temperature, and biotic stressors such as bacteria, oomycetes, fungi, and other pests. Emerging diseases like the oomycete Globisporangium ultimum, and the fungal pest Fusarium oxysporum are of particular interest as they cause significant damage to important crops for the global market such as soybeans, corn, and wheat.
Current agricultural practices, while increasing crop production, have demonstrated negative impacts on the environment, and we need natural and sustainable alternatives with minimal deleterious effects. Biologics have emerged from decades of research on microorganisms that have a positive impact on plants through growth-promotion and disease control mechanisms among many others. Most of these organisms are isolated from soils and plant roots, such as plant growth-promoting rhizobacteria or PGPR. Soils are teeming with fascinating forms of microbial life and may harbor organisms that can be the future for improving plant health and meeting the rising demand for food. A major challenge often faced with using PGPR is its loss in efficacy when transitioning from greenhouse to field conditions. Therefore, it is imperative that we find new innovations to increase PGPR propagation and soil competitiveness when applied in the field.
While many different species of PGPR persist in soils, in recent years the genera Bacillus has emerged as a prime candidate due to its plant growth-promotion activity, biocontrol capacity, and endospore production. These spores are incredibly durable and can tolerate harsh abiotic conditions, such as extreme heat or cold, while also persisting for long periods of time without any available nutrients. Furthermore, they are relatively inexpensive to produce and would be an affordable alternative for agricultural use. Recent studies have suggested that the PGPR Bacillus velezensis also has the unique capacity to break down the complex carbohydrate pectin as a sole carbon source.
Pectin is integral to the structure of plants cellular matrix and can be found at high concentrations in the peels of fruits such as apples and citrus. Among the citrus fruits, oranges contain the highest amount of pectin in their peels. These peels are a byproduct of the orange fruit processing industry and can be utilized as a cheap and readily accessible waste product serving as a nutrient source for PGPR in soils and enhancing their beneficial effects on plants. It has been demonstrated that Bacillus velezensis, in combination with a pectin-rich prebiotic, can increase nodulation and root architecture in soybeans.
The purpose of this dissertation is to evaluate plant-associated soil microorganisms using culture dependent, or growing and experimenting with them under laboratory conditions, and culture independent, or non-culture-based approaches such as PCR or DNA sequencing, methods.
In Chapter 1, I discuss the history of agriculture, the environmental conditions and pests that have plagued successful crop production, and the innovations and uses of agrichemicals leading to their negative downstream effects. Then I discuss the biological alternatives, specifically PGPR, and the multiple plant beneficial functions they perform in soils. I discuss the use and production of pectin, and the benefits of using these molecules in conjunction with PGPR to enhance their efficacy across greenhouse and field trials. In Chapter 2, I present collaborative research with our colleagues in Crop Sciences where we utilized a culture-dependent approach to combine Bacillus velezensis AP193 and a pectin-rich prebiotic, orange peel, to study their independent and combined impact across commercially available soybean cultivar under greenhouse and field conditions. Additionally, we expand the list of PGPR to evaluate if a specific relationship exists between responsive cultivar and PGPR under greenhouse and field conditions. In Chapter 3, I present collaborative research with our colleagues in the Departments of Plant Pathology, Chemistry and Biochemistry, and Chemical Engineering utilizing a culture-dependent approach to study the biocontrol effects of PGPR in vitro and under growth chamber and greenhouse conditions against the potent plant pathogens G. ultimum and F. oxysporum. We also used a pectin-rich prebiotic, CitraFiber, in combination with the PGPR to evaluate enhanced biocontrol efficacy in soybeans. Furthermore, we developed a pectin-rich growth medium (i.e. broth, gel, and agar), used as both to culture and study the effects of a synbiotic seed treatment for soybean. In Chapter 4, I use a culture-independent approach to studying the Candidate Phylum Radiation, or ultra-small bacteria, in soils and how changes in soil amendments change their population structure. Collectively, my dissertation work describes natural alternatives to traditional agricultural practices for plant growth and provides evidence that these new methods can be effective with minimal harm to humans and the environment. | en_US |
