Plants exist in a constantly evolving microbial environment that significantly influences their growth, development, and overall well-being. Within this microbial milieu, certain bacteria play a pivotal role in enhancing plant health and growth. These beneficial bacteria are collectively referred to as plant growth-promoting bacteria (PGPB). They offer valuable services to plants, including improved nutrient absorption, heightened growth stimulation, and increased resilience against pathogens and the other environmental adversities. PGPB engage with plants through diverse modes of interaction, such as root colonization, endophytic association, or rhizosphere competence. An in-depth comprehension of the molecular mechanisms and ecological dynamics governing these interactions is essential for unlocking the potential of PGPB in promoting sustainable agriculture and environmental remediation.
In Chapter 1, I provide an overview of current methods used to detect and diagnose Pseudomonas syringae. This encompasses traditional approaches like culture isolation and microscopy, as well as modern techniques such as PCR and ELISA. Furthermore, I explore the upcoming advancements in this domain, emphasizing the necessity for highly sensitive and specific methods to detect pathogens even at low concentrations. Additionally, I delve into approaches for diagnosing P. syringae infections when they coexist with other pathogens. Chapter 1 Figures can be found in Appendix A.
In Chapter 2, I present a significant protocol for monitoring the progression of gray mold fungal infection at various developmental stages of strawberries. I detail three distinct in vivo inoculation methods for Botrytis cinerea on strawberry plants, focusing on early, middle, and late stages of strawberry growth. Chapter 2 Figures can be found in Appendix B.
In Chapter 3, I introduce Bacillus proteolyticus OSUB18 as a novel inducer of ISR (Induced Systemic Resistance). This bacterium enhances plants' resistance against bacterial and fungal pathogens through mechanisms such as promoting callose deposition, accumulating ROS (Reactive Oxygen Species), increasing the production of essential phytohormones and other metabolites related to plant defense, and elevating the expression of key genes associated with plant defense. Chapter 3 Figures can be found in Appendix C.
In Chapter 4, I provide additional evidence supporting the positive impact of OSUB18 on plant growth. This effect is likely due to the alteration of root architecture and the effective reduction of numerous plant diseases. OSUB18 achieves this dual effect by directly countering pathogens and by indirectly enhancing the plant's systemic resistance. Our in-depth analysis, including thorough phenotypic observations and transcriptome analysis, supports these findings. Chapter 4 Figures can be found in Appendix D.
In Chapter 5, I present the genome sequence of Bacillus amyloliquefaciens strain GD4a, a Gram-positive bacterial endophyte. Initially isolated from switchgrass plants (Panicum virgatum) in a restored coal-mining site in Kentucky, this genome is approximately 3.8 megabases (Mbp) in size and comprises 3,760 protein-coding genes, boasting a GC content of 46.55%. Chapter 5 Figures can be found in Appendix E.
In Chapter 6, I investigate the effectiveness of GD4a, extracted from switchgrass, in elevating plant growth, mitigating plant diseases, and fortifying host plant immunity. The study reveals that GD4a generates a potent bacterial extracellular exudate (BEE), disrupting the pathogenicity of B. cinerea by impeding fungal conidium germination and hypha formation. Furthermore, I pinpoint Benzocaine (BEN) as a novel small molecule triggering basal defense, ISR, and SAR responses in Arabidopsis plants. Chapter 6 Figures can be found in Appendix F.
This dissertation collectively underscores the effectiveness of beneficial bacteria, namely B. proteolyticus OSUB18 and B. amyloliquefaciens GD4a. These microbes not only enhance plant growth and yield but also effectively curb both bacterial and fungal plant diseases. This research points towards promising applications and future trajectories in the realm of Plant Growth-Promoting Bacteria (PGPB) research, particularly for fostering sustainable agriculture and environmental remediation.