Semester of Graduation

Summer 2026

Degree

Master of Science (MS)

Department

Plant Pathology and Crop Physiology

Document Type

Thesis

Abstract

Soybean (Glycine max L.) is one of the world's most important commodities, yet its productivity is increasingly threatened by abiotic stresses such as drought and by soilborne pathogens, including Rhizoctonia solani. Seed priming offers a practical strategy to enhance further seed and plant performance. Chemical seed priming preactivated defensive mechanisms prior to stress exposure. Among chemical agents, Benzothiadiazole (BTH), a functional analog of salicylic acid, activates systemic acquired resistance (SAR), while acibenzolar-S-methyl (ASM) represents its commercially available formulation. However, information remains limited regarding the optimal concentrations of these compounds on soybean seed priming, their effects on drought tolerance, and their influence on beneficial root-associated bacterial communities. This study evaluated the effects of BTH and ASM seed priming through three integrated objectives.  First, optimal ASM concentrations and seed priming protocols (single vs double seed drying) were determined by assessing germination and early seedling development. High ASM completely inhibited germination, whereas 0.05 mM ASM maintained high germination while minimizing phytotoxic effects. Priming procedure also influenced priming performance, with shorter soaking and single seed drying improving seedling establishment. Compared with ASM, BTH exhibited greater stability across priming procedures. The second objective integrated the effects of chemical seed priming under greenhouse conditions on plant growth, drought tolerance, and resistance against Rhizoctonia solani AG-4. Under non-stressed conditions, BTH and ASM produced minimal effects on vegetative growth, indicating limited growth promotion in the absence of external stress. During drought, chemical priming did not prevent reductions in shoot growth, although ASM increased shoot fresh biomass relative to hydropriming. Following pathogen inoculation, all chemical treatments reduced disease severity, while 0.05 mM BTH consistently produced the greatest reduction in lesion size without compromising shoot growth. Disease incidence remained high across treatments, suggesting that chemical priming primarily restricted disease progression rather than preventing infection. On BTH-, ASM-treated plants, lesions tended to show a re-brownish coloration, which was associated with hypersensitive response. Quantification of hydrogen peroxide (H₂O₂) demonstrated treatment-specific oxidative responses, with 0.05 mM BTH maintaining stable H₂O₂ accumulation over time. The third objective examined whether BTH seed priming altered soybean root-associated bacterial communities using 16S rRNA amplicon sequencing of endosphere and rhizosphere samples collected at early vegetative (V3–V4) and late reproductive (R5–R6) growth stages. Plant developmental stage was the primary driver of microbial community succession in both compartments, resulting in reduced alpha diversity and significant shifts in community composition over time. Early-stage communities were enriched with diverse soil-associated taxa, whereas late-stage communities showed enrichment with groups associated with beneficial traits. BTH effects were temporal- and compartment-specific, altering the relative abundance of bacterial taxa within the early endosphere without changing overall community composition. These treatment effects were largely reduced by the reproductive stage, indicating that host developmental filtering ultimately overrides chemical priming effects on the soybean microbiome. Collectively, these findings demonstrate that low-concentration BTH seed priming enhances soybean defense against R. solani while maintaining normal plant growth and exerting only minor, transient effects on beneficial bacterial communities. The results support BTH as a promising component of integrated soybean disease management while highlighting plant developmental stage as the dominant factor shaping root microbiome assembly.

Date

7-16-2026

Committee Chair

Ham. Jong Hyun

LSU Acknowledgement

1

LSU Accessibility Acknowledgment

1

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