Document Type

Honors Thesis

Semester of Graduation

Spring 2026

Abstract

Maize (Zea mays) is a major global crop cultivated for food and feed. Most commercial varieties grown in the United States contain herbicide-resistant traits to facilitate weed management. One such herbicide trait is Liberty-Link, which contains the bialaphos resistance (bar) gene from Streptomyces hygroscopicus that confers resistance to glufosinate-ammonium, the active
ingredient in Liberty herbicide. This bar gene is also widely used as a selectable marker in plant transformation. In non-transgenic plants, glufosinate-ammonium inhibits plant glutamine synthetase by acting as a glutamate analog, thereby disrupting amino acid biosynthesis and leading
to plant death. In contrast, transgenic plants expressing the bar gene produce phosphinothricin acetyltransferase, which detoxifies glufosinate and provides herbicide resistance. Determining whether transgenic plants are homozygous or heterozygous for the bar gene typically requires real-time PCR molecular genotyping, which is costly, labor-intensive, and impractical for large-scale
breeding efforts. To circumvent this need, the present study investigates a practical, LibertyⓇ herbicide-based approach for distinguishing homozygous and heterozygous transgenic maize plants without the need for molecular analysis. Homozygous transgenic maize lines and non-transgenic controls were used and grown under laboratory and field conditions. Seedlings were
subjected to a leaf-painting assay using varying concentrations of LibertyⓇ herbicide, and leaf injury symptoms were visually scored. Phenotypic responses were compared with quantitative PCR results to evaluate accuracy. Glutamine synthetase activity was measured in leaf extracts from non-transgenic and homozygous plants. Liberty herbicide leaf painting assays at 2.0 µL mL-1 reliably distinguished homozygous transgenic and heterozygous maize with visual injury scores strongly correlating with qPCR ΔCT values (R2=0.96). Homozygous plants exhibited a mean ΔCT of 0.89, whereas heterozygous plants showed a higher mean ΔCT of 5.79. There was a 98% concordance for homozygous transgenic seedlings. Homozygous transgenic plants were ranked 1, heterozygous plants were ranked 2 to 4, and non-transgenic plants were ranked 5. Glutamine synthetase activity increased in all genotypes following treatment.

Awardee Name

Isabel Hau

Academic Major

Biochemistry

Project Mentor

Zhi-Yuan Chen

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