Graduate Studies
Embargoed Master's Theses
First Advisor
Joe Louis
Committee Members
Scott Sattler, Kyle Koch
Date of this Version
8-2025
Document Type
Thesis
Citation
A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science
Major: Entomology
Under the supervision of Professor Joe Louis
Lincoln, Nebraska, August 2025
Abstract
Although sugarcane aphids (SCA; Melanaphis sacchari (Zehntner)) have been present in the United States (U.S.) since the 1970s, they did not emerge as a significant pest until 2013, when they began infesting sorghum (Sorghum bicolor (L.) Moench). Since then, SCAs have spread to all sorghum-growing states in the U.S., causing significant yield losses. Previous studies have shown that modifying monolignol synthesis can alter sorghum’s resistance to SCA, offering potential targets for developing innately SCA-resistant sorghum plants and reducing reliance on chemical insecticides. Ferulate-5-hydroxylase (F5H), a key gene in monolignol biosynthesis, modifies lignin composition and provides substrates for Brown midrib 12 (Bmr12), a gene whose mutation has been linked to enhanced SCA resistance through elevated levels of the auxin conjugate indole-3-acetic acid–aspartate (IAA-Asp). In this study, we investigated the role of F5H in regulating sorghum defense against SCA by using F5H-overexpressed (F5H-OE) lines in comparison to RTx430 (wild-type) plants. No-choice and choice bioassays showed significantly greater SCA colonization and settling on F5H-OE plants compared to RTx430 plants. Electrical Penetration Graph (EPG) analysis further revealed that the SCA spent more time feeding from the sieve elements of F5H-OE plants than from RTx430 plants, indicating a dampened phloem-based defense. Biochemical analyses demonstrated that F5H-OE plants had comparable basal levels of total lignin but reduced basal flavonoid levels than RTx430 plants. To understand the molecular basis of this susceptibility, we examined the expression of the regulatory gene SbMyb60 and other monolignol genes located upstream and downstream of F5H. Interestingly, exogenous application of IAA–Asp restored SCA resistance in F5H-OE plants to RTx430 levels. We also examined the interaction between F5H-OE and the bmr12 mutation through a no-choice bioassay. While bmr12 showed strong resistance to the SCA, the combination of F5H-OE with bmr12 (bmr12-F5H-OE1) significantly attenuated this resistance. Overall, these findings identify F5H as a negative regulator of sorghum defense and demonstrate that its overexpression enhances SCA susceptibility in sorghum.
Advisor: Joe Louis
Comments
Copyright 2025, Kashish Verma. Used by permission