Degree Type

Dissertation

Date of Award

2016

Degree Name

Doctor of Philosophy

Department

Agronomy

Major

Plant Breeding

First Advisor

Michelle A. Graham

Second Advisor

Silvia R. Cianzio

Abstract

Breeding for pathogen resistance is an important objective to improve and protect soybean yields. In 2010, 14.4% of total soybean yield was suppressed by diseases. Brown stem rot (BSR), caused by the fungus Phialophora gregata, reduces yield by as much as 38%. To date, three dominant BSR resistance genes have been identified: Rbs1, Rbs2, and Rbs3. The objectives of my research were 1) to determine if plant introductions contained novel BSR resistance genes, 2) to determine the correlation between P. gregata hyphae growth and foliar symptoms as well as characterize the response of the three BSR resistance genes to P. gregata infection, and 3) to identify and characterize the gene networks regulating defense responses to BSR using RNA-Seq and Virus Induced Gene Silencing (VIGS).

To identify novel sources of resistance, four plant introductions with unknown sources of resistance (PI 594637, PI 594638B, PI 594650A, and PI 594858B) were each crossed to three genotypes with the three known BSR resistance genes, developing 12 populations for allelism studies. BSR symptoms were assessed under growth chamber conditions five weeks after inoculation by measuring foliar and stem severities, and recovery of P. gregata from stem sections. Allelism tests of plants from the crosses PI 594638B, PI 594858B, and PI 594650A with each of the resistant sources fit a 15:1 ratio, indicating that the resistant gene possessed by each of the PIs was non-allelic to Rbs1, Rbs2, and Rbs3. The three PIs contain at least one novel BSR-resistance gene, and have the potential to serve as donors to elite germplasm increasing stability of host resistance to P. gregata.

To understand the relationship between P. gregata hyphae growth and foliar symptom development, BSR severity was recorded for individual leaves on infected plants of five different genotypes: L78-4097 (Rbs1), PI 437833 (Rbs2), PI 437970 (Rbs3), Corsoy 79 (susceptible control), and BSR 101 (resistant control). Microscopy was used to count hyphae in longitudinal sections made from stem and petiole segments corresponding to the rated leaves. Based on observed hyphal colonization patterns and significant correlations between the amount of hyphae present and the severity rating of the leaf, it was determined that Rbs1 has a different mechanism of resistance than Rbs2 and Rbs3. Further, the presence of hyphae is not a prerequisite for foliar BSR symptom development suggesting something other than hyphae, such as a toxin, may be responsible for foliar symptoms. This research provides insight into the mechanism of resistance to P. gregata for each of the three known BSR resistance genes.

To identify the gene networks regulating defense responses to P. gregata, we conducted RNA-Seq analyses of P. gregata infected and mock infected leaf, stem, and root tissues of both a resistant (PI 437970, Rbs3) and a susceptible (Corsoy 79) soybean genotype. Samples collected one-week post infection were used for RNA-Seq analyses. Results indicate that resistant and susceptible genotypes respond differently when infected with P. gregata. In all tissues, the resistant genotype induced greater numbers of defense-related genes with greater differential gene expression while the susceptible genotype induced a large number of general stress response genes. From the RNA-Seq results, novel SNPs were identified in the Rbs3 locus on Chromosome 16. Virus Induced Gene Silencing (VIGS) was used to characterize the candidate resistance genes and downstream defense responses. These results provide additional information about mechanisms of BSR resistance and will allow us to develop markers to screen lines for resistance earlier than current phenotyping protocols allow.

DOI

https://doi.org/10.31274/etd-180810-5398

Copyright Owner

Chantal Elaine McCabe

Language

en

File Format

application/pdf

File Size

135 pages

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