Investigating Mechanisms of Resistance to Sclerotinia Homoeocarpa Using Arabidopsis Thaliana As a Model.

Dollar spot, caused by Sclerotinia homoeocarpa (F.T. Bennett), is among the most devastating diseases of turfgrasses worldwide.  Fungicides are typically needed to control this disease, yet resistance of the pathogen to commonly applied fungicides has necessitated the development of resistant cultivars.  Previous research on bentgrasses has demonstrated a range of resistance phenotypes and identified a number of quantitative trait loci that appear to contribute to improved dollar spot resistance.  Little is known, however, about the defense pathways and molecular mechanisms that contribute to this phenomenon.  The availability of T-DNA insertion lines for many defense-associated genes make the model plant Arabidopsis thaliana a perfect tool for initial studies on the molecular mechanisms of pathogen resistance.  In this project, it first was determined that S. homoeocarpa is able to infect the wild-type A. thaliana ecotype Columbia (Col-0).  Infection assays were then performed on Col-0 lines with mutations in various defense-associated pathways.  One-week-old S. homoeocarpa cultures were scraped from the surface of potato dextrose agar plates and homogenized in ¼ potato dextrose broth (PDB).  Inoculum was diluted to 1 x 10^4 mycelial fragments/ml and lightly sprayed onto two-and-a-half-month-old plants arranged in a randomized complete block design.  Control plants were sprayed with plain ¼ PDB.  Disease severity was calculated as the number of necrotic leaves over the total number of leaves per plant.  Mutant plants rar1-21 and sid2-2, which have mutations in R gene-mediated defense and salicylic acid signaling, respectively, were significantly less resistant to S. homoeocarpa than wild-type Col-0.  The defense overexpression mutant dnd1-1, was more resistant to S. homoeocarpa when compared to wild-type Col-0.  Other mutants tested were not statistically different from control plants in disease resistance.  These results indicate a possible biotrophic phase in disease development and implicate the salicyclic acid signaling pathway as a key component in defense against S. homoeocarpa.