Gene Expression Clusters Suggest Potential Mechanisms of Resistance to Powdery Mildew in Hop (Humulus lupulus)

Abstract

Powdery mildew, caused by Podosphaera macularis, is one of the most important diseases of hop in the Pacific Northwest of the United States. Resistant cultivars exist; however, recently, the pathogen has overcome two sources of resistance in popular cultivars. Breeding for new, emerging races of the pathogen is thus a continuing priority. We used RNA-seq to understand gene expression trends in a resistant (cv. USDA Nugget) and a susceptible (cv. Symphony) hop cultivar along a time series following inoculation. We found stronger transcriptional reprogramming in the resistant cultivar than the susceptible cultivar. Transcripts that increased in expression within 12 h post-inoculation in the resistant cultivar included genes in the jasmonic acid biosynthesis pathway, as well as phenylpropanoid biosynthesis. Transcripts that had a positive correlation with time post-inoculation in the resistant cultivar included genes involved in phosphorylation, cell-surface signaling pathways, and response to jasmonic acid. We used untargeted HPLC–HR-MS/MS analysis and confirmed an increase in jasmonic acid and several phenolic compounds. We hypothesize that resistance to powdery mildew in cv. Nugget may be at least partially due to perception of the pathogen outside of the cell, and transcriptional and metabolic reprogramming is mediated by jasmonic acid signaling. At least part of that transcriptional and metabolic reprogramming involves changes to lignin production, which might reinforce the cell wall. The multi-omics approach to describe the contrasting responses of a resistant and a susceptible hop cultivar following inoculation with this important pathogen may indicate new strategies for developing resistant germplasm and control methods. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .