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Effect of Hop stunt viroid on host (Humulus lupulus) transcriptome and its interactions with hop powdery mildew (Podospheara macularis)

Madhu Kappagantu: Washington State University

<div>Hop stunt viroid (HSVd) and hop powdery mildew fungus (<em>Podospheara macularis</em>) (HPM) are the most economically important pathogens that infect hops. Interactions of these two pathogens during mixed infections were investigated, and changes in the hop transcriptome during HSVd infection were quantified. In a detached leaf assay, HSVd-free and HSVd-positive hop leaves of cultivar Willamette were inoculated with HPM, which revealed reduced growth of HPM on HSVd infected leaves. To identify the changes in hop transcriptome during HSVd infection that may cause HSVd antagonism towards HPM, RNA from hop leaves representative of each treatment (i.e. HSVd/ HPM-free control, HSVd infected, HPM infected; and leaves infected with HSVd and HPM) were subjected to high throughput sequencing. Transcriptome analysis of HSVd-infected leaves revealed dynamic changes in host gene expression. Expression of genes related to defense, and to lipid and terpenoid metabolism were most strongly altered as a result of HSVd infection. Further analysis by quantitative reverse transcription-polymerase chain reaction analysis showed that transcript levels of pathogenesis-related (PR) genes such as PR 1, chitinase, and thaumatin-like protein genes were induced in leaves infected with HPM alone, but were suppressed in leaves previously infected with HSVd, thus indicating that HSVd acts as a negative regulator of certain PR genes which are typically elicited by other pathogen infections.</div>