The <i>Synchytrium endobioticum</i> AvrSen1 Triggers a Hypersensitive Response in <i>Sen1</i> Potatoes While Natural Variants Evade Detection

Abstract

<i>Synchytrium endobioticum</i> is an obligate biotrophic fungus of division Chytridiomycota. It causes potato wart disease, has a worldwide quarantine status and is included on the Health and Human Services and United States Department of Agriculture Select Agent list. <i>S. endobioticum</i> isolates are grouped in pathotypes based on their ability to evade host resistance in a set of differential potato varieties. Thus far, 39 pathotypes are reported. A single dominant gene (<i>Sen1</i>) governs pathotype 1 (D1) resistance and we anticipated that the underlying molecular model would involve a pathogen effector (AvrSen1) that is recognized by the host. The <i>S. endobioticum</i>-specific secretome of 14 isolates representing six different pathotypes was screened for effectors specifically present in pathotype 1 (D1) isolates but absent in others. We identified a single <i>AvrSen1</i> candidate. Expression of this candidate in potato <i>Sen1</i> plants showed a specific hypersensitive response (HR), which cosegregated with the <i>Sen1</i> resistance in potato populations. No HR was obtained with truncated genes found in pathotypes that evaded recognition by <i>Sen1</i>. These findings established that our candidate gene was indeed <i>Avrsen1.</i> The <i>S. endobioticum AvrSen1</i> is a single-copy gene and encodes a 376-amino-acid protein without predicted function or functional domains, and is the first effector gene identified in Chytridiomycota, an extremely diverse yet underrepresented basal lineage of fungi.