An Alternative Bioassay forSynchytrium endobioticumDemonstrates the Expression of Potato Wart Resistance in Aboveground Plant Parts

Abstract

The obligate biotrophic chytrid species Synchytrium endobioticum is the causal agent of potato wart disease. Currently, 39 pathotypes have been described based on their interaction with a differential set of potato varieties. Wart resistance and pathotyping is performed using bioassays in which etiolated tuber sprouts are inoculated. Here, we describe an alternative method in which aboveground plant parts are inoculated. Susceptible plants produced typical wart symptoms in developing but not in fully expanded aboveground organs. Colonization of the host by S. endobioticum was verified by screening for resting spores by microscopy and by molecular techniques using TaqMan polymerase chain reaction and RNAseq analysis. When applied to resistant plants, none of these symptoms were detectable. Recognition of S. endobioticum pathotypes by differentially resistant potato varieties was identical in axillary buds and the tuber-based bioassays. This suggests that S. endobioticum resistance genes are expressed in both etiolated “belowground” sprouts and green aboveground organs. RNAseq analysis demonstrated that the symptomatic aboveground materials contain less contaminants compared with resting spores extracted from tuber-based assays. This reduced microbial contamination in the aboveground bioassay could be an important advantage to study this obligate biotrophic plant–pathogen interaction. Because wart resistance is active in both below- and aboveground organs, the aboveground bioassay can potentially speed up screening for S. endobioticum resistance in potato breeding programs because it omits the requirement for tuber formation. In addition, possibilities arise to express S. endobioticum effectors in potato leaves through agroinfiltration, thereby providing additional phenotyping tools for research and breeding. [Formula: see text] Copyright © 2019 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

TheSynchytrium endobioticumAvrSen1 triggers a Hypersensitive Response in Sen1 potatoes while natural variants evade detection

“…Pathotype grouping of S. endobioticum isolates is based on tuber-based bioassays but agroinfiltration experiments are typically performed on leaves. We previously demonstrated that plant R genes for the EPPO differential set (6) were equally expressed in aboveground plant parts compared to the tuber-based assays (35). This justifies the use of the leaf-based agroinfiltration assay to test the interaction between candidate Avr genes and R genes.…”

Synchytrium endobioticum, the potato wart disease pathogen

“…In contrast, fresh warts containing summer sporangia, which are produced asexually, are used as inoculum in the Glynne–Lemmerzahl assay. Recently, a new infection procedure was described based on infection of aboveground plant parts or auxiliary buds demonstrating that R genes are expressed in both aetiolated belowground sprouts and green aboveground organs (van de Vossenberg et al, 2019b). Although the methods are different, the resulting scores are mostly well correlated although sometimes consistent differences appear to exist (Prodhomme et al, 2020b).…”

“…Normal development seems to be halted in the case of leaf malformation, whereas it is more or less continued in the case of shoot malformations. Also, aerial root formation was observed after infection of shoots, which could indicate a change in the auxin transport (van de Vossenberg et al, 2019b). Many plant pathogens synthesize plant hormones, toxins, and effectors to manipulate hormonal cross-talk and facilitate infection (Ma & Ma, 2016).…”