Powdery scab caused by Spongospora subterranea f. sp. subterranea (Sss) has recently become one of the most devastating potato diseases of economic importance in South Africa. The use of resistant cultivars has long been considered the most effective and sustainable strategy to manage the pathogen. However, little is known about the molecular mechanisms underlying resistance of potato tubers to Sss. Using RNA‐sequencing (RNA‐seq), 2058 differentially expressed genes (DEGs) were identified from two potato cultivars (tolerant and susceptible) in response to Sss infection. Analysis of the expression patterns of 10 selected defence‐response genes was carried out at two different stages of tuber growth using RT‐qPCR to validate the RNA‐seq data. Several defence‐related genes showed contrasting expression patterns between the tolerant and susceptible cultivars, including marker genes involved in the salicylic acid hormonal response pathway (StMRNA, StUDP and StWRKY6). Induction of six defence‐related genes (StWRKY6, StTOSB, StSN2, StLOX, StUDP and StSN1) persisted until harvest of the tubers, while three other genes (StNBS, StMRNA and StPRF) were highly up‐regulated during the initial stages of disease development. The results of this preliminary study suggest that the tolerant potato cultivar employs quantitative resistance and salicylic acid pathway hormonal responses against tuber infection by Sss. The identified genes have the potential to be used in the development of molecular markers for selection of powdery scab resistant potato lines in marker‐assisted breeding programmes.
Figure S1. Relative expression levels of selected potato defence genes in tolerant and susceptible potato cultivars. Expression levels of StTOSB, StSN2, StUDP, StSN1, StPRF, StLOX, StWRKY6, StMRNA, StDEF and StNBS were measured in Innovator (tolerant) and Vanderplank (susceptible) cultivars inoculated and uninoculated with Spongospora subterranea f. sp. subterranea. Amplification of StEFα‐1 and β‐tubulin gene expression was used to normalize the expression value in each sample. The relative expression values were determined against the average values of the uninoculated control samples. Data represent fold change of gene expression at 7 weeks after emergence and at 15 weeks after emergence. The average of three replicates is shown and different letters show significant differences (Student's t‐test: P < 0.05) between the two cultivars.
Figure S2. Mean powdery scab disease severity and disease index of two potato cultivars evaluated for Spongospora subterranea f. sp. subterranea tuber infection. Values are log 10 transformed means of the three biological replicates for each cultivar. Bars represent the standard error.
Table S1. List of differentially expressed genes in tolerant potato cultivar Innovator, inoculated with Spongospora subterranea f. sp. subterranea.
Table S2. List of differentially expressed genes in susceptible potato cultivar Vanderplank, inoculated with Spongospora subterranea f. sp. subterranea.
Table S3. List of genes up‐regulated in tolerant and susceptible potato cultivars inoculated/uninoculated with Spongospora subterranea f. sp. subterranea.
Table S4. List of genes down‐regulated in tolerant and susceptible potato cultivars inoculated/uninoculated with Spongospora subterranea f. sp. subterranea.