Physiological and genetic basis of Spongospora subterranea zoospore attachment on potato roots

The plasmodiophorid, Spongospora subterranea f. sp. subterranea is an economically significant pathogen of potato (Solanum tuberosum L.) that is responsible for tuber powdery scab, root zoosporangium infection, and root galling diseases. Powdery scab has long been recognised as a major economic issue decreasing crop productivity while affected tubers have reduced market value. Options for managing Spongospora diseases are limited, and largely rely on using resistant cultivars, although no cultivar has been found to be completely resistant to the pathogen.

Traditionally, the identification of host resistance to S. subterranea has been based on the assessment of tuber powdery scab and/or root galling, in large, replicated field or glasshouse challenge trials, which are both time and resource intensive. Additionally, environmental factors and polycyclic infection issues can confound the accuracy of these evaluations. A potential alternative rapid in vitro assessment method for screening of cultivar resistance to potato tuber and root diseases is to observe relative propensity of initial zoospore root attachment at the very start of the infection process that would avoid issues of polycyclic infection.

The importance of zoospore root attachment as the initial stage of infection has been addressed in this study. The optimal conditions for zoospore release in Hoagland's solution and then the attachment of zoospores to plant hosts were first examined. Incubation of resting spores showed that the most sporosori germinated at 20^oC The extent of zoospore root attachment to host root hairs and other epidermis cells varied with cultivar, region of the root maturation zone, and temperature (with the most zoospore root attachment occurring at 15^oC. Based on these preliminary experiments, a rapid and robust in vitro bioassay was developed to assess the resistance to zoospore root attachment for a total of 153 potato lines and cultivars. The presence of a continuum of varietal responses to zoospore root attachment provided evidence for the existence of polygenic regulation of resistance to this characteristic. Further comparisons revealed that the efficacy of zoospore root attachment was also substantially associated with known cultivar resistance to powdery scab, zoosporangium infection severity, and root galling, but there were notable exceptions to this generalisation.

Somatic cell selection was used in a further study to generate variants of potato with enhanced resistance to diseases. From the in vitro zoospore root attachment assessments, 31 (33%) of the variants, from five potato cultivars and one clone, exhibited greater resistance to zoospore root attachment in comparison to their unselected parents. A glasshouse pathogenicity trial showed that most variants with enhanced resistance to zoospore root attachment also showed reduced powdery scab tuber disease than their parents, although these differences were not always statistically significant. This study demonstrated that somatic cell selection can be an effective approach for selecting disease-resistant variants to root infection, with the variants also showing reduced tuber disease.

To identify the potential basis for host resistance to S. subterranea at the molecular level, 12 potato cultivars differing in resistance to zoospore root attachment were used for comparative proteomic analysis. A total of 3723 proteins were quantified in root samples across the 12 cultivars using a data-independent acquisition mass spectrometry approach. Analysis identified 454 proteins that were more abundant, and 626 proteins were less abundant, in the resistant cultivars than in the susceptible cultivars. In the resistant cultivars, functional annotation of proteome data indicated that the gene ontology terms related to oxidative stress and metabolic processes were increased. KEGG pathway analysis confirmed the presence of phenylpropanoid biosynthesis pathway in the resistant cultivars, suggesting the potential role of lignin biosynthesis in host resistance to S. subterranea. Several enzymes involved in pectin biosynthesis and remodelling, such as pectinesterase and pectin acetylesterase, were more abundant in the resistant cultivars. Further investigation of the potential role of root cell wall pectin using the enzyme pectinase at a range of concentrations showed that pectinase reduced in zoospore root attachment both in the resistant and susceptible cultivars. These results suggest a unique role of cell wall pectin in regulating S. subterranea zoospore root attachment. This study provides the first insights into potato resistance to the zoospore root attachment at a proteomic level.

The enzyme study investigated the potential roles of surface root cell wall polysaccharides and proteins in zoospore root attachment by combining transcriptomic and proteomic datasets of a resistant and a susceptible potato cultivar. Effects of enzymatic removal of root cell wall proteins, N-linked glycans or polysaccharides on S. subterranea attachment to root tissue of a resistant and a susceptible potato cultivar. Subsequently, mass spectrometry analysis of peptides released by trypsin shaving (TS) of root segments identified a total of 1235 proteins of which 262 were differentially abundant between the resistant and susceptible cultivars. In particular, proteins associated with glutathione metabolism and lignin biosynthesis, were more abundant in the resistant cultivar. Comparison with whole-root proteomic analysis of the same resistant and susceptible cultivars led to identification of 226 proteins that were unique to the TS dataset, of which 188 were significantly different between cultivars. Among these, the pathogen defence-related cell wall protein stem 28 kDa glycoprotein and two major latex proteins were identified as significantly less abundant in the resistant cultivar compared to the susceptible cultivar. A further major latex protein was detected at reduced levels in the resistant cultivar in both TS and whole-root proteomic datasets. In contrast, in the TS-specific dataset, three glutathione S-transferase (GST) proteins were more abundant in the resistant cultivar, while the protein glucan endo-1,3-beta-glucosidase was detected at increased levels in both the TS and whole-root datasets. These results imply particular roles of major latex proteins and glucan endo-1,3-beta-glucosidase in the regulation of host susceptibility to S. subterranea.

This study expands knowledge of the physiological and molecular basis of S. subterranea zoospore attachment to potato roots. S. subterranea zoospore root attachment and host resistance, which are important components of potato tuber and root disease management and potato breeding programmes, are now making significant progresses. Additional research is required, but the information provided by the present study will aid future investigations of potato Spongospora host/pathogen relationships.