An in Vitro Pathogenicity Model-System to Study the Molecular Basis of the Rhizoctonia Solani Infection Process

Abstract

The soil-borne fungus Rhizoctonia solani is world-wide distributed and has a broad host range. The infection of a host-plant by R. solani is described to proceed in distinct stages: i) recognition, ii) attachment, iii) development of infection structures, iv) penetration and v) colonization (Rhizoctonia solani: Biology and Pathology, J. R. Parmeter, Ed., 1970). We decided to study the molecular mechanisms that regulate the infection process, to obtain an insight in the factors that determine disease initiation by R. solani. We started with the development of an in vitro assay, in which the pathogenicity of R. solani to seedlings can be studied. A range of 10 host plants (Arabidopsis thaliana, Brassica oleracea, Capsicum annuum, Linum usitatissimum, Lycopersicon lycopersicum, Raphanus sativus, Sesamwn indicun, Sinapis alba, Solanum melongena and Solanum tuberosum) was tested with several R. solani isolates, belonging to 8 different anastomosis groups (AG: 1, 2 (2–1, 2–2), 3, 4, 5, 6, 7, BI). Pathogenicity was studied both on a macroscopical and microscopical level, to determine which step in the infection process is most suitable for further molecular studies. Within the range of host plants and R. solani isolates tested, macroscopic observations revealed: a) that plant species are either susceptible to every R. solani isolate (5 plant species) or show a differential response, i.e. these plants responded to most R. solani isolates with the development of symptoms, but to some with no symptoms at all (5 plant species), b) that plant species either develop an increasing resistance to R. solani as the seedlings age increases (6 plant species) or have a steady resistance level (1 plant species), c) that R. solani isolates either show host plant specificity (15 isolates) or show no host plant specificity (9 isolates) d) that between AG’s, but also within AG’s, R. solani isolates are encountered that behave differently (pathogenic versus non-pathogenic) towards a single plant species.