Histological analysis of spindle and spheroid root galls caused by Plasmodiophora brassicae

Abstract

Clubroot of crucifers, caused by the obligate parasite Plasmodiophora brassicae, is characterized by the formation of conspicuous root galls. These galls usually have a club- or spindle-shaped morphology, and interfere with water and nutrient uptake by infected plants. Smaller galls, historically regarded as resistance structures and distinct from the typical spindle-shaped galls, have also been identified and termed ‘spheroid galls’ because of their spherical or nearly spherical form. An assessment of various Brassica species and varieties revealed that spheroid galling could be observed in all genotypes investigated, but occurred regularly only in a few particular host/P. brassicae combinations. While spindle gall formation was coincident with the expansion of the stele and infection of secondary tissues by P. brassicae, spheroid galls typically had a region of proliferating tissue that corresponded to the secondary cortex and periderm of the healthy plants, with the outer proliferating tissue less infected than the inner portions. The underlying host tissue showed limited secondary tissue development, was largely uninfected, and, where infection occurred, a continuous stele was maintained. An active host defensive reaction, in the form of cell lignification or the hypersensitive response, was not observed, while pathogen resting spores were visible in one longitudinal section of a spheroid gall. These findings suggest that while the proliferation of P. brassicae is restricted in spheroid galls, these structures are not indicative of complete resistance to clubroot.