Live cell imaging of Plasmodiophora brassicae-host plant interactions based on a two-step axenic culture system

Plasmodiophora brassicae, a parasitic protist, induces club-shaped tumor-like growth of host Brassicas roots and hypocotyls after infection. Due to its soil-borne nature and intracellular, biotrophic parasitism the infection biology and early pathogenesis remains in doubt. In this study, we have established a new protocol, based on a two-step axenic culture of P. brassicae with its host tissues, for easy and in planta observation of cellular interactions between P. brassicae and host plants: first, coculture of P. brassicae with infected canola root tissues, on growth-medium plates, enables the propagation of P. brassicae that serves as pure inoculum for pathogenicity assays, and second, the pure inoculum is subsequently used for pathogenicity tests on both canola and Arabidopsis seedlings grown on medium plates in Petri dishes. During the first axenic culture, we established a staining protocol by which the pathogen was fluorescently labeled with Nile red and calcofluor white, thus allowing in planta observation of pathogen development. In the pathogenicity assays, our results showed that axenic cultures of P. brassicae, in calli, remains fully virulent and completes its life cycle in both canola and Arabidopsis roots grown in Petri dishes. Combining visualization of fluorescent probe-labeled P. brassicae structures with fluorescent protein tagging of Arabidopsis cellular components, further revealed dynamic responses of host cells at the early stages of P. brassicae infection. Thus, established protocols for in planta detection of P. brassicae structures and the live cell imaging of P. brassicae-Arabidopsis interactions provide a novel strategy for improving our detailed knowledge of P. brassicae infection in host tissues.