A new phenotype of Polymyxa betae in Arabidopsis thaliana Nicolas Desoignies & Celine Stocco & Claude Bragard & Anne Legrève Accepted: 30 March 2011 # KNPV 2011 Abstract The understanding of the molecular biology of Polymyxa betae, the protist vector of Beet necrotic yellow vein virus, remains limited because of the obligate nature of this root endoparasite and the limited data on the genome of Beta vulgaris, its most common host plant. The aim of this work was to assess the infection of P. betae in Arabidopsis thaliana in order to learn more about the P. betae genome and its interaction with the host. The susceptibility of a set of ecotypes of various origins to a monosporosorus and aviruliferous isolate of P. betae was analyzed in a series of bioassays conducted under controlled con- ditions. P. betae was detected in roots of A. thaliana using light microscopy and PCR. The infection severity was relatively low in this species compared with B. vulgaris, but the different stages of the life cycle were present. The phenotype of P. betae in A. thaliana root cells differed from the phenotype in B. vulgaris: the spore-forming phase was more prevalent in comparison with the sporangial phase, and the sporosori contained a lower number of spores. The compatible interaction between P. betae and A. thaliana obtained after the inoculation of zoospores and optimal conditions for the development of P. betae provide a new model system that can be used to improve the knowledge on the P. betae genome and on the mechanisms of the spore-forming phase of P. betae. Keywords Arabidopsis thaliana . Host compatibility . Phenotype . Plant-pathogen interactions . Polymyxa betae . Spore-forming phase Introduction The understanding of the complex interactions between obligate parasites and their host plants benefits from development of model systems that overcome the difficulties associated with their specific parasitism. Arabidopsis thaliana is one of the most studied model plants. The knowledge of the complete genome offers new molecular tools for studies of plant-pathogen interactions. The aim of this work was to develop an Arabidopsis-based experimental model that can offer new resources and tools to understand mechanisms involved in Polymyxa spp.-plant interactions. P. betae Keskin is a natural soil-borne parasite of the roots of Chenopodiaceae (Keskin 1964). It is classified within the plasmodiophorids, a monophyletic group of obli- gate biotrophic parasites that includes 10 genera previously considered to belong to the Fungi, but which are now included in the Protozoa (Braselton 2001). Its life cycle in the roots of Beta vulgaris is Eur J Plant Pathol DOI 10.1007/s10658-011-9783-5 N. Desoignies : C. Stocco : C. Bragard : A. Legrève (*) Université catholique de Louvain, Earth and Life Institute, Phytopathology, Croix du sud 2 bte 3, 1348 Louvain-la-Neuve, Belgium e-mail: anne.legreve@uclouvain.be