Polymyxa betae - Beta vulgaris : understanding the molecular interactions through transcriptome and plant defense analysis

(eng) Polymyxa betae is a biotrophic obligate endoparasite of sugar beet roots, belonging to the Plasmodiophorids (Cercozoa, Protozoa). The infection by this parasite is asymptomatic, excepted when P. betae transmits to a susceptible plant the Beet necrotic yellow vein virus (BNYVV), the causal agent of the rhizomania. This disease is controlled by the use of BNYVV tolerant varieties. But the emergence of viral resistance breaking isolates requires the development of alternate control strategies. The prevention of vector infection represents an interesting alternative. In this view, we investigated P. betae genome and molecular interactions between P. betae and its host. Two model studies, using Arabidopsis thaliana and sugar beet hairy roots, were developed and evaluated to investigate molecular biology of P. betae. A differential transcript analysis between healthy and P. betae infected sugar beets allowed the identification of 76 new ESTs from P. betae and 120 ESTs from sugar beet. The dynamic of expression of selected genes revealed the potential role of two P. betae proteins, the profilin and a Von Willebrand factor domain containing protein, in the first phase of infection. This study also highlighted an overexpression of some sugar beet genes involved in the defense, such as PR proteins, stress resistance proteins or lectins, especially during the plasmodial stage. The induction of systemic acquired resistance (SAR) by P. betae was confirmed in a further bioassay showing the protection offered by the protist against Cercospora beticola. P. betae seems thus to by-pass the SAR. In contrast, the protist is affected by the induced systemic resistance. Indeed, the elicitation of sugar beet by Bacillus lipopeptides allowed a significant reduction of infection by the protist. This thesis highlighted the biological processes involved in the P. betae-sugar beet interactions, potential new targets for sugar beet selection and rhizomania control. The results concerning defense ways enabled to consider P. betae not only as the target of the plant defenses but also as an actor of these defenses.