Interactions between Polymyxa betae and plant systemic defense ways

Polymyxa betae is the vector of Beet necrotic yellow vein virus (BNYVV), the causal agent of sugar beet rhizomania disease. Because of the widespread use of cultivars partially resistant to BNYVV, resistance breaking BNYVV isolates have been reported. In order to develop alternative control strategies, we investigated interactions between P. betae and plant defenses. A first set of bioassays was conducted in order to assess P. betae infection after the elicitation of inducible defenses in sugar beet. The systemic acquired resistance (SAR) or the induced systemic resistance (ISR) was induced by a treatment of the plants with an analogue of the salicylic acid (SA) or with lipopeptides (LPs) from Bacillus amyloliquefaciens, respectively. While P. betae infection, quantified by real time PCR, was not reduced in plants treated with SA, the intensity of infection was significantly lower (up to 95 % of reduction) in plants after a treatment with LPs. LPs were shown to effectively induce systemic resistance in both roots and leaves of sugar beet, resulting in a reduction in P. betae infection. The absence of any effect of SAR against P. betae could be explained by the coevolution of P. betae and sugar beet, allowing the biotrophic endoparasite to bypass self-induced SAR. This hypothesis was validated in another bioassay. SAR was efficiently induced in sugar beet after the infection by P. betae, allowing the plants to resist (75% reduction) to a further infection by Cercospora beticola. These results suggest that P. betae has to be considered not only as a target but also as an actor of plant defenses.