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Flagella Mediate Endophytic Competence Rather Than Act as MAMPS in Rice–Azoarcus sp. Strain BH72 Interactions

Azoarcus sp. strain BH72 is an endophytic betaproteobacterium able to colonize rice roots without induction of visible disease symptoms. BH72 possesses one polar flagellum. The genome harbors three copies of putative fliC genes, generally encoding the major structural protein flagellin. It is not clear whether, in endophytic interactions, flagella mediate endophytic competence or act as MAMPs (microbe-asscociated molecular patterns) inducing plant defense responses. Therefore, possible functions of the three FliC proteins were investigated. Only fliC3 was found to be highly expressed in pure culture and in association with rice roots and to be required for bacterial motility, suggesting that it encodes the major flagellin. Endophytic colonization of rice roots was significantly reduced in the in-frame deletion mutant, while the establishment of microcolonies on the root surface was not affected. Moreover, an elicitation of defense responses related to FliC3 was not observed. In conclusion, our data support the hypothesis that FliC3 does not play a major role as a MAMP but is required for endophytic colonization in the Azoarcus-rice interaction, most likely for spreading inside the plant.