A sub-compatible rhizobium strain reveals infection duality in Lotus.

Lotus species develop infection threads to guide rhizobia into nodule cells. However, there is evidence that some species have a genetic repertoire to allow other modes of infection. Conducting confocal and electron microscopy, quantification of marker gene expression, and phenotypic analysis of transgenic roots infected with mutant rhizobia we elucidated the infection mechanism used by Rhizobium leguminosarum Norway to colonise Lotus burttii. R. leguminosarum Norway induces a distinct host transcriptional response compared to Mesorhizobium loti. It infects L.burttii utilising an epidermal and trans-cellular infection thread-independent mechanism at high frequency. The entry into plant cells occurs directly from the apoplast and is primarily mediated by "peg"-like structures, the formation of which is dependent on the production of Nod factor by the rhizobia. These results demonstrate that Lotus species can exhibit duality in their infection mechanisms depending on the rhizobial strain that they encounter. This is especially relevant in the context of interactions in the rhizosphere where legumes do not encounter single strains, but complex rhizobial communities. Additionally, our findings support a perception mechanism at the nodule cell entry interface reinforcing the idea that there are successive checkpoints during rhizobial infection.