The actinorhizal host Discaria trinervis orchestrates intercellular root entry of its filamentous Frankia microsymbiont

FOURNIER J; IMANISHI L; CHABAUD M; ABDOU-PAVY I; GENRE A; BRICHET L; VAYSSAYRE V; PIRROLLES E; BROTTIER L; GHERBI H; HOCHER V; SVISTOONOFF S; BARKER DG; WALL LG

Stockholm

Congreso; 13th European Congress on Nitrogen Fixation; 2018

ECNF

The establishment of nitrogen-fixing root nodule symbiotic associations between either legumes and rhizobia or between actinorhizal hosts and filamentous Frankia requires entry of the microsymbionts within host tissues to colonize the developing nodules. In contrast to model legumes or actinorhizal hosts such as Casuarina glauca and Alnus glutinosa that use a root hair intracellular pathway, the actinorhizal host Discaria trinervis is initially colonized by Frankia via an exclusively intercellular pathway (Valverde and Wall, 1999). This mode of microsymbiont entry, although quite frequent in both types of root nodule symbiosis, has received little attention until now. Promoter sequences of the infection-associated Cg12 subtilisin-like apoplastic serine protease gene (C. glauca) (Svistoonoff et al., 2003) and its D. trinervis ortholog Dt12 have been used to drive the expression of fluorescent cellular reporters in composite D. trinervis plants and thus facilitate the identification of intercellular infection sites in intact roots. In vivo imaging of the initial stages of Frankia colonization of the D. trinervis root has revealed that intercellular Frankia colonization is systematically associated with both nucleo-cytoplasmic reorganisation in adjacent host cells and a major remodeling of the intercellular apoplastic compartment. These observations suggest that the actinorhizal host root modifies both the size and composition of the intercellular apoplast in order to accommodate the filamentous microsymbiont, thus providing strong evidence that the host plant plays a determinant role during intercellular colonization. Mechanistic analogies between inter- and intracellular modes of host colonization by symbiotic microbes will be discussed.