Site-specific subtilase gene expression reveals major plant cellular remodeling associated with intercellular Frankia root colonization of the actinorhizal host Discaria trinervis

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

Hammamet

Simposio; 19th International Meeting on Frankia and Actinorhizal Plants; 2018

Different processes associated with symbiotic microbe root entry have been described for both legume and actinorhizal host plants. The most common and the best studied of these in model legumes is referred to as the root hair ?intracellular? pathway, that starts with root hair tip curling as the first plant morphological response that identifies future infection sites. Alternatively, the microsymbiont can penetrate the intact epidermis via the apoplastic interface between adjacent host cells, and this is termed ?intercellular? colonization. However, in this latter case there are no visible plant morphological responses that identify potential sites of root entry. The current lack of knowledge about the molecular and cellular mechanisms associated with intercellular colonization incited us to focus on the Discaria trinervis - Frankia symbiotic association (Valverde & Wall, 1999). The development of Agrobacterium rhizogenes-mediated root transformation for D. trinervis makes this actinorhizal host an attractive model for studying the mechanisms of apoplastic intercellular root colonization (Imanishi et al., 2011). In this way we were able to show that the promoters of both the Casuarina glauca subtilase-encoding Cg12 gene and the orthologous D. trinervis gene (Dt12) are transcriptionally activated in D. trinervis root tissues associated with the earliest stages of Frankia colonization. Furthermore, SYTO 9-staining of Frankia filaments within root sections expressing fluorescent reporters under the control of the subtilase gene promoter underscored the correlation between reporter expression and intercellular Frankia progression within the D. trinervis root outer cortex. Finally, confocal microscopy of intact roots undergoing Frankia colonization of the outer root tissues has revealed a major cellular reorganisation of the host cells immediately adjacent to sites of Frankia colonization, associated with structural modifications to the cell-cell interface. Together, these observations provide strong evidence that the host plant plays a key role during colonization of the root intercellular apoplast by symbiotic filamentous nitrogen-fixing Frankia. These findings point to likely mechanistic analogies between inter- and intracellular modes of host colonization by symbiotic microbes. Future studies will now need to investigate to what extent microbe-host molecular signaling and associated activation of the Common Symbiotic Signaling Pathway are required for intercellular Frankia colonization of actinorhizal plants.