Identification of the endophypte of Dryas and Rubus (Rosaceae)

Root nodules of Dryas drummondii are of the coralloid type (Alnus type). The endophyte is present in the middle cortical cells of the root-nodule tissue. Transmission electron micrographs revealed an actinorhizal endophyte with septate hyphae and non-septate spherical or ovoid vesicles. Vesicles always possess at the base a septum; septa formation in the endophyte is always associated with the presence of mesosomes. Branching of the endophyte is not necessarily correlated with septum formation. Hyphal structures are more prominent in the apical part of the root nodule and vesicles are more numerous in a broad zone below this. In the middle and towards the base of the root nodule the endophytic structures appear in a stage of disintegration. Vesicles appear in a broad region near the periphery of the host cell and regularly show no strict orientation towards the host-cell wall. In the center of the host cells only hyphae occur. In the intercellular spaces between the host cells the Frankia endophyte produces spore-like structures although the outline of the sporangia is often faint. The coralloid root of Rubus ellipticus shows characteristically a basal rootlet initiated below the dichotomous branching of the nodular lobes, but extending beyond the root nodule. The endophyte is only present in the outer cortex of the root nodule in a 1-2 cell wide layer. This endophytic layer is bounded, internally as well as externally, with a 4-5 cell wide layer of tannin-filled host cells. The implications of this situation are discussed. Tannin-filled cells occur regularly in Rubus species and their arrangement has been used for taxonomic purposes within the genus. The Rubus endophyte is a Frankia species with septate hyphae and distinctly septate spherical vesicles. The ultrastructure of the vesicles of the Rubus endophyte is very similar to that of the Alnus endophyte.