Ultrastructural and immunological demonstration of the nodulation of the European Alnus glutinosa (L.) Gaertn. host plant by the North-American Alnus crispa var. mollis Fern, root nodule endophyte

Abstract

The inoculation of the European A. glutinosa (L.) Gaertn. host plant by a crushed-nodule inoculum, prepared with the North American A. crispa var. mollis Fern. root nodule, was successful. Fluorescein- and ferritin-labeled antibodies, specific against the A. crispa var. mollis root nodule endophyte, demonstrated the identity of this endophyte in the resulting nodules. The nodulation process of this abnormal host-endophyte system was studied by light microscopy and EM. An excretion of host blebs containing electron-dense polysaccharide material, resulting in the formation of exo-encapsulation threads containing presumptive endophytic bacterial cells, was associated with deformed root hairs. Originating from an exoencapsulation thread, the endophyte penetrates the root hair cell and then migrates as a hypha toward the cortical cells of the root. Its migration in the cortical cells of the primary nodule results in the induction of a lateral root which develops as the true nodule. The ultrastructure of the A. crispa var. mollis endophyte developing in the primary and true nodule of the abnormal A. glutinosa host was similar to the one induced inside its normal A. crispa var. mollis host. The actinomycetal intruder was a branched and septate hypha able to produce septate vesicles. The endophyte was always encapsulated in an electron-dense polysaccharide material surrounded by a host plasma membrane envelope. However, in this abnormal host-endophyte system, the number of primary nodules formed per root system was drastically reduced, and their appearance was delayed by 1-2 wk. The delayed nodules were effective in fixing N and were able to support satisfactory plant growth in a N-free medium.