The Root Nodule Symbiosis between Rhizobium 'hedysari' and its drought-tolerant host Hedysarum coronarium.

The symbiotic interaction occurring between the legume Hedysarum coronarium, a drought-tolerant forage crop relevant for the mediterranean basin, and its natural host-specific, nitrogen-fixing bacterial symbiont, Rhizobium 'hedysari',was studied by combined light and transmission electron microscopy. Several unusual morphological features of this root nodule symbiosis were found: in nature the plant is able to form modified pseudo-roots called 'shovels'; we observed that these structures can form independently from the presence of bacteria. Computer-assisted image analyses suggested three size classes of root hair lengths, one of which was longer than 600µm. The primary infection route is through infection threads in deformed root hairs. Transmission electron microscopy of infection threads within the nodule revealed an atypical double layered wall. Nodules were indeterminant with an apical meristem, infection zone, and central bacteroid tissue. The predominant shape of differentiated R.'hedysari' bacteroids is cruciform.