Entry of Rhizobia into Roots of Mimosa scabrella Bentham Occurs between Epidermal Cells

SUMMARY: The nitrogen-fixing symbiosis between Mimosa scabrella and Rhizobium Br 3454 was initiated by an infection sequence consisting of (a) surface colonization of roots; (b) penetration of mucigel and primary layers of radial walls of epidermal cells; (c) further penetration through primary wall layers and intercellular air spaces; (d) occasional intracellular penetration of epidermal and cortical cells, always occurring from a boundary between two neighbouring cells. Root hairs, when present, were never seen to be infected by the well-known infection thread mechanism, nor were infections seen to occur through wounds. This newly described method of infection may be common in legumes.