ELECTRON MICROSCOPY OF INFECTION THREADS AND BACTERIA IN YOUNG ROOT NODULES OF MEDICAGO SATIVA

Abstract

Ultrathin sections of alfalfa nodules (Medicago sativa-L.) revealed that initially the infection thread consisted of a naked mucoid strand, containing imbedded bacteria, migrating between the walls of the cortical cells of the host plant. At certain locations, the plant cell walls were forced into a funnel-shaped structure which elongated to form a tubular sheath containing bacterial cells and gum. Lateral and terminal vesicles formed on the ensheathed infection thread, and these burst to liberate the bacteria into the host cell cytoplasm[long dash]the process being similar to that proposed by certain of the earlier workers. In some cases, the ensheathed thread migrated entirely across a host cell without rupturing, and, in this case, the tip of the thread fused into the host cell wall opposite the point of entry. The bacteria, after their release from the thread, became bacteroidal; and ultimately each bacterial cell became surrounded by a double-layered membrane. It is suggested that this enclosing membrane may be derived from the tubules of the endoplasmic reticulum, although the possibility of the formation of these membranes de novo as a protective mechanism of the host plant cannot entirely be disregarded.