Bacterium release into host cells of nitrogen-fixing soybean nodules: the symbiosome membrane comes from three sources.

The release process of bacteria into the cytoplasm of soybean nodule cells has been studied, and three functional zones of the infection thread are delineated. Zone 1 is found over the greatest length of very long infection threads. Zone 2 is a short region where membrane mobilization by exocytosis of endoplasmic reticulum (ER) into the infection-thread membrane takes place; the result is that much new membrane and wall degradation enzymes can be provided. In addition, de novo membrane formation takes place inside the infection thread in apposition to the bacterial outer membrane. Zone 3 is the endocytic region where both bacteria and infection-thread wall degradation vesicles are released into the host cytoplasm and constitute a second product of endocytosis at the infection thread tip. Evidence is presented indicating that the symbiosome membrane, even at its time of origin, is composed of membrane from three sources: the host infection-thread membrane, ER, and de novo synthesis; the membrane formation that is so large for these purposes is probably carried out both from the ER directly and also through the Golgi-apparatus synthesis. Evidence is also given that the bacteria have lost their exopolysaccharide coatings before release into symbiosomes.