Immuno-gold localization of leghaemoglobin in cytoplasm in nitrogen-fixing root nodules of pea

A unique feature of nitrogen-fixing nodules formed by association of species of Rhizobium and Frankia with various legumes and non-legumes is that they contain leghaemoblogin (Lb), an oxygen-carrying myoglobin-like protein1–6. Elucidation of the function of Lb, which is thought to facilitate the flow of oxygen to the nitrogen-fixing bacteroid forms of Rhizobium within infected nodule cells7,8, has been hindered by uncertainty regarding its precise intracellular location9. Several workers have reported that Lb is located exclusively in the plant cytoplasm in nodules of various legumes10–12, while others13–15 have concluded that some of the Lb is located in the peribacteroid spaces in some legumes. Using immuno-gold staining16 of Lb on thin sections of pea nodules, we demonstrate here that Lb can be detected in the plant cytoplasm and the nucleus of infected cells but not in the peribacteroid spaces