Iron Transport across Symbiotic Membranes of Nitrogen-Fixing Legumes

Abstract

Iron is an essential nutrient for the legume-rhizobia symbiosis and nitrogen-fixing bacteroids within root nodules of legumes have a very high demand for the metal. Within the infected cells of nodules, the bacteroids are surrounded by a plant membrane to form an organelle-like structure called the symbiosome. In this review, we focus on how iron is transported across the symbiosome membrane and accessed by the bacteroids.

Keywords: iron; legumes; nitrogen fixation; symbiosomes.

Figure 1

(A) nodules on the root of a soybean plant. (B) detail of infected cells. V = vacuole; as = air space; uc = uninfected cell; ic = infected cell; pbm = peribacteroid membrane, now known as the symbiosome membrane; bact = bacteroid.

Figure 2

Schematic representation of proposed iron transport in a symbiosome. A symbiosome membrane (SM) surrounding two bacteroids is shown. Ferrous iron is transported across the symbiosome membrane via two avenues: VTL1a (Vacuolar iron Transporter Like protein) in soybean and M. truncatula and FPN (ferroportin) in M. truncatula. Ferric iron is also transported into the symbiosome, but the protein responsible has not been identified. The symbiosome space is a storage compartment for iron, in much the same manner as a vacuole. Ferric iron in the symbiosome space can be reduced by a ferric chelate reductase (FRO) on the SM and transported either into the bacteroid via the freoB transporter or back into the infected cell’s cytosol via DMT1 (Divalent Metal Transporter). See text for more details.