Biogenesis of the Peribacteroid Membrane in Root Nodules: Roles of Dynamev and Phosphatidyl-Inositol 3-Kinase

Abstract

Rhizobia in the infected cells of root nodules are compartmentalized inside a membrane, peribacteroid membrane (PBM), where they are differentiated into bacteroids and begin fixing nitrogen. Since bacteria are never free inside the plant cell during symbiosis, the PBM serves as an interface between the two organisms and its integrity is vital for symbiotic nitrogen fixation. As bacteria are released from the infection thread they are endocytosed by the plasma membrane of the host and this membrane proliferates to enclose bacteria individually or in a group to eight per envelope. This necessitates a tight coordination of PBM biogenesis with the bacterial cell division and the number of bacteroids per envelope reflects when cell division ceases following endocytosis. This event is determined by the host. Although derived from the plasma membrane (PM), the PBM undergoes significant chandes and becomes a mosaic membrane having properties in common with the vacuolar and the PM (Verma, 1992). Thus, vesicular traffic in root nodules must be unique to direct appropriate PBM nodulins and the peribacteroid fluid proteins to their proper sites. The plant genetics data (Tikhonovich et al., 1995) is consistent with this interpretation. The mechanism controlling PBM biogenesis and proliferation is poorly understood. Recent studies on plant and bacterial genetics have shed some lights on biogenesis of this compartment and suggest that both bacteria and the host plant control the synthesis of PBM and its effectiveness in symbiosis.