Biosynthesis of Rhizobial Exopolysaccharides and Their Role in the Root Nodule Symbiosis of Leguminous Plants

Abstract

Bacteria belonging to the genera Rhizobium, Bradyrhizobium, Sinorhizobium, Mesorhizobium, and Azorhizobium, commonly called rhizobia, are able to elicit the formation of nodules on the roots of leguminous plants which can be considered as new plant organs. Inside these organs, the rhizobia are present as dinitrogen-fixing organelles maintained by the plant in return for ammonia. Characteristic of this symbiosis is that rhizobia infect living plant cells by an endocytotical process. For a detailed survey of the Rhizobium infection process, we refer to a review by Kijne.¹ Briefly, the rhizobia attach to growing root hairs and provoke tight root hair curls in which they are trapped. Simultaneously, rhizobia induce cell divisions in the root cortex, resulting in the formation of a nodule primordium. Starting from the root hair curl, the rhizobia invade the plant through infection threads (tip-growing tubular structures containing rhizobia), are endocytosed by young nodule cells, and differentiate into dinitrogen-fixing bacteroids. This symbiotic association is host-plant-specific, i.e., one rhizobial strain can infect only a limited number of different hosts. For example, R. leguminosarum bv. viciae can induce formation of dinitrogen-fixing nodules on Pisum, Vicia, Lathyrus, and Lens, whereas R. leguminosarum bv. trifolii nodulates Trifolium species (see Table 1).