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Publisher Summary This chapter discusses the biology of legume–Rhizobium interactions in nodule formation. Bacteria of the genus Rhizobium are capable of invading the roots of many legumes and forming nodules that fix nitrogen. In most legumes, infection occurs through a complicated series of processes that begin with rhizobial growth in the rhizosphere. It then proceeds with formation of infection threads within root hairs and ends with the release of rhizobia into cortical cells of the plant host. Another mode of infection has been reported among the Aeschynomenae. In this case, cortical cells of the root are invaded directly by the Rhizobium forming a nodule in which the cells containing bacteria are capable of division. During nodule formation, rhizobia are transformed into vegetative bacteroids that are virtually nitrogen-fixing organelles. In some species, rhizobia within their membrane envelope divide after release from the infection thread. In this way, several bacteroids may be enclosed in a single envelope. Infected cells are relatively large and are almost completely full of bacteroids. Often the cells contain a central vacuole. Nodules in which little or no nitrogen is fixed are normally termed ineffective. |
