Symbiotic Nitrogen Fixation
1. Rhizobium - Legume Symbiosis
Many leguminous plant species can enter into a symbiotic relationship with root-nodule bacteria, collectively referred to as rhizobia. The legumes belong to the order Fabales, family Leguminosae (alternatively Fabaceae), in eurosid clade I (Doyle and Luckow 2003). Traditionally, three main subfamilies are distinguished: Caesalpinoidae, Mimosoidae and Papilionidae. Only one nonlegume, the woody plant Parasponia sp., can be nodulated by rhizobia and utilize nitrogen fixed by the bacteria. Numerous leguminous plants may also occur in deserts, with rhizobium in their root nodules such as the Parkinsonia species (“paloverde”). Soybeans (Glycine max) and the cowpea group (which includes the Acacia mangium and Acacia senegal) are infected by the Bradyrhizobium bacteria. By supplying the host with nitrogen, an individual rhizobium can enhance host photosynthesis, presumably increasing the rhizobium’s own access to photosynthate. The interaction between a particular strain of rhizobia and the "appropriate" legume is mediated by: *A "Nod factor" secreted by the rhizobia and transmembrane receptors on the cells of the root hairs of the legume. *Different strains of rhizobia produce different Nod factors, and different legumes produce receptors of different specificity. If the combination is correct, the bacteria enter an epithelial cell of the root; then migrate into the cortex. Their path runs within an intracellular channel that grows through one cortex cell after another. This infection thread is constructed by the root cells, not the bacteria, and is formed only in response to the infection. When the infection thread reaches a cell deep in the cortex, it bursts and the rhizobia are engulfed by endocytosis into membrane-enclosed symbiosomes within the cytoplasm. At this time the cell goes through several rounds of mitosis — without cytokinesis — so the cell becomes polyploid. The cortex cells then...
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