Beringer J E, Brewin N, Johnston A W, Schulman H M, Hopwood D A
Proc R Soc Lond B Biol Sci. 1979 Apr 11;204(1155):219-33. doi: 10.1098/rspb.1979.0024.
The rhizobia are soil microorganisms that can interact with leguminous plants to form root nodules within which conditions are favourable for bacterial nitrogen fixation. Legumes allow the development of very large rhizobial populations in the vicinity of their roots. Infections and nodule formation require the specific recognition of host and Rhizobium, probably mediated by plant lectins. Penetration of the host by a compatible Rhizobium species usually provokes host root cell division to form the nodule, and a process of differentiation by both partners then ensues. In most cases the rhizobia alter morphologically to form bacteroids, which are usually larger than the free-living bacteria and have altered cell walls. At all stages during infection, the bacteria are bounded by host cell plasmalemma. The enzyme nitrogenase is synthesized by the bacteria and, if leghaemoglobin is present, nitrogen fixation will occur. Leghaemoglobin is a product of the symbiotic interaction, since the globin is produced by the plant while the haem is synthesized by the bacteria. In the intracellular habitat the bacteria are dependent upon the plant for supplies of energy and the bacteroids, in particular, appear to differentiate so that they are no longer able to utilize the nitrogen that they fix. Regulation of the supply of carbohydrate and the use of the fixed nitrogen thus appear to be largely governed by the host.
根瘤菌是土壤微生物，能与豆科植物相互作用形成根瘤，根瘤内的环境有利于细菌固氮。豆科植物能使根际附近形成大量根瘤菌群体。感染和根瘤形成需要宿主与根瘤菌的特异性识别，这可能由植物凝集素介导。相容的根瘤菌物种侵入宿主通常会引发宿主根细胞分裂以形成根瘤，随后双方都会发生分化过程。在大多数情况下，根瘤菌形态发生改变形成类菌体，类菌体通常比自由生活的细菌大，且细胞壁发生了改变。在感染的各个阶段，细菌都被宿主细胞质膜包围。细菌合成固氮酶，如果存在豆血红蛋白，就会发生固氮作用。豆血红蛋白是共生相互作用的产物，因为球蛋白由植物产生，而血红素由细菌合成。在细胞内环境中，细菌依赖植物提供能量，特别是类菌体似乎发生了分化，以至于它们不再能够利用所固定的氮。因此，碳水化合物供应的调节和固定氮的利用似乎在很大程度上由宿主控制。
