The Development of Rhizobium Genetics: The Fourth Fleming Lecture

Abstract

The rhizobia are Gram-negative bacteria of major agricultural importance because of their ability to form nitrogen-fixing nodules on the roots of leguminous plants. The nitrogen fixation genes are carried by the rhizobia and are possibly only expressed in nature within the root nodule. Usually a given strain of Rhizobium can nodulate only a limited number of legume species. For temperate species this has given rise to a taxonomic classification based on the range of host plants that are nodulated. For example, Rhizobium trifolii includes those rhizobia which (ideally) nodulate only clovers (Trifolium spp.) which, in turn, are (ideally) nodulated only by strains of R. trfolii. However, while this grouping works reasonably well with temperate legumes, the far more numerous tropical legumes and rhizobia appear not to show the same host-Rhizobium specificity. A more useful characterization is on the basis of growth rate which reflects basic metabolic differences : temperate rhizobia on the whole are fast growers (taking 3 to 7d to form colonies on agar plates) while the others are generally slow growers (7 to 21 d).Root nodules are formed as the result of a series of poorly understood interactions between the appropriate Rhizobium species and its host plant. There is evidence that the population of rhizobia in the soil is stimulated by legumes (Robinson, 1967) but insufficient data are available to determine whether this is specific for a given host-Rhizobium combination, or is a general response of rhizobia to the presence of leguminous plants. Recognition, which may involve lectin binding (Dazzo & Hubbell, 1975), probably occurs before entry into the root, usually through root hairs or, more rarely, through breaks in the root occurring, for example, at the emergence of lateral roots. Within root hairs the bacteria are always enclosed within a tube, the infection thread, which is surrounded by a pectic and cellulose wall material that is continuous with the plant cell wall. The infection thread branches and penetrates through cells within the cortex. This growth is correlated with (stimulates?) cell division in the inner root cortex and the growth of the root nodule. Nodule formation, unlike tumour formation by Agrobacterium tumefaciens (another member of the Rhizobiaceae), is controlled, and a differentiated structure is formed.Within the nodule the rhizobia differentiate into bacteroids, which are the nitrogen-fixing forms. The plant produces globin (Sidloi-Lumbroso et al., 1978); this combines with haem produced by the rhizobia (Cutting & Schulman, 1969) to form leghaemoglobin, which appears to be a prerequisite for a nitrogen-fixing legume nodule. Once infection has occurred the plant provides the rhizobia with all their nutrient requirements and, in turn, receives fixed nitrogen when a nitrogen-fixing strain of Rhizobium is involved.We know that rhizobia can produce indole-acetic acid (Dullaart, 1970) and very small amounts of cytokinins (Phillips & Torrey, 1972), both of which are important plant hormones. ...

Construction of a genetic map for Caulobacter crescentus.

“…This chromosome mobilizati ilar in several respects to the chrc mobilization mediated by RP4 or R68. eral species of the Rhizobiaceae (2). ) teristic of R68.45-mediated conjugati( mobilization of large segments of ti chromosome.…”

Biochemical genetics of nitrogen fixation.

“…Several books on N2 fixation have been published recently (32,67,90,101,154,178), and a variety of specialized topics concerning N2 fixation have been reviewed, including biochemistry (62,71,126,147,249), the Rhizobium infection process (16,27,53,190), nonlegume symbioses (1), cyanobacteria (95,250), and genetics and regulation (14,25,201).…”

Rhizobium-Leguminosae Symbiosis: The Bacterial Point of View

“…The isolation of various classes of bacterial mutants unable to carry out the symbiotic process has demonstrated that a number of different steps are involved in the establishment of effective nitrogen fixation (Vincent, 1980;Beringer, 1980). However, the genetic basis of this bacterial-plant interaction is poorly understood and the characterization and manipulation of symbiotic genes has been hampered by two main problems.…”