Inputs of biologically fixed nitrogen derived from the symbiotic relationship between legumes and their root-nodule bacteria into terrestrial ecosystems amount to at least 70 million metric tons per year. It is obvious that this enormous quantity will need to be augmented as the world's population increases and as the natural resources that supply fertilizer nitrogen diminish. This objective will be achieved through the development of superior legume varieties, improvement in agronomic practice, and increased efficiency of the nitrogen fixation process itself by better management of the symbiotic relationship between plant and bacteria. This paper considers ways and means by which populations of root-nodule bacteria, established and introduced, can be manipulated ecologically, agronomically, edaphically and genetically to improve legume productivity and, as a consequence, soil fertility.

Manipulation of rhizobia microflora for improving legume productivity and soil fertility: a critical assessment

The influence of the legume in root‐nodule symbiosis

Biochemical and symbiotic properties of the rhizobia

When a single host plant is infected by more than one strain of rhizobia, they face a tragedy of the commons. Although these rhizobia benefit collectively from nitrogen fixation, which increases host-plant photosynthesis, each strain might nonetheless increase its own reproduction, relative to competing strains, by diverting resources away from nitrogen fixation. Host sanctions can limit the evolutionary success of such rhizobial cheaters (strains that would otherwise benefit by fixing less nitrogen). Host sanctions have been shown in soybean (Glycine max) nodules, where the next generation of symbiotic rhizobia is descended from bacteroids (the differentiated cells that can fix nitrogen). Evidence for sanctions is less clear in legume species that induce rhizobial dimorphism inside their nodules. There, bacteroids are swollen and cannot reproduce regardless of how much nitrogen they fix, but sanctions could reduce reproduction of their undifferentiated clonemates within the same nodule. This rhizobial dimorphism can affect rhizobial evolution, including cheating options, in ways that may affect future generations of legumes. Both the importance of sanctions to hosts and possible physiological mechanisms for sanctions may depend on whether bacteroids are potentially reproductive.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2009.02941.x

Controlling the reproductive fate of rhizobia: how universal are legume sanctions?

SUMMARY: Strains of agrobacteria, rhizobia and other organisms were collected from various sources and tested for their ability to utilize 61 compounds as sole carbon sources, oxidative or fermentative glucose metabolism, hydrolysis of starch, casein and gelatin, anaerobic arginine metabolism and production of 3-ketolactose. The resulting data was used in a numerical analysis and the strains were grouped on the basis of overall similarity.
The resultant groupings confirmed the idea that the genera Agrobacterium and Rhizobium be combined, and four species of fast-growing rhizobia were recognized: Group I, Rhizobium radiobacter; Group II, Rhizobium meliloti; Group III, related to Agrobacterium rhizogenes and the Biotype 2 strains of Keane, Kerr & New (1970); Group IV, Rhizobium leguminosarum. Crown-gall organisms were associated with groups I and III. The four species were defined without reference to pathogenicity or nodulating ability and it appears at present that crown-gall organisms can only be distinguished by pathogenicity testing.
Agrobacterium rubi H36 probably belongs in the genus Rhizobium whereas Agrobacterium gypsophilae 1948 and Agrobacterium pseudotsugae 180 do not.

The Taxonomy of the Crown-gall Organism Agrobaterium tumefaciens and Its Relationship to Rhizobia and Other Agrobacteria

Strains of Rhizobium may be distinguished in their behavior in symbiosis by their ability to infect a given host plant, as well as by the types and numbers of nodules produced and by the nodules' effectiveness in benefiting the host through nitrogen fixation. Allen and Baldwin (1931) have reviewed the changes which strains of Rhizobium have been observed to undergo, and the more important of these may be summarized. Nobbe and Hiltner (1893) and Frank (1899) claimed that growth of Rhizobium on gelatin media could induce ineffectivity in nitrogen fixation in the nodules or even complete loss of ability to infect the roots. Other workers have claimed that cultivation on nitrogen-rich media leads to a reduction in effectivity and vice versa, and Simon (1908), Snieszko (1929), and Hutchinson (1924) present evidence that culture in soil restores lost virulence and effectivity. Finally, Wuinschik (1925) and Allen and Baldwin (1931) claimed that repeated plant passage led to changes in the effectivity of strains, although neither Stapp (1929) nor Virtanen (1945) was able to confirm Wulnschik's results. In all these cases it is not clear whether the observed changes were due to the selective action of the treatment on an already heterogeneous population or whether new variants had appeared during the experiments. The gradual changes often observed could thus be attributed either (1) to the selective increase of a small number of cells originally present in the culture, and bearing the newly observed character, (2) to the development of a few variants that subsequently increased differentially, or (3) to a general change occurring throughout the bacterial population. Further, the majority of tests were made as pot experiments, and under these conditions the possibility of contamination cannot be excluded. The object of the present work was to study, with pure culture technique, the effects of plant passage and of various cultural environments in inducing variation in a population of Rhizobium known to be homogeneous at the commencement of the tests.

P. S. Nutman

Papers

Variation within Strains of Clover Nodule Bacteria in Size of Nodule Produced and in “Effectivity” of Symbiosis