Natural variation in symbiotic nitrogen-fixingRhizobium andFrankia spp.

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Abstract
A description is given of the natural variation in nitrogen-fixingRhizobium andFrankia spp. strains and the ability to form root nodules on compatible host plants. Arguments are given for the hypothesis that co-evolution has taken place through mutual interaction of host plants and indigenousRhizobium andFrankia populations in the soil leading to most efficient symbiotic associations. The significance of root nodules as selective enrichment cultures of particular strains in natural and cultivated soils is exemplified byRhizobium leguminosarum on various ecotypes ofPisum sativum and withFrankia sp. on various actinorhizal plants, in particularAlnus spp., in different geographic regions. The importance of a host-dependent distribution ofRhizobium andFrankia spp. for agriculture and forestry is discussed.