CONFORMITY AND DIVERSITY AMONG FIELD ISOLATES OF RHIZOBIUM-LEGUMINOSARUM BV-VICIAE, BV-TRIFOLII, AND BV-PHASEOLI REVEALED BY DNA HYBRIDIZATION USING CHROMOSOME AND PLASMID PROBES

A study was made of 113 bacterial isolates from root nodules of peas, lentils, red clover, and French beans, which had been grown in the same soil. Plasmid band profiles visualized in Eckhardt gels were analysed in relation to DNA hybridization patterns obtained by probing restricted total cellular DNA in Southern blots. Rhizobium leguminosarum chromosomal probes (plac12, pCOS309.1) and various symbiotic plasmid (nod gene region) probes were used. Dominant plasmid DNA hybridization patterns and more frequent combinations of plasmid patterns and chromosomal types were found among the isolates of each host plant species; the occurrence of alternative combinations indicated that genetic transfer and recombination among members of this soil population had taken place. About 40% of all isolates belonged to the same chromosomal type. Isolates of the same chromosomal type were often found with cryptic plasmids of the same size in different host plant species. Although isolates could not be assigned to their respective plant host groups using chromosomal probes alone, this was generally possible using symbiotic plasmid probes and the results were in complete accordance with plant tests. However, there was a group of bean isolates in which no homology to any of the R. leguminosarum probes was detected under the conditions of high stringency used. Other exceptional isolates of beans conformed in probe tests and subsequent plant host specificity tests better to biovars viciae or trifolii than to biovar phaseoli; thus, the nodulation of beans (i.e., Phaseolus vulgaris) in the field appears less subject to stringent control of specificity than that of other host plant species. It was also noted that the nod gene regions probed showed greater diversity in isolates of biovars viciae and trifolii than in biovar phaseoli.