RENODULATION AND CHARACTERIZATION OF RHIZOBIUM ISOLATES FROM CICER MILKVETCH (ASTRAGALUS-CICER L)

In 1993 and 1994, 12 bacterial isolates were isolated from root nodule s of cicer milkvetch (Astragalus cicer). In the tests for nodulation o f A. cicer by these bacterial isolates, five were found to form hypert rophic structures, while only two formed true nodules. These true nodu les were formed in a sterilized soil system. This system might be able to act as a DNA donor to provide residual DNA to other microbes in th e soil. The rhizobial isolates were thought to have lost genetic mater ial crucial to nodulation during the isolation process. This hypothesi s was supported by an experiment in which isolate B2 was able to nodul ate A. cicer in vermiculite culture after being mixed with heat-killed rhizobia, Rhizobium leguminosarum by. trifolii and R. loti. The nodul ation would not occur in vermiculite culture system without the heat-k illed rhizobia. Based on the biochemical data, the B2 and 9462L, which formed true nodules with A. cicer, were closely related. The rhizobia type cultures that nodulate A. cicer include Bradyrhizobium japonicum , Rhizobium leguminosarum by. trifolii, R. leguminosarum by. viceae, a nd R. loti. All of these rhizobia were from different cross-inoculatio n groups. The B2 and 9462L isolates could only nodulate Medicago sativ a, Phaseolus vulgaris, and Melilotus officinalis, but not these specie s within the genus from which they were isolated: Astragalus. The trad itional cross-inoculation group concept obviously does not fit well in the classification of rhizobia associated with Astragalus. The rhizob ia isolated from A. cicer can be quite different, and the rhizobia abl e to renodulate A. cicer also quite diverse.