Effect of mutations in aromatic catabolism on the persistence and competitiveness of Rhizobium leguminosarum bv. trifolii
1994
Rynne, F.G. | Glenn, A.R. | Dilworth, M.J.
Rhizobium leguminosarum bv. trifolii WU95 degrades aromatic compounds only via the protocatechuate branch of the 3-oxoadipate pathway. Transposon Tn5-233-induced mutants of WU95 defective in 4-hydroxybenzoate hydroxylase or protocatechuate 3,4-dioxygenase or 3-oxoadipate succinyl-CoA transferase were symbiotically effective when used as single inoculants on Trifolium subterraneum cv. Seaton Park. Since metabolism of all types of aromatic compounds was therefore blocked in some of these strains, they allowed us to test whether access to phenolic compounds in soil was important for survival or competitiveness of R. leguminosarum bv. trifolii. The three mutants and the wild type were inoculated into a sandy soil either as single inoculants or in mixtures with the wild type WU95, and the soils were then planted with seedlings of subterranean clover, either immediately or after incubation in the glasshouse for 3 or 6 months. There were no consistent differences in plant parameters (including nodule number or weight per plant) with respect to the inoculant type or the length of time the inoculant had been in the soil before planting. When mixtures of WU95 plus a mutant strain containing Tn5-233 were used and the nodules typed, all of the aromatic mutants formed ca 50% of the nodules, indicating that they were equally competitive. It therefore appears that, at least for 6 months, the inability to catabolise aromatic compounds in these mutants did not affect either their survival or competitiveness. Another mutant of WU95 (MNF9070), contains Tn5-233 inserted in an unknown site as well as a spontaneous mutation to rifampicin-resistance; the latter results in very poor competitiveness, as it does in R. meliloti.
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