Mimosine, a toxin produced by the tree-legume leucaena provides a nodulation competition advantage to mimosine-degrading rhizobium strains

The trees and shrubs of different Leucaena spp. contain a non-protein amino acid, mimosine which is toxic to animals and microorganisms. We found small amounts of mimosine in the nodules and the root exudate of Leucaena leucocephala. Some Leucaena-nodulating strains of Rhizobium degrade mimosine (Mid+) and use it as a source of nutrients. To determine if the ability to catabolize mimosine by Rhizobium has a role in symbiosis, we constructed mutants that cannot degrade mimosine (Mid−) from a Mid+ competitive strain and tested them for growth in the presence of mimosine. When mimosine was added to a complete medium, it stimulated the growth of the Mid+ wild-type strain but inhibited the growth of a mutant in which the gene encoding a mimosine-degrading enzyme was disrupted. L. leucocephala and Phaseolus vulgaris plants inoculated with these mutants formed effective nodules showing that mimosine catabolism by the microsymbiont has no direct effect on symbiosis. When mixtures of the wild-type strain and the Mid− mutants were used to inoculate L. leucocephala and P. vulgaris seedlings, most nodules in L. leucocephala were occupied by the wild-type strain, whereas, the nodule occupancies in P. vulgaris were proportional to the inoculant mixtures. These results show that the Mid− mutants are defective in nodulation competition on Leucaena but not on beans. This indicates that mimosine present in Leucaena provides a nodulation competition advantage to the mimosine degrading Rhizobium strains.