Factors affecting co-inoculation with antibiotic-producing bacteria to enhance rhizobial colonization and nodulation

Abstract

Co-inoculation with antibiotic-producing bacteria and rhizobia resistant to those antibiotics has been proposed as a means of promoting colonization and nodulation of legumes by root-nodule bacteria. A study was conducted to establish some of the factors affecting co-inoculation with antibiotic-producing strains of Bacillus and Streptomyces griseus. The stimulation of Rhizobium meliloti and yield and N uptake by alfalfa was enhanced with increasing inoculum size of Bacillus sp. S. griseus and chitin added to soil increased nodulation of soybeans by Bradyrhizobium japonicum and increased nodulation, yield, and number of pods on a second crop grown in the same soil. Bacillus sp. persisted in soil in sufficient numbers for at least 51 days to increase colonization of soybean roots by B. japonicum. The populations of S. griseus, Bacillus sp., and antibiotic-resistant isolates of R. meliloti and B. japonicum fell after their addition to seeds. Nevertheless, a benefical effect by the antibiotic-producing bacteria was evident on R. meliloti colonization of the rhizosphere, nodulation, and yield of alfalfa grown from seeds stored 94 days and on B. japonicum colonization, nodule number, yield, and seed weight of soybeans grown from seeds stored 90 days. Because non-antibiotic-producing derivatives of Bacillus sp. and S. griseus did not promote colonization or nodulation of alfalfa roots by R. meliloti, the benefit of this co-inoculation is a result of antibiotic formation.