Submitted to: Canadian Journal of Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/9/2000
Publication Date: 8/15/2000
Citation: Oehrle, N.W., Karr, D.B., Kremer, R.J., Emerich, D.W. 2000. Enhanced attachment of Bradyrhizobium japonicum to soybean through reduced root colonization of internally-seedborne microorganisms. Canadian Journal of Microbiology. 46(7):600-606. Interpretive Summary: Many bacteria are found inside healthy soybean seeds, some of which potentially can be detrimental toward other root microorganisms that are beneficial to soybean growth. We developed a method to reduce growth of these seedborne bacteria on germinating soybean seeds and seedling roots and assessed the effect of this method on the activity of a beneficial bacterium. The beneficial bacterium Bradyrhizobium japonicum, or rhizobium, when enclosed in special structures (nodules) on roots, makes atmospheric nitrogen available to soybean, which eventually uses the nitrogen to form proteins. Seedborne bacteria that grow on soybean roots often interfere with the growth of rhizobium, which leads to reduced nodule growth and ultimately to low protein production in the plant. In this study, we discovered that treating seeds with the widely available antibiotic penicillin virtually eliminated growth of seedborne bacteria on soybean seedling roots and increased the numbers of rhizobium growing on the roots by over three times, assuring optimum protein production. Scientists, producers, and the seed industry can use this information for improving seed treatment methods to enhance the natural process of fixing atmospheric nitrogen for soybean nutrient demands and protein production. Also, use of these methods could reduce use of chemical nitrogen fertilizers, a potential contributor to reduced environmental quality and a needless expense in soybean production.
Technical Abstract: Internal-seedborne microorganisms are those surviving common surface- sterilization procedures. These microbes often colonize the surface of the radicle of a germinating soybean (Glycine max) seed, introducing an undefined parameter into studies on attachment and infection by Bradyrhizobium japonicum. Bacterial isolates from surface-sterilized soybean seed, cv Williams 82 and cv Maverick, were identified as Agrobacterium radiobacter, Aeromonas sp., Bacillus spp., Chryseomonas luteola, Flavimonas oryzihabitans, and Sphingomonas paucimobilis. Growth of these microbes during seed germination was reduced by treating seeds with 500 ug/ml Penicillin G. The effects of this antibiotic on seedling development and on B. japonicum 2143 attachment, nodulation, and nitrogen fixation were studied. Penicillin G treatment did not reduce seed germination or root tip growth, and did not affect seedling development. No differences in nodulation kinetics, nitrogen fixation onset or rates were observed. However, the number of B. japonicum attached to treated intact seedlings was enhanced 250 to 325%, demonstrating that other root-colonizing bacteria can interfere with rhizobial attachment. Penicillin G treatment of soybean seeds reduces the populations of root colonizing microbes that survive standard seed sterilization without affecting plant development.