Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2000
Publication Date: N/A
Citation: Interpretive Summary: Farmers need new and improved crops that can help produce feed for grazing livestock. Pasture legumes are especially important because they improve pasture quality, produce steady amounts of forage during summer, and obtain much of the nitrogen they need from the atmosphere. Legumes can get nitrogen from the air because certain bacteria live in specialized structures, called nodules, on the legume roots. These bacteria are the 'work horses' that transform nitrogen gas into amino acids for the plant. We studied a new pasture legume called Kura clover, which others have found to be a long-lived, high quality forage crop. The use of Kura clover in the USA is currently limited because of difficulties in establishing the seedlings. One limitation appears to be that the seedlings are slow to form nodules on their roots, and one reason for slow nodule establishment might be the bacteria in nodules. We want to find the best bacteria for Kura clover and first had to determine whether there are many different bacteri present in our soils. In this study we determined the diversity of these bacteria in soils from Kura clover fields in North America and from one field in Russia near the area where this crop evolved. We found that the diversity of bacteria in North America was quite limited compared to the diversity found at the Russian site. Therefore, to find the best bacteria to stimulate growth of Kura clover in the USA, it is crucial that collections of these bacteria be made from Russia and neighboring areas of southeastern Europe and southwestern Asia. Such a discovery could help farmers achieve more sustainable grazing systems for milk and meat production.
Technical Abstract: Kura clover (Trifolium ambiguum M.B.) is a persistent rhizomatous forage legume, whose use in the USA is limited by establishment difficulties in part attributable to nodulation problems. In this study, soil was collected from established stands of Kura clover growing in nine diverse North American environments. Rhizobia were plant trapped using Kura clover cv. 'Endura' as host, then rhizobia from nodules fingerprinted using BOX-PCR. The diversity of isolates from North America was then contrasted to that of rhizobia from a single Caucasian environment (Russia), its center of origin. Populations were characterized using clustering methods, and genetic diversity estimated using the Shannon-Weaver diversity index. The genetic diversity of the North American populations was extremely limited, all isolates being closely related to two of the strains found in a locally available commercial inoculant. In contrast, Russian isolates formed a distinct cluster with significant internal genetic diversity. Genetic diversity index for the North American and Russian populations was 3.5 and 10.76, respectively. The implication of this and other studies is that Kura clover is highly specific in Rhizobium requirement. If the performance of this legume in the USA is to be improved, either by modifying current establishment practices or plant breeding, it is essential that these studies be paralleled by more collections and evaluation of rhizobia from its center of origin, given the extremely limited diversity of rhizobia found in North America.