Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/5/2000
Publication Date: 9/2/2000
Citation: JIANG, G., KRISHNAN, H.B. SINORHIZOBIUM FREDII USDA257, A CULTIVAR-SPECIFIC SOYBEAN SYMBIONT, CARRIES TWO COPIES OF Y4YA AND Y4YB, TWO GENES THAT ARE LOCATED IN A REGION THAT ENCODES THE TYPE III PROTEIN SECRETION SYSTEM. MOLECULAR PLANT-MICROBE INTERACTIONS. V. 13(9). P. 1010-1014. Interpretive Summary: A rhizobium called Sinorhizobium fredii forms nodules on the roots of soybean plants. The nodules are specialized structures where atmospheric nitrogen is fixed by the bacterium, which in turn, is utilized by the soybean plants for growth and development. This process is termed biological nitrogen fixation and it enables soybean plants to grow in nitrogen-poor soils. We studied why some combinations of soybean cultivars and Sinorhizobium yield nitrogen-fixing root-nodules, while others do not. This information will enable manipulation of biological nitrogen fixation so that farmers can increase the yield of soybean with minimal use of nitrogen fertilizers.
Technical Abstract: Sinorhizobium fredii USDA257 forms nitrogen-fixing nodules on primitive soybean [Glycine max (L.) Merr.] cultivar 'Peking' but fails to nodulate improved cultivar 'McCall.' The cultivar-specificity is governed by a plasmid-borne locus, nolXBTUV. By DNA sequence analysis, we have identified two open-reading frames, y4yA and y4yB, immediately downstream of nolX. Northern blot analysis indicated that the expression of the y4yA and y4yB is inducible by isoflavonoids and an intact copy of nolX is required for their expression. Two copies of y4yA and y4yB were present in S. fredii USDA257, one of them on the sym plasmid, and the other on the chromosome. Only the plasmid copy of y4yA and y4yB was found in the cultivar non- specific strain USDA191. Introduction of the chromosomal copy of y4yA and y4yB from USDA257 into USDA191 did not influence the ability of the latter strain to nodulate McCall soybean. Unlike nolX, the inactivation of y4yA and y4yB of USDA257 did not extend the host range of this strain. We have cloned both the chromosomal and plasmid copies of the y4yA and y4yB from USDA257 and determined their nucleotide sequence. nolX, which is located immediately upstream of y4yB in the plasmid copy, was missing in the chromosomal copy. Single mutants, with deletions in the plasmid or chromosomal copies of y4yA and y4yB, or double mutants, where both the plasmid and chromosomal copies were mutated, had no observable effect on symbiotic ability of these strains. Unlike the nolX mutant, the y4yA and y4yB mutants did not influence the flavonoid-dependent, extra-cellular protein production. DNA sequences homologous to S. fredii USDA257 y4yA and y4yB were found in Rhizobium sp. NGR234 and S. saheli USDA4893, but were absent in Bradyrhizobium spp., the traditional symbiont of soybean.