1992 Volume 38 Issue 4 Pages 327-342
Plasmid pRt032 which has a 14-kb HindIII fragment containing nodulation genes from Rhizobium leguminosarum bv. trifolii ANU843 was transferred into R. fredii USDA 193. A transconjugant CT24 was isolated whose host range was extended to white clover. However, nodulation on the original host soybean was strongly inhibited in CT24 as compared with that of wild-type R. fredii USDA 193. The inhibition of nodulation on soybean was investigated with R. fredii USDA 193 carrying a derivate of pRt032 (nod218) in which the lac operon and Tn5 (MudI1734) was inserted downstream of the nodA promoter. This strain, AF218, restored efficient nodulation on the soybean.
The NodD-mediated induction of the R. leguminosarum bv. trifolii nodA-lacz fusion in R. fredii USDA 193 was also monitored during the growth cell cycle following exposure to 7, 4′-dihydroxyflavone (DHF), clover extracts, and soybean extracts for 3h. The nodA induction was observed in ratios of 11:7:3, respectively. However, nodA induction was inhibited using 7, 4′-dihydroxyisoflavone (daidzein), an inducer of R. fredii nod genes.
The factor that induces the R. leguminosarum bv. trifolii nod gene expression in soybean extract is not the daidzein, but other unidentified plant factors. In strain CT24, heterologous common nod genes are expressed. These genes interfere with R. fredii nod gene products, causing poor nodulation on its original host soybean.