Nodulation of Rhizobium fredii USDA192 containing Rhizobium leguminosarum bv. trifolii ANU843 nod genes on homologous host soybean and heterologous host clover

Abstract

Plasmids pRt032 and pRKR9032, both carrying a 14 kb HindIII fragment from Rhizobium leguminosarum bv. trifolii ANU843 that harbours genes essential for clover modulation, were transferred by conjugation to Rhizobium fredii USDA192 which nodulates soybean roots. The transconjugant strains NA102 and YA101 induced nodules on clover roots in addition to soybean roots. This fact indicates that conjugal transfer of the nodulation genes of R. l. bv. trifolii to R. fredii USDA192 extended the host range of R. fredii USDA192 to clover, the host of R. l. bv. trifolii. However, the clover nodules induced by the transconjugants were small, whitish and nitrogen fixation was not detected. Moreover nitrogen fixation activity also could not be detected in the soybean nodules induced by the transconjugant strain NA102. The Nod factors produced by transconjugants in the presence of flavonoids such as apigenin and genistein differed from those of their recipient strains. These results indicate that the unusual Nod factors may cause the induction of ineffective nodules.