Microscopic analysis of the effect of Rhizobium leguminosarum biovar trifolii host specific nodulation genes in the infection of white clovers

A microscopic assessment is presented of the comparative infection capacity of wild-type and hybrid strains of Rhizobium leguminosarum bv. viciae with R. l. bv. trifolii strain ANU 843 on white clover seedlings. The R. l. bv. viciae hybrid strains contained defined DNA segments coding for different combinations of R. l. bv. trifolii host-specific nodulation genes. White clover plants were examined over a 72 h period to assess Rhizobium infectivity, the morphological changes in root hair growth; colonisation ability of rhizobia; infection thread initiation and the ability to induce cortical cell division. R. l. bv. viciae strain 300 induced root hair curling more slowly than strain ANU 843 or any of the hybrid strain 300 bacteria, and when curling had taken place, there was poorer colonization by strain 300 within the folded hair cell, no evidence of infection thread formation and only limited cortical cell division 72 h after inoculation. The addition of the host-specific nodulation genes of R. l. bv. trifolii to strain 300 was necessary to induce infection threads and establish a normal pattern of nodulation of the roots of white clovers.