THE RHIZOBIUM-LEGUMINOSARUM BIOVAR VICIAE NODO GENE CAN ENABLE A NODE MUTANT OF RHIZOBIUM-LEGUMINOSARUM BIOVAR TRIFOLII TO NODULATE VETCH

Analysis of the nodulation characteristics of transposon-induced mutants of Rhizobium leguminosarum bv. viciae revealed that nodO and the closely-linked rhi genes contribute to nodulation of peas (Pisum sativum) and the vetch Vicia hirsuta. Although mutation of nodO alone had no significant effect on nodulation of either legume. a double mutant lacking both nodO and nodE nodulated both legumes very poorly. Similarly, a double mutant lacking nodE and either rhiA or rhiB nodulated peas less efficiently than a nodE mutant. Thus, although mutations affecting only the rhi genes normally have no observed effect on nodulation, these genes do appear to contribute to pea nodulation. When transferred to a wild-type strain of Rhizobium leguminosarum bv. trifolii, neither nodO nor the rhi gene region conferred pea or vetch nodulating ability. However, in a nodE mutant of R. I. bv. trifolii, nodO did confer a significant level of vetch nodulating ability, indicating that the secreted NodO protein can play a role in determining legume recognition by R. I. bv. viciae.