Parasitic origins of nitrogen-mixing Rhizobium-legume symbioses. A review of the evidence.

This paper is divided into two sections. The first part (I) reviews the literature on the legume-Rhizobium association with emphasis on the processes leading to the establishment of the association. In the second part (II) it is proposed that the legume-Rhizobium association was originally necrotrophic , beginning when the free-living, nitrogen-fixing, saprotrophic Rhizobium developed the ability to infect the plant. The pre-infection events, infection processes and nodulation in the colonization of the legumes by the Rhizobium are similar to those of other parasitic associations. Likewise, the host responses to the Rhizobium entry, infection thread synthesis and bacteroid formation are comparable to those of other plants when they encounter phytopathogens . Evolutionary processes acted in the selection of biotrophy , the fine control and regulation of the extracellular enzymes of the necrotrophic Rhizobium converted the association into biotrophy . The nutritional dependence of the Rhizobium on the legume, the requirement of the plant for combined nitrogen and the Rhizobium potential to meet this requirement drove the biotrophic association into mutualism . This became possible when regulation of the nitrogen-fixing system of the Rhizobium was modified and the oxygen carrying protein leghemoglobin was acquired or evolved by the legume to enhance nitrogen fixation.