Casuarina glauca prenodule cells display the same differentiation as the corresponding nodule cells

Recent phylogenetic studies have implied that all plants able to enter root nodule symbioses with nitrogen-fixing bacteria go back to a common ancesto r (D. E. Soltis, P. S. Soltis, D. R. Morgan, S. M. Swensen, B. C. Mullin, J . M. Dowd, and P. G. Martin, Proc. Natl. Acad. Sci. USA, 92:2647-2651, 1995 ), However, nodules formed by plants from different groups are distinct in nodule organogenesis and structure. In most groups, nodule organogenesis in volves the induction of cortical cell divisions. In legumes these divisions lead to the formation of a nodule primordium, while in non-legumes they le ad to the formation of a so-called prenodule consisting of infected and uni nfected cells. Nodule primordium formation does not involve prenodule cells , and the function of prenodules is not known. Here, we examine the differe ntiation of actinorhizal prenodule cells in comparison to nodule cells with regard to both symbionts, Our findings indicate that prenodules represent primitive symbiotic organs whose cell types display the same characteristic s as their nodule counterparts. The results are discussed in the contest of the evolution of root nodule symbioses.