XVI International Botanical Congess

Nodulation is a process whereby rhizobia infect legume roots, trigger nodule formation, fix atmospheric N2 and convert it to NH4+ which is available to the plant. The bacterial establishment and functioning of the nodule appear to be under the control of ethylene, a plant hormone. The role of C2H4 in nodulation will be reviewed and new insights into the problem will be presented by introducing R50, a nodulation pea mutant. It has been known for some time that C2H4 is a nodulation inhibitor, but it is only recently that it has been shown to act at the boundary between epidermis and outer cortex and during nodule primordium formation (Guinel and LaRue, 1991, 1992). Using molecular tools, Heidstra et al. (1997) have shown that C2H4 is likely to be produced in the cortex facing phloem poles where it probably suppresses nodule formation. C2H4 role in nodulation regulation was further emphasized by Penmetsa et al. (1997) who isolated a C2H4-insensitive Medicago mutant able to hypernodulate. Another role proposed for C2H4, in Sesbania, is that of determining the fate of nodule development (Fernandez-Lopez et al., 1998). R50 opens new avenues to the study of the roles of plant hormones since, in this mutant, C2H4 and cytokinin appear to be linked components of the signaling pathway controlling rhizobial infection.