Early Nodulin Expression During Spontaneous Nodulating in Alfalfa

Twenty-nine new Medicago truncatula nodulin genes, expressed during Rhizobium-induced nodule development and designated MtN1 to 29, have been identified by a subtractive hybridization approach (Gamas et al., 1996). Expression of M. sativa homologs was studied in the context of spontaneous nodulation, triggered in the absence of Rhizobium by nitrogen starvation (Truchet et al., 1989). This system provides an interesting material where nodule organogenetic processes are uncoupled from infection or nitrogen fixation processes. Total RNA was extracted from alfalfa roots harvested after 0, 3 or 14 days of nitrogen starvation and from isolated spontaneous nodules of different sizes. Expression of 18 MtN clones was studied by northern hybridization. Most of them have homologs expressed in M. sativa, and they provide useful markers associated with different steps of spontaneous nodulation. Four classes can be distinguished, depending on their kinetics of expression during spontaneous nodulation.