Expression profiles of 22 novel molecular markers for organogenetic pathways acting in alfalfa nodule development

During symbiotic nodule development, a variety of molecular signals of rhiz obia and plant origin are likely to be involved in the control of the expre ssion of specific genes in the legume Medicago sativa (alfalfa), Twenty-two new, nodule-associated expressed sequence fags (ESTs, MsNod clones) as wel l as 16 clones for previously reported alfalfa nodulins were identified by cold-plaque screening, Protein homologs were found for 10 of the 22 MsNod-e ncoded polypeptides, revealing putative novel functions associated with thi s symbiosis, Expression of these MsNod genes was investigated in spontaneou s nodules (generated in the absence of bacteria), in nodules induced by a S inorhizobium meliloti wild-type strain and Eps(-) and Bac(-) mutant derivat ives, as well as in roots inoculated with a Nod(-) mutant strain. This anal ysis enabled us to correlate plant gene expression with the different stage s of nodule ontogeny and invasion. The effect of phytohormones on MsNod gen e expression was analyzed in cytokinin- and auxin-treated alfalfa roots. Cy tokinin induced the accumulation of seven MsNod transcripts, four of them w ere also regulated by the synthetic auxin 2,4-D (2,4-dichlorophenoxyacetic acid). Comparison of MsNod expression profiles in wild-type and transgenic M. truncatula roots overexpressing the early nodulin Enod40 suggested that one clone, the M. sativa L3 ribosomal protein homolog (MsNod377), is a puta tive component of an Enod40-dependent pathway acting during nodule developm ent. These novel molecular markers may help in the investigation of gene ne tworks and regulatory circuits controlling nodule organogenesis.