Nodule organogenesis is triggered by the bacterial Nod signal molecule the Rhizobium-legume symbiosis. Plant early induced genes participating in the Nod signalling cascade are most likely of low abundance and therefore, have been difficult to identify by usual cDNA libraries which have allowed cloning of the most abundant mRNAs. To overcome this limitation, we will then use the differential display retrotranscriptase PCR technique that allows to separate and clone individual mRNAs by means of the sensitive PCR reaction. On the basis of the facts tha. early nodulins can be induced by cytokinins we will compare the RNA populations derived from Medicago roots treated with either active or inactive Nod factor to those treated with cytokinin. Amplified cDNA fragments corresponding to differentially expressed mRNAs are shown to be will be analysed by sequencing and identification of full length genes in the Medicago cDNA or genomic library.