SYMBIOTIC INDUCTION OF A MADS-BOX GENE DURING DEVELOPMENT OF ALFALFA ROOT-NODULES

In response to infection by Rhizobium, highly differentiated organs ca lled nodules form on legume roots, Within these organs, the symbiotic association between the host plant and bacteria is established, A puta tive plant transcription factor, NMH7, has been identified in alfalfa root nodules. nmh7 contains a MADS-box DNA-binding region and shows ho mology to flower homeotic genes. This gene is a member of a multigene family in alfalfa and was identified on the basis of nucleic acid homo logy to plant regulatory protein genes (MADS-box-cantaining genes) fro m Antirrhinum and Arabidopsis. RNA analysis and in situ hybridization showed that expression of this class of regulatory genes is limited to the infected cells of alfalfa root nodules and is likely to be involv ed in the signal transduction pathway initiated by the bacterial symbi ont, Rhizobium meliloti, The expression of nmh7 in a root-derived orga n is unusual for this class of regulatory genes.