Plant gene expression in early stages of Rhizobium-legume symbiosis.

Upon infection of the root of a leguminous plant with Rhizobium or Bradyrhizobium bacteria, root nodules are formed. During root nodule formation a set of genes, called nodulin genes, is specifically expressed. Based upon the stage of nodule development in which they become expressed, they are divided into early and late nodulin genes. Early nodulin genes become expressed just before or during infection and during the formation of the nodule structure. Late nodulin genes are first expressed at the onset of dinitrogen fixation, when the nodule structure is essentially ready. The role of plant and bacterial genes involved in root nodule morphogenesis from stages preceding actual infection, through infection until the start of dinitrogen fixation is reviewed.