Factors controlling root nodule induction by Rhizobium meliloti

The establishment of nitrogen-fixing root nodules by Rhizobium requires multiple interactions between the bacterium and the plant. These interactions involve transmission of signal molecules, activating sets of genes determining symbiotic nodule development in the partner organism. This type of signaling between the host plant and its microsymbiont has been recently elucidated for the earliest interactions leading to the induction of root nodules (for reviews see Kondorosi and Kondorosi 1986; Rolfe and Gresshoff 1988). It was demonstrated that the host plants exude different flavonoids and isoflavones and some of these compounds activate (or inhibit) the expression of nodulation (nod) genes in Rhizobium. The activated nod genes are then involved in determining the production of diffusible signal molecules, evoking the earliest plant responses, root hair curling and division of cortical root cells. In this paper we discuss some aspects of the genetic control and specificity of nodule induction and signaling by R. meliloti.