Signal Transduction in Rhizobium-Induced Nodule Formation

During a deficiency of reduced nitrogen sources, legu- minous plants are able to live symbiotically with bacteria of the genera Rhizobium, Bradyrhizobium, or Azorhizobium (here collectively called rhizobia). The interaction starts with an attachment of the rhizobia to the plant root hairs. The plant enables the bacteria to invade its roots by the formation of tubular structures, the infection threads, that grow through the root hairs into the cortex. Simulta- neously, cortical cells are mitotically activated, giving rise to a nodule primordium. Infection threads grow toward the primordium. Upon arrival, the bacteria are released into the cytoplasm of the primordial cells and become surrounded by a plant-derived peribacteroid membrane. The nodule primordium develops into a mature nodule, and the bacteria differentiate into their endosymbiotic form, the bacteroids. These bacteroids are able to fix nitro- gen into ammonia, which can subsequently be utilized by the plant. The establishment of a nodule is thus achieved collabo- ratively from both participants and it can be anticipated that a flow of signals is being transmitted between plant and bacteria; this Update highlights the present knowledge of this communication.