Nodulation in the absence of Rhizobium

A wide variety of legumes are able to establish symbiotic association with nitrogenfixing Rhizobium and Bradyrhizobium bacterial species. Through a complex series of interactions these soil bacteria infect the roots of the host plant and induce the formation of a specialized organ, the nodule (Caetano-Anollés and Gresshoff, 1991). To initiate this process the host root secretes substances that act as chemoattractants and inducers of gene expression in the bacteria. In turn, the bacteria move towards and associate with the host root surface, secrete factors that alter the pattern of the original plant inducers and release nodulation signals that elicit the first two visible morphological responses in the host, root hair curling and cortical cell division. The bacteria are then entrapped between cell wall surfaces, usually by the pronounced curling of a root hair, and penetrate through the underlying host cell wall to produce a tubular structure called the infection thread. The continuous deposition of cell wall-like material around the bacterial cells and their multiplication results in elongation and branching of the infection thread towards the dividing cortical root cells. In the meantime, the dividing cortical cells give rise to a meristematic center beneath the infection site, the nodule primordium. The bacteria are finally released into dividing host cells and packaged within host plasmamembrane where they differentiate into bacteroids and fix atmospheric nitrogen into ammonia in exchange for fixed carbon and other nutrients.