Root nodulation: a developmental program involving cell fate conversion triggered by symbiotic bacterial infection

• Nodule development involves cell fate conversion of differentiated cortical cells.
• Progress of nodule organogenesis is coordinated by a rhizobial infection process.
• Activation of a transcriptional cascade is crucial to nodule development.
• Cytokinin signaling has a key role in nodule development.

Root nodulation is a unique developmental process that predominantly occurs in leguminous plants. In this process, signaling initiated by symbiotic bacterial infection alters the fate of differentiated cortical cells and causes formation of new organs. Two qualitatively different regulatory events, namely bacterial infection and nodule organogenesis, need to be coordinated in the epidermis and cortical cells to establish proper nodule formation. Recent studies have determined the tissue-specific requirements of known symbiotic genes and also detailed a direct molecular link between the two regulatory pathways. Additionally, the detailed function of cytokinin signaling has been identified and the downstream genes have been isolated, providing greater understanding of the genetic mechanisms underlying nodule organogenesis.