Abstract:

The symbiosis between leguminous plants and rhizobia leads to the formation of a novel root organ, the nodule. In mature nodules, rhizobia provide the host plant with ammonium, which is produced through bacterial nitrogen fixation. The symbiotic interactions involve the perception of bacterial signaling factors called Nod factors (NFs) by plant host, the NF signaling pathway, the formation of infection threads and the development of nodule in the cortex. Although this nodule formation is beneficial for host plants to secure a nitrogen resource, overproduction of nodules could deleteriously affect plant growth. Legume plants avoid this by utilizing a negative feedback regulation known as autoregulation of nodulation (AON), in which earlier formed nodules suppress further nodulation through shoot-root communication. We summarize nodule formation and types and highlight recent studies on the molecular basis underlying NF signaling cascade, AON and effects of environmental nitrogen conditions on nodulation. We also discuss current research problems and reflect on the future of this field.

Key words: legumes, nodule formation, nodulin genes, signaling, symbiotic nitrogen fixation