抄録
In the endosymbiosis with nitrogen-fixing bacteria, rhizobia, legume plants form lateral organs called nodules on their roots. Although this symbiosis gives legumes an advantage in surviving on the land with limited nitrogen, nodule development associated with cell division and nitrogen-fixation cost appreciable amounts of morphological and metabolic energy. To keep the balance of symbiosis and homeostasis, the number of nodules is regulated by host plants. Nodules developed at early stage suppress nodule formation at later stage. This systemic negative regulation of nodule formation has been termed autoregulation of nodulation (AON). One of most important mechanisms of AON involves long-distance signaling via shoot-functioning receptor-like kinases and proteins (RLKs and RLPs) such as HAR1, KLV and LjCLV2 in Lotus japonicus, SUNN in Medicago truncatula, NARK in Glycine max, and SYM29, PsCLV2 in Pea sativum. These RLKs and RLPs have high homologies with other RLKs that regulate stem cell proliferation in shoot apical meristem in non-legume plants, indicating similarity and functional divergence of molecular mechanisms between the regulation of nodulation in legumes and the regulation of shoot apical meristem in non-legume plants. Based on recent studies, the following model is proposed to explain AON. First, several CLE peptides are induced by rhizobial infection or nitrogen compounds and function as root-derived signals (RDS). RDSs are transported to shoot and perceived by shoot-functioning RLKs and RLPs. Then the signals are converted into shoot-derived signals (SDS) and SDSs are in turn transported to root to suppress further nodulation.
