Nitrate sensing and signaling during soybean nodulation

Legume nodulation and subsequent nitrogen fixation represent fascinating biological systems involving development, signaling and sensing (1) . We have cloned the responsible genes for two induced non-nodulation mutations (nod49/rj1 and nod139/nn5) in soybean. They control the synthesis of the two components of the putative nodulation (Nod) factor receptor in legumes, namely two divergent LysM receptor kinases (2,3). Gene assignment was confirmed by complementation. Gene duplication and functional divergence between duplicated genes was analysed. Significantly we discovered that axenic overexpression of one component (namely GmNFR1a) led to increased acid-soil tolerance for nodulation. This supports the conclusion that Nod factor recognition and signalling are the key target for acid soil, a globally significant problem. We also recognised that localised inhibition of nodulation by nitrate results from expression of a CLV3-like CLE peptide (NIC1). Transgenic overexpression of the gene in soybean led to substantial inhibition of local nodulation, but not distant nodulation on non-transgenic roots (4). This inhibition did not occur when tested in a NARK-deficient (supernodulating) background, suggesting that NARK functions in both roots and shoots, using related peptide ligands, but subject to different regulatory clues (see diagram in refs 1 and 4.) These discoveries may have significance for future soybean improvement