Figure 3.

Biological nitrogen fixation in leguminous plants. (A) Genes involved in nodule initiation, development and signal recognition pathway. (B) Protein sequence alignment of Nod related genes identified in A. ipaensis and A. duranensis. (C) Phylogenetic tree of nodule development genes and their homologs from A. ipaensis and A. duranensis. (D) Identification of high conserved domains of leucine-rich repeat (LRR) receptor kinases. Red dashed boxes represent LRR conserved motif. (A) The rhizobium (blue) attach to the surface of root hair cell. After swelling, deformation, curling and infection thread, the bacteria are released into cells via endocytosis then a vacuole-like structures (symbiosomes), in which the bacteria convert N₂ to NH₃, formed. But how is the Nod signal transmitted? Initially, the rhizobia-derived signal is perceived by LysM-type protein receptor kinases, such as NRF1 and 5 (Radutoiu et al., 2003) and SYM10 (Schneider et al., 2002) identified in L. japonicus and P. sativuml, followed by a downstream leucine-rich receptor kinase, for example SYMRK (Stracke et al., 2002 and Capoen et al., 2005), NORK (Endre et al., 2002), DMI2 (Catoira et al., 2000), and SYM19 (Stracke et al., 2004) from L. japonicus, Sesbania rostrata, M. sativa, M. truncatula, and P. sativuml, respectively. Then, the Nod factor (NF) signal is processed through a signal transduction cascade involving proteins including ion channels [MDI1(Ané et al., 2004), CASTOR (Imaizumi-Anraku et al., 2005), POLLUX (Imaizumi-Anraku et al., 2005), and SYM8 (Edwards et al., 2007)], calcium-calmodulin-dependent kinase (CaCaDK) (MDI3 and SYM9) (Lévy et al., 2004) and transcription factors [NSP1 (Smit et al., 2005), NSP2 (Kaló et al., 2005), SYM7 (Kaló et al., 2005), NIN (Schauser et al., 1999), and SYM35 (Borisov et al., 2003)]. Finally, rhizobia infection occurred primarily through uncharacterized target genes that may be activated by these TFs.