**6. Soybean: A model species for legume research**

**Figure 2.** Soybean plants growing in a field in Toowoomba, Queensland, Australia. Mutants unable to form nodules

A Comprehensive Survey of International Soybean Research - Genetics, Physiology, Agronomy and Nitrogen

The AON pathway involves long-distance root-shoot signaling initiated during nodule de‐ velopment by the synthesis of a root-derived signal (Gresshoff and Delves, 1986; Delves et al, 1986; Reid et al., 2011a). Recent work has indicated that this signal is likely a CLV3/ESRrelated (CLE) peptide(s) hormone (Okamoto et al., 2009; Mortier et al., 2010; Reid et al., 2011b; Lim et al., 2011). In soybean, these CLE peptides are called Rhizobia Induced CLE1 (RIC1) and RIC2 (Reid et al 2011b; Lim et al., 2011). Grafting and over-expression experi‐ ments have shown that these signals travel to the shoot (Delves et al., 1986; Reid et al., 2011b), likely via the xylem, where they, or a product of their action, are perceived by a LRR receptor kinase, called the Nodulation Autoregulation Receptor Kinase (NARK) in soybean (*e. g.* , Searle et al., 2003). NARK may act in a complex with other receptors, such as CLAVA‐ TA2 and KLAVIER (Miyazawa et al., 2010; Krusell et al., 2011). This perception results in the production of a novel Shoot-Derived Inhibitor (SDI). The SDI signal subsequently travels from the shoot back down to the roots, likely via the phloem, where it acts to inhibit further nodulation events (Reid et al 2011a). It has recently been established in soybean that SDI is

stature as a result of investing too much energy into forming nodule structures.

) are stunted and pale compared with wild-type (WT) plants due to their inability to establish a symbiotic interac‐ tion with nitrogen-fixing *Bradyrhizobium japonicum*. Supernodulating mutants (nod++) are significantly stunted in

(nod-

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Soybean has been the subject of a great deal of research in an effort to identify unique traits and to isolate superior cultivars offering improved growth and yields (Gresshoff 2012). It al‐ so represents an excellent model species for legumes in general (Ferguson and Gresshoff 2009), with outcomes frequently extrapolated to the other important food and feed legume crops, such as bean, pea, chickpea, faba bean, lentil, peanut, clover and lucerne (*e. g.* , Rispail et al., 2010).

Soybean represents one of the best characterized legumes species, both physiologically and biochemically. It grows quickly, is high yielding, and has a size and stature that are well suited for most field and laboratory studies (Figure 3). Its relatively large size enables the harvest of large quantities of tissues and it is ideal for studies involving grafting (*e. g.* , Delves et al., 1986; Reid et al., 2011b), xylem sap analyses (*e. g.* , Djordjevic et al., 2007), *Agro‐ bacterium rhizogenes*-mediated transformation for gene over-expression and RNA interfer‐ ence (*e. g.* , Kereszt et al., 2007; Reid et al., 2011b; Lin et al., 2011b) and Virus-Induced Gene

Silencing (VIGS) for functional genomics approaches (*e. g.* , Zhang and Ghabrial, 2006). Fur‐ thermore, soybean has a large germplasm, including vast mutant (Figure 2; *e. g.* , Carroll et al., 1985; Bolon et al., 2011) and TILLING populations (*e. g.* , Cooper et al., 2008; Batley et al., 2012).

**Figure 3.** Glasshouse grown soybean plants 1 and 3 weeks after germination. The fast, uniform growth of soybean, together with the availability of its genome sequence and its amenability to most physiological, molecular and bio‐ chemical analyses makes it an ideal model species for legume research.

Recently, the soybean genome was sequenced (Schmutz et al., 2010) and complete transcrip‐ tome analyses have been performed, including the generation of transcriptome atlases (Li‐ bault et al., 2010a,b; Severin et al., 2010; Reid et al., 2012; Hayashi et al., 2012). Together, these resources provide an efficient non-targeted tool to identify new genes and patterns of gene expression. Analyses between the genome of soybean and those available for other le‐ gumes species, including *M. truncatula*, *L. japonicus*, pigionpea and bean, also provide an ex‐ cellent opportunity for comparative legume genomics (Cannon et al., 2009).

Understanding the genes and genomes of legumes will help to establish elite cultivars that benefit sustainable farming practices. Integrating central regulators of nodulation is essential for such targeted legume crop improvement. Indeed, outcomes derived via soybean re‐ search could help to underpin future advances in managing the legume-rhizobia symbiosis (*e. g.* , Rispail et al., 2010). This could lead to improved nitrogen use efficiency and reduced nitrogen-fertilizer inputs, thus helping to reduce the monetary and environmental costs as‐ sociated with nitrogen-fertilizer use.
