Mapping of QTLs associated with biological nitrogen fixation traits in soybean

摘要

Nitrogen (N), the element most limiting for crop growth, is usually supplied by application of fertilizer, but at substantial costs to farmers and with potentially adverse effects on the environment. Soybean Glycine max (L.) Merr., among other legumes, can obtain N through biological nitrogen fixation (BNF), and breeding for yield improvement can be performed considering BNF, inorganic-N supply, or both (Coale et al. 1985; Kumudini et al. 2008). The BNF results from the symbiosis between legumes with a diverse group of nitrogen fixing soil bacteria collectively known as rhizobia. This association is mutually beneficial: while bacteria provide a source of nitrogen to plant development, plants provide carbon sources for maintaining bacteria metabolism.The establishment of the symbioses involves complex mechanisms starting with the mutual exchange of diffusible signal molecules. Plants secrete seed and root molecules – mainly flavonoids – that are sensed by specific rhizobia. In response, the rhizobia produce lipochitooligosaccharides namely Nod factors (NFs) that are recognized by LysM receptor-like kinase of the legumes. A new organ – the nodule – is then formed on the root and hosts the nitrogen fixing bacteria (Subramanian et al. 2006; Kouchi et al. 2010).The elicitation and the development of an effective nodule are accompanied by the expression of specific genes in both rhizobia and their host-plants. The genetic mechanisms of the bacteria are best understood and several rhizobial genes called nod, nol and noe have been described (Masson-Boivin et al. 2009). On the other side, studies with the host-plants are more complex, due to biological characteristics and genome sizes. More recently, the integration of genetic and genomic approaches and the adoption of legume models with Medicago truncatula and Lotus japonicum opened a framework to the investigation of host genes essential for the BNF process. Twenty-six genes involved with Nod perception and different steps of nodule formation were characterized based on mutants, cloning and mapping strategies (Kouchi et al. 2010). Furthermore, dozens of genes involved in nodule formation and in the BNF process called nodulin genes have been prospected by high throughput tools, such as expressed sequence tags and microarray analysis (Brechenmacher et al. 2008; Libault et al. 2010).In soybean, several loci controlling nodulation have been described since the 1950s. Williams and Lynch (1954) reported the recessive gene rj1 in a spontaneous non-nodulating mutant. Genes rj2 (Caldwell 1966), rj3 (Vest 1970) and rj4 (Vest and Caldwell 1972) have natural occurrence and control strain specific restricted nodulation. In addition, genes rj5 and rj6 (Harper and Nickell 1995) – conferring non-nodulation ability – have been identified by chemical induced mutation and correspond to the same loci of the rj1. Supernodulating mutants (rj7 and rj8) have also been generated by chemical mutagenesis (Vuong et al. 1996).Some of these genes controlling nodulation in soybean have been cloned and their products have been described. The first gene related to nodulation control in soybean was cloned by Searle et al. (2003) using the rj7 and rj8 supernodulating mutants. By similar approaches the Nod factor receptor genes called NFR1α and NFR1β (Indrasumunar et al. 2011) and NFR5α and NFR5β (Indrasumunar et al. 2010) were cloned. Similar genes were first identified by mutant directed cloning in Medicago truncatula (Amor et al. 2003; Limpens et al. 2003), L. japonicus (Madsen et al. 2003; Radutoiu et al. 2003) and pea (Pisum sativum) (Zhukov et al. 2008).These discoveries provide important clues to understand the molecular mechanisms of host regulation of the symbiosis. However, more information with field orientation is necessary to access the potential of breeding for BNF. The occurrence of natural variability on nodulation and efficiency of BNF in soybean have been reported (Neuhausen et al. 1988; Sinclair et a