Abstract
Biological nitrogen fixation (BNF) has global economic and environmental importance, but has often not been considered in soybean [Glycine max (L.) Merrill] breeding programs. Knowing the genetic diversity and structure of a population within a germoplasm represent a key step for breeding programs. This study aimed at determining the structure of the population and diversity of soybean with regard to BNF and protein content in grain. In total, 191 accessions were evaluated, including 171 commercial soybean cultivars, developed and released by public institutions and private companies in Brazil, and 20 ancestral lines. The genotypes were chosen to represent four genetic groups: 128 Brazilian public genotypes, 20 exotic, and 43 genotypes from private companies. Soybeans were genotyped with 22 SSR markers, previously described as associated with BNF and protein content. Genetic diversity was evaluated using the DARwin 5.0 software. Population structure was inferred by principal component analysis and by the STRUCTURE software. The accessions were distributed in two groups: one clustering approximately 50 % of the accessions, from Brazilian public and private companies; the other one clustering 45 % of the accessions, including Brazilian, exotic and private germoplasms. Some accessions (5 %) were not grouped in any cluster. Principal component analysis explained 29 % of the total variance and there was a tendency to cluster the accessions into two groups. Similar results were obtained with the STRUCTURE, clearly showing two subpopulations. There is variability for BNF and protein content amongst both modern germoplasms cultivated in Brazil and ancestral lines. This variability could be better explored in soybean breeding programs to improve these traits.
Highlights
A difficulty encountered in soybean [Glycine max (L.) Merrill] breeding programs is how to increase protein content without lowering yield (Chung et al, 2003)
They were clearly separated into two distinct clusters: one grouping approximately 50 % of the accessions, including public Brazilian compamies (BR) and one private company (N) germplasms; the other one grouping 45 % of the accessions, including Brazilian (BR), exotic (E) and two private companies (N and ST)
Large-scale soybean cultivation in Brazil started in the 1960s, with few lines coming from the United States
Summary
A difficulty encountered in soybean [Glycine max (L.) Merrill] breeding programs is how to increase protein content without lowering yield (Chung et al, 2003). An alternative to circumvent this problem is to improve biological nitrogen fixation (BNF) efficiency, because nitrogen (N) is the most limiting nutrient for crop growth and is directly related to protein content in grains (Hungria et al, 2006; Santos et al, 2013). Several studies performed worldwide have demonstrated that the association between soybean and Bradyrhizobium japonicum, Bradyrhizobium diazoefficiens and Bradyrhizobium elkanii strains is economically and environmentally important (Hungria and Mendes, 2014). Savings on N-fertilizer by the replacement of BNF are estimated at about US$15 billion per season in Brazil (Hungria and Mendes, 2014). Benefits can be achieved in grain protein content, as the N derived from BNF is preferentially shifted to grains when compared to the Nfertilizer (Hungria and Neves, 1987; Israel et al, 1985)
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