|Natarajan, Savithiry - Savi|
Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2006
Publication Date: 5/31/2006
Citation: Mahmoud, A.A., Natarajan, S.S., Bennett, J.O., Mawhinney, T.P., Wiebold, W.J., Krishnan, H.B. 2006. Effect of six decades of selective breeding on soybean protein composition and quality: a biochemical and molecular analysis. Journal of Agricultural and Food Chemistry. 54(11):3916-3922. Interpretive Summary: Securing and maintaining a market for the 35.7 million metric tons of soybean meal produced annually in the United States requires diligent attention be paid to the quality of the product. Manufacture of superior meal begins with soybean seed, which contain a high quality protein. In the past 60 years of selective breeding, the primary goal has been for yield enhancement. In this study, we grew Mukden, Lincoln, Mandarin, and A. K. soybeans, which are among the limited number of cultivars forming the genetic base of US soybeans side by side with cultivars representing consecutive releases during the past 60 years. Our results show a definitive decrease in protein content of soybean cultivars during the past six decades. Analysis of the seed protein showed that the relative accumulation of the major seed storage proteins had not changed significantly and the proportions of each amino acid in seeds over the years had remained stable. Our results emphasize the need for the introgression of diverse soybean germplasm from distinct geographical locations to broaden the genetic base of soybeans. Exploitation of this diversity by breeders holds promise for the continued improvement of North American soybean cultivars.
Technical Abstract: To evaluate the extent of the genetic change and its effects on seed protein composition of soybean cultivars released during the past 60 years, representative ancestral cultivars and those derived from selective breeding were grown in a side-by-side comparison. Total seed protein content, determined by combustion analysis of nitrogen, revealed a decline in the protein content after decades of selection and breeding. Sodium dodecyl sulfate polyacrylamide gel electrophoresis and high-resolution two-dimensional electrophoresis comparison of protein profiles of soybean cultivars indicated that relative expression of most of seed storage proteins had not varied substantially from that of the ancestral lines to the present commercial cultivars. There was noticeably less beta-subunit of beta-conglycinin, a protein devoid of sulfur amino acids, in the modern cultivars represented by Mustang, Pioneer 93B09, and Asgrow 3602. Comparison of the amino acid profile of soybean seed, a benchmark of the protein’s nutritional quality, revealed that the ancestral progenitor, G. soja, was significantly higher in cysteine, glutamic acid, histidine, and arginine than either the ancestral or the modern cultivars. Selective breeding over the past 60 years had no substantial effect on the overall amino acid composition. The degree of divergence in the DNA sequence of the genes encoding the glycinin and beta-conglycinin in the ancestral and modern cultivars was investigated using Southern hybridization and polymerase chain reaction. Even though some restriction fragment polymorphisms could be detected, overall the banding pattern was remarkably similar among the ancestral cultivars and those derived from them, suggesting a high degree of conservation of seed-storage protein genes. The results of our study suggest that selection and breeding for yield during the past 60 years had no major influence on the protein composition ostensibly due to limited genetic diversity among the parental lines.