2010 Annual Report
1a.Objectives (from AD-416)
1) Correlate metabolites during seed development with final seed composition; and.
2)Explore the basis for variation in seed composition as a function of canopy position.
1b.Approach (from AD-416)
The proposed studies will focus on three broad aims. First, we will build on our observation that the free pool of amino acids (in particular asparagine) during development is correlated positively with seed protein content at maturity. Free amino acids will be extracted from rapidly growing seeds and quantitated using GC-MS. We will attempt to identify quantitative trait loci associated with the free asparagine pool in developing seeds of a segregating inbred population that has previously been genotyped. Concurrent studies will examine the levels of enzymes involved in asparagine metabolism (asparaginase and asparagine synthetase) using custom anti-peptide antibodies for immunoblotting analysis. Second, we will initiate a new effort to identify soybean genotypes containing high levels of sulfur amino acids (methionine and cysteine) in mature seeds. If preliminary findings can be confirmed, appropriate crosses will be made to begin to move the high-sulfur trait into enhanced germplasm lines. Third, we will continue to examine the effect of canopy position on seed composition. In general, seeds that develop at the top of the canopy have more protein, less oil, and fatty acids contain more monounsaturated oleic (18:1), compared to seeds at the bottom of the canopy. Our current results suggest that quantitative differences exist among genotypes in these responses. In the proposed studies, we will determine whether these canopy position effects may simply reflect sensitivity to environmental parameters within the canopy and concurrently, appropriate crosses will be made to identify controlling genes.
Important determinants of soybean seed quality are the concentration of protein and oil and the composition of each of these storage products. For example, soybeans tend to be low in S-containing amino acids and seeds with a higher S/N ratio are considered higher protein quality. We observed that all aspects of the composition of mature soybean seeds varied as a function of position on the main stem. In studies with ten indeterminate genotypes conducted over a 4-year period in Urbana, IL, seed % protein generally increased while % oil decreased from the bottom to the top of the canopy. The difference in protein content comparing seeds from the top quadrant to the bottom quadrant can be substantial (2 to 6 percentage points) and determines the ‘bulk seed’ protein content, which ranges from ~40% in the traditional cultivars to ~50+% in the high-protein genotypes. There were also positional effects on protein quality. In general, %S tended to track %N and generally increased from base to top of the canopy. However, the S/N ratio varied and was higher in the bottom quadrant of seeds (compared to top quadrant) for 8 of the 10 genotypes studied. Thus, seeds that developed lower in the canopy had reduced total protein but the quality (S-content) was higher. Two-dimensional electrophoresis of proteins from seeds produced at different positions indicated that the content of ß-conglycinin subunits, which are low in S-containing amino acids, was higher in seeds from the top of the canopy. The positional effects on seed composition appear to reflect genetic and environmental factors, and we are speculating that temperature maybe one of the critical environmental parameters.