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.
We determined that seed composition varies with pod position on the main stem and that the canopy positional effects can be substantial, with seed protein content higher at the top of the canopy by 2.5 to 5 percentage points compared to the bottom of the canopy. Conversely, oil content tends to be higher at the bottom of the canopy (1.5 to 3 percentage points) relative to the top of the canopy. These differences clearly impact the bulk seed composition for a given genotype. In 8 of the 10 cultivars studied, canopy position also affected seed protein quality, with seeds produced at the bottom of the canopy having a higher relative sulfur content (S/N ratio). Substantial differences in relative sulfur content were also noted among genotypes suggesting the potential for improvement in seed quality.