Submitted to: Journal of Experimental Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/25/2012
Publication Date: 2/22/2012
Citation: Panduranganl, S., Pajak, A., Molnar, S., Cober, E., Dhaubhadel, S., Hernandez-Sebastia, C., Kaiser, W.M., Nelson, R.L., Huber, S.C., Marsolais, F. 2012. Relationship between asparagine metabolism and protein concentration in soybean seed. Journal of Experimental Botany. 63(8):3173-3184. Interpretive Summary: Seed composition is a major focus for many soybean breeding programs and therefore an understanding of the biological and genetic mechanisms that control the accumulation of protein and oil in soybean seeds is important. A major question is whether seed protein content is controlled by the availability of specific amino acids, which are the building blocks for the storage proteins, or whether other factors within the developing seeds (e.g. intrinsic capacity to synthesize storage proteins versus oil) are critical. To address this, we evaluated the content of free amino acids in developing seeds of two recombinant inbred lines segregating for seed protein content. A positive correlation between free asparagine content during development and seed protein concentration at maturity was observed establishing the free asparagine trait as a physiological marker associated with high seed protein content. Moreover, two of the four quantitative trait loci (QTL) known to control seed protein concentration were found to be associated with the asparagine trait. Of particular interest is the QTL on linkage group I (chromosome 20) that is known to be a major determinant of seed protein level. Knowledge of the association with asparagine content will ultimately help to identify the gene(s) underlying this major QTL. We also show that high asparagine content and high seed protein content at maturity is associated with increased expression of specific asparaginase isoforms in the seed coat (elevated ASPGB1 but reduced ASPGB2), which may facilitate higher rates of assimilate movement into the developing cotyledons. Collectively, these results firmly establish physiological markers associated with seed composition and will ultimately help to identify the gene(s) underlying major QTLs controlling protein concentration.
Technical Abstract: The relationship between asparagine metabolism and protein concentration was investigated in soybean seed. Phenotyping of a population of recombinant inbred lines adapted to Illinois confirmed a positive correlation between free asparagine levels in developing seeds and protein concentration at maturity. Analysis of a second population of recombinant inbred lines adapted to Ontario associated the elevated free asparagine trait with two of four quantitative trait loci determining protein concentration, including a major one on chromosome 20 (linkage group I) which has been reported in multiple populations. In the seed coat, levels of asparagine synthetase were high at 50 mg and progressively declined until 150 mg seed weight suggesting that nitrogenous assimilates are preconditioned at early developmental stages to enable a high concentration of asparagine in the embryo. The levels of asparaginase B1 showed an opposite pattern, being low at 50 mg and progressively increased until 150 mg, coinciding with an active phase of storage reserve accumulation. In a pair of genetically related cultivars, higher levels of asparaginase B1 protein and activity in the seed coat, by approximately 2-fold, were associated with high protein concentration, reflecting enhanced flux of nitrogen. Transcript expression analyses attributed this difference to a specific asparaginase gene, ASPGB1a. These results contribute to our understanding of the processes determining protein concentration in soybean seed.