Relationship between asparagine metabolism and protein concentration in soybean seed

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 population variation for 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 pre-conditioned 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, ∼2-fold higher levels of asparaginase B1 protein and activity in seed coat, 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.

Fig. 1.

Relationships between biochemical parameters in seed in a population of 73 recombinant inbred lines adapted to Illinois, derived from Williams 82 and LG00-13365: (A) protein and oil concentration at maturity; (B) asparagine concentration in developing seed and protein concentration at maturity; (C) asparagine concentration in developing seed and oil concentration at maturity; (D) protein concentration at maturity and sucrose concentration in developing seed; (E) total free amino acids and sucrose concentration in developing seed; (F) phosphate concentration in developing seed and protein concentration in mature seed; (G) protein and phytate concentration in mature seed.

Fig. 2.

Western blots of asparagine synthetase (AS) and asparaginase (ASPG) in seed coat and embryo of developing seeds from Maple Arrow (low protein) and AC Proteus (high protein). The molecular mass (MM) of markers is indicated on the left; p, polypeptide precursor; α, α-subunit.