β-Conglycinins in Developing Soybean Seeds

Abstract

The temporal sequence of development of the major proteins of seeds of soybean (Meff.) has been studied during development of cotyledons from flowering to maturity. A well-defrned difference occurred in the tmes of appearance and the periods of maximum accumulation of a-, a'-, and /8-subunits of betaconglycinin. Whereas a-and a'-subunits appeared 15 to 17 days after flowering, accumulation of ,8-subunit did not commence until 22 days after flowering. Such alterations in subunit composition infer that changes also occurred in the amino acid composition of betaconglycinin during maturation, particularly in the content of methionine which is low in the l8-subunit.Marked changes occur in the proteins accumulating in cotyledons during seed development in many legume species (4,5,10,18). The yield and quality of the proteins eventually stored in mature seeds depends both on the relative rates of synthesis of these proteins and on the length of time for which such synthesis occurs. Variations both in the rates of synthesis of the major proteins, legumin and vicilin, and in the times at which their syDtheses commence have been demonstrated in developing cotyledons of Pisum sativum (9, 10). An ordered sequence of appearance also occurs among the group of proteins antigenically related to the mature vicilin fraction (10). Differences have also been shown between legumin and vicilin fractions in Viciafaba both in the time at which accumulation commences and also in the rates of synthesis (18).In soybean the major equivalent proteins are glycinin (11 S) and /?-conglycinin (7 S) (1, 6). Previous studies on developing soybean pods have shown that although proteins of both the 11 S and 7 S types appear early in cotyledonary development, there are major differences between the 7 S proteins in immature and mature cotyledons (5). During development a complex series of changes has been shown to occur in the proportions of three fractions obtained from the 7 S proteins by electrophoresis on polyacrylamide gels (5). Since it is now known that fi-conglycinin is composed of at least six isomers in mature tissue (13,17) the significance of these developmental changes is unclear.All the isomers of fl-conglycinin are combinations of only three major polypeptides, the a-, a'-, and ,8-subunits combined as trimers (14,15). In this paper, therefore, the 7 S proteins in developing soybean seeds have been reexamined under conditions which dissociate the secondary complexes. It is expected that the component polypeptides studied in this way should be related more directly to the primary products of protein synthesis.

Developmental Regulation of β-Conglycinin in Soybean Axes and Cotyledons

“…f3-Conglycinin is a trimeric molecule of 180,000 to 240,000 D and is composed, primarily, of various combinations of the a', a, and # subunits (22), as well as lesser amounts of polypeptides related to a' and a-subunits that have been designated the y subunits (J Bryant, unpublished data). The a' and a subunits begin to accumulate soon after the cell division phase of seed development is completed (about 18 to 20 d after anthesis); the ,3 subunit accumulates 5 to 7 d later (8,17,21). The accumulation of j3-conglycinin mRNA molecules in seed was found to increase just prior to that of the protein subunits (9, 17).…”

“…Using the developmental timetable presented in Table I, soybean embryos at specific developmental stages were selected and seed protein extracts were prepared. Seed proteins were extracted by homogenizing the immature seeds, fully mature dry seeds, or the radicle portion of the embryonic axes in 35 mm K-phosphate (pH 7.3), 10 (8,17,21). Meinke et al (17) also reported that the accumulation of each subunit differs in a tissue dependent manner.…”

Expression of a Soybean (Glycine max [L.] Merr.) Seed Storage Protein Gene in Transgenic Arabidopsis thaliana and Its Response to Nutritional Stress and to Abscisic Acid Mutations

“…The genes encoding these three subunits are a11 expressed during the middle and latter stages of seed development. However, detailed analyses of subunit gene expression revealed variation in temporal (Gayler and Sykes, 1981; Meinke et al, 1981; Naito et al, 1988) and spatial (Ladin et al, 1987) pattems of expression during soybean seed maturation. Nutritional stresses of whole plants such as sulfate deficiency or potassium deficiency change the accumulation of the subunit proteins (Gayler and Sykes, 1985).…”

Temporary application of nitrate to nitrogen-deficient soybean plants at the mid- to late-stages of seed development increased the accumulation of the β-Subunit of β-conglycinin, a major seed storage protein

“…Storage protein accumulation is expressed throughout the seed develop-ental stages. All the three subunit genes of the fl-conglycinin are expressed specifically at the middle to late stages of soybean seed development, although the expression of each subunit gene is regulated differently in terms of time and tissue specificity (Gayler and Sykes 1981;Meinke et al 1981;Ladin et al 1987;Naito et al 1988). …”