The contents of seed storage compounds, protein and oil, determine the best use of soybean seeds, namely materials for food processing and oil production. Genetic and environmental factors could affect the chemical compositions of soybean seeds. However, the mechanisms of how the accumulation of these primary seed compounds is regulated are mostly unclear. In this chapter, we describe the different effects of nodulation on the protein and oil contents in soybean seeds and the crucial role of

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... sphoenolpyruvate carboxylase (PEPC) in the protein accumulation of soybean seeds. Based on our previous studies on soybean seeds, we introduce five manners deduced; (1) protein accumulation is independent of oil accumulation, (2) nitrogen fixation results in decreasing oil amount per seed and decreased seed oil content, (3) a high pseudo negative correlation between protein and oil contents in seeds is likely to be observed under less nitrogen supply from the soil, (4) nitrogen absorbed from soil during the late growth stage promote seed production, (5) plant-type PEPC, ex. Gmppc2 in soybean could play a role in amino acid biosynthesis for storage protein accumulation in seeds during the late maturation period.