Assimilate-Dependent and Genotype-Intrinsic Pathways Differentially Regulate Early Seed Development and Determine...

Soybean (Glycine max L.) cultivars exhibit substantial variation in seed weight; however, the developmental and physiological mechanisms contributing to this variation remain incompletely characterized. In this study, we investigated how early seed developmental dynamics relate to final seed weight by comparing large-seeded and small-seeded cultivars under control and depodding conditions. Depodding, achieved by retaining a single pod per node, minimizes assimilate competition. Final seed weight was positively correlated with cotyledon cell number and with the duration of the lag phase, a key early stage of seed development. Large-seeded cultivars exhibited significantly longer lag phases and higher cotyledon pavement cell numbers than small-seeded cultivars, indicating that extended lag phases promote enhanced cell proliferation, thereby contributing to greater seed weight. Although depodding further increased the cotyledon cell number, this effect was associated with accelerated embryo development rather than a prolonged lag phase. These findings demonstrate that both the duration of the lag phase and the rate of early embryo development influence cotyledon cell proliferation and ultimately seed weight. Furthermore, genotype-intrinsic developmental programs and assimilate supply availability regulate these processes through distinct pathways. Together, our results highlight the important role of early seed developmental timing in determining final seed weight and provide new insights into the developmental basis of yield-related traits in soybean.