Quantitative analysis of the 3D cell shape changes driving soybean germination.

Seed germination is central to plant establishment and is the starting point for the majority of world agriculture. This transition from seed to seedling has been extensively studied at an organ level, while few studies have examined the cellular events which underlie it. Reports in the model species Arabidopsis have identified a radicle-derived wave of cell expansion underlying the germination process. Whether this spatiotemporal pattern of cell expansion is specific to this model plant or conserved in other species remains unknown. Here we examined the 3D cell anisotropy driving germination in soybean. By examining changes in cell shape at two positions along the length of the axis over time, preferential growth was observed in the portion of the axis closest to the radicle. A gradient of cell size was observed across the cortical cell layers of the soybean axis, and differences in starting cell size translated into differential relative growth rates across cell layers where larger cells showed greater relative growth rates than smaller cells. Differences in cell position-specific cell anisotropy were also observed. These data demonstrate that a radicle-derived growth pattern is present in the crop species soybean, and reveal the presence of a complex cellular organization in this hypocotyl which show cell type-specific anisotropy diving germination.