Abstract
BACKGROUND: Endosperm is a triploid tissue in seed resulting from a sperm nucleus fused with the binucleate central cell after double fertilization. Endosperm may be involved in metabolite production, solute transport, nutrient storage, and germination. In the legume family (Fabaceae), with the greatest number of domesticated crops, approximately 60% of genera contain well-differentiated endosperm in mature seeds. Soybean seeds, the most important legume crop in the worlds, have endosperm surrounding embryos during all stages of seed development. However, the function of soybean endosperm is still unknown. RESULTS: Flow cytometry assay confirmed that soybean endosperm was triploid. Cytobiological observation showed that soybean endosperm cells were alive with zigzag-shape cell wall. Soybean endosperm cells allowed fusion proteins (42 kDa) to move from bombarded cells to adjacent unbombarded-cells. Such movement is not simple diffusion because the fusion proteins failed to move into dead cells. We used symplastic tracers to test the transport potential of soybean endosperm. Small organic dye and low-molecular-weight symplastic tracers revealed fast symplastic transport. After a treatment of an inhibitor of ATPase, N,N'-dicyclohexylcarbodiimide (DCCD), symplastic transport was blocked, but all tracers still showed fast apolopastic transport. The transport speed of 8-hydroxypyrene-1,3,6-trisulfonic acid in endosperm was 1.5 to 3 times faster than in cotyledon cells or Arabidopsis embryos. CONCLUSIONS: Soybean endosperm is a membrane-like, semi-transparent, and fully active tissue located between the seed coat and cotyledon. Soybean endosperm cells allowed macromolecules to move fast via plasmodesmata transport. The size exclusion limit is larger for soybean endosperm cells than its cotyledon or even Arabidopsis embryo cells. Soybean endosperm may be involved in fast and horizontal transport during the mid-developmental stage of seeds.