Changes in transfer cell distribution in the ovule of soybean after fertilization

Abstract

The presence of transfer cells in various regions of the postfertilization ovule of soybean is described. A developmental study shows that transfer cells, occurring in the micropylar nucellus, are formed after fertilization but destroyed by expansion of the embryo sac during transition from the zygote to the two-celled embryo. Subsequently wall ingrowths appear in five additional sites: (i) in the region where the embryonic basal cell wall is associated with the degenerated synergid, projecting into the basal cell; (ii) on the chalazal embryo sac wall projecting into the central cell; (iii) on the embryo sac wall projecting into the basal cell; (iv) on common walls of micropylar suspensor cells; and (v) on some cell walls at the micropylar end of the inner integuments. It is our opinion that these transfer cells are all involved in augmenting metabolite transport and that their orderly appearance in different areas of the ovule signifies changes in the nutritional environment of the young embryo and endosperm of soybean. Because these transfer cells are closely associated with the embryo sac wall, it is proposed that this wall is a common apoplast functioning as both a sink for metabolites from the nucellus and source for all solutes taken up by the cells of the embryo sac.