Sucrose transport into developing seeds of Pisum sativum L.

The anatomy of developing pea seeds is characterized by transfer cells pres ent in both coats and cotyledons at the maternal/filial interface. To deter mine the nature and cellular localization of sucrose transporters in pea se eds, a full-length clone of a sucrose/H+ symporter (PsSUT1) was isolated fr om a cotyledon cDNA library. Northern blot analyses of different organs sho wed that PsSUT1 is expressed in non-seed tissues, including sucrose sinks a nd sources. Within developing seeds, transcripts of PsSUT1 and PsAHA1 genes were detected in all tissues, while transcripts of a sucrose binding prote in (GmSBP) were confined to cotyledon epidermal transfer cells. Signal inte nsities of PsSUT1 and PsAHA1 transcripts and protein products were most pro nounced in the thin-walled parenchyma cells of seed coats and epidermal tra nsfer cells of cotyledons. For cotyledons, the highest transporter densitie s were localized to those portions of plasma membranes lining the wall ingr owth regions of epidermal transfer cells. Responses of [C-14]sucrose influx to metabolic inhibitors indicated that proton-coupled sucrose transport wa s operative in both seed coats and cotyledons. Cotyledon epidermal transfer cells were shown to support the highest sucrose flux. Maximal transport ac tivity was found to account for the sucrose flux differences between seed t issues. Intercellular movement of the symplasmic tracer, 5-(6)-carboxyfluor escein (CF), demonstrated that symplasmic pathways interconnect the vascula r tissues to thin-walled parenchyma transfer cells of seed coats and, for c otyledons, epidermal transfer cells to storage parenchyma cells.