Cell specific expression of three genes involved in plasma membrane sucrose transport in developing Vicia faba seed

In developing seeds of Vicia faba, transfer cells line the inner surface of the seed coat and the juxtaposed epidermal surface of the cotyledons. Circumstantial evidence, derived from anatomical and physiological studies, indicates that these cells are the likely sites of sucrose efflux to, and influx from, the seed apoplasm, respectively. In this study, expression of an H+/sucrose symporter-gene was found to be localised to the epidermal-transfer cell complexes of the cotyledons. The sucrose binding protein (SEP) gene was expressed in these cells as well as in the thin-walled parenchyma transfer cells of the seed coat. SEP was immunolocalised exclusively to the plasma membranes located in the wall ingrowth regions of the transfer cells. In addition, a plasma membrane H+-ATPase was most abundant in the wall ingrowth regions with decreasing levels of expression at increasing distance from the transfer cell layers. The observed co-localisation of high densities of a plasma membrane H+-ATPase and sucrose transport proteins to the wall ingrowths of the seed coat and cotyledon transfer cells provides strong evidence that these regions are the principal sites of facilitated membrane transport of sucrose to and from the seed apoplasm.