Pathway of Photosynthate Transfer in the Developing Seed ofVicia fabaL: A Structural Assessment of the Role of Transfer Cells in Unloading from the Seed Coat

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Abstract
Photosynthate movement within the coat of the developing seed of Vicia faba occurs radially inward from the restricted vascular system and laterally through the non-vascularized region of the seed coat prior to exchange to the seed apoplast. Thin-walled parenchyma/transfer cells line the entire inner surface of the seed coat and thus are located at the terminus of the photosynthate transfer pathway. The principal cellular route of transfer within the seed coat and the role of the thin-walled parenchyma/transfer cells in membrane exchange to the seed apoplast has been investigated. Sucrose fluxes, computed from estimates of the plasma membrane surface areas of the cell types of the pathway, the plasmodesmatal cross-sectional areas interconnecting contiguous cells and the observed rate of sucrose delivery to the embryo indicate that sieve element unloading and subsequent transfer to the thin-walled parenchyma/transfer cells is through the symplast. For the cells of the ground tissue, plasmodesmatal density is consistently higher on their anticlinal walls. This observation supports the reported pattern of lateral transfer through these tissues in the non-vascular regions of the seed coat. Wall ingrowths are initiated sequentially in the thin-walled parenchyma cells to maintain 1–3 rows of thin-walled parenchyma/transfer cells. The development of these wall ingrowths results in a 58% increase in the plasma membrane surface area of these cells and provides them with the capacity to act as the principal cellular site for membrane exchange of sucrose to the seed apoplast. This cellular route of symplastic transfer from the sieve elements to the ground tissues where membrane exchange to the seed apoplast occurs is consistent with that reported for Phaseolus vulgaris

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