The cellular pathway of photosynthate transfer in the developing wheat grain. I. Delineation of a potential transfer pathway using fluorescent dyes

A potential cellular pathway for photosynthate transfer between the crease phloem and the starchy endosperm of the developing wheat grain has been delineated using fluorescent dyes. Membrane permeable and impermeable dyes have been introduced into the grain through the crease phloem, the endosperm cavity or the dorsal surface of the starchy endosperm. The movement of the symplastic tracer 5-(6)-6-carboxyfluorescein (CF) derived from 5-(6)-6-carboxyfluorescein diacetate (CFDA), from either direction between the crease phloem and the endosperm cavity, indicated that the symplastic pathway was operative from the crease phloem to the nucellar projection. Furthermore, the inward movement of apoplastic tracer trisodium, 3-hydroxy-5,8,10-pyrentrisulphonate (PTS) from the endosperm cavity and that of CF following plasmolysis showed that there was a high resistance to solute transfer within the apoplast of the pigment strand. All dyes entered the modified aleurone and adjacent sub-aleurone bordering the endosperm cavity. Subsequent movement of the symplastic tracers CF and sulphorhodamine G (SRG) into and through the endosperm was rapid. However, the movement of apoplastic tracers PTS and Calcofluor White (CFW) was relatively slow and with tissue plasmolysis, CF was confined to the cytoplasm of the modified aleurone and subaleurone cells. Together, these results demonstrate that there is a high resistance to solute movement within the apoplast of the cells bordering the endosperm cavity. We propose that photosynthate transfer is via the symplast to the nucellar projection where membrane exchange to the endosperm cavity occurs. Uptake from the cavity is by the modified aleurone and small endosperm cells prior to transfer through the symplast to and through the starchy endosperm.