Sucrose transport-related genes are expressed in both maternal and filial tissues of developing wheat grains

In developing wheat grains (Triticum turgidum var. durum cv. Fransawi), post-sieve element transport of phloem-imported photoassimilates (sucrose) includes membrane transport, to and from the grain apoplasm, between symplasmically-isolated maternal and filial tissues. The cellular location and mechanism of these membrane transport steps were explored during rapid grain fill. Genomic Southern analysis indicated the presence of a multigene family of sucrose/H+ symporters (SUTs). One or more SUTs were highly expressed in developing grains, as were P-type H+/ATPase(s) and a sucrose binding protein (SBP). Transcripts of these genes were detected in vascular parenchyma, nucellar projection and aleurone cells. Antibodies, raised against a SUT, an H+/ATPase and a SBP, were selectively bound to plasma membranes of vascular parenchyma cells, nucellar projection transfer cells and modified aleurone/sub-aleurone transfer cells. The nucellar projection transfer cells and modified aleurone/sub-aleurone transfer cells exhibited strong proton pumping activity. In contrast, SUT transport function was restricted to filial tissues containing modified aleurone/sub-aleurone transfer cells. Based on these findings, we conclude that SUTs expressed in maternal tissues do not function as sucrose/H+ symporters. Membrane exchange from nucellar projection transfer cells to the endosperm cavity occurs by an as yet unresolved mechanism. Sucrose uptake from the endosperm cavity into filial tissues is mediated by a SUT localised to plasma membranes of the modified aleurone/sub-aleurone transfer cells.