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), pos t-sieve element transport of phloem-imported photoassimilates (sucrose) inc ludes membrane transport, to and from the grain apoplasm, between symplasmi cally-isolated maternal and filial tissues. The cellular location and mecha nism of these membrane transport steps were explored during rapid grain fil l. Genomic Southern analysis indicated the presence of a multigene family o f sucrose/H+ symporters (SUTs). One or more SUTs were highly expressed in d eveloping grains, as were P-type H+/ATPase(s) and a sucrose binding protein (SBP). Transcripts of these genes were detected in vascular parenchyma, nu cellar 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 mo dified aleurone/sub-aleurone transfer cells exhibited strong proton pumping activity. In contrast, SUT transport function was restricted to filial tis sues containing modified aleurone/sub-aleurone transfer cells. Based on the se findings, we conclude that SUTs expressed in maternal tissues do not fun ction as sucrose/H+ symporters. Membrane exchange from nucellar projection transfer cells to the endosperm cavity occurs by an as yet unresolved mecha nism. Sucrose uptake from the endosperm cavity into filial tissues is media ted by a SUT localised to plasma membranes of the modified aleurone/sub-ale urone transfer cells.