Sucrose transport into barley seeds: molecular characterization of two transporters and implications for seed development and starch accumulation

In order to understand sucrose transport in developing seeds of cereals at the molecular level, we cloned from a caryopses library two cDNAs encoding sucrose transporters, designated HvSUT1. and HvSUT2. Sucrose uptake activit y was confirmed by heterologous expression in yeast. Both transporter genes are expressed in maternal as well as filial tissues. In a series of in sit u hybridizations we analysed the cell type-specific expression in developin g seeds. HvSUT1 is preferentially expressed in caryopses in the cells of th e nucellar projection and the endospermal transfer layer, which represent t he sites of sucrose exchange between the maternal and the filial generation and are characterized by transfer cell formation. HvSUT2 is expressed in a ll sink and source tissues analysed and may have a general housekeeping rol e. The rapid induction of HvSUT1 gene expression in caryopses at approximat ely 5-6 days after fertilization coincides with increasing levels of sucros e as well as sucrose synthase mRNA and activity, and occurs immediately bef ore the onset of rapid starch accumulation within the endosperm. Starch bio synthesis requires sucrose to be imported into the endosperm, as direct pre cursor for starch synthesis and to promote storage-associated processes. We discuss the possible role of HvSUT1 as a control element for the endosperm al sucrose concentration.