The role of invertases and hexose transporters in controlling sugar ratios in maternal and filial tissues of barley caryopses during early development

To analyse carbohydrate metabolism and its role during early seed development of barley we characterised genes encoding two cell wall-bound invertases (HvCWINV1 and HvCWINV2) and two putative hexose transporter-like genes (HvSTP1 and HvSTP2 ). No typical vacuolar invertase gene could be identified. Instead, a gene encoding sucrose:fructan 6-fructosyltransferase (HvSF6FT1 ), an enzyme with soluble acid invertase activity, was isolated and characterised. Furthermore, enzyme activities and sugar levels were measured. HvSF6FT1 -mRNA levels and acid soluble invertase activity are highest in the maternal pericarp 1-2 days after flowering (DAF). HvSF6FT1 is strongly expressed in regions flanking the main vascular bundle and to a lower extent in filial endospermal transfer cells, which persist until maturity and never accumulate starch. In contrast, cell wall-bound invertase HvCWINV2 is expressed early in development mainly in the style region and later on in pericarp areas which transiently accumulate starch and undergo degradation later in development. The hexose transporter HvSTP2 shows a temporal and spatial expression pattern similar to HvCWINV2. Transcripts of HvCWINV1 have been localised within the first row of endospermal cells and in the outermost area of the nucellar projection as well as in endospermal transfer cells before starch filling; the same regions of the endosperm are labelled with a hexose transporter HvSTP1 -probe. HvSTP1 is expressed at very low levels within the pericarp but much higher in the syncytial endosperm at 3 DAF and in endospermal transfer cells 7 DAF. The temporal and spatial association of HvCWINV1 and HvSTP1 expression indicates that hexoses liberated by the invertase within the endospermal cavity are taken up by the transporter to be delivered into the central uncellularised space of the endosperm to supply mitotically active endosperm cells with hexoses. The results are discussed and compared with published data on the role of soluble sugars as signal molecules in seed developmental processes.