Fructan biosynthesis and fructosyltransferase evolution: Expression of the 6-SFT (sucrose : fructan 6-fructosyltransferase) gene in crested wheatgrass (Agropyron cristatum)

Crested wheatgrass (Agropyron cristatum), an important cool-season grass, contains a complexity of fructans, including levans, inulins and graminans. Fructan types are characterized by unique combinations of carbon-carbon linkages. This study established the DNA sequence for the 6-SFT gene (sucrose : fructan 6-fructosyltransferase) in A. cristatumand compared the sequence with invertases and fructosyltransferases from various species. Detachment with illumination, as well as cool treatment of crested wheatgrass leaves induced the accumulation of fructans. All three types of fructans, β-2,1; β-2,6 and graminans (combination of both β-2,1 and β-2,6 linkages) were detected. The cDNA of 6-SFT was cloned from A. cristatum. The deduced amino acid sequences showed 85 percnt; identity with barley (Hordeum vulgare) 6-SFT. It also exhibited high homology with other fructosyltransferases and invertases. The expression of 6-SFT mRNA was analyzed with Northern hybridization. mRNA of the 6-SFT gene accumulated after 8 h of illumination in detached leaves and remained at a steady state for 36 hours. Under low temperature (10 °C/5 °C, day/night), the mRNA of 6-SFT began to accumulate after 8 h and reached the highest level at 2 d and then declined after 8 days of treatment. A coincidence of fructan and 6-SFT mRNA accumulation in crested wheatgrass suggests that 6-SFT has a role in fructan biosynthesis. A cladogram comparison of the amino acid sequences of most known sequences of fructosyltransferases and 23 invertases shows separations into various clades of like genes. 6-SFT does not contain the unique NDPNG amino acid sequence characteristic of invertases. Sequence analysis indicated that fructosyltransferases likely evolved separately in dicot and monocot species.