Fructan formation in transgenic white clover expressing a fructosyltransferase from Streptococcus salivarius

Abstract

White clover (Trifolium repens L.) is an important pasture legume that does not normally accumulate fructan as a storage carbohydrate. We have generated transgenic white clover plants that accumulate fructan, by expressing the fructosyltransferase (Ftf) enzyme from the bacterium Streptococcus salivarius under the control of the cauliflower mosaic virus (CaMV) 35S promoter. Fructan accumulated in leaves, petioles, stolons, flowers, and roots of transgenic plants. Levels of fructan up to approximately 2% dry weight were measured in leaves. The fructan was of high molecular mass ( > 5000 kDa), typical of bacterial fructans. Ftf enzyme activity up to 120 nmol min^–1 g^–1 fresh weight was determined in leaf extracts of the transformed plants, and appeared to be stable throughout leaf development. Most transformed lines appeared normal, flowered and produced seed, but the growth rate of some transformed lines decreased. Photosynthetic carbon assimilation and levels of endogenous carbohydrates (hexoses, sucrose and starch) were not substantially changed in a clonal line with relatively low fructan. However, in a clonal line with relatively high fructan accumulation, plant growth was reduced, leaf photosynthesis was decreased by 60%, and carbohydrate contents were reduced. The results are discussed in the context of manipulating soluble carbohydrate composition in pasture species to improve nutritive quality for grazing animals.