A novel rice accumulating fructan in the vacuole as a temporary storage carbohydrate and its evaluation for forage

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The feasibility of using rice of forage as whole crop silage has been recently investigated in Japan because of the decrease in self-sufficiency of domestic forage and the over-production of rice. Fructan is a water-soluble oligo- and polysaccharide. Temperate grasses accumulate fructan in their vacuoles as a temporary storage carbohydrate, whereas rice is not able to synthesize fructan. It has been reported that fructan improves the rumen condition as well as the digestibility. In order to increase the quality of rice forage, we introduced wheat fructosyltransferase genes into rice and succeeded in obtaining transgenic rice plants that accumulate fructan in tissues such as the leaves and stems. A rice transformant with 1-SST (sucrose: sucrose 1-fructosyltransferase) gene accumulates β-(2, 1) linkage oligomer (DP3-5), and a rice transformant with 6-SFT (sucrose: fructan 6-fructosyltransferase) gene accumulates mainly β-(2, 6) linkage fructan. A rice plant carrying both 1-SST and 6-SFT genes showed an additional accumulation of fructan with β-(2, 6) linked fructosyl-units attached to bifurcose. When the plants were grown in a greenhouse under summer light conditions, fructan contents in leaves of the transgenic lines with wheat 1-SST and with both two genes reached about 20 mg and 30 mg/g fresh weight, respectively. The content of total soluble sugar also increased in these lines. On the other hand, fructan accumulation levels in the transgenic lines with wheat 6-SFT gene were low (2-7 mg/g fresh weight). Moreover, most of the transgenic lines showed no morphological abnormalities. This fructan-accumulating rice has the potential for utilization for forage. Investigation will be continued.