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

由于国内饲草自给率下降和水稻产量过剩，日本最近研究了将饲草大米用作全作物青贮料的可行性。果聚糖是一种水溶性低聚多糖。温带牧草在液泡中积累果聚糖作为临时储存碳水化合物，而水稻不能合成果聚糖。据报道，果聚糖可以改善瘤胃条件和消化率。为了提高水稻饲料品质，我们将小麦果糖转移酶基因导入水稻，成功地获得了叶片、茎等组织中积累果聚糖的转基因水稻植株。转1-Sst(蔗糖：蔗糖1-果糖基转移酶)基因的转化子主要积累β-(2，1)连接寡聚体(DP3-5)，转6-SFT(蔗糖：果聚糖6-果糖基转移酶)基因的转化子主要积累β-(2，6)连接果聚糖。同时携带1-SST和6-SFT基因的水稻植株表现出额外的果聚糖积累，其中β-(2，6)连接的果糖基团连接在二叉糖上。在温室夏季光照条件下，转1-SST和双基因转基因植株叶片果糖含量分别达到20 mg/g鲜重和30 mg/g鲜重左右。这些品系的总可溶性糖含量也有所增加。转6-SFT基因小麦品系的果聚糖积累水平较低(2-7 mg/g鲜重)。此外，大多数转基因株系没有表现出形态上的异常。这种富含果聚糖的大米具有作为饲料的潜力。调查工作将继续进行。

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.