Analysis on the molecular mechanism of the accumulation of seed storage materials and molecular breeding of phosphate storage

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Phosphates in seeds are stored as phytic acid (myo-inositol -1,2,3,4,5,6-hexakisphosphate). Phytic acid is not readily digested by monogastric animals, so that grains and legumes in feed result in a large amount of phosphorus in animal wastes. Reducing the phytic acid content of seeds is a major objective of breeding, both to increase the available nutrients and to reduce the environmental load. Myo-inositol 3-phosphate synthase (EC 5.5.1.4) catalyzes the first step of phytic acid biosynthesis. We isolated that gene from rice (RINO1), and demonstrated that it plays a key role in the synthetic pathway during seed development. In this study, we attempted to reduce the phytic acid content of grain by reducing RINO1 expression in developing rice seeds.To repress RINO1 expression, we introduced RINO1 cDNA in sense and antisense orientations under control of either the CaMV 35S, RINO1, or rice major storage protein glutelin (GluB-1) promoters, or the promoter of a transcriptional factor (RISBZ1) responsible for GluB-1 expression, into rice plants, using Agrobacterium tumefaciens-mediated transformation. To determine the effect of the transgene on phytic acid biosynthesis in seeds, the inorganic (Pi) and total phosphorus contents of the grain were examined. In non-transgenic rice seeds, the concentration of Pi is around 5% of the total phosphorus and the majority of the remaining phosphorus is phytic acid phosphorus. Several transgenic plants contained elevated Pi levels in T_1 seeds, varying from 12 to 63% Pi. The total phosphorus content of the seeds of transgenic plants remained constant, and equaled that of non-transgenic plants. The results suggest that the phytic acid content was reduced in these transgenic plants.