Phosphorus is an important nutrient for crop productivity. More than 60% of the total phosphorus in cereal crops is finally allocated into the grains and is therefore removed at harvest. This removal accounts for 85% of the phosphorus fertilizers applied to the field each year. However, because humans and non-ruminants such as poultry, swine and fish cannot digest phytate, the major form of phosphorus in the grains, the excreted phosphorus causes eutrophication of waterways. A reduction in phosphorus accumulation in the grain would contribute to sustainable and environmentally friendly agriculture. Here we describe a rice transporter, SULTR-like phosphorus distribution transporter (SPDT), that controls the allocation of phosphorus to the grain. SPDT is expressed in the xylem region of both enlarged- and diffuse-vascular bundles of the nodes, and encodes a plasma-membrane-localized transporter for phosphorus. Knockout of this gene in rice (Oryza sativa) altered the distribution of phosphorus, with decreased phosphorus in the grains but increased levels in the leaves. Total phosphorus and phytate in the brown de-husked rice were 20-30% lower in the knockout lines, whereas yield, seed germination and seedling vigour were not affected. These results indicate that SPDT functions in the rice node as a switch to allocate phosphorus preferentially to the grains. This finding provides a potential strategy to reduce the removal of phosphorus from the field and lower the risk of eutrophication of waterways.

A motor learning mechanism called saccadic adaptation ensures accuracy of saccades throughout life despite growth, aging, and some pathologies of the oculomotor plant or nervous system. The present study investigates effects of preceding adaptation on the speed of subsequent adaptation during single experiments. Adaptive changes in gain (movement size divided by target eccentricity) were induced by intrasaccadic step (ISS) of the target. After the gain was altered (control block), we reversed the direction of ISS to bring the gain back to approximately 1.0 (recovery). We then reversed ISS direction again to induce another adaptation (test block). Analyses revealed that the gain changed at a higher rate in the early part of test adaptation than in the corresponding part of control. After approximately 100-300 saccades in the test block, adaptation slowed down. The gain value at which adaptation slowed was correlated with the gain achieved in the control. We further examined effects of a 30 min intervention inserted between recovery and test blocks. When zero-visual-error trials ( approximately 700 saccades) were repeated during this period, the rate of test adaptation was similar to that of control. In contrast, when the animal was deprived of visual inputs during this period, test adaptation was still influenced by preceding learning. We conclude that a memory of previous learning remains during recovery to facilitate subsequent adaptation and that such a memory does not disappear merely with time but is erased actively by repeated zero-error movements. Our results, which cannot be explained by a single mechanism, suggest that the saccadic system is equipped with more than one plasticity process.

Myo-inositol-1-phosphate (I[1]P) synthase (EC 5.5.1.4) catalyzes the reaction from glucose 6-phosphate to I(1)P, the first step of myo-inositol biosynthesis. Among the metabolites of I(1)P is inositol hexakisphosphate, which forms a mixed salt called phytin or phytate, a storage form of phosphate and cations in seeds. We have isolated a rice (Oryza sativa L.) cDNA clone, pRINO1, that is highly homologous to the I(1)P synthase from yeast and plants. Northern analysis of total RNA showed that the transcript accumulated to high levels in embryos but was undetectable in shoots, roots, and flowers. In situ hybridization of developing seeds showed that the transcript first appeared in the apical region of globular-stage embryos 2 d after anthesis (DAA). Strong signals were detected in the scutellum and aleurone layer after 4 DAA. The level of the transcript in these cells increased until 7 DAA, after which time it gradually decreased. Phytin-containing particles called globoids appeared 4 DAA in the scutellum and aleurone layer, coinciding with the localization of the RINO1 transcript. The temporal and spatial patterns of accumulation of the RINO1 transcript and globoids suggest that I(1)P synthase directs phytin biosynthesis in rice seeds.Myo-inositol and I(1)P are essential for the survival of eukaryotic cells, because various metabolic routes diverge from these two compounds. A number of possible roles for myo-inositol and I(1)P metabolites in plant development have been demonstrated (for review, see Bohnert et al., 1995). I(1)P is utilized for the synthesis of plasma membrane phosphoinositides, which are involved in signal transduction as sources of second messengers (Gross and Boss, 1993). Myo-inositol combined with Gal is incorporated into the raffinose family of the vegetative storage form of carbohydrates (Kandler and Hopf, 1982). Oxidized inositols serve as noncellulosic cell wall components (Loewus and Loewus, 1983), and methylated inositols were shown to be involved in osmoprotection in a halophytic plant (Ishitani et al., 1996). In addition, I(1)P is further phosphorylated to inositol hexakisphosphoric acid, or phytic acid, which strongly binds metallic cations such as K, Mg, Mn, Ca, Fe, and Zn via ionic associations with negatively charged phosphates (Maga, 1982;Lott et al., 1995) to form a mixed salt called phytin or phytate (Ashton, 1976). The synthesis of phytin as a storage form of phosphate and cations is a metabolic pathway unique to plants (Loewus and Dickinson, 1982).Phytin is mainly stored in protein bodies in seeds as spherical inclusions called globoids (Lott et al., 1995). Cereal grains and oil seeds are particularly rich sources of phytin, and the phosphorus of phytin represents 80% to 90% of total phosphorus in these seeds (Maga, 1982). In rice (Oryza sativa) seeds phytin accumulates in the aleurone and scutellum cells but not in the starchy endosperm cells (Tanaka et al., 1973;Ogawa et al., 1977). During germination phytin is digested by an acid phosphatase called phytase (Laboure et al....

Phytic acid (inositol hexakisphosphate [InsP 6 ]) is the storage compound of phosphorus in seeds. As phytic acid binds strongly to metallic cations, it also acts as a storage compound of metals. To understand the mechanisms underlying metal accumulation and localization in relation to phytic acid storage, we applied synchrotron-based x-ray microfluorescence imaging analysis to characterize the simultaneous subcellular distribution of some mineral elements (phosphorus, calcium, potassium, iron, zinc, and copper) in immature and mature rice (Oryza sativa) seeds. This fine-imaging method can reveal whether these elements colocalize. We also determined their accumulation patterns and the changes in phosphate and InsP 6 contents during seed development. While the InsP 6 content in the outer parts of seeds rapidly increased during seed development, the phosphate contents of both the outer and inner parts of seeds remained low. Phosphorus, calcium, potassium, and iron were most abundant in the aleurone layer, and they colocalized throughout seed development. Zinc was broadly distributed from the aleurone layer to the inner endosperm. Copper localized outside the aleurone layer and did not colocalize with phosphorus. From these results, we suggest that phosphorus translocated from source organs was immediately converted to InsP 6 and accumulated in aleurone layer cells and that calcium, potassium, and iron accumulated as phytic acid salt (phytate) in the aleurone layer, whereas zinc bound loosely to InsP 6 and accumulated not only in phytate but also in another storage form. Copper accumulated in the endosperm and may exhibit a storage form other than phytate.

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