Temporal and Spatial Patterns of Accumulation of the Transcript of Myo-Inositol-1-Phosphate Synthase and Phytin-Containing Particles during Seed Development in Rice1

Abstract

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....

Comparative Expression and Cellular Localization of Myo-inositol Phosphate Synthase (MIPS) in the Wild Type and in an EMS Mutant During Common Bean (Phaseolus vulgaris L.) Seed Development

“…No signal transcripts of Pv_BAT93 MIPS were revealed in stems or roots; this could be due to the relatively very low occurrence or absence of the biosynthesis of inositol or phytic acid in these tissues compared with developing seeds and leaves (Yoshida et al 1999). The mRNA levels of the PvMIPSs gene were higher in developing seeds than in other tissues, including leaves, flowers, roots, cotyledons, and stems.…”

“…6). This expression pattern could be correlated with the accumulation of phytic acid, which is a storage reserve of phosphorus in seeds (Yoshida et al 1999). We suggest that the accumulation of MIPS transcripts during the early stages of common-bean seed development must have a greater role not only in plant basic metabolism, but also during seed development.…”

“…In A. thaliana, three MIPS genes were reported (Johnson and Sussex 1995;Mitsuhashi et al 2008). Three were found in S. indicum (Chun et al 2003), four in G. max (Hegman et al 2001;Chappell et al 2006), seven in Z. mays (Larson and Raboy 1999), and two MIPS genes encoding the RINO1 and RINO2 genes were present in the rice genome (Yoshida et al 1999;Suzuki et al 2007). …”

“…Interestingly, MIPS has also been shown to be highly expressed in developing seeds, suggesting that myo-inositol has an important role in seed development. To date, the localization of the MIPS transcript in developing seed has been characterized only for O. sativa (Yoshida et al 1999), G. max (Chiera and Grabau 2007), and A. thaliana (Mitsuhashi et al 2008). In O. sativa, in situ hybridization showed that the MIPS (RINO1) transcript appeared initially in the apical region of the embryo and accumulated to a high level in the scutellum and aleurone layers (Yoshida et al 1999).…”

“…To date, the localization of the MIPS transcript in developing seed has been characterized only for O. sativa (Yoshida et al 1999), G. max (Chiera and Grabau 2007), and A. thaliana (Mitsuhashi et al 2008). In O. sativa, in situ hybridization showed that the MIPS (RINO1) transcript appeared initially in the apical region of the embryo and accumulated to a high level in the scutellum and aleurone layers (Yoshida et al 1999). In G. max, many MIPS isoforms have been shown to be expressed, mainly in developing seeds (Chappell et al 2006).…”