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In animal cells, myo-inositol is an important regulatory molecule in several physiological and biochemical processes, including signal transduction and membrane biogenesis. However, the fundamental biological functions of myo-inositol are still far from clear in plants. Here, we report the genetic characterization of three Arabidopsis thaliana genes encoding D-myo-inositol-3-phosphate synthase (MIPS), which catalyzes the rate-limiting step in de novo synthesis of myo-inositol. Each of the three MIPS genes rescued the yeast ino1 mutant, which is defective in yeast MIPS gene INO1, and they had different dynamic expression patterns during Arabidopsis embryo development. Although single mips mutants showed no obvious phenotypes, the mips1 mips2 double mutant and the mips1 mips2 mips3 triple mutant were embryo lethal, whereas the mips1 mips3 and mips1 mips2^sup +/-^ double mutants had abnormal embryos. The mips phenotypes resembled those of auxin mutants. Indeed, the double and triple mips mutants displayed abnormal expression patterns of DR5:green fluorescent protein, an auxin-responsive fusion protein, and they had altered PIN1 subcellular localization. Also, membrane trafficking was affected in mips1 mips3. Interestingly, overexpression of PHOSPHATIDYLINOSITOL SYNTHASE2, which converts myoinositol to membrane phosphatidylinositol (PtdIns), largely rescued the cotyledon and endomembrane defects in mips1 mips3. We conclude that myo-inositol serves as the main substrate for synthesizing PtdIns and phosphatidylinositides, which are essential for endomembrane structure and trafficking and thus for auxin-regulated embryogenesis.
INTRODUCTION
myo-inositol is the most important and abundant stereoisomer of the six-carbon cyclitol inositol. It exists in all eukaryotes and is also found in some prokaryotes (Eagle et al., 1957; Bachhawat and Mande, 2000). myo-inositol emerged early during evolution and evolved into diverse derivatives, such as phosphoinositides (PIs), inositol phosphates, glycosylphosphatidylinositol, and various inositol conjugates (Michell, 2008). Inositol and its derivatives are involved in various biochemical and physiological processes, including intracellular signal transduction (Irvine and Schell, 2001), membrane construction and trafficking (Cullen et al., 2001), membrane-related protein anchoring (Tiede et al., 1999; Peskan et al., 2000; Borner et al., 2005), and cell wall construction (Loewus 1973). They also act as protein cofactors (Macbeth et al., 2005; Tan et al., 2007) and play roles in auxin storage and trafficking in plant seeds (Hall and Bandurski, 1986).
In mammalian cells, myo-inositol and its derivatives play important roles in reproduction (Chiu et al., 2003; Papaleo et al., 2009), embryo...