Abstract
Backgroundmyo-Inositol (Ins) metabolism during early stages of seed development plays an important role in determining the distributional relationships of some seed storage components such as the antinutritional factors, sucrose galactosides (also known as raffinose oligosaccharides) and phytic acid (PhA) (myo-inositol 1,2,3,4,5,6-hexakisphosphate). The former is a group of oligosaccharides, which plays a role in desiccation at seed maturation. They are not easily digested by monogastric animals, hence their flatulence-causing properties. Phytic acid is highly negatively charged, which chelates positive ions of essential minerals and decreases their bioavailability. It is also a major cause of phosphate-related water pollution. Our aim was to investigate the influence of competitive diversion of Ins as common substrate on the biosynthesis of phytate and sucrose galactosides.ResultsWe have studied the initial metabolic patterns of Ins in developing seeds of Brassica napus and determined that early stages of seed development are marked by rapid deployment of Ins into a variety of pathways, dominated by interconversion of polar (Ins phosphates) and non-polar (phospholipids) species. In a time course experiment at early stages of seed development, we show Ins to be a highly significant constituent of the endosperm and seed coat, but with no phytate biosynthesis occurring in either tissue. Phytate accumulation appears to be confined mainly within the embryo throughout seed development and maturation. In our approach, the gene for myo-inositol methyltransferase (IMT), isolated from Mesembryanthemum crystallinum (ice plant), was transferred to B. napus under the control of the seed-specific promoters, napin and phaseolin. Introduction of this new metabolic step during seed development prompted Ins conversion to the corresponding monomethyl ether, ononitol, and affected phytate accumulation. We were able to produce homozygous transgenic lines with 19% - 35% average phytate reduction. Additionally, changes in the raffinose content and related sugars occurred along with enhanced sucrose levels. Germination rates, viability and other seed parameters were unaffected by the IMT transgene over-expression.ConclusionsCompetitive methylation of Ins during seed development reduces seed antinutritional components and enhances its nutritional characteristics while maintaining adequate phosphate reserves. Such approach should potentially raise the canola market value and likely, that of other crops.
Highlights
Background myoInositol is an essential component in the biosynthesis of an array of derivatives ranging from simple inositol phosphates to complex membrane-associated products with important cellular functions
We studied the effect of over-expressing the gene encoding inositol methyltransferase (IMT) (EC 2.1.1.129) from M. crystallinum [20] on seed phytic acid (PhA) accumulation in transgenic B. napus under two different seed-specific promoters, napin and phaseolin [21]
Early stages of Brassica seed development appear to be dominated by reciprocal interconversions of polar Ins phosphates and non-polar species
Summary
Inositol is an essential component in the biosynthesis of an array of derivatives ranging from simple inositol phosphates to complex membrane-associated products with important cellular functions. It can be isomerized and (or) methylated to form a variety of species-specific epimers and methyl ethers. A number of these, including Ins, have been noted to accumulate mostly in osmotically challenged plants and have since been recognized as osmoprotectant metabolites [1]. The synthesis of PhA predominates in developing seeds, and constitutes the major storage form of seed phosphorus. Both Ins and its bound phosphates are released by hydrolysis upon germination
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