Perturbing the metabolic dynamics of myo-inositol in developing Brassica napus seeds through in vivo methylation impacts its utilization as phytate precursor and affects downstream metabolic pathways

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.

Faba Bean: Transcriptome Analysis from Etiolated Seedling and Developing Seed Coat of Key Cultivars for Synthesis of Proanthocyanidins, Phytate, Raffinose Family Oligosaccharides, Vicine, and Convicine

“…For example, the gene for L‐ myo ‐inositol 1‐phosphate synthase (MIPS), the enzyme responsible for the de novo biosynthesis of myo ‐inositol 1‐phosphate (initial substrate in the primary pathway of phytate biosynthesis) was relatively little expressed in seedlings but showed a large increase in seed coat. This agrees with the observation that myo ‐inositol accumulates at high levels in developing seed coat of other crops, although with no corresponding phytate accumulation (Dong et al, 2013). Expression of the successive kinases that generate the various myo ‐inositol polyphosphates was observed in all tissues examined at a steady, low level (Table 2), while in embryo tissue of Windsor, we previously found few MIPS or kinase transcripts (Ray and Georges, 2010).…”

Accumulation of Phosphorus-Containing Compounds in Developing Seeds of Low-Phytate Pea (Pisum sativum L.) Mutants

“…Embryo-specific silencing of expression of an ATP binding cassette (ABC) transporter in maize produced seeds with low-phytic acid with no adverse effect on seed weight [ 33 ]. When the myo-inositol methyltransferase (IMT) gene was transferred to Brassica napus through a transgenic approach, a 19% to 35% reduction in phytate was achieved without affecting the seed parameters [ 34 ]. In addition, rice low-phytate mutants produced through RNAi mediated seed-specific silencing of the inositol pentakisphosphate 2-kinase (IPK1) gene, had no undesirable agronomic characters [ 35 ].…”

Seed Biofortification and Phytic Acid Reduction: A Conflict of Interest for the Plant?

“…An inverse relationship between MIPS expression and seed myo -inositol content is also observed in two allelic mrp mutations of rice and in three barley lpa mutants ( lpa2-1 , lpa3-3 and M955 ), for which the molecular defects are still unknown [ 81 , 107 ]. Changes in myo -inositol content also affect the synthesis of derived metabolites, such as galactinol and raffinosaccharides ( Table S1 ) [ 43 , 76 , 107 , 108 , 109 , 110 ]. Seed myo -inositol content has also been suggested to correlate with response to ABA during seed germination [ 19 , 76 , 111 ].…”