Biosynthesis of planteose in Sesamum indicum

Novel constituents of Salvia hispanica L. (chia) nutlet mucilage and the improved in vitro fermentation of nutlets when ground

Upon wetting, chia (Salvia hispanica L.) nutlets produce a gel-like capsule of polysaccharides called mucilage that comprises a significant part of their dietary fibre content. Seed/nutlet mucilage is often used as a texture modifying hydrocolloid and bulking dietary fibre due to its water-binding ability, though the utility of mucilage from different sources is highly structure-function dependent. The composition and structure of chia nutlet mucilage is poorly defined, and a better understanding will aid in exploiting its dietary fibre functionality, particularly if, and how, it is utilised by gut microbiota. In this study, microscopy, chromatography, mass spectrometry and glycome profiling techniques showed that chia nutlet mucilage is highly complex, layered, and contains several polymer types. The mucilage comprises a novel xyloamylose containing both β-linked-xylose and α-linked-glucose, a near-linear xylan that may be sparsely substituted, a modified cellulose domain, and abundant alcohol-soluble oligosaccharides. To assess the dietary fibre functionality of chia nutlet mucilage, an in vitro cumulative gas production technique was used to determine the fermentability of different chia nutlet preparations. The complex nature of chia nutlet mucilage led to poor fermentation where the oligosaccharides appeared to be the only fermentable substrate present in the mucilage. Of note, ground chia nutlets were better fermented than intact whole nutlets, as judged by short chain fatty acid production. Therefore, it is suggested that the benefits of eating chia as a "superfood", could be notably enhanced if the nutlets are ground rather than being consumed whole, improving the bioaccessibility of key nutrients including dietary fibre.

Involvement of α-galactosidase OmAGAL2 in planteose hydrolysis during seed germination of Orobanche minor

Root parasitic weeds of the Orobanchaceae, such as witchweeds (Striga spp.) and broomrapes (Orobanche and Phelipanche spp.), cause serious losses in agriculture worldwide, and efforts have been made to control these parasitic weeds. Understanding the characteristic physiological processes in the life cycle of root parasitic weeds is particularly important to identify specific targets for growth modulators. In our previous study, planteose metabolism was revealed to be activated soon after the perception of strigolactones in germinating seeds of O. minor. Nojirimycin inhibited planteose metabolism and impeded seed germination of O. minor, indicating a possible target for root parasitic weed control. In the present study, we investigated the distribution of planteose in dry seeds of O. minor by matrix-assisted laser desorption/ionization-mass spectrometry imaging. Planteose was detected in tissues surrounding-but not within-the embryo, supporting its suggested role as a storage carbohydrate. Biochemical assays and molecular characterization of an α-galactosidase family member, OmAGAL2, indicated that the enzyme is involved in planteose hydrolysis in the apoplast around the embryo after the perception of strigolactones, to provide the embryo with essential hexoses for germination. These results indicate that OmAGAL2 is a potential molecular target for root parasitic weed control.

The effect of nojirimycin on the transcriptome of germinating Orobanche minor seeds

Orobanchaceae root parasitic weeds cause serious agricultural damage worldwide. Although numerous studies have been conducted to establish an effective control strategy for the growth and spread of root parasitic weeds, no practical method has been developed so far. Previously, metabolomic analyses were conducted on germinating seeds of a broomrape, Orobanche minor, to find novel targets for its selective control. Interestingly, planteose metabolism was identified as a possible target, and nojirimycin (NJ) selectively inhibited the germination of O. minor by intercepting planteose metabolism, although its precise mode of action was unclear. Here, transcriptome analysis by RNA-Seq was conducted to obtain molecular insight into the effects of NJ on germinating O. minor seeds. Differential gene expression analysis results suggest that NJ alters sugar metabolism and/or signaling, which is required to promote seed germination. This finding will contribute to understanding the effect of NJ and establishing a novel strategy for parasitic weed control.

Planteose is a short-term storage carbohydrate in Actinidia leaves

The polyol myo-inositol constitutes 10-20% of soluble carbohydrates in mature leaves of Actinidia deliciosa (A.Chev.) C.F. Liang et A.R. Ferguson var. deliciosa 'Hayward' and A. arguta (Sieb. et Zucc.) Planch. ex Miq. var. arguta. In contrast with other non-structural carbohydrates, myo-inositol concentrations in A. deliciosa leaves increase only slightly during development from sink to source, and are not affected in source leaves by increased sink demand upon fruit set. In mature fruit-bearing leaves myo-inositol concentrations fluctuate diurnally, increasing during the night and declining towards morning, but in plants with less sink demand a diurnal pattern is not observed. In potted A. arguta seedlings subjected to extended dark periods, leaf concentrations of sugars and starch decline rapidly while myo-inositol concentrations are maintained. Labelling studies with ¹⁴CO₂ revealed that myo-inositol in leaves is not a primary photosynthetic product and is turned over more slowly than other soluble carbohydrates. A suggested role of myo-inositol as a precursor in mucilage synthesis was not substantiated, as radioactivity was incorporated into mucilage more rapidly than into free myo-inositol. Planteose, a trisaccharide comprising sucrose and galactose, incorporated substantial amounts of radioactivity and accumulated to high levels, indicating a role in short-term storage of sucrose. Planteose was synthesised during the day and degraded during the night in a manner that was opposite to that of sucrose while starch and myo-inositol levels remained relatively constant. Planteose has been reported in Cyclamen persicum, ash and sesame seed. This is the first report of planteose in Actinidia, and the first time it has been identified as a major short-term storage carbohydrate in Actinidia leaves.

Fructans, but not the sucrosyl-galactosides, raffinose and loliose, are affected by drought stress in perennial ryegrass

The aim of this study was to evaluate the putative role of the sucrosyl-galactosides, loliose [alpha-D-Gal (1,3) alpha-D-Glc (1,2) beta-D-Fru] and raffinose [alpha-D-Gal (1,6) alpha-D-Glc (1,2) beta-D-Fru], in drought tolerance of perennial ryegrass and to compare it with that of fructans. To that end, the loliose biosynthetic pathway was first established and shown to operate by a UDP-Gal: sucrose (Suc) 3-galactosyltransferase, tentatively termed loliose synthase. Drought stress increased neither the concentrations of loliose and raffinose nor the activities of loliose synthase and raffinose synthase (EC 2.4.1.82). Moreover, the concentrations of the raffinose precursors, myoinositol and galactinol, as well as the gene expressions of myoinositol 1-phosphate synthase (EC 5.5.1.4) and galactinol synthase (EC 2.4.1.123) were either decreased or unaffected by drought stress. Taken together, these data are not in favor of an obvious role of sucrosyl-galactosides in drought tolerance of perennial ryegrass at the vegetative stage. By contrast, drought stress caused fructans to accumulate in leaf tissues, mainly in leaf sheaths and elongating leaf bases. This increase was mainly due to the accumulation of long-chain fructans (degree of polymerization > 8) and was not accompanied by a Suc increase. Interestingly, Suc but not fructan concentrations greatly increased in drought-stressed roots. Putative roles of fructans and sucrosyl-galactosides are discussed in relation to the acquisition of stress tolerance.

Galactokinase of Vicia faba seeds

Galactokinase (EC 2.7.1.6) from the dormant seeds of Vicia faba was purified approximately 1300-fold with an 18% recovery through an eight-step procedure. The preparation showed the presence of only minor contaminations as judged by disc-gel electrophoresis. The native enzyme displayed a molecular weight of approximately 60 000 (determined by Sephadex G-100 gel-filtration) and the subunit value was 30 000. The isoelectric point of the enzyme was 5.3 and the amino acid analysis showed high percentage of acidic amino acids. The pH optimum of the enzyme was 7.3 at 25 degrees C. The relative activity for phosphorylating various monosaccharides followed the order, D-galactose greater than 2-deoxy-D-galactose greater than D-galactosamine; D-fucose, L-arabinose, L-galactose and D-glucose were not phosphorylated. Whereas ATP acted as an efficient phosphate donor, ADP, GTP and UTP were unable to act in this capacity. The Km and the V values of the substrates were determined. The metal ion requirement for the enzymic activity followed the order, Mg2+ greater than Co2+ greater than Mn2+ greater than Ni2+ greater than Ca2+. The enzymic reaction was inhibited by heavy metal ions and sulphydryl reagents indicating the participation of -SH group(s) in enzymic catalysis. Product inhibition was observed; galactose 1-phosphate and ADP were competitive and non-competitive inhibitors, respectively. Seed germination showed an increase in galactokinase level up to 24 h followed by a rapid decrease. The level of raffinose and stachyose decreased continually. The galactokinase level was found to be sufficiently high to phosphorylate the liberated galactose. No free galactose was observed at any stage of germination.