Planteose as a storage carbohydrate required for early stage of germination of Orobanche minor and its metabolism as a possible target for selective control

Cycloheximide in the nanomolar range inhibits seed germination of Orobanche minor

From the 992 samples of culture extracts of microorganisms isolated from soil in Japan, we found that the extract of Streptomyces sp. no. 226 inhibited Orobanche minor seed germination without significantly affecting the seed germination of Trifolium pratense and the growth of Aspergillus oryzae and Escherichia coli. Using ESI-MS, 1H-NMR, and 13C-NMR, we identified the active compound as cycloheximide. Cycloheximide had half-maximum inhibitory concentrations of 2.6 ng/mL for the inhibition of seed germination of O. minor and 2.5 µg/mL for that of the conidial germination of A. oryzae. Since cycloheximide is known to inhibit translation by interacting with ribosomal protein L28 (RPL28) in yeast, we investigated whether RPL protein of O. minor plays a critical role in the inhibition of O. minor seed germination. Our data suggested that O. minor RPL27A was not sensitive to cycloheximide by comparing it to the strain expressing S. cerevisiae RPL28. These findings suggest the presence of an unidentified mechanism by which cycloheximide hinders O. minor seed germination.

Discrimination of raffinose and planteose based on porous graphitic carbon chromatography in combination with mass spectrometry

Raffinose and planteose are non-reducing, isomeric trisaccharides present in many higher plants. Structurally, they differ in the linkage of α-D-galactopyranosyl to either glucose C(6) or to C (6') of fructose, respectively and thus differentiating each other is very challenging. The negative ion mode mass spectrometric analysis is shown to distinguish planteose and raffinose. However, to facilitate the robust identification of planteose in complex mixtures, herein, we have demonstrated the use of porous graphitic carbon (PGC) chromatography combined with QTOF-MS² analysis. The separation of planteose and raffinose was achieved on PGC, wherein both have recorded different retention time. Detection through MS² analysis revealed the specific fragmentation patterns for planteose and raffinose that are distinctive to each other. The applicability of this method on oligosaccharides pool extracted from different seeds showed clear separation of planteose that allowed unambiguous identification from complex mixtures. Therefore, we propose PGC-LC-MS/MS can be employed for sensitive, throughput screening of planteose from wider plant sources.

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.

The Effect of 10 Crop Plants That Served as Hosts on the Primary Metabolic Profile of the Parasitic Plant Phelipanche aegyptiaca

Phelipanche aegyptiaca Pers. is a holoparasitic plant that parasitizes various types of host plants. Its penetration into host roots causes a massive reduction in the yield of many crop plants worldwide. The nature of the compounds taken by the parasite from its host is still under debate in the scientific literature. To gain more knowledge about the effect of the hosts on the parasite's primary metabolic profile, GC-MS analyses were conducted on the parasites that developed on 10 hosts from four plant families. There are three hosts from each family: Brassicaceae, Apiaceae and Solanaceae and one host from Fabaceae. The results showed significant differences in the metabolic profiles of P. aegyptiaca collected from the different hosts, indicating that the parasites rely strongly on the host's metabolites. Generally, we found that the parasites that developed on Brassicaceae and Fabaceae accumulated more amino acids than those developed on Apiaceae and Solanaceae that accumulated more sugars and organic acids. The contents of amino acids correlated positively with the total soluble proteins. However, the aromatic amino acid, tyrosine, correlated negatively with the accumulation of the total phenolic compounds. This study contributes to our knowledge of the metabolic relationship between host and parasite.

Comparative transcriptomic analysis provides insights into the molecular basis underlying pre-harvest sprouting in rice

BACKGROUND

Pre-harvest sprouting (PHS) is one of the most serious rice production constraints in areas where prolonged rainfall occurs during harvest. However, the molecular mechanisms of transcriptional regulation underlying PHS remain largely unknown.

RESULTS

In the current study, comparative transcriptome analyses were performed to characterize the similarities and differences between two rice varieties: PHS-sensitive Jiuxiangzhan (JXZ) and PHS-resistant Meixiangxinzhan (MXXZ). The physiological experimental results indicated that PHS causes a significant decrease in starch content and, in contrast, a significant increase in soluble sugar content and amylase activity. The extent of change in these physiological parameters in the sensitive variety JXZ was greater than that in the resistant variety MXXZ. A total of 9,602 DEGs were obtained from the transcriptome sequencing data, and 5,581 and 4,021 DEGs were identified in JXZ and MXXZ under high humidity conditions, respectively. The KEGG pathway enrichment analysis indicated that many DEGs under high humidity treatment were mainly linked to plant hormone signal transduction, carbon metabolism, starch and sucrose metabolism, and phenylpropanoid biosynthesis. Furthermore, the number of upregulated genes involved in these pathways was much higher in JXZ than in MXXZ, while the number of downregulated genes was higher in MXXZ than in JXZ. These results suggest that the physiological and biochemical processes of these pathways are more active in the PHS-sensitive JXZ than in the PHS-resistant MXXZ.

CONCLUSION

Based on these results, we inferred that PHS in rice results from altered phytohormone regulation, more active carbon metabolism and energy production, and enhanced phenylpropanoid biosynthesis. Our study provides a theoretical foundation for further elucidation of the complex regulatory mechanism of PHS in rice and the molecular breeding of PHS-resistant rice varieties.

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 a Host on the Primary Metabolic Profiling of Cuscuta Campestris’ Main Organs, Haustoria, Stem and Flower

Cuscuta campestris (dodder) is a stem holoparasitic plant without leaves or roots that parasitizes various types of host plants and causes damage to certain crops worldwide. This study aimed at gaining more knowledge about the effect of the hosts on the parasite’s levels of primary metabolites. To this end, metabolic profiling analyses were performed on the parasite’s three main organs, haustoria, stem and flowers, which developed on three hosts, Heliotropium hirsutissimum, Polygonum equisetiforme and Amaranthus viridis. The results showed significant differences in the metabolic profiles of C. campestris that developed on the different hosts, suggesting that the parasites rely highly on the host’s metabolites. However, changes in the metabolites’ contents between the organs that developed on the same host suggest that the parasite can also self-regulate its metabolites. Flowers, for example, have significantly higher levels of most of the amino acids and sugar acids, while haustoria and stem have higher levels of several sugars and polyols. Determination of total soluble proteins and phenolic compounds showed that the same pattern is detected in the organs unrelated to the hosts. This study contributes to our knowledge about the metabolic behavior of this parasite.

The composition of Australian Plantago seeds highlights their potential as nutritionally-rich functional food ingredients

When wetted, Plantago seeds become covered with a polysaccharide-rich gel called mucilage that has value as a food additive and bulking dietary fibre. Industrially, the dry husk layer that becomes mucilage, called psyllium, is milled off Plantago ovata seeds, the only commercial-relevant Plantago species, while the residual inner seed tissues are either used for low value animal feed or discarded. We suggest that this practice is potentially wasting a highly nutritious resource and here describe the use of histological, physicochemical, and chromatographic analyses to compare whole seed composition/characteristics of P. ovata with 11 relatives already adapted to harsh Australian conditions that may represent novel commercial crop options. We show that substantial interspecific differences in mucilage yield and macromolecular properties are mainly a consequence of differences in heteroxylan and pectin composition and probably represent wide differences in hydrocolloid functionality that can be exploited in industry. We also show that non-mucilage producing inner seed tissues contain a substantial mannan-rich endosperm, high in fermentable sugars, protein, and fats. Whole seed Plantago flour, particularly from some species obtained from harsh Australian environments, may provide improved economic and health benefits compared to purified P. ovata psyllium husk, by retaining the functionality of the seed mucilage and providing additional essential nutrients.

Germination stimulatory activity of bacterial butenolide hormones from Streptomyces albus J1074 on seeds of the root parasitic weed Orobanche minor

Damage caused by Orobanchaceae root parasitic weeds is a substantial agricultural problem for global food security. Many studies have been conducted to establish practical methods of control, but efforts are still required for successful management. Seed germination of root parasitic weeds requires host-derived germination stimulants including strigolactones (SLs). Studies on SLs have revealed that a butenolide ring is the essential moiety for SL activity as a germination stimulant. Interestingly, recent studies have revealed that butenolide hormones regulate the biosynthesis of secondary metabolites and mediate communication in actinomycete bacteria. Because of the structural similarity between SLs and the bacterial butenolides, we evaluated the germination stimulatory activity of butenolides isolated from Streptomyces albus J1074 on root parasitic weeds. These butenolides were found to specifically induce seed germination of Orobanche minor. Our findings contribute to understanding the molecular mechanisms of germination stimulant perception and to the development of a method for their biological control.

Integrated Analysis of the Transcriptome and Metabolome Revealed Candidate Genes Involved in GA3-Induced Dormancy Release in Leymus chinensis Seeds

Leymus chinensis is a perennial forage grass that has good palatability, high yield and high feed value, but seed dormancy is a major problem limiting the widespread cultivation of L. chinensis. Here, we performed transcriptomic and metabolomic analysis of hulled and de-hulled seeds of L. chinensis treated with or without GA₃ to investigate the changes in gene and metabolites associated with dormancy release induced by GA₃. The germination test revealed that the optimum concentration of GA₃ for disruption of L. chinensis seed dormancy was 577 μM. A total of 4327 and 11,919 differentially expressed genes (DEGs) and 871 and 650 differentially abundant metabolites were identified in de-hulled and hulled seeds treated with GA₃, respectively, compared with seeds soaked in sterile water. Most of the DEGs were associated with starch and sucrose metabolism, protein processing in the endoplasmic reticulum, endocytosis and ribosomes. Furthermore, isoquinoline alkaloid biosynthesis, tyrosine metabolism, starch and sucrose metabolism, arginine and proline metabolism, and amino sugar and nucleotide sugar metabolism were significantly enriched pathways. Integrative analysis of the transcriptomic and metabolomic data revealed that starch and sucrose metabolism is one of the most important pathways that may play a key role in providing carbon skeletons and energy supply for the transition of L. chinensis seeds from a dormant state to germination by suppressing the expression of Cel61a, egID, cel1, tpsA, SPAC2E11.16c and TPP2, enhancing the expression of AMY1.1, AMY1.2, AMY1.6 and GLIP5, and inhibiting the synthesis of cellobiose, cellodextrin, and trehalose while promoting the hydrolysis of sucrose, starch, cellobiose, cellodextrin, and trehalose to glucose. This study identified several key genes and provided new insights into the molecular mechanism of seed dormancy release induced by GA₃ in L. chinensis. These putative genes will be valuable resources for improving the seed germination rate in future breeding studies.

Metabolic Analysis Reveals Cry1C Gene Transformation Does Not Affect the Sensitivity of Rice to Rice Dwarf Virus

Metabolomics is beginning to be used for assessing unintended changes in genetically modified (GM) crops. To investigate whether Cry1C gene transformation would induce metabolic changes in rice plants, and whether the metabolic changes would pose potential risks when Cry1C rice plants are exposed to rice dwarf virus (RDV), the metabolic profiles of Cry1C rice T1C-19 and its non-Bt parental rice MH63 under RDV-free and RDV-infected status were analyzed using gas chromatography–mass spectrometry (GC-MS). Compared to MH63 rice, slice difference was detected in T1C-19 under RDV-free conditions (less than 3%), while much more metabolites showed significant response to RDV infection in T1C-19 (15.6%) and in MH63 (5.0%). Pathway analysis showed biosynthesis of lysine, valine, leucine, and isoleucine may be affected by RDV infection in T1C-19. No significant difference in the contents of free amino acids (AAs) was found between T1C-19 and MH63 rice, and the free AA contents of the two rice plants showed similar responses to RDV infection. Furthermore, no significant differences of the RDV infection rates between T1C-19 and MH63 were detected. Our results showed the Cry1C gene transformation did not affect the sensitivity of rice to RDV, indicating Cry1C rice would not aggravate the epidemic and dispersal of RDV.

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.

Developmental patterns of enzyme activity, gene expression, and sugar content in sucrose metabolism of two broomrape species

A better understanding of broomrape physiological features opens up new perspectives for developing specific management strategies. For this purpose, activities of key enzymes involved in osmoregulation (SAI1, CWI, M6PR, and SUS1) were considered at developmental stages of two important broomrape species (Egyptian and branched broomrape) on tomato. While Egyptian broomrape tubercles had high activities of invertases, branched broomrape shoots revealed high activities of M6PR and SUS1 during both pre- and post-emergence stages except for M6PR at post-emergence stages of P. aegyptiaca. Interestingly, the main accumulation of total reducing sugars was detected in tubercle during pre- and in shoot during post-emergence. Unlike low levels of genes expression (except for CWI) before parasite emergence, significantly higher expression levels of SAI1, SUS1 and M6PR were detected after parasite emergence. Matching the expression levels of SAI1 and SUS1 genes with their corresponding enzymes activities makes them as the suitable candidates for gene silencing strategies.

Comparative Metabolomics of Early Development of the Parasitic Plants Phelipanche aegyptiaca and Triphysaria versicolor

Parasitic weeds of the family Orobanchaceae attach to the roots of host plants via haustoria capable of drawing nutrients from host vascular tissue. The connection of the haustorium to the host marks a shift in parasite metabolism from autotrophy to at least partial heterotrophy, depending on the level of parasite dependence. Species within the family Orobanchaceae span the spectrum of host nutrient dependency, yet the diversity of parasitic plant metabolism remains poorly understood, particularly during the key metabolic shift surrounding haustorial attachment. Comparative profiling of major metabolites in the obligate holoparasite Phelipanche aegyptiaca and the facultative hemiparasite Triphysaria versicolor before and after attachment to the hosts revealed several metabolic shifts implicating remodeling of energy and amino acid metabolism. After attachment, both parasites showed metabolite profiles that were different from their respective hosts. In P. aegyptiaca, prominent changes in metabolite profiles were also associated with transitioning between different tissue types before and after attachment, with aspartate levels increasing significantly after the attachment. Based on the results from 15N labeling experiments, asparagine and/or aspartate-rich proteins were enriched in host-derived nitrogen in T. versicolor. These results point to the importance of aspartate and/or asparagine in the early stages of attachment in these plant parasites and provide a rationale for targeting aspartate-family amino acid biosynthesis for disrupting the growth of parasitic weeds.

Enhanced production of nojirimycin via Streptomyces ficellus cultivation using marine broth and inhibitory activity of the culture for seeds of parasitic weeds

Root parasitic weeds, such as Orobanche spp. and Striga spp., cause serious damage to crops. Recently, it was demonstrated that nojirimycin (NJ) selectively inhibits seed germination in these weeds. In this study, we modified the medium for Streptomyces ficellus to increase its production of NJ and evaluated the culture as an antiparasitic weed agent. We screened alternatives to Pharmamedia™, an additive in the original medium, and found that marine broth stimulated NJ production. Moreover, soluble starch-depleted medium could maintain S. ficellus NJ production. The NJ concentration reached 710 mg/L after four-day batch culture in starch-depleted marine broth medium, which was 17-fold higher than that in the Pharmamedia™ medium. The culture in the marine broth medium inhibited seed germination of Orobanche spp. and Striga spp. as effectively as a standard solution of NJ.

Isolation and structural elucidation by 2D NMR of planteose, a major oligosaccharide in the mucilage of chia (Salvia hispanica L.) seeds

An oligosaccharide was isolated in high purity and excellent yield from the water-extractable mucilage of chia (Salvia hispanica L.) seeds using an optimized solid-phase extraction method. LC-MS analysis showed that the compound presents a molecular mass of 504Da and trifluoroacetic acid hydrolysis revealed that it consists of galactose, glucose and fructose. Glycosidic linkage analysis showed that the oligosaccharide contains two non-reducing ends corresponding to terminal glucopyranose and terminal galactopyranose, respectively. The oligosaccharide was identified as planteose by the complete assignment of a series of 2D NMR spectra (COSY, TOCSY, ROESY, HSQC, and HMBC). The significance of the presence of planteose in chia seeds is discussed in the context of nutrition and food applications.

Seed Metabolome Analysis of a Transgenic Rice Line Expressing Cholera Toxin B-subunit

Plant-based human vaccines have been actively developed in recent years, and rice (Oryza sativa L.) is one of the best candidate crops for their production and delivery. By expressing a modified cholera toxin B (CTB) subunit, we previously developed MucoRice-CTB, a rice-based vaccine against cholera, which is caused by infection of the intestine with the bacteria Vibrio cholerae. MucoRice-CTB lines have been extensively characterized by whole-genome sequencing and proteome analyses to evaluate the mutation profiles and proteome status, respectively. Here, we report non-targeted metabolomic profiling of the MucoRice-CTB transgenic rice line 51A (MR-CTB51A), MucoRice-RNAi (MR-RNAi), and their non-transgenic parent line by using gas chromatography-time-of-flight mass spectrometry. The levels of several amino acids, organic acids, carbohydrates, lipids, and secondary metabolites were significantly increased in MR-CTB51A compared with the non-transgenic parent line. These metabolomics results complement essential information obtained by genome sequencing and proteomics approaches, thereby contributing to comprehensive understanding of the properties of MucoRice-CTB as a plant-based vaccine.

Primary metabolic profiling of Egyptian broomrape (Phelipanche aegyptiaca) compared to its host tomato roots

Broomrape (Phelipanche aegyptiaca) is a root holoparasitic plant considered among the most destructive agricultural weeds worldwide. In order to acquire more knowledge about the metabolism of broomrape and its interaction with its tomato host, we performed primary metabolic profiling using GCMS analysis for the early developmental stage of the parasite and of infected and non-infected roots. The analysis revealed that out of 59 metabolites detected, the levels of 37 significantly increased in the parasite while the levels of 10 significantly decreased compared to the infected roots. In addition, the analysis showed that the levels of total protein in the albumin fraction, reducing sugars (representing starch) and total phenols increased by 9.8-, 4.6- and 3.3-fold, respectively, in the parasite compared to the roots. These changes suggest that P. aegyptiaca has its own metabolism that differs significantly in its regulation from those found in their host. In addition, the results have shown that the levels of most of the metabolites in the infected roots were similar to levels detected in the non-infected roots, except for seven metabolites whose levels increased in the infected versus the non-infected roots. This suggests that the parasite did not significantly affect the host primary metabolic pathways.