Semi-targeted metabolomic analysis provides the basis for enhanced antioxidant capacities in pigmented rice grains

Histochemical, metabolic and ultrastructural changes in leaf patelliform nectaries explain extrafloral nectar synthesis and secretion in Clerodendrum chinense

BACKGROUND AND AIMS

Extrafloral nectaries are nectar-secreting structures present on vegetative parts of plants which provide indirect defences against herbivore attack. Extrafloral nectaries in Clerodendrum chinense are patelliform-shaped specialized trichomatous structures. However, a complete understanding of patelliform extrafloral nectaries in general, and of C. chinense in particular, has not yet been established to provide fundamental insight into the cellular physiological machinery involved in nectar biosynthesis and secretory processes.

METHODS

We studied temporal changes in the morphological, anatomical and ultrastructural features in the architectures of extrafloral nectaries. We also compared metabolite profiles of extrafloral nectar, nectary tissue, non-nectary tissue and phloem sap. Further, both in situ histolocalization and normal in vitro activities of enzymes related to sugar metabolism were examined.

KEY RESULTS

Four distinct tissue regions in the nectar gland were revealed from histochemical characterization, among which the middle nectariferous tissue was found to be the metabolically active region, while the intermediate layer was found to be lipid-rich. Ultrastructural study showed the presence of a large number of mitochondria along with starch-bearing chloroplasts in the nectariferous region. However, starch depletion was noted with progressive maturation of nectaries. Metabolite analysis revealed compositional differences among nectar, phloem sap, nectary and non-nectary tissue. Invertase activity was higher in secretory stages and localized in nectariferous tissue and adjacent region.

CONCLUSIONS

Our study suggests extrafloral nectar secretion in C. chinense to be both eccrine and merocrine in nature. A distinct intermediate lipid-rich layer that separates the epidermis from nectary parenchyma was revealed, which possibly acts as a barrier to water flow in nectar. This study also revealed a distinction between nectar and phloem sap, and starch could act as a nectar precursor, as evidenced from enzymatic and ultrastructural studies. Thus, our findings on changing architecture of extrafloral nectaries with temporal secretion revealed a cell physiological process involved in nectar biosynthesis and secretion.

Metabolomic analysis for disclosing nutritional and therapeutic prospective of traditional rice cultivars of Cauvery deltaic region, India

Traditional rice is gaining popularity worldwide due to its high nutritional and pharmaceutical value, as well as its high resistance to abiotic and biotic stresses. This has attracted significant attention from breeders, nutritionists, and plant protection scientists in recent years. Hence, it is critical to investigate the grain metabolome to reveal germination and nutritional importance. This research aimed to explore non-targeted metabolites of five traditional rice varieties, viz., Chinnar, Chithiraikar, Karunguruvai, Kichili samba, and Thooyamalli, for their nutritional and therapeutic properties. Approximately 149 metabolites were identified using the National Institute of Standards and Technology (NIST) library and Human Metabolome Database (HMDB) and were grouped into 34 chemical classes. Major classes include fatty acids (31.1-56.3%), steroids and their derivatives (1.80-22.4%), dihydrofurans (8.98-11.6%), prenol lipids (0.66-4.44%), organooxygen compounds (0.12-6.45%), benzene and substituted derivatives (0.53-3.73%), glycerolipids (0.36-2.28%), and hydroxy acids and derivatives (0.03-2.70%). Significant variations in metabolite composition among the rice varieties were also observed through the combination of univariate and multivariate statistical analyses. Principal component analysis (PCA) reduced the dimensionality of 149 metabolites into five principle components (PCs), which explained 96% of the total variance. Two clusters were revealed by hierarchical cluster analysis, indicating the distinctiveness of the traditional varieties. Additionally, a partial least squares-discriminant analysis (PLS-DA) found 17 variables important in the projection (VIP) scores of metabolites. The findings of this study reveal the biochemical intricate and distinctive metabolomes of the traditional therapeutic rice varieties. This will serve as the foundation for future research on developing new rice varieties with traditional rice grain metabolisms to increase grain quality and production with various nutritional and therapeutic benefits.

Simultaneous Quantitation and Discovery (SQUAD) Analysis: Combining the Best of Targeted and Untargeted Mass Spectrometry-Based Metabolomics

Untargeted and targeted approaches are the traditional metabolomics workflows acquired for a wider understanding of the metabolome under focus. Both approaches have their strengths and weaknesses. The untargeted, for example, is maximizing the detection and accurate identification of thousands of metabolites, while the targeted is maximizing the linear dynamic range and quantification sensitivity. These workflows, however, are acquired separately, so researchers compromise either a low-accuracy overview of total molecular changes (i.e., untargeted analysis) or a detailed yet blinkered snapshot of a selected group of metabolites (i.e., targeted analysis) by selecting one of the workflows over the other. In this review, we present a novel single injection simultaneous quantitation and discovery (SQUAD) metabolomics that combines targeted and untargeted workflows. It is used to identify and accurately quantify a targeted set of metabolites. It also allows data retro-mining to look for global metabolic changes that were not part of the original focus. This offers a way to strike the balance between targeted and untargeted approaches in one single experiment and address the two approaches' limitations. This simultaneous acquisition of hypothesis-led and discovery-led datasets allows scientists to gain more knowledge about biological systems in a single experiment.

Geographical Origin Differentiation of Rice by LC–MS-Based Non-Targeted Metabolomics

Many factors, such as soil, climate, and water source in the planting area, can affect rice taste and quality. Adulterated rice is common in the market, which seriously damages the production and sales of high-quality rice. Traceability analysis of rice has become one of the important research fields of food safety management. In this study, LC–MS-based non-targeted metabolomics technology was used to trace four rice samples from Heilongjiang and Jiangsu Provinces, namely, Daohuaxiang (DH), Huaidao No. 5 (HD), Songjing (SJ), and Changlixiang (CL). Results showed that the discrimination accuracy of the partial least squares discriminant analysis (PLS-DA) model was as high as 100% with satisfactory prediction ability. A total of 328 differential metabolites were screened, indicating significant differences in rice metabolites from different origins. Pathway enrichment analysis was carried out on the four rice samples based on the KEGG database to determine the three metabolic pathways with the highest enrichment degree. The main biochemical metabolic pathways and signal transduction pathways involved in differential metabolites in rice were obtained. This study provides theoretical support for the geographical origins of rice and elucidates the change mechanism of rice metabolic pathways, which can shed light on improving rice quality control.

Formulation and Characterization of Gum Arabic Stabilized Red Rice Extract Nanoemulsion

Interest in the utilization of plant-based bioactive compounds in foods has increased due to their biochemical activities and as alternatives in the reduction of high concentrations of chemical utilization. However, some of these additives are hydrophobic, thus being harder to disperse into the hydrophilic food matrix. Therefore, an oil-in-water nanoemulsion (RRE1-RRE10) was formulated with different concentrations of red rice extract (1-10% w/v). Nanoemulsion showed droplet sizes within the range of 157.33-229.71 nm and the best formulation (RRE5) was selected based on the creaming index which was stable to flocculation over a range of temperatures (30-90 °C), pH (2-9), and salt concentration (100-600 mM). It showed significantly improved antioxidant and anti-inflammatory activity as compared to its other counterparts. Potential antimicrobial activity against Staphylococcus aureus was attributed to RRE5 nanoemulsion as compared to Escherichia coli. Therefore, due to the potential bioactivity of RRE5 nanoemulsion, it can be scaled up at the industrial level.

Metabolite Profiling Reveals Distinct Modulation of Complex Metabolic Networks in Non-Pigmented, Black, and Red Rice (Oryza sativa L.) Cultivars

Comprehensive profiling of primary and secondary metabolites was performed to understand metabolic differences associated with color formation in pigmented rice (Oryza sativa L.). Overall, 110 metabolites from non-pigmented, black, and red rice cultivars were identified. Black and red rice contained high levels of flavonoids associated with plant color. Black rice also contained high levels of terpenoids (carotenoids, tocopherols, phytosterols, and monoterpenes). The non-pigmented rice contained relatively low levels of secondary metabolites. Multivariate and pathway analyses were performed to data-mine the metabolite profiles. Hierarchical clustering analysis of correlation coefficients revealed metabolite clusters based on nitrogen and carbon sources. These clusters suggested a negative correlation between nitrogen and carbon. Pathway analysis revealed that black rice was rich in carbon-based secondary metabolites, with relatively low levels of primary metabolites compared with other rice cultivars. These data highlight the complex interactions between nitrogen and carbon metabolism of primary and secondary metabolites in rice. For the first time, the relationships and metabolic differences in terpenoid content (monoterpenes, triterpenes, and tetraterpenes) of non-pigmented and pigmented rice cultivars were analyzed. These findings should greatly contribute to the understanding of pigmented rice metabolome and inform breeding programs for new rice cultivars.

Glandular trichomes of the flowers and leaves in Millingtonia hortensis (Bignoniaceae)

Three types of the glandular trichomes are developed on the flowers and leaves of Millingtonia hortensis. Morphology, cell ultrastructure and content of the volatile compounds are specific to each trichome type. The aim of this study was to characterize the structural and histochemical features of the glandular trichomes (GTs) of two types localized on the different flower parts and leaves in Millingtonia hortensis, as well as to identify the composition of the internal pool of metabolites. The peltate GTs are most common; they are founded on peduncle, calyx, ovary, and leaves. GTs consist of 12-24-cell disk-shaped head and a single-celled neck. The capitate GTs are located on corolla tube and have four to eight-cell head, single-celled neck and a wide multicellular stalk. A series of histochemical reactions and fluorescent microscopy revealed the various substances in the chemical composition of GTs. Acid polysaccharides are predominately identified in the capitate trichomes of the corolla tube and peltate trichomes of calyx, terpenes present in larger quantity in the trichomes of the corolla tube and ovary, whilst phenolic substances prevail in the trichomes of the calyx and ovary. GTs of each type are characterized by specific ultrastructural traits. Smooth endoplasmic reticulum (SER) and leucoplasts prevail in the peltate trichomes of peduncle, calyx and ovary; Golgi apparatus is the common organelle in the capitate trichomes of the corolla tube and peltate trichomes of calyx; the huge aggregates of the RER cisterns there are in cytoplasm of all leaf trichomes. Synthesized secretion accumulates in the subcuticular cavity of all GTs except the leaf peltate trichomes. In the trichomes of the leaves secretion is stored in the thick upper cell wall with the wide cutinized layer. For the first time content of the internal pool of metabolites from the flowers and leaves was identified by GC-MS. Seventeen compounds, including alcohols, fatty acid derivatives, monoterpenes, sesquiterpenes, and benzenoids were identified. 1-octen 3-ol, 3-carene, methyl salicylate, p-hydroxybenzeneethanol and 1-hydroxy-2,4-di-tertbutyl-benzene were the main compounds of the flower scent. We consider GTs of the reproductive organs in M. hortensis synthesizing acid polysaccharides and volatile compounds as secretory structures attracting of pollinators, whereas the leaf peltate trichomes accumulating predominately non-volatile phenols, protect young vegetative shoots against small herbivorous insects and pathogens.