Transgenic rice seed synthesizing diverse flavonoids at high levels: a new platform for flavonoid production with associated health benefits

Biofortified Rice Provides Rich Sakuranetin in Endosperm

Sakuranetin plays a key role as a phytoalexin in plant resistance to biotic and abiotic stresses, and possesses diverse health-promoting benefits. However, mature rice seeds do not contain detectable levels of sakuranetin. In the present study, a transgenic rice plant was developed in which the promoter of an endosperm-specific glutelin gene OsGluD-1 drives the expression of a specific enzyme naringenin 7-O-methyltransferase (NOMT) for sakuranetin biosynthesis. The presence of naringenin, which serves as the biosynthetic precursor of sakuranetin made this modification feasible in theory. Liquid chromatography tandem mass spectrometry (LC-MS/MS) validated that the seeds of transgenic rice accumulated remarkable sakuranetin at the mature stage, and higher at the filling stage. In addition, the panicle blast resistance of transgenic rice was significantly higher than that of the wild type. Specially, the matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) imaging was performed to detect the content and spatial distribution of sakuranetin and other nutritional metabolites in transgenic rice seeds. Notably, this genetic modification also did not change the nutritional and quality indicators such as soluble sugars, total amino acids, total flavonoids, amylose, total protein, and free amino acid content in rice. Meanwhile, the phenotypes of the transgenic plant during the whole growth and developmental periods and agricultural traits such as grain width, grain length, and 1000-grain weight exhibited no significant differences from the wild type. Collectively, the study provides a conceptual advance on cultivating sakuranetin-rich biofortified rice by metabolic engineering. This new breeding idea may not only enhance the disease resistance of cereal crop seeds but also improve the nutritional value of grains for human health benefits.

Tartary buckwheat rutin: Accumulation, metabolic pathways, regulation mechanisms, and biofortification strategies

Rutin is a significant flavonoid with strong antioxidant property and various therapeutic effects. It plays a crucial role in disease prevention and human health maintenance, especially in anti-inflammatory, antidiabetic, hepatoprotective and cardiovascular effects. While many plants can synthesize and accumulate rutin, tartary buckwheat is the only food crop possessing high levels of rutin. At present, the rutin content (RC) is regarded as the key index for evaluating the nutritional quality of tartary buckwheat. Consequently, rutin has become the focus for tartary buckwheat breeders and has made considerable progress. Here, we summarize research on the rutin in tartary buckwheat in the past two decades, including its accumulation, biosynthesis and breakdown pathways, and regulatory mechanisms. Furthermore, we propose several strategies to increase the RC in tartary buckwheat seeds based on current knowledge. This review aims to provide valuable references for elevating the quality of tartary buckwheat in the future.

A universal design of restructured dimer antigens: Development of a superior vaccine against the paramyxovirus in transgenic rice

The development of vaccines, which induce effective immune responses while ensuring safety and affordability, remains a substantial challenge. In this study, we proposed a vaccine model of a restructured "head-to-tail" dimer to efficiently stimulate B cell response. We also demonstrate the feasibility of using this model to develop a paramyxovirus vaccine through a low-cost rice endosperm expression system. Crystal structure and small-angle X-ray scattering data showed that the restructured hemagglutinin-neuraminidase (HN) formed tetramers with fully exposed quadruple receptor binding domains and neutralizing epitopes. In comparison with the original HN antigen and three traditional commercial whole virus vaccines, the restructured HN facilitated critical epitope exposure and initiated a faster and more potent immune response. Two-dose immunization with 0.5 μg of the restructured antigen (equivalent to one-127th of a rice grain) and one-dose with 5 μg completely protected chickens against a lethal challenge of the virus. These results demonstrate that the restructured HN from transgenic rice seeds is safe, effective, low-dose useful, and inexpensive. We provide a plant platform and a simple restructured model for highly effective vaccine development.

Engineered coumarin accumulation reduces mycotoxin-induced oxidative stress and disease susceptibility

Coumarins can fight pathogens and are thus promising for crop protection. Their biosynthesis, however, has not yet been engineered in crops. We tailored the constitutive accumulation of coumarins in transgenic Nicotiana benthamiana, Glycine max and Arabidopsis thaliana plants, as well as in Nicotiana tabacum BY-2 suspension cells. We did so by overexpressing A. thaliana feruloyl-CoA 6-hydroxylase 1 (AtF6'H1), encoding the key enzyme of scopoletin biosynthesis. Besides scopoletin and its glucoside scopolin, esculin at low level was the only other coumarin detected in transgenic cells. Mechanical damage of scopolin-accumulating tissue led to a swift release of scopoletin, presumably from the scopolin pool. High scopolin levels in A. thaliana roots coincided with reduced susceptibility to the root-parasitic nematode Heterodera schachtii. In addition, transgenic soybean plants were more tolerant to the soil-borne pathogenic fungus Fusarium virguliforme. Because mycotoxin-induced accumulation of reactive oxygen species and cell death were reduced in the AtF6'H1-overexpressors, the weaker sensitivity to F. virguliforme may be caused by attenuated oxidative damage of coumarin-hyperaccumulating cells. Together, engineered coumarin accumulation is promising for enhanced disease resilience of crops.

Geographic differences and variation of functional components of brown rice in 690 mini-core collections from global germplasms

Objective

To understand the geographic differences and variations in the functional components of brown rice cores collected from global rice germplasms.

Methods

Four functional components, γ-aminobutyric acid (GABA), resistant starch (RS), total flavonoids, and alkaloids, in brown rice from 690 mini-core collections from 31 countries from five continents and the International Rice Research Institute, were analyzed using a spectrophotometry colorimetric method, and the results were statistically validated.

Conclusion

The highest average amounts of functional components were obtained in Asian germplasms, except for GABA, and total flavonoids were highest in brown rice from Europe and Oceania, followed by Asia. The highest coefficient of variation for GABA was observed in Asia; that for RS and total flavonoids was observed in Africa, followed by Asia; and that for alkaloids was observed in America, followed by Asia. Overall, Asian countries were the most prominent and representative zones with the highest genotypic potential for functional components of brown rice. Forty-one rice accessions with enriched functional components originated mostly from biodiversity-rich areas in China, followed by those in the Philippines. Late sowing favored the enrichment of these components in brown rice. The current study provides a reference for rice breeding with enriched functional constituents, and guidelines for screening functional rice that could be used for human chronic disease research.

Strategies to fortify the nutritional values of polished rice by implanting selective traits from brown rice: A nutrigenomics-based approach

Whole-grain cereals are important components of a healthy diet. It reduces the risk of many deadly diseases like cardiovascular diseases, diabetes, cancer, etc. Brown rice is an example of whole grain food, which is highly nutritious due to the presence of various bioactive compounds (flavonoids, phenolics, vitamins, phytosterols, oils, etc.) associated with the rice bran layer of brown rice. White rice is devoid of the nutritious rice bran layer and thus lacks the bioactive compounds which are the major attractants of brown rice. Therefore, to confer health benefits to the public at large, the nutrigenomic potential of white rice may be improved by integrating the phytochemicals associated with the rice bran layer of brown rice into it via biofortification processes like conventional breeding, agronomic practices, metabolic engineering, CRISPR/Cas9 technology, and RNAi techniques. Thus, this review article focuses on improving the nutritional qualities of white/polished rice through biofortification processes, utilizing new breeding technologies (NBTs).

Biofortification: an approach to eradicate micronutrient deficiency

Micronutrient deficiency also known as "hidden hunger" refers to a condition that occurs when the body lacks essential vitamins and minerals that are required in small amounts for proper growth, development and overall health. These deficiencies are particularly common in developing countries, where a lack of access to a varied and nutritious diet makes it difficult for people to get the micronutrients they need. Micronutrient supplementation has been a topic of interest, especially during the Covid-19 pandemic, due to its potential role in supporting immune function and overall health. Iron (Fe), zinc (Zn), iodine (I), and selenium (Se) deficiency in humans are significant food-related issues worldwide. Biofortification is a sustainable strategy that has been developed to address micronutrient deficiencies by increasing the levels of essential vitamins and minerals in staple crops that are widely consumed by people in affected communities. There are a number of agricultural techniques for biofortification, including selective breeding of crops to have higher levels of specific nutrients, agronomic approach using fertilizers and other inputs to increase nutrient uptake by crops and transgenic approach. The agronomic approach offers a temporary but speedy solution while the genetic approach (breeding and transgenic) is the long-term solution but requires time to develop a nutrient-rich variety.

Metabolic engineering of Escherichia coli for 2,4-dinitrotoluene degradation

2,4-Dinitrotoluene (2,4-DNT) as a common industrial waste has been massively discharged into the environment with industrial wastewater. Due to its refractory degradation, high toxicity, and bioaccumulation, 2,4-DNT pollution has become increasingly serious. Compared with the currently available physical and chemical methods, in situ bioremediation is considered as an economical and environmentally friendly approach to remove toxic compounds from contaminated environment. In this study, we relocated a complete degradation pathway of 2,4-DNT into Escherichia coli to degrade 2,4-DNT completely. Eight genes from Burkholderia sp. strain were re-synthesized by PCR-based two-step DNA synthesis method and introduced into E. coli. Degradation experiments revealed that the transformant was able to degrade 2,4-DNT completely in 12 h when the 2,4-DNT concentration reached 3 mM. The organic acids in the tricarboxylic acid cycle were detected to prove the degradation of 2,4-DNT through the artificial degradation pathway. The results proved that 2,4-DNT could be completely degraded by the engineered bacteria. In this study, the complete degradation pathway of 2,4-DNT was constructed in E. coli for the first time using synthetic biology techniques. This research provides theoretical and experimental bases for the actual treatment of 2,4-DNT, and lays a technical foundation for the bioremediation of organic pollutants.

Designing plant flavonoids: harnessing transcriptional regulation and enzyme variation to enhance yield and diversity

Plant synthetic biology has emerged as a powerful and promising approach to enhance the production of value-added metabolites in plants. Flavonoids, a class of plant secondary metabolites, offer numerous health benefits and have attracted attention for their potential use in plant-based products. However, achieving high yields of specific flavonoids remains challenging due to the complex and diverse metabolic pathways involved in their biosynthesis. In recent years, synthetic biology approaches leveraging transcription factors and enzyme diversity have demonstrated promise in enhancing flavonoid yields and expanding their production repertoire. This review delves into the latest research progress in flavonoid metabolic engineering, encompassing the identification and manipulation of transcription factors and enzymes involved in flavonoid biosynthesis, as well as the deployment of synthetic biology tools for designing metabolic pathways. This review underscores the importance of employing carefully-selected transcription factors to boost plant flavonoid production and harnessing enzyme promiscuity to broaden flavonoid diversity or streamline the biosynthetic steps required for effective metabolic engineering. By harnessing the power of synthetic biology and a deeper understanding of flavonoid biosynthesis, future researchers can potentially transform the landscape of plant-based product development across the food and beverage, pharmaceutical, and cosmetic industries, ultimately benefiting consumers worldwide.

Biotechnologically Engineered Plants

The development of recombinant DNA technology during the past thirty years has enabled scientists to isolate, characterize, and manipulate a myriad of different animal, bacterial, and plant genes. This has, in turn, led to the commercialization of hundreds of useful products that have significantly improved human health and well-being. Commercially, these products have been mostly produced in bacterial, fungal, or animal cells grown in culture. More recently, scientists have begun to develop a wide range of transgenic plants that produce numerous useful compounds. The perceived advantage of producing foreign compounds in plants is that compared to other methods of producing these compounds, plants seemingly provide a much less expensive means of production. A few plant-produced compounds are already commercially available; however, many more are in the production pipeline.

Heterologous mogrosides biosynthesis in cucumber and tomato by genetic manipulation

Mogrosides are widely used as high-value natural zero-calorie sweeteners that exhibit an array of biological activities and allow for vegetable flavour breeding by modern molecular biotechnology. In this study, we developed an In-fusion based gene stacking strategy for transgene stacking and a multi-gene vector harbouring 6 mogrosides biosynthesis genes and transformed it into Cucumis sativus and Lycopersicon esculentum. Here we show that transgenic cucumber can produce mogroside V and siamenoside I at 587 ng/g FW and 113 ng/g FW, respectively, and cultivated transgenic tomato with mogroside III. This study provides a strategy for vegetable flavour improvement, paving the way for heterologous biosynthesis of mogrosides.

Molecular farming using transgenic rice endosperm

Plant expression platforms are low-cost, scalable, safe, and environmentally friendly systems for the production of recombinant proteins and bioactive metabolites. Rice (Oryza sativa L.) endosperm is an ideal bioreactor for the production and storage of high-value active substances, including pharmaceutical proteins, oral vaccines, vitamins, and nutraceuticals such as flavonoids and carotenoids. Here, we explore the use of molecular farming from producing medicines to developing functional food crops (biofortification). We review recent progress in producing pharmaceutical proteins and bioactive substances in rice endosperm and compare this platform with other plant expression systems. We describe how rice endosperm could be modified to design metabolic pathways and express and store stable products and discuss the factors restricting the commercialization of transgenic rice products and future prospects.

Tangeretin inhibits fungal ferroptosis to suppress rice blast

Flavonoids are polyphenolic secondary metabolites that function as signaling molecules, allopathic compounds, phytoalexins, detoxifying agents and antimicrobial defensive compounds in plants. Blast caused by the fungus Magnaporthe oryzae is a serious disease affecting rice cultivation. In this study, we revealed that a natural flavonoid, tangeretin, substantially delays the formation of M. oryzae appressoria and blocks the development of blast lesions on rice plants. Our data suggest that tangeretin has antioxidant activity that interferes with conidial cell death/ferroptosis, which is critical for M. oryzae pathogenicity. Tangeretin showed a ferroptosis inhibition efficacy comparable to the well-established liproxstatin-1. Furthermore, overexpression of the NADPH oxidases NOX1 or NOX2 significantly decreased sensitivity toward tangeretin treatment, suggesting Nox-mediated lipid peroxidation as a possible target for tangeretin in regulating redox signaling and ferroptosis in M. oryzae. Our nursery and field tests showed that application of tangeretin can effectively mitigate overall disease symptoms and prevent leaf blast. Our study reveals the plant-derived fungal ferroptosis inhibitor tangeretin as a potential and novel antifungal agrochemical for the sustainable prevention of the devastating blast disease in important cereal crops.

Biofortification of Cereals and Pulses Using New Breeding Techniques: Current and Future Perspectives

Cereals and pulses are consumed as a staple food in low-income countries for the fulfillment of daily dietary requirements and as a source of micronutrients. However, they are failing to offer balanced nutrition due to deficiencies of some essential compounds, macronutrients, and micronutrients, i.e., cereals are deficient in iron, zinc, some essential amino acids, and quality proteins. Meanwhile, the pulses are rich in anti-nutrient compounds that restrict the bioavailability of micronutrients. As a result, the population is suffering from malnutrition and resultantly different diseases, i.e., anemia, beriberi, pellagra, night blindness, rickets, and scurvy are common in the society. These facts highlight the need for the biofortification of cereals and pulses for the provision of balanced diets to masses and reduction of malnutrition. Biofortification of crops may be achieved through conventional approaches or new breeding techniques (NBTs). Conventional approaches for biofortification cover mineral fertilization through foliar or soil application, microbe-mediated enhanced uptake of nutrients, and conventional crossing of plants to obtain the desired combination of genes for balanced nutrient uptake and bioavailability. Whereas, NBTs rely on gene silencing, gene editing, overexpression, and gene transfer from other species for the acquisition of balanced nutritional profiles in mutant plants. Thus, we have highlighted the significance of conventional and NBTs for the biofortification of cereals and pulses. Current and future perspectives and opportunities are also discussed. Further, the regulatory aspects of newly developed biofortified transgenic and/or non-transgenic crop varieties via NBTs are also presented.

Tricin Biosynthesis and Bioengineering

Tricin (3',5'-dimethoxyflavone) is a specialized metabolite which not only confers stress tolerance and involves in defense responses in plants but also represents a promising nutraceutical. Tricin-type metabolites are widely present as soluble tricin O-glycosides and tricin-oligolignols in all grass species examined, but only show patchy occurrences in unrelated lineages in dicots. More strikingly, tricin is a lignin monomer in grasses and several other angiosperm species, representing one of the "non-monolignol" lignin monomers identified in nature. The unique biological functions of tricin especially as a lignin monomer have driven the identification and characterization of tricin biosynthetic enzymes in the past decade. This review summarizes the current understanding of tricin biosynthetic pathway in grasses and tricin-accumulating dicots. The characterized and potential enzymes involved in tricin biosynthesis are highlighted along with discussion on the debatable and uncharacterized steps. Finally, current developments of bioengineering on manipulating tricin biosynthesis toward the generation of functional food as well as modifications of lignin for improving biorefinery applications are summarized.

Genetic mapping identifies a rice naringenin O-glucosyltransferase that influences insect resistance

Naringenin, the biochemical precursor for predominant flavonoids in grasses, provides protection against UV damage, pathogen infection and insect feeding. To identify previously unknown loci influencing naringenin accumulation in rice (Oryza sativa), recombinant inbred lines derived from the Nipponbare and IR64 cultivars were used to map a quantitative trait locus (QTL) for naringenin abundance to a region of 50 genes on rice chromosome 7. Examination of candidate genes in the QTL confidence interval identified four predicted uridine diphosphate-dependent glucosyltransferases (Os07g31960, Os07g32010, Os07g32020 and Os07g32060). In vitro assays demonstrated that one of these genes, Os07g32020 (UGT707A3), encodes a glucosyltransferase that converts naringenin and uridine diphosphate-glucose to naringenin-7-O-β-d-glucoside. The function of Os07g32020 was verified with CRISPR/Cas9 mutant lines, which accumulated more naringenin and less naringenin-7-O-β-d-glucoside and apigenin-7-O-β-d-glucoside than wild-type Nipponbare. Expression of Os12g13800, which encodes a naringenin 7-O-methyltransferase that produces sakuranetin, was elevated in the mutant lines after treatment with methyl jasmonate and insect pests, Spodoptera litura (cotton leafworm), Oxya hyla intricata (rice grasshopper) and Nilaparvata lugens (brown planthopper), leading to a higher accumulation of sakuranetin. Feeding damage from O. hyla intricata and N. lugens was reduced on the Os07g32020 mutant lines relative to Nipponbare. Modification of the Os07g32020 gene could be used to increase the production of naringenin and sakuranetin rice flavonoids in a more targeted manner. These findings may open up new opportunities for selective breeding of this important rice metabolic trait.

A multi-omics approach to lignocellulolytic enzyme discovery reveals a new ligninase activity from Parascedosporium putredinis NO1

Lignocellulose, the structural component of plant cells, is a major agricultural byproduct and the most abundant terrestrial source of biopolymers on Earth. The complex and insoluble nature of lignocellulose limits its conversion into value-added commodities, and currently, efficient transformation requires expensive pretreatments and high loadings of enzymes. Here, we report on a fungus from the Parascedosporium genus, isolated from a wheat-straw composting community, that secretes a large and diverse array of carbohydrate-active enzymes (CAZymes) when grown on lignocellulosic substrates. We describe an oxidase activity that cleaves the major β-ether units in lignin, thereby releasing the flavonoid tricin from monocot lignin and enhancing the digestion of lignocellulose by polysaccharidase mixtures. We show that the enzyme, which holds potential for the biorefining industry, is widely distributed among lignocellulose-degrading fungi from the Sordariomycetes phylum.

Polyphenols in foods: Classification, methods of identification, and nutritional aspects in human health

Polyphenols widely exists in various foods, including main crops, fruits, beverages and some wines. Famous representatives of polyphenols, such as resveratrol in red wine, (-)-epigallocatechin gallate in green tea, chlorogenic acid in coffee, anthocyanins in colored fruits, procyanidins in grape seed have become hot research topics in food science and nutrition. There have been thousands of papers on the biochemistry, chemistry, nutritional values and population-based investigations of dietary polyphenols. In this chapter, we reviewed the published articles and database of dietary polyphenols to draw a profile for the classification, structural identification, and biological activities mainly based on enzymes, cell bioassay and animal models, as well as the population-based investigation results. The typical compound and its health benefits for each category of polyphenols was also introduced. The identification of dietary polyphenols could be solved by combined spectroscopy methods, of which the liquid chromatography tandem mass spectrometry is highlighted to greatly increase the efficiency on structural identification. Although the population-based investigation showed some controversial results for health benefits, the multi-functions of dietary polyphenols on preventing metabolic syndromes, various cancers and neurodegenerative disease have attracted much attention.

Metabolic Engineering of Isoflavones: An Updated Overview

Isoflavones are ecophysiologically active secondary metabolites derived from the phenylpropanoid pathway. They were mostly found in leguminous plants, especially in the pea family. Isoflavones play a key role in plant-environment interactions and act as phytoalexins also having an array of health benefits to the humans. According to epidemiological studies, a high intake of isoflavones-rich diets linked to a lower risk of hormone-related cancers, osteoporosis, menopausal symptoms, and cardiovascular diseases. These characteristics lead to the significant advancement in the studies on genetic and metabolic engineering of isoflavones in plants. As a result, a number of structural and regulatory genes involved in isoflavone biosynthesis in plants have been identified and characterized. Subsequently, they were engineered in various crop plants for the increased production of isoflavones. Furthermore, with the advent of high-throughput technologies, the regulation of isoflavone biosynthesis gains attention to increase or decrease the level of isoflavones in the crop plants. In the review, we begin with the role of isoflavones in plants, environment, and its benefits in human health. Besides, the main theme is to discuss the updated research progress in metabolic engineering of isoflavones in other plants species and regulation of production of isoflavones in soybeans.

Nutrition and antioxidant profiling in the unpolished and polished grains of eleven indigenous aromatic rice cultivars

The present study emphasized on the yet-unexplored exhaustive analyses of nutritional and antioxidant parameters in the unpolished and polished grains of eleven indigenous aromatic rice varieties. Tulaipanji appeared to be a highly demanding variety by virtue of having sufficient levels of micronutrients like Fe, Zn and Cu (linked with higher expression of fer2, ZIP and NAS3), inorganic phosphorus, hexose sugars, total amino acids and lysine (correlated with higher expression of glutelin and RLRH1), tocopherol (due to higher HGGT expression), total phenolic content, flavonoids, anthocyanins (concomitant with higher expression of PPO, PAL and ANS), LOX activity and LOX1 gene expression, and overall higher total antioxidant capacity, particularly in the polished grains. The importance of IET-21261, with regard to higher content of phytic acid and total phosphorus (with high IPK1 expression), β-carotene (with high PSY expression) and tocopherol (with high HGGT expression), and of Kalonunia, with respect to cysteine and γ-oryzanol in the polished grains, was also significant. Lower α-amylase enzyme activity and α-amylase expression led to considerable starch accumulation, with lower sucrose content, in the unpolished grains of Radhunipagal and polished grains of Pusa Basmati-1. Paramanya registered the highest content of thiamine and TH1 expression, together with minimum methylglyoxal level (low TPI expression). Paramanya and Radhunipagal maintained a higher pool of majority of the nutritional and antioxidant components in their unpolished grains. The polished grains of all the genotypes showed strikingly lower nutritional constituents, as compared to unpolished grains. The knowledge gained from this study will largely provide a road map to the farmers and rice consumers for making proper choice of the aromatic genotypes for large-scale cultivation and dietary consumption to derive maximum nutritional benefits.

Improving nutritional quality of rice for human health

This review surveys rice nutritional value, mainly focusing on breeding achievements via adoption of both genetic engineering and non-transgenic strategies to improve key nutrients associated with human health. Rice (Oryza sativa) is an essential component of the diets and livelihoods of over 3.5 billion people. Polished rice is mostly consumed as staple food, fulfilling daily energy demands and part of the protein requirement. Brown rice is comparatively more nutritious, containing more lipids, minerals, vitamins, dietary fiber, micronutrients, and bioactive compounds. In this article, we review the nutritional facts about rice including the level of γ-aminobutyric acid, resistant starch, lysine, iron, zinc, β-carotene, folate, anthocyanin, various carotenoids, and flavonoids, focusing on their synthesis and metabolism and the advances in their biofortification via adoption of both conventional and genetic engineering strategies. We conclude that besides representing a staple food, rice has the potential to become a source of various essential nutrients or bioactive compounds through appropriate genetic improvements to benefit human health and prevent certain chronic diseases. Finally, we discuss the available, non-genetically engineering strategies for the nutritional improvement of rice, including their main strengths and constraints.

Comprehensive construction strategy of bidirectional green tissue-specific synthetic promoters

Bidirectional green tissue-specific promoters have important application prospects in genetic engineering and crop genetic improvement. However, there is no report on the application of them, mainly due to undiscovered natural bidirectional green tissue-specific promoters and the lack of a comprehensive approach for the synthesis of these promoters. In order to compensate for this vacancy, the present study reports a novel strategy for the expression regulatory sequence selection and the bidirectional green tissue-specific synthetic promoter construction. Based on this strategy, seven promoters were synthesized and introduced into rice by agrobacterium-mediated transformation. The functional identification of these synthetic promoters was performed by the expression pattern of GFP and GUS reporter genes in two reverse directions in transgenic rice. The results indicated that all the synthetic promoters possessed bidirectional expression activities in transgenic rice, and four synthetic promoters (BiGSSP2, BiGSSP3, BiGSSP6, BiGSSP7) showed highly bidirectional expression efficiencies specifically in green tissues (leaf, sheath, panicle, stem), which could be widely applied to agricultural biotechnology. Our study provided a feasible strategy for the construction of synthetic promoters, and we successfully created four bidirectional green tissue-specific synthetic promoters. It is the first report on bidirectional green tissue-specific promoters that could be efficiently applied in genetic engineering.

Plant Synthetic Metabolic Engineering for Enhancing Crop Nutritional Quality

Nutrient deficiencies in crops are a serious threat to human health, especially for populations in poor areas. To overcome this problem, the development of crops with nutrient-enhanced traits is imperative. Biofortification of crops to improve nutritional quality helps combat nutrient deficiencies by increasing the levels of specific nutrient components. Compared with agronomic practices and conventional plant breeding, plant metabolic engineering and synthetic biology strategies are more effective and accurate in synthesizing specific micronutrients, phytonutrients, and/or bioactive components in crops. In this review, we discuss recent progress in the field of plant synthetic metabolic engineering, specifically in terms of research strategies of multigene stacking tools and engineering complex metabolic pathways, with a focus on improving traits related to micronutrients, phytonutrients, and bioactive components. Advances and innovations in plant synthetic metabolic engineering would facilitate the development of nutrient-enriched crops to meet the nutritional needs of humans.

Momilactones A, B, and Tricin in Rice Grain and By-Products are Potential Skin Aging Inhibitors

We previously reported the inhibitory potentials of momilactones A (MA) and B (MB) against key enzymes related to type 2 diabetes and obesity. In this study, antioxidant and anti-skin-aging activities of MA and MB were investigated and compared with tricin, a well-known antioxidant and antiaging flavonoid in rice. MA, MB, and tricin were purified from rice husk by column chromatography and their biological activities were subsequently assayed by in vitro trials. The contents of MA, MB, and tricin of different commercial rice cultivars in Japan were quantified and confirmed by ultra-performance liquid chromatography-electrospray ionization-mass spectrometry (UPLC-ESI-MS) and high-performance liquid chromatography (HPLC) analyses. The antioxidant assays revealed a synergistic activity of the mixture MA and MB (MAB, 1:1, v/v). In addition, in 2,2'-azino-bis (ABTS) assay, IC₅₀ values of MAB (0.3 mg/mL) and tricin (0.3 mg/mL) was 4-fold and 9-fold greater than that of individual MB (1.3 mg/mL) or MA (2.8 mg/mL), respectively. The in vitro enzymatic assays on pancreatic elastase and tyrosinase indicated that MA and MB were potential to relief skin wrinkles and freckles. In detail, MA exerted higher inhibition on both enzymatic activities (30.9 and 37.6% for elastase and tyrosinase inhibition, respectively) than MB (18.5 and 12.6%) and MAB (32.0 and 19.7%) at a concentration of 2.0 mg/mL. Notably, MA and the mixture MAB exhibited stronger inhibitions on elastase and tyrosinase in comparison with tricin and vanillin. MA, MB, and tricin in rice are potential to develop cosmetics as well as supplements for skin aging treatments.

Transcriptomic analysis reveals flavonoid biosynthesis of Syringa oblata Lindl. in response to different light intensity

BACKGROUND

Hazy weather significantly increase air pollution and affect light intensity which may also affect medicinal plants growth. Syringa oblata Lindl. (S. oblata), an effective anti-biofilm medicinal plants, is also vulnerable to changes in plant photoperiods and other abiotic stress responses. Rutin, one of the flavonoids, is the main bioactive ingredient in S. oblata that inhibits Streptococcus suis biofilm formation. Thus, the present study aims to explore the biosynthesis and molecular basis of flavonoids in S. oblata in response to different light intensity.

RESULTS

In this study, it was shown that compared with natural (Z0) and 25% ~ 35% (Z2) light intensities, the rutin content of S. oblata under 50% ~ 60% (Z1) light intensity increased significantly. In addition, an integrated analysis of metabolome and transcriptome was performed using light intensity stress conditions from two kinds of light intensities which S. oblata was subjected to: Z0 and Z1. The results revealed that differential metabolites and genes were mainly related to the flavonoid biosynthetic pathway. We found out that 13 putative structural genes and a transcription factor bHLH were significantly up-regulated in Z1. Among them, integration analysis showed that 3 putative structural genes including 4CL1, CYP73A and CYP75B1 significantly up-regulated the rutin biosynthesis, suggesting that these putative genes may be involved in regulating the flavonoid biosynthetic pathway, thereby making them key target genes in the whole metabolic process.

CONCLUSIONS

The present study provided helpful information to search for the novel putative genes that are potential targets for S. oblata in response to light intensity.

Molecular and Functional Characterization of Oryza sativa Flavonol Synthase (OsFLS), a Bifunctional Dioxygenase

Flavonol synthase (FLS) belongs to the 2-oxoglutarate-dependent dioxygenase (2-ODD) superfamily. We isolated OsFLS from the rice ( Oryza sativa) cultivar "Ilmi" OsFLS includes highly conserved 2-ODD-specific motifs and FLS-specific regions. Recombinant OsFLS exhibited both FLS and flavanone 3β-hydroxylase (F3H) activities, converting dihydroflavonols into flavonols and flavanones into dihydroflavonols, respectively, and more efficiently used dihydrokaempferol than dihydroquercetin as a substrate. OsFLS was expressed in both nonpigmented and pigmented rice seeds and was developmentally regulated during seed maturation. Transgenic tobacco ( Nicotiana tabacum) plants expressing OsFLS produced pale pink or white flowers with significantly increased levels of kaempferol-3- O-rutinoside and dramatically reduced levels of anthocyanin in their petals. Additionally, pod size and weight were reduced compared to the wild type. Several early and late biosynthetic genes of flavonoid were downregulated in the transgenic flowers. We demonstrated that OsFLS is a bifunctional 2-ODD enzyme and functions in flavonol production in planta.

Tricin levels and expression of flavonoid biosynthetic genes in developing grains of purple and brown pericarp rice

The methylated flavone tricin has been associated with numerous health benefits, including reductions in intestinal and colon cancers in animal models. Tricin is found in a wide range of plant species and in many different tissues. However, whole cereal grains, such as rice, barley, oats, and wheat, are the only food sources of tricin, which is located in the bran portion of the grain. Variation in tricin levels was found in bran from rice genotypes with light brown, brown, red, and purple pericarp color, with the purple pericarp genotypes having the highest levels of tricin. Here, we analyzed tricin and tricin derivative levels in developing pericarp and embryo samples of a purple pericarp genotype, IAC600, that had high tricin and tricin derivative levels in the bran, and a light brown pericarp genotype, Cocodrie, that had no detectable tricin or tricin derivatives in the bran. Tricin and tricin derivatives were detected in both the pericarp and embryo of IAC600 but only in the embryo of Cocodrie. The purple pericarp rice had higher total levels of free tricin plus tricin derivatives than the light brown pericarp rice. When expressed on a per grain basis, most of the tricin component of IAC600 was in the pericarp. In contrast, Cocodrie had no detectable tricin in the pericarp samples but did have detectable chrysoeriol, a precursor of tricin, in the pericarp samples. We also used RNA-Seq analysis of developing pericarp and embryo samples of the two cultivars to compare the expression of genes involved in the flavonoid biosynthetic pathway. The results presented here suggest that understanding the basis of tricin accumulation in rice pericarp may lead to an approach to increasing tricin levels in whole grain rice. From analysis of gene expression levels in the pericarp samples it appears that regulation of the flavone specific genes is independent of regulation of the anthocyanin biosynthetic genes. It therefore may be feasible to develop brown pericarp rice cultivars that accumulate tricin in the pericarp.

Stepwise pathway engineering to the biosynthesis of zeaxanthin, astaxanthin and capsanthin in rice endosperm

Carotenoid pigments are valuable components of the human diet. A notable example is β-carotene, or provitamin A, which is converted into the derivatives astaxanthin and capsanthin, via the common intermediate zeaxanthin. To generate rice varieties producing diverse carotenoids beyond β-carotene, we specifically used a Capsicum β-carotene hydroxylase gene, B (CaBch) and a codon optimized version of the same gene, stB (stBch) to increase zeaxanthin synthesis. We also used a recombinant BAK gene (CaBch-2A-HpBkt), consisting of the CaBch sequence and a Haematococcus β-carotene ketolase gene (HpBkt) linked by a bicistronic 2 A sequence, as well as a codon optimized recombinant stBAK gene (stBch-2A-stBkt) to create astaxanthin synthesis. The four cassettes to seed-specifically express the B, stB, BAK and stBAK genes were individually combined with a PAC gene (CaPsy-2A-PaCrtI) cassette to previously impart β-carotene-enriched trait in rice endosperm. The single T-DNA vectors of B-PAC, stB-PAC, BAK-PAC and stBAK-PAC resulted in the accumulation of zeaxanthin and astaxanthin in the endosperm of the transgenic rice seeds. In addition, an extended version on the carotenoid pathway was introduced into rice to allow the production of capsanthin, by intercrossing a B-PAC rice line with a Ccs rice line, which harbors a Capsicum capsanthin-capsorubin synthase gene. Ultimately, we developed three functional rice varieties: B-PAC (0.8 μg/g zeaxanthin, deep yellow), stBAK-PAC (1.4 μg/g ketocarotenoids, including astaxanthin, pinkish red) and B-PAC x Ccs (0.4 μg/g of ketoxanthophylls, including capsanthin, orange-red) with the similar levels of total carotenoids to PAC rice, suggesting the capacity was dependent on β-carotene levels. Collectively, a combination of genetic engineering and conventional breeding is effective for multi-step metabolic engineering and biochemical pathway extension.

Biofortified Crops Generated by Breeding, Agronomy, and Transgenic Approaches Are Improving Lives of Millions of People around the World

Biofortification is an upcoming, promising, cost-effective, and sustainable technique of delivering micronutrients to a population that has limited access to diverse diets and other micronutrient interventions. Unfortunately, major food crops are poor sources of micronutrients required for normal human growth. The manuscript deals in all aspects of crop biofortification which includes-breeding, agronomy, and genetic modification. It tries to summarize all the biofortification research that has been conducted on different crops. Success stories of biofortification include lysine and tryptophan rich quality protein maize (World food prize 2000), Vitamin A rich orange sweet potato (World food prize 2016); generated by crop breeding, oleic acid, and stearidonic acid soybean enrichment; through genetic transformation and selenium, iodine, and zinc supplementation. The biofortified food crops, especially cereals, legumes, vegetables, and fruits, are providing sufficient levels of micronutrients to targeted populations. Although a greater emphasis is being laid on transgenic research, the success rate and acceptability of breeding is much higher. Besides the challenges biofortified crops hold a bright future to address the malnutrition challenge.

Risks and benefits related to alimentary exposure to xenoestrogens

Xenoestrogens are widely diffused in the environment and in food, thus a large portion of human population worldwide is exposed to them. Among alimentary xenoestrogens, phytoestrogens (PhyEs) are increasingly being consumed because of their potential health benefits, although there are also important risks associated to their ingestion. Furthermore, other xenoestrogens that may be present in food are represented by other chemicals possessing estrogenic activities, that are commonly defined as endocrine disrupting chemicals (EDCs). EDCs pose a serious health concern since they may cause a wide range of health problems, starting from pre-birth till adult lifelong exposure. We herein provide an overview of the main classes of xenoestrogens, which are classified on the basis of their origin, their structures and their occurrence in the food chain. Furthermore, their either beneficial or toxic effects on human health are discussed in this review.

Metabolomics-assisted biotechnological interventions for developing plant-based functional foods and nutraceuticals

Today, the dramatic changes in types of food consumed have led to an increased burden of chronic diseases. Therefore, the emphasis of food research is not only to ensure quality food that can supply adequate nutrients to prevent nutrition related diseases, but also to ensure overall physical and mental-health. This has led to the concept of functional foods and nutraceuticals (FFNs), which can be ideally produced and delivered through plants. Metabolomics can help in getting the most relevant functional information, and thus has been considered the greatest -OMICS technology to date. However, metabolomics has not been exploited to the best potential in plant sciences. The technology can be leveraged to identify the health promoting compounds and metabolites that can be used for the development of FFNs. This article reviews (i) plant-based FFNs-related metabolites and their health benefits; (ii) use of different analytic platforms for targeted and non-targeted metabolite profiling along with experimental considerations; (iii) exploitation of metabolomics to develop FFNs in plants using various biotechnological tools; and (iv) potential use of metabolomics in plant breeding. We have also provided some insights into integration of metabolomics with latest genome editing tools for metabolic pathway regulation in plants.

Development of "Purple Endosperm Rice" by Engineering Anthocyanin Biosynthesis in the Endosperm with a High-Efficiency Transgene Stacking System

Anthocyanins have high antioxidant activities, and engineering of anthocyanin biosynthesis in staple crops, such as rice (Oryza sativa L.), could provide health-promoting foods for improving human health. However, engineering metabolic pathways for biofortification remains difficult, and previous attempts to engineer anthocyanin production in rice endosperm failed because of the sophisticated genetic regulatory network of its biosynthetic pathway. In this study, we developed a high-efficiency vector system for transgene stacking and used it to engineer anthocyanin biosynthesis in rice endosperm. We made a construct containing eight anthocyanin-related genes (two regulatory genes from maize and six structural genes from Coleus) driven by the endosperm-specific promoters,plus a selectable marker and a gene for marker excision. Transformation of rice with this construct generated a novel biofortified germplasm "Purple Endosperm Rice" (called "Zijingmi" in Chinese), which has high anthocyanin contents and antioxidant activity in the endosperm. This anthocyanin production results from expression of the transgenes and the resulting activation (or enhancement) of expression of 13 endogenous anthocyanin biosynthesis genes that are silenced or expressed at low levels in wild-type rice endosperm. This study provides an efficient, versatile toolkit for transgene stacking and demonstrates its use for successful engineering of a sophisticated biological pathway, suggesting the potential utility of this toolkit for synthetic biology and improvement of agronomic traits in plants.

Pyramiding of tea Dihydroflavonol reductase and Anthocyanidin reductase increases flavan-3-ols and improves protective ability under stress conditions in tobacco

Tea (Camellia sinensis) is one of the richest sources of flavan-3-ols, an important class of flavonoids. The expression level of gene-encoded key regulatory enzymes of flavan-3-ol/anthocyanin biosynthetic pathway, dihydroflavonol 4-reductase (DFR) and anthocyanidin reductase (ANR), has been highly correlated with the flavan-3-ol contents and antioxidant activity in tea plant. In the present study, pyramiding of CsDFR and CsANR in tobacco was achieved. However, single transgenic tobacco overexpressing either CsDFR or CsANR was documented earlier. In continuation, pyramided transgenic lines were evaluated for the possible, either same or beyond, effect on flavan-3-ol accumulation and protective ability against biotic and abiotic stresses. The pyramided transgenic lines showed early flowering and improved seed yield. The transcript levels of flavan-3-ol/anthocyanin biosynthetic pathway and related genes in pyramided transgenic lines were upregulated as compared to control tobacco plants. The accumulations of flavan-3-ols were also found to be higher in pyramided transgenic lines than control tobacco plants. In contrast, anthocyanin content was observed to be decreased in pyramided transgenic lines, while DPPH activity was higher in pyramided transgenic lines. In pyramided transgenic lines, strong protective ability against feeding by Spodoptera litura was documented. The seeds of pyramided transgenic lines were also found to have better germination rate under aluminum toxicity as compared to control tobacco plants. Interestingly, the synergistic effect of these two selected genes are not beyond from transgenic lines expressing either CsDFR and CsANR alone as published earlier in terms of flavan-3-ols accumulation. However, the unique flower color and better seed germination rate are some interestingly comparable differences that were reported in pyramided lines in relation to individual transgenic plants. In conclusion, the present results reveal an interesting dynamic between CsDFR and CsANR in modulating flavan-3-ol/anthocyanin levels and functional analysis of stacked CsDFR and CsANR transgenic tobacco lines.

Genome-wide identification and phylogenetic analysis of the chalcone synthase gene family in rice

The enzymes of the chalcone synthase family are also known as type III polyketide synthases (PKS), and produce a series of secondary metabolites in bacteria, fungi and plants. In a number of plants, genes encoding PKS comprise a large multigene family. Currently, detailed reports on rice (Oryza sativa) PKS (OsPKS) family genes and tissue expression profiling are limited. Here, 27 candidate OsPKS genes were identified in the rice genome,and 23 gene structures were confirmed by EST and cDNA sequencing; phylogenetic analysis has indicated that these 23 OsPKS members could be clustered into three groups (I-III). Comparative analysis has shown OsPKS08 and OsPKS26 could be classified with the CHS genes of other species. Two members OsPKS10 and OsPKS21 were grouped into anther specific chalcone synthase-like (ASCL) clade. Intron/exon structure analysis revealed that nearly all of the OsPKS members contained one phase-1 intron at a conserved Cys. Analysis of chromosomal localization and genome distribution showed that some of the members were distributed on a chromosome as a cluster. Expression data exhibited widespread distribution of the rice OsPKS gene family within plant tissues, suggesting functional diversification of the OsPKS genes. Our results will contribute to future study of the complexity of the OsPKS gene family in rice.

Expression of Flavone Synthase II and Flavonoid 3'-Hydroxylase Is Associated with Color Variation in Tan-Colored Injured Leaves of Sorghum

Sorghum (Sorghum bicolor L. Moench) exhibits various color changes in injured leaves in response to cutting stress. Here, we aimed to identify key genes for the light brown and dark brown color variations in tan-colored injured leaves of sorghum. For this purpose, sorghum M36001 (light brown injured leaves), Nakei-MS3B (purple), and a progeny, #7 (dark brown), from Nakei-MS3B × M36001, were used. Accumulated pigments were detected by using high-performance liquid chromatography: M36001 accumulated only apigenin in its light brown leaves; #7 accumulated both luteolin and a small amount of apigenin in its dark brown leaves, and Nakei-MS3B accumulated 3-deoxyanthocyanidins (apigeninidin and luteolinidin) in its purple leaves. Apigenin or luteolin glucoside derivatives were also accumulated, in different proportions. Differentially expressed genes before and after cutting stress were identified by using RNA sequencing (RNA-seq). Integration of our metabolic and RNA-seq analyses suggested that expression of only flavone synthase II (FNSII) led to the synthesis of apigenin in M36001, expression of both FNSII and flavonoid 3'-hydroxylase (F3'H) led to the synthesis of apigenin and luteolin in #7, and expression of both flavanone 4-reductase and F3'H led to the synthesis of 3-deoxyanthocyanidins in Nakei-MS3B. These results suggest that expression of FNSII is related to the synthesis of flavones (apigenin and luteolin) and the expression level of F3'H is related to the balance of apigenin and luteolin. Expression of FNSII and F3'H is thus associated with dark or light brown coloration in tan-colored injured leaves of sorghum.

Molecular and Biochemical Analysis of Two Rice Flavonoid 3'-Hydroxylase to Evaluate Their Roles in Flavonoid Biosynthesis in Rice Grain

Anthocyanins and proanthocyanidins, the major flavonoids in black and red rice grains, respectively, are mainly derived from 3',4'-dihydroxylated leucocyanidin. 3'-Hydroxylation of flavonoids in rice is catalyzed by flavonoid 3'-hydroxylase (F3'H: EC 1.14.13.21). We isolated cDNA clones of the two rice F3'H genes (CYP75B3 and CYP75B4) from Korean varieties of white, black, and red rice. Sequence analysis revealed allelic variants of each gene containing one or two amino acid substitutions. Heterologous expression in yeast demonstrated that CYP75B3 preferred kaempferol to other substrates, and had a low preference for dihydrokaempferol. CYP75B4 exhibited a higher preference for apigenin than for other substrates. CYP75B3 from black rice showed an approximately two-fold increase in catalytic efficiencies for naringenin and dihydrokaempferol compared to CYP75B3s from white and red rice. The F3'H activity of CYP75B3 was much higher than that of CYP75B4. Gene expression analysis showed that CYP75B3, CYP75B4, and most other flavonoid pathway genes were predominantly expressed in the developing seeds of black rice, but not in those of white and red rice, which is consistent with the pigmentation patterns of the seeds. The expression levels of CYP75B4 were relatively higher than those of CYP75B3 in the developing seeds, leaves, and roots of white rice.

Multigene synergism increases the isoflavone and proanthocyanidin contents of Medicago truncatula

Isoflavones and proanthocyanidins (PAs), which are flavonoid derivatives, possess many health benefits and play important roles in forage-based livestock production. However, the foliage of Medicago species accumulates limited levels of both isoflavones and PAs. In this study, biosynthesis of isoflavone and PA in Medicago truncatula was enhanced via synergy between soya bean isoflavone synthase (IFS1); two upstream enzymes, chalcone synthase (CHS) and chalcone isomerase (CHI); and the endogenous flavanone 3-hydroxylase (F3H). Constitutive expression of GmIFS1 alone resulted in ectopic accumulation of the isoflavone daidzein and large increases in the levels of the isoflavones formononetin, genistein and biochanin A in the leaves. Furthermore, coexpression of GmIFS1 with GmCHS7 and GmCHI1A generally increased the available flux to flavonoid biosynthesis and resulted in elevated isoflavone, flavone and PA contents. In addition, down-regulation of MtF3H combined with coexpression of GmIFS1, GmCHS7 and GmCHI1A led to the highest isoflavone levels (up to 2 μmol/g fresh weight in total). Taken together, our results demonstrate that multigene synergism is a powerful means to enhance the biosynthesis of particular flavonoids and can be more broadly applied to the metabolic engineering of forage species.

Isolation and Functional Characterization of Bidirectional Promoters in Rice

Bidirectional promoters, which show great application potential in genetic improvement of plants, have aroused great research interest recently. However, most bidirectional promoters were cloned individually in the studies of single genes. Here, we initiatively combined RNA-seq data and cDNA microarray data to discover the potential bidirectional promoters in rice genome. Based on the expression level and correlation of each adjacent and oppositely transcribed gene pair, we selected four candidate gene pairs. Then, the intergenic region between each pair was isolated and cloned into a dual reporter vector pDX2181 for functional identification. GUS and GFP assays of the transgenic plants indicated that all the intergenic regions showed bidirectional expression activity in various tissues. Through 5' and 3' deletion analysis on one of the above bidirectional promoters, we identified the enhancing region which sharply increased its bidirectional expression efficiency and the essential regions respectively responsible for its 5' and 3' basic expression activity. The bidirectional arrangement of the four gene pairs in six gramineous plants was also analyzed, showing the conserved characteristics of the four bidirectional promoters identified in our study. In addition, two novel cis-sequences conserved in the four bidirectional promoters were discovered by bioinformatic identification. Our study proposes a feasible method for selecting, cloning, and functionally identifying bidirectional promoters as well as for discovering their bidirectional regulatory regions and conserved sequences in rice.

Transgenic rice seed expressing flavonoid biosynthetic genes accumulate glycosylated and/or acylated flavonoids in protein bodies

Plant-specialized (or secondary) metabolites represent an important source of high-value chemicals. In order to generate a new production platform for these metabolites, an attempt was made to produce flavonoids in rice seeds. Metabolome analysis of these transgenic rice seeds using liquid chromatography-photodiode array-quadrupole time-of-flight mass spectrometry was performed. A total of 4392 peaks were detected in both transgenic and non-transgenic rice, 20-40% of which were only detected in transgenic rice. Among these, 82 flavonoids, including 37 flavonols, 11 isoflavones, and 34 flavones, were chemically assigned. Most of the flavonols and isoflavones were O-glycosylated, while many flavones were O-glycosylated and/or C-glycosylated. Several flavonoids were acylated with malonyl, feruloyl, acetyl, and coumaroyl groups. These glycosylated/acylated flavonoids are thought to have been biosynthesized by endogenous rice enzymes using newly synthesized flavonoids whose biosynthesis was catalysed by exogenous enzymes. The subcellular localization of the flavonoids differed depending on the class of aglycone and the glycosylation/acylation pattern. Therefore, flavonoids with the intended aglycones were efficiently produced in rice seeds via the exogenous enzymes introduced, while the flavonoids were variously glycosylated/acylated by endogenous enzymes. The results suggest that rice seeds are useful not only as a production platform for plant-specialized metabolites such as flavonoids but also as a tool for expanding the diversity of flavonoid structures, providing novel, physiologically active substances.

Tricin, a flavonoid monomer in monocot lignification

Tricin was recently discovered in lignin preparations from wheat (Triticum aestivum) straw and subsequently in all monocot samples examined. To provide proof that tricin is involved in lignification and establish the mechanism by which it incorporates into the lignin polymer, the 4'-O-β-coupling products of tricin with the monolignols (p-coumaryl, coniferyl, and sinapyl alcohols) were synthesized along with the trimer that would result from its 4'-O-β-coupling with sinapyl alcohol and then coniferyl alcohol. Tricin was also found to cross couple with monolignols to form tricin-(4'-O-β)-linked dimers in biomimetic oxidations using peroxidase/hydrogen peroxide or silver (I) oxide. Nuclear magnetic resonance characterization of gel permeation chromatography-fractionated acetylated maize (Zea mays) lignin revealed that the tricin moieties are found in even the highest molecular weight fractions, ether linked to lignin units, demonstrating that tricin is indeed incorporated into the lignin polymer. These findings suggest that tricin is fully compatible with lignification reactions, is an authentic lignin monomer, and, because it can only start a lignin chain, functions as a nucleation site for lignification in monocots. This initiation role helps resolve a long-standing dilemma that monocot lignin chains do not appear to be initiated by monolignol homodehydrodimerization as they are in dicots that have similar syringyl-guaiacyl compositions. The term flavonolignin is recommended for the racemic oligomers and polymers of monolignols that start from tricin (or incorporate other flavonoids) in the cell wall, in analogy with the existing term flavonolignan that is used for the low-molecular mass compounds composed of flavonoid and lignan moieties.

Cytochrome P450 93G1 Is a Flavone Synthase II That Channels Flavanones to the Biosynthesis of Tricin O-Linked Conjugates in Rice

Flavones are a major class of flavonoids with a wide range of physiological functions in plants. They are constitutively accumulated as C-glycosides and O-linked conjugates in vegetative tissues of grasses. It has long been presumed that the two structural modifications of flavones occur through independent metabolic routes. Previously, we reported that cytochrome P450 93G2 (CYP93G2) functions as a flavanone 2-hydroxylase (F2H) that provides 2-hydroxyflavanones for C-glycosylation in rice (Oryza sativa). Flavone C-glycosides are subsequently formed by dehydratase activity on 2-hydroxyflavanone C-glycosides. On the other hand, O-linked modifications were proposed to proceed after the flavone nucleus is generated. In this study, we demonstrate that CYP93G1, the closest homolog of CYP93G2 in rice, is a bona fide flavone synthase II (FNSII) that catalyzes the direct conversion of flavanones to flavones. In recombinant enzyme assays, CYP93G1 desaturated naringenin and eriodictyol to apigenin and luteolin, respectively. Consistently, transgenic expression of CYP93G1 in Arabidopsis (Arabidopsis thaliana) resulted in the accumulation of different flavone O-glycosides, which are not naturally present in cruciferous plants. Metabolite analysis of a rice CYP93G1 insertion mutant further demonstrated the preferential depletion of tricin O-linked flavanolignans and glycosides. By contrast, redirection of metabolic flow to the biosynthesis of flavone C-glycosides was observed. Our findings established that CYP93G1 is a key branch point enzyme channeling flavanones to the biosynthesis of tricin O-linked conjugates in rice. Functional diversification of F2H and FNSII in the cytochrome P450 CYP93G subfamily may represent a lineage-specific event leading to the prevalent cooccurrence of flavone C- and O-linked derivatives in grasses today.

Co-expression of Arabidopsis transcription factor, AtMYB12, and soybean isoflavone synthase, GmIFS1, genes in tobacco leads to enhanced biosynthesis of isoflavones and flavonols resulting in osteoprotective activity

Isoflavones, a group of flavonoids, restricted almost exclusively to family Leguminosae are known to exhibit anticancerous and anti-osteoporotic activities in animal systems and have been a target for metabolic engineering in commonly consumed food crops. Earlier efforts based on the expression of legume isoflavone synthase (IFS) genes in nonlegume plant species led to the limited success in terms of isoflavone content in transgenic tissue due to the limitation of substrate for IFS enzyme. In this work to overcome this limitation, the activation of multiple genes of flavonoid pathway using Arabidopsis transcription factor AtMYB12 has been carried out. We developed transgenic tobacco lines constitutively co-expressing AtMYB12 and GmIFS1 (soybean IFS) genes or independently and carried out their phytochemical and molecular analyses. The leaves of co-expressing transgenic lines were found to have elevated flavonol content along with the accumulation of substantial amount of genistein glycoconjugates being at the highest levels that could be engineered in tobacco leaves till date. Oestrogen-deficient (ovariectomized, Ovx) mice fed with leaf extract from transgenic plant co-expressing AtMYB12 and GmIFS1 but not wild-type extract exhibited significant conservation of trabecular microarchitecture, reduced osteoclast number and expression of osteoclastogenic genes, higher total serum antioxidant levels and increased uterine oestrogenicity compared with Ovx mice treated with vehicle (control). The skeletal effect of the transgenic extract was comparable to oestrogen-treated Ovx mice. Together, our results establish an efficient strategy for successful pathway engineering of isoflavones and other flavonoids in crop plants and provide a direct evidence of improved osteoprotective effect of transgenic plant extract.