Metabolic engineering of high carotenoid potato tubers containing enhanced levels of beta-carotene and lutein

Prospects of microgreens as budding living functional food: Breeding and biofortification through OMICS and other approaches for nutritional security

Nutrient deficiency has resulted in impaired growth and development of the population globally. Microgreens are considered immature greens (required light for photosynthesis and growing medium) and developed from the seeds of vegetables, legumes, herbs, and cereals. These are considered "living superfood/functional food" due to the presence of chlorophyll, beta carotene, lutein, and minerals like magnesium (Mg), Potassium (K), Phosphorus (P), and Calcium (Ca). Microgreens are rich at the nutritional level and contain several phytoactive compounds (carotenoids, phenols, glucosinolates, polysterols) that are helpful for human health on Earth and in space due to their anti-microbial, anti-inflammatory, antioxidant, and anti-carcinogenic properties. Microgreens can be used as plant-based nutritive vegetarian foods that will be fruitful as a nourishing constituent in the food industryfor garnish purposes, complement flavor, texture, and color to salads, soups, flat-breads, pizzas, and sandwiches (substitute to lettuce in tacos, sandwich, burger). Good handling practices may enhance microgreens'stability, storage, and shelf-life under appropriate conditions, including light, temperature, nutrients, humidity, and substrate. Moreover, the substrate may be a nutritive liquid solution (hydroponic system) or solid medium (coco peat, coconut fiber, coir dust and husks, sand, vermicompost, sugarcane filter cake, etc.) based on a variety of microgreens. However integrated multiomics approaches alongwith nutriomics and foodomics may be explored and utilized to identify and breed most potential microgreen genotypes, biofortify including increasing the nutritional content (macro-elements:K, Ca and Mg; oligo-elements: Fe and Zn and antioxidant activity) and microgreens related other traits viz., fast growth, good nutritional values, high germination percentage, and appropriate shelf-life through the implementation of integrated approaches includes genomics, transcriptomics, sequencing-based approaches, molecular breeding, machine learning, nanoparticles, and seed priming strategiesetc.

Biochemical Characterization of Orange-Colored Rice Calli Induced by Target Mutagenesis of OsOr Gene

We generated an orange-colored (OC) rice callus line by targeted mutagenesis of the orange gene (OsOr) using the CRISPR-Cas9 system. The OC line accumulated more lutein, β-carotene, and two β-carotene isomers compared to the WT callus line. We also analyzed the expression levels of carotenoid biosynthesis genes by qRT-PCR. Among the genes encoding carotenoid metabolic pathway enzymes, the number of transcripts of the PSY2, PSY3, PDS, ZDS and β-LCY genes were higher in the OC line than in the WT line. In contrast, transcription of the ε-LCY gene was downregulated in the OC line compared to the WT line. In addition, we detected increases in the transcript levels of two genes involved in carotenoid oxidation in the OC lines. The developed OC lines also showed increased tolerance to salt stress. Collectively, these findings indicate that targeted mutagenesis of the OsOr gene via CRISPR/Cas9-mediated genome editing results in β-carotene accumulation in rice calli. Accordingly, we believe that this type of genome-editing technology could represent an effective alternative approach for enhancing the β-carotene content of plants.

Fortification and bioaccessibility of saffron apocarotenoids in potato tubers

Carotenoids are C40 isoprenoids with well-established roles in photosynthesis, pollination, photoprotection, and hormone biosynthesis. The enzymatic or ROS-induced cleavage of carotenoids generates a group of compounds named apocarotenoids, with an increasing interest by virtue of their metabolic, physiological, and ecological activities. Both classes are used industrially in a variety of fields as colorants, supplements, and bio-actives. Crocins and picrocrocin, two saffron apocarotenoids, are examples of high-value pigments utilized in the food, feed, and pharmaceutical industries. In this study, a unique construct was achieved, namely O6, which contains CsCCD2L, UGT74AD1, and UGT709G1 genes responsible for the biosynthesis of saffron apocarotenoids driven by a patatin promoter for the generation of potato tubers producing crocins and picrocrocin. Different tuber potatoes accumulated crocins and picrocrocin ranging from 19.41–360 to 105–800 μg/g DW, respectively, with crocetin, crocin 1 [(crocetin-(β-D-glucosyl)-ester)] and crocin 2 [(crocetin)-(β-D-glucosyl)-(β-D-glucosyl)-ester)] being the main compounds detected. The pattern of carotenoids and apocarotenoids were distinct between wild type and transgenic tubers and were related to changes in the expression of the pathway genes, especially from PSY2, CCD1, and CCD4. In addition, the engineered tubers showed higher antioxidant capacity, up to almost 4-fold more than the wild type, which is a promising sign for the potential health advantages of these lines. In order to better investigate these aspects, different cooking methods were applied, and each process displayed a significant impact on the retention of apocarotenoids. More in detail, the in vitro bioaccessibility of these metabolites was found to be higher in boiled potatoes (97.23%) compared to raw, baked, and fried ones (80.97, 78.96, and 76.18%, respectively). Overall, this work shows that potatoes can be engineered to accumulate saffron apocarotenoids that, when consumed, can potentially offer better health benefits. Moreover, the high bioaccessibility of these compounds revealed that potato is an excellent way to deliver crocins and picrocrocin, while also helping to improve its nutritional value.

Recent Advances in Molecular Improvement for Potato Tuber Traits

Potato is an important crop due to its nutritional value and high yield potential. Improving the quality and quantity of tubers remains one of the most important breeding objectives. Genetic mapping helps to identify suitable markers for use in the molecular breeding, and combined with transgenic approaches provides an efficient way for gaining desirable traits. The advanced plant breeding tools and molecular techniques, e.g., TALENS, CRISPR-Cas9, RNAi, and cisgenesis, have been successfully used to improve the yield and nutritional value of potatoes in an increasing world population scenario. The emerging methods like genome editing tools can avoid incorporating transgene to keep the food more secure. Multiple success cases have been documented in genome editing literature. Recent advances in potato breeding and transgenic approaches to improve tuber quality and quantity have been summarized in this review.

Phytoene Synthase: The Key Rate-Limiting Enzyme of Carotenoid Biosynthesis in Plants

Phytoene synthase (PSY) catalyzes the first committed step in the carotenoid biosynthesis pathway and is a major rate-limiting enzyme of carotenogenesis. PSY is highly regulated by various regulators and factors to modulate carotenoid biosynthesis in response to diverse developmental and environmental cues. Because of its critical role in controlling the total amount of synthesized carotenoids, PSY has been extensively investigated and engineered in plant species. However, much remains to be learned on its multifaceted regulatory control and its catalytic efficiency for carotenoid enrichment in crops. Here, we present current knowledge on the basic biology, the functional evolution, the dynamic regulation, and the metabolic engineering of PSY. We also discuss the open questions and gaps to stimulate additional research on this most studied gene/enzyme in the carotenogenic pathway.

Omics Technologies to Enhance Plant Based Functional Foods: An Overview

Functional foods are natural products of plants that have health benefits beyond necessary nutrition. Functional foods are abundant in fruits, vegetables, spices, beverages and some are found in cereals, millets, pulses and oilseeds. Efforts to identify functional foods in our diet and their beneficial aspects are limited to few crops. Advances in sequencing and availability of different omics technologies have given opportunity to utilize these tools to enhance the functional components of the foods, thus ensuring the nutritional security. Integrated omics approaches including genomics, transcriptomics, proteomics, metabolomics coupled with artificial intelligence and machine learning approaches can be used to improve the crops. This review provides insights into omics studies that are carried out to find the active components and crop improvement by enhancing the functional compounds in different plants including cereals, millets, pulses, oilseeds, fruits, vegetables, spices, beverages and medicinal plants. There is a need to characterize functional foods that are being used in traditional medicines, as well as utilization of this knowledge to improve the staple foods in order to tackle malnutrition and hunger more effectively.

Carotenoids and Apocarotenoids in Planta: Their Role in Plant Development, Contribution to the Flavour and Aroma of Fruits and Flowers, and Their Nutraceutical Benefits

Carotenoids and apocarotenoids are diverse classes of compounds found in nature and are important natural pigments, nutraceuticals and flavour/aroma molecules. Improving the quality of crops is important for providing micronutrients to remote communities where dietary variation is often limited. Carotenoids have also been shown to have a significant impact on a number of human diseases, improving the survival rates of some cancers and slowing the progression of neurological illnesses. Furthermore, carotenoid-derived compounds can impact the flavour and aroma of crops and vegetables and are the origin of important developmental, as well as plant resistance compounds required for defence. In this review, we discuss the current research being undertaken to increase carotenoid content in plants and research the benefits to human health and the role of carotenoid derived volatiles on flavour and aroma of fruits and vegetables.

Potato biofortification: an effective way to fight global hidden hunger

Hidden hunger is leading to extensive health problems in the developing world. Several strategies could be used to reduce the micronutrient deficiencies by increasing the dietary uptake of essential micronutrients. These include diet diversification, pharmaceutical supplementation, food fortification and crop biofortification. Among all, crop biofortification is the most sustainable and acceptable strategy to overcome the global issue of hidden hunger. Since most of the people suffering from micronutrient deficiencies, have monetary issues and are dependent on staple crops to fulfil their recommended daily requirements of various essential micronutrients. Therefore, increasing the micronutrient concentrations in cost effective staple crops seems to be an effective solution. Potato being the world's most consumed non-grain staple crop with enormous industrial demand appears to be an ideal candidate for biofortification. It can be grown in different climatic conditions, provide high yield, nutrition and dry matter in lesser time. In addition, huge potato germplasm have natural variations related to micronutrient concentrations, which can be utilized for its biofortification. This review discuss the current scenario of micronutrient malnutrition and various strategies that could be used to overcome it. The review also shed a light on the genetic variations present in potato germplasm and suggest effective ways to incorporate them into modern high yielding potato varieties.

Antioxidants in Potatoes: A Functional View on One of the Major Food Crops Worldwide

With a growing world population, accelerating climate changes, and limited arable land, it is critical to focus on plant-based resources for sustainable food production. In addition, plants are a cornucopia for secondary metabolites, of which many have robust antioxidative capacities and are beneficial for human health. Potato is one of the major food crops worldwide, and is recognized by the United Nations as an excellent food source for an increasing world population. Potato tubers are rich in a plethora of antioxidants with an array of health-promoting effects. This review article provides a detailed overview about the biosynthesis, chemical and health-promoting properties of the most abundant antioxidants in potato tubers, including several vitamins, carotenoids and phenylpropanoids. The dietary contribution of diverse commercial and primitive cultivars are detailed and document that potato contributes much more than just complex carbohydrates to the diet. Finally, the review provides insights into the current and future potential of potato-based systems as tools and resources for healthy and sustainable food production.

Metabolomics Intervention Towards Better Understanding of Plant Traits

The majority of the most economically important plant and crop species are enriched with the availability of high-quality reference genome sequences forming the basis of gene discovery which control the important biochemical pathways. The transcriptomics and proteomics resources have also been made available for many of these plant species that intensify the understanding at expression levels. However, still we lack integrated studies spanning genomics–transcriptomics–proteomics, connected to metabolomics, the most complicated phase in phenotype expression. Nevertheless, for the past few decades, emphasis has been more on metabolome which plays a crucial role in defining the phenotype (trait) during crop improvement. The emergence of modern high throughput metabolome analyzing platforms have accelerated the discovery of a wide variety of biochemical types of metabolites and new pathways, also helped in improving the understanding of known existing pathways. Pinpointing the causal gene(s) and elucidation of metabolic pathways are very important for development of improved lines with high precision in crop breeding. Along with other-omics sciences, metabolomics studies have helped in characterization and annotation of a new gene(s) function. Hereby, we summarize several areas in the field of crop development where metabolomics studies have made its remarkable impact. We also assess the recent research on metabolomics, together with other omics, contributing toward genetic engineering to target traits and key pathway(s).

Ubiquitination of phytoene synthase 1 precursor modulates carotenoid biosynthesis in tomato

Carotenoids are natural pigments that are indispensable to plants and humans, whereas the regulation of carotenoid biosynthesis by post-translational modification remains elusive. Here, we show that a tomato E3 ubiquitin ligase, Plastid Protein Sensing RING E3 ligase 1 (PPSR1), is responsible for the regulation of carotenoid biosynthesis. PPSR1 exhibits self-ubiquitination activity and loss of PPSR1 function leads to an increase in carotenoids in tomato fruit. PPSR1 affects the abundance of 288 proteins, including phytoene synthase 1 (PSY1), the key rate-limiting enzyme in the carotenoid biosynthetic pathway. PSY1 contains two ubiquitinated lysine residues (Lys380 and Lys406) as revealed by the global analysis and characterization of protein ubiquitination. We provide evidence that PPSR1 interacts with PSY1 precursor protein and mediates its degradation via ubiquitination, thereby affecting the steady-state level of PSY1 protein. Our findings not only uncover a regulatory mechanism for controlling carotenoid biosynthesis, but also provide a strategy for developing carotenoid-enriched horticultural crops.

Wang et al. report on the role of a novel E3 ubiquitin ligase, Plastid Protein Sensing RING E3 ligase 1 (PPSR1), during tomato fruit ripening and find that it interacts with phytoene synthase 1 (PSY1) precursor protein and mediates its degradation via ubiquitination. This affects the steady-state level of PSY1 protein, the key rate-limiting enzyme in the carotenoid biosynthetic pathway. This study may provide a strategy for developing carotenoid-enriched horticultural crops.

Investigation of Streptomyces scabies Causing Potato Scab by Various Detection Techniques, Its Pathogenicity and Determination of Host-Disease Resistance in Potato Germplasm

Streptomyces scabies is a Gram-positive bacterial pathogen that causes common scab disease to several crops, particularly in the potato. It is a soil borne pathogen, a very devastating scab pathogen and difficult to manage in the field. Streptomyces has several species that cause common scab such as S. scabiei, S. acidiscabies, S. europaeiscabiei, S. luridiscabiei, S. niveiscabiei, S. puniciscabiei, S. reticuliscabiei, S. stelliscabiei, S. turgidiscabies, S. ipomoeae. Common scab disease harmfully affects potato economic and market value due to the presence of black spots on the tuber. Owing to its genetic diversity and pathogenicity, the determination of pathogen presence in potato fields is still challenging. In this study, S. scabies genetic diversity was measured by surveying five potato-growing areas of Pakistan during the growing season 2019. A total of 50 Streptomyces isolates, including S. scabies, S. acidiscabies, S. griseoflavus were isolated and identified based on morphologic, biochemical and molecular analysis. Virulent confirmation assays confirmed ten virulent strains of Streptomyces spp. On the potato cultivars Cardinal and Santee. Among the Streptomyces species, S. scabies showed the highest scab index, followed by S. acidiscabies and S. griseoflavus by exhibiting the scab-like lesions on potato tubers. Ten potato cultivars were screened against these virulent isolates of Streptomyces. The Faisalabad white variety showed the highest scab index followed By Cardinal, Tourag, Kuroda, Santee, Lady Rosetta, Asterix, Diamant, Faisalabad red and Sadaf. Moreover, genetic diversity and pathogenicity of Streptomyces spp. on potato tubers were also likely diverse in different geographical regions and also potato cultivars. This study represents a contribution to understanding the local interaction between potatoes and Streptomyces spp. in Pakistan. It will aid in supporting a solution for the management of this pathogen around the world.

Differential Regulation of Phytoene Synthase PSY1 During Fruit Carotenogenesis in Cultivated and Wild Tomato Species (Solanum section Lycopersicon)

In plants, carotenoids define fruit pigmentation and are involved in the processes of photo-oxidative stress defense and phytohormone production; a key enzyme responsible for carotene synthesis in fruit is phytoene synthase 1 (PSY1). Tomatoes (Solanum section Lycopersicon) comprise cultivated (Solanum lycopersicum) as well as wild species with different fruit color and are a good model to study carotenogenesis in fleshy fruit. In this study, we identified homologous PSY1 genes in five Solanum section Lycopersicon species, including domesticated red-fruited S. lycopersicum and wild yellow-fruited S. cheesmaniae and green-fruited S. chilense, S. habrochaites and S. pennellii. PSY1 homologs had a highly conserved structure, including key motifs in the active and catalytic sites, suggesting that PSY1 enzymatic function is similar in green-fruited wild tomato species and preserved in red-fruited S. lycopersicum. PSY1 mRNA expression directly correlated with carotenoid content in ripe fruit of the analyzed tomato species, indicating differential transcriptional regulation. Analysis of the PSY1 promoter and 5′-UTR sequence revealed over 30 regulatory elements involved in response to light, abiotic stresses, plant hormones, and parasites, suggesting that the regulation of PSY1 expression may affect the processes of fruit senescence, seed maturation and dormancy, and pathogen resistance. The revealed differences between green-fruited and red-fruited Solanum species in the structure of the PSY1 promoter/5′-UTR, such as the acquisition of ethylene-responsive element by S. lycopersicum, could reflect the effects of domestication on the transcriptional mechanisms regulating PSY1 expression, including induction of carotenogenesis during fruit ripening, which would contribute to red coloration in mature fruit.

Metabolic engineering of ketocarotenoids biosynthetic pathway in Chlamydomonas reinhardtii strain CC-4102

In Chlamydomonas reinhardtii, ketocarotenoid biosynthesis is limited to the diploid zygospore stage. In this study, we attempted to engineer the ketocarotenoid pathway into Chlamydomonas haploid vegetative green cells by overexpressing the key enzyme ß-carotene ketolase (CrBKT). We chose strain CC-4102 for the approach; competitive pathways, α-carotene biosynthesis and xanthophyll cycle are silenced in this strain. Driven by the strong constitutive HSP70/RBCS2 promoter CrBKT overexpression resulted in the production of canthaxanthin, the ketolation product from ß-carotene as well as a drastic reduction in the chlorophyll concentration. Intriguingly, these phenotypes could only be detected from lines transformed and grown heterotrophically in the dark. Once exposed to light, these transformants lost the aforementioned phenotypes as well as their antibiotic resistance. This phenomenon is in agreement with the fact that we were unable to recover any canthaxanthin-producing line among light-selected transformants.

Genetic Engineering for Global Food Security: Photosynthesis and Biofortification

Increasing demands for food and resources are challenging existing markets, driving a need to continually investigate and establish crop varieties with improved yields and health benefits. By the later part of the century, current estimates indicate that a >50% increase in the yield of most of the important food crops including wheat, rice and barley will be needed to maintain food supplies and improve nutritional quality to tackle what has become known as 'hidden hunger'. Improving the nutritional quality of crops has become a target for providing the micronutrients required in remote communities where dietary variation is often limited. A number of methods to achieve this have been investigated over recent years, from improving photosynthesis through genetic engineering, to breeding new higher yielding varieties. Recent research has shown that growing plants under elevated [CO2] can lead to an increase in Vitamin C due to changes in gene expression, demonstrating one potential route for plant biofortification. In this review, we discuss the current research being undertaken to improve photosynthesis and biofortify key crops to secure future food supplies and the potential links between improved photosynthesis and nutritional quality.

Transcriptome Profiling Provides Insight into the Genes in Carotenoid Biosynthesis during the Mesocarp and Seed Developmental Stages of Avocado (Persea americana)

Avocado (Persea americana Mill.) is an economically important crop because of its high nutritional value. However, the absence of a sequenced avocado reference genome has hindered investigations of secondary metabolism. For next-generation high-throughput transcriptome sequencing, we obtained 365,615,152 and 348,623,402 clean reads as well as 109.13 and 104.10 Gb of sequencing data for avocado mesocarp and seed, respectively, during five developmental stages. High-quality reads were assembled into 100,837 unigenes with an average length of 847.40 bp (N50 = 1725 bp). Additionally, 16,903 differentially expressed genes (DEGs) were detected, 17 of which were related to carotenoid biosynthesis. The expression levels of most of these 17 DEGs were higher in the mesocarp than in the seed during five developmental stages. In this study, the avocado mesocarp and seed transcriptome were also sequenced using single-molecule long-read sequencing to acquired 25.79 and 17.67 Gb clean data, respectively. We identified 233,014 and 238,219 consensus isoforms in avocado mesocarp and seed, respectively. Furthermore, 104 and 59 isoforms were found to correspond to the putative 11 carotenoid biosynthetic-related genes in the avocado mesocarp and seed, respectively. The isoform numbers of 10 out of the putative 11 genes involved in the carotenoid biosynthetic pathway were higher in the mesocarp than those in the seed. Besides, alpha- and beta-carotene contents in the avocado mesocarp and seed during five developmental stages were also measured, and they were higher in the mesocarp than in the seed, which validated the results of transcriptome profiling. Gene expression changes and the associated variations in gene dosage could influence carotenoid biosynthesis. These results will help to further elucidate carotenoid biosynthesis in avocado.

Crop biofortification for iron (Fe), zinc (Zn) and vitamin A with transgenic approaches

Micronutrient malnutrition is an important issue in the developing countries especially in Asia and Africa where millions of school-going children and pregnant women are affected. Poor people are more exposed to risks of malnutrition and hidden hunger due to intake of carbohydrate rich but micronutrient deficient plant based food. The expansion of high yielding but micronutrient poor cultivars further intensified the malnutrition. The existing approaches viz., supplementation and food fortification of staple food with minerals and vitamins can address the issue of adequate nutrition security. But supplementation and fortification is neither feasible for each nutrient specially iron nor viable due to recurrent cost. Recently, genetic bio-fortification of crops is emerged as self-targeted and non-recurrent approach to address the micronutrient malnutrition. Most of the traditional breeding approaches were limited due to non-availability of enough genetic variation in the crossable genepools. Additionally, it also lacks the modulation of target gene expression underlying the micronutrient accumulation. At this juncture, genetic engineering based food biofortification is promising way to address the hidden hunger especially, where breeding is not rewarding due to lack of genetic variability. Genetic modification through gene technology is swift and accurate method to develop nutrient denser crops without any recurrent investment as compared to different strategies.

The Bacterial Phytoene Desaturase-Encoding Gene (CRTI) is an Efficient Selectable Marker for the Genetic Transformation of Eukaryotic Microalgae

Genetic manipulation shows great promise to further boost the productivity of microalgae-based compounds. However, selection of microalgal transformants depends mainly on the use of antibiotics, which have raised concerns about their potential impacts on human health and the environment. We propose the use of a synthetic phytoene desaturase-encoding gene (CRTIop) as a selectable marker and the bleaching herbicide norflurazon as a selective agent for the genetic transformation of microalgae. Bacterial phytoene desaturase (CRTI), which, unlike plant and algae phytoene desaturase (PDS), is not sensitive to norflurazon, catalyzes the conversion of the colorless carotenoid phytoene into lycopene. Although the expression of CRTI has been described to increase the carotenoid content in plant cells, its use as a selectable marker has never been testedin algae or in plants. In this study, a version of the CRTI gene adapted to the codon usage of Chlamydomonas has been synthesized, and its suitability to be used as selectable marker has been shown. The microalgae were transformed by the glass bead agitation method and selected in the presence of norflurazon. Average transformation efficiencies of 550 colonies µg-1 DNA were obtained. All the transformants tested had incorporated the CRTIop gene in their genomes and were able to synthesize colored carotenoids.

Elevated carotenoids in staple crops: The biosynthesis, challenges and measures for target delivery

Poverty eradication and global food security are among the targets of world leaders, most especially combating the scourge of hidden hunger. Provitamin A carotenoids cannot be synthesized de novo by human and so it must be taken as part of the diet. The deficiency of which is causing almost 6000 sights to be lost daily in most developing countries because of the monotonous starchy diets lacking substantial amount of carotenoid. Conventional breeding as well as genetic engineering have been used to increase the level of carotenoid in many staples including rice, potato, maize and cassava. While products from genetic engineering are still been subjected to strict regulatory measures preventing the delivery of the products to target consumers, some of the products from conventional breeding are already on the table of consumers. Interestingly, both technologies are crucial to tackling micronutrient deficiencies. This review discusses the role of carotenoid in human, the biosynthesis in plant and some of the staple crops that have been modified for increased carotenoid. Some measures expected of the leaders of the countries in need of these products for safe delivery to the target population after two decades is also highlighted.

The Role of Orange Gene in Carotenoid Accumulation: Manipulating Chromoplasts Toward a Colored Future

Carotenoids are isoprenoid pigments synthesized in plants, algae, and photosynthetic bacteria and fungus. Their role is essential in light capture, photoprotection, pollinator attraction, and phytohormone production. Furthermore, they can regulate plant development when they are processed as small signaling molecules. Due to their importance for human health, as promoters of the immune system and antioxidant activity, carotenoids have been used in the pharmaceutical, food, and nutraceutical industries. Regulation of carotenoid synthesis and accumulation has been extensively studied. Excellent work has been done unraveling the mode of action of phytoene synthase (PSY), a rate-limiting enzyme of carotenoid biosynthesis pathway, in model species and staple crops. Lately, interest has been turned to Orange protein and its interaction with PSY during carotenoid biosynthesis. Discovered as a dominant mutation in Brassica oleracea, Orange protein regulates carotenoid accumulation by posttranscriptionally regulating PSY, promoting the formation of carotenoid-sequestering structures, and also preventing carotenoid degradation. Furthermore, Orange protein contributes to homeostasis regulation, improving plant tolerance to abiotic stress. In this mini review, the focus is made on recent evidence that elucidates Orange protein mode of action and expression in different plant species. Additionally, strategies are proposed to modify Orange gene by utilization of genome editing techniques. A better understanding of carotenoid biosynthesis and accumulation will lead to a positive impact on the development of healthy food for a growing population.

Transcriptome Sequencing and Biochemical Analysis of Perianths and Coronas Reveal Flower Color Formation in Narcissus pseudonarcissus

Narcissus pseudonarcissus is an important bulbous plant with white or yellow perianths and light yellow to orange-red coronas, but little is known regarding the biochemical and molecular basis related to flower color polymorphisms. To investigate the mechanism of color formation, RNA-Seq of flower of two widely cultured cultivars (‘Slim Whitman’ and ‘Pinza’) with different flower color was performed. A total of 84,463 unigenes were generated from the perianths and coronas. By parallel metabolomic and transcriptomic analyses, we provide an overview of carotenoid biosynthesis, degradation, and accumulation in N. pseudonarcissus. The results showed that the content of carotenoids in the corona was higher than that in the perianth in both cultivars. Accordingly, phytoene synthase (PSY) transcripts have a higher abundance in the coronas than that in perianths. While the expression levels of carotenoid biosynthetic genes, like GGPPS, PSY, and LCY-e, were not significantly different between two cultivars. In contrast, the carotenoid degradation gene NpCCD4 was highly expressed in white-perianth cultivars, but was hardly detected in yellow-perianth cultivars. Silencing of NpCCD4 resulted in a significant increase in carotenoid accumulation, especially in all-trans-β-carotene. Therefore, we presume that NpCCD4 is a crucial factor that causes the low carotenoid content and color fading phenomenon of ‘Slim Whitman’ by mediating carotenoid turnover. Our findings provide mass RNA-seq data and new insights into carotenoid metabolism in N. pseudonarcissus.

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.

Potential of golden potatoes to improve vitamin A and vitamin E status in developing countries

Potato (Solanum tuberosum L.) is the third most widely consumed plant food by humans. Its tubers are rich in starch and vitamin C, but have low or null levels of essential nutrients such as provitamin A and vitamin E. Transformation of potato with a bacterial mini-pathway for β-carotene in a tuber-specific manner results in a “golden” potato (GP) tuber phenotype resulting from accumulation of provitamin A carotenoids (α- and β-carotene) and xanthophylls. Here, we investigated the bioaccessibility of carotenoids and vitamin E as α-tocopherol (αTC) in boiled wild type and golden tubers using in vitro digestion. Golden tubers contained up to 91 μg provitamin A carotenes (PAC)/g D, increased levels of xanthophylls, phytoene and phytofluene, as well as up to 78 μg vitamin E/g DW. Cubes from wild type and GP tubers were boiled and subjected to simulated digestion to estimate bioaccessibility of carotenoids and αTC. Retention in boiled GPs exceeded 80% for β-carotene (βC), α-carotene (αC), lutein, phytoene ± and αTC, but less than 50% for phytofluene. The efficiency of partitioning of total βC, αC, E-lutein, phytoene, phytofluene and αTC in the mixed micelle fraction during small intestinal digestion was influenced by genotype, tuber content and hydrophobicity. Apical uptake of the compounds that partitioned in mixed micelles by monolayers of human intestinal Caco-2 cells during incubation for 4h was 14–20% for provitamin A and xanthophylls, 43–45% for phytoene, 23–27% for phytofluene, and 53% for αTC. These results suggest that a 150 g serving of boiled golden potatoes has the potential to contribute 42% and 23% of the daily requirement of retinol activity equivalents (RAE), as well as 34 and 17% of the daily vitamin E requirement for children and women of reproductive age, respectively.

Genetic Engineering: A Possible Strategy for Protein-Energy Malnutrition Regulation

Protein-energy malnutrition (PEM) has adversely affected the generations of developing countries. It is a syndrome that in severity causes death. PEM generally affects infants of 1-5 age group. This manifestation is maintained till adulthood in the form of poor brain and body development. The developing nations are continuously making an effort to curb PEM. However, it is still a prime concern as it was in its early years of occurrence. Transgenic crops with high protein and enhanced nutrient content have been successfully developed. Present article reviews the studies documenting genetic engineering-mediated improvement in the pulses, cereals, legumes, fruits and other crop plants in terms of nutritional value, stress tolerance, longevity and productivity. Such genetically engineered crops can be used as a possible remedial tool to eradicate PEM.

Manipulation of Metabolic Pathways to Develop Vitamin-Enriched Crops for Human Health

Vitamin deficiencies are major forms of micronutrient deficiencies, and are associated with huge economic losses as well as severe physical and intellectual damages to humans. Much evidence has demonstrated that biofortification plays an important role in combating vitamin deficiencies due to its economical and effective delivery of nutrients to populations in need. Biofortification enables food plants to be enriched with vitamins through conventional breeding and/or biotechnology. Here, we focus on the progress in the manipulation of the vitamin metabolism, an essential part of biofortification, by the genetic modification or by the marker-assisted selection to understand mechanisms underlying metabolic improvement in food plants. We also propose to integrate new breeding technologies with metabolic pathway modification to facilitate biofortification in food plants and, thereby, to benefit human health.

Ketocarotenoid Production in Soybean Seeds through Metabolic Engineering

The pink or red ketocarotenoids, canthaxanthin and astaxanthin, are used as feed additives in the poultry and aquaculture industries as a source of egg yolk and flesh pigmentation, as farmed animals do not have access to the carotenoid sources of their wild counterparts. Because soybean is already an important component in animal feed, production of these carotenoids in soybean could be a cost-effective means of delivery. In order to characterize the ability of soybean seed to produce carotenoids, soybean cv. Jack was transformed with the crtB gene from Pantoea ananatis, which codes for phytoene synthase, an enzyme which catalyzes the first committed step in the carotenoid pathway. The crtB gene was engineered together in combinations with ketolase genes (crtW from Brevundimonas sp. strain SD212 and bkt1 from Haematococcus pluvialis) to produce ketocarotenoids; all genes were placed under the control of seed-specific promoters. HPLC results showed that canthaxanthin is present in the transgenic seeds at levels up to 52 μg/g dry weight. Transgenic seeds also accumulated other compounds in the carotenoid pathway, such as astaxanthin, lutein, β-carotene, phytoene, α-carotene, lycopene, and β-cryptoxanthin, whereas lutein was the only one of these detected in non-transgenic seeds. The accumulation of astaxanthin, which requires a β-carotene hydroxylase in addition to a β-carotene ketolase, in the transgenic seeds suggests that an endogenous soybean enzyme is able to work in combination with the ketolase transgene. Soybean seeds that accumulate ketocarotenoids could potentially be used in animal feed to reduce or eliminate the need for the costly addition of these compounds.

Effect of an Introduced Phytoene Synthase Gene Expression on Carotenoid Biosynthesis in the Marine Diatom Phaeodactylum tricornutum

Carotenoids exert beneficial effects on human health through their excellent antioxidant activity. To increase carotenoid productivity in the marine Pennales Phaeodactylum tricornutum, we genetically engineered the phytoene synthase gene (psy) to improve expression because RNA-sequencing analysis has suggested that the expression level of psy is lower than other enzyme-encoding genes that are involved in the carotenoid biosynthetic pathway. We isolated psy from P. tricornutum, and this gene was fused with the enhanced green fluorescent protein gene to detect psy expression. After transformation using the microparticle bombardment technique, we obtained several P. tricornutum transformants and confirmed psy expression in their plastids. We investigated the amounts of PSY mRNA and carotenoids, such as fucoxanthin and β-carotene, at different growth phases. The introduction of psy increased the fucoxanthin content of a transformants by approximately 1.45-fold relative to the levels in the wild-type diatom. However, some transformants failed to show a significant increase in the carotenoid content relative to that of the wild-type diatom. We also found that the amount of PSY mRNA at log phase might contribute to the increase in carotenoids in the transformants at stationary phase.

Arabidopsis OR proteins are the major posttranscriptional regulators of phytoene synthase in controlling carotenoid biosynthesis

Carotenoids are indispensable natural pigments to plants and humans. Phytoene synthase (PSY), the rate-limiting enzyme in the carotenoid biosynthetic pathway, and ORANGE (OR), a regulator of chromoplast differentiation and enhancer of carotenoid biosynthesis, represent two key proteins that control carotenoid biosynthesis and accumulation in plants. However, little is known about the mechanisms underlying their posttranscriptional regulation. Here we report that PSY and OR family proteins [Arabidopsis thaliana OR (AtOR) and AtOR-like] physically interacted with each other in plastids. We found that alteration of OR expression in Arabidopsis exerted minimal effect on PSY transcript abundance. However, overexpression of AtOR significantly increased the amount of enzymatically active PSY, whereas an ator ator-like double mutant exhibited a dramatically reduced PSY level. The results indicate that the OR proteins serve as the major posttranscriptional regulators of PSY. The ator or ator-like single mutant had little effect on PSY protein levels, which involves a compensatory mechanism and suggests partial functional redundancy. In addition, modification of PSY expression resulted in altered AtOR protein levels, corroborating a mutual regulation of PSY and OR. Carotenoid content showed a correlated change with OR-mediated PSY level, demonstrating the function of OR in controlling carotenoid biosynthesis by regulating PSY. Our findings reveal a novel mechanism by which carotenoid biosynthesis is controlled via posttranscriptional regulation of PSY in plants.

Functional analysis of the Brassica napus L. phytoene synthase (PSY) gene family

Phytoene synthase (PSY) has been shown to catalyze the first committed and rate-limiting step of carotenogenesis in several crop species, including Brassica napus L. Due to its pivotal role, PSY has been a prime target for breeding and metabolic engineering the carotenoid content of seeds, tubers, fruits and flowers. In Arabidopsis thaliana, PSY is encoded by a single copy gene but small PSY gene families have been described in monocot and dicotyledonous species. We have recently shown that PSY genes have been retained in a triplicated state in the A- and C-Brassica genomes, with each paralogue mapping to syntenic locations in each of the three “Arabidopsis-like” subgenomes. Most importantly, we have shown that in B. napus all six members are expressed, exhibiting overlapping redundancy and signs of subfunctionalization among photosynthetic and non photosynthetic tissues. The question of whether this large PSY family actually encodes six functional enzymes remained to be answered. Therefore, the objectives of this study were to: (i) isolate, characterize and compare the complete protein coding sequences (CDS) of the six B. napus PSY genes; (ii) model their predicted tridimensional enzyme structures; (iii) test their phytoene synthase activity in a heterologous complementation system and (iv) evaluate their individual expression patterns during seed development. This study further confirmed that the six B. napus PSY genes encode proteins with high sequence identity, which have evolved under functional constraint. Structural modeling demonstrated that they share similar tridimensional protein structures with a putative PSY active site. Significantly, all six B. napus PSY enzymes were found to be functional. Taking into account the specific patterns of expression exhibited by these PSY genes during seed development and recent knowledge of PSY suborganellar localization, the selection of transgene candidates for metabolic engineering the carotenoid content of oilseeds is discussed.

Comparative proteomic analysis of labellum and inner lateral petals in Cymbidium ensifolium flowers

The labellum in orchids shares homology with the inner lateral petals of the flower. The labellum is a modified petal and often distinguished from other petals and sepals due to its large size and irregular shape. Herein, we combined two-dimensional gel electrophoresis (2-DE) and matrix assisted laser desorption/ionization time of flight/time of flight (MALDI-TOF/TOF) approaches to identify the differentially expressed proteome between labellum and inner lateral petal in one of Orchid species (C. ensifolium). A total of 30 protein spots were identified, which showed more than a two-fold significant difference (p < 0.05) in their expression. Compared with C. ensifolium transcriptome (sequenced in house), 21 proteins matched the translated nucleotide. The proteins identified were classified into 48 categories according to gene ontology (GO). Additionally, these proteins were involved in 18 pathways and 9 possible protein-protein interactions. Serine carboxypeptidase and beta-glucosidase were involved in the phenylpropanoid pathway, which could regulate biosynthesis of floral scent components. Malate dehydrogenase (maeB) and triosephosphate isomerase (TPI) in carbon fixation pathway could regulate the energy metabolism. Xyloglucan endotransglucosylase/hydrolase (XET/XTH) could promote cell wall formation and aid the petal’s morphogenesis. The identification of such differentially expressed proteins provides new targets for future studies; these will assess the proteins’ physiological roles and significance in labellum and inner lateral petals.

Genetic stability developed for β-carotene synthesis in BR29 rice line using dihaploid homozygosity

Obtaining transgenic crop lines with stable levels of carotenoids is highly desirable. We addressed this issue by employing the anther culture technique to develop dihaploid lines containing genes involved in β-carotene metabolism. First, we used Agrobacterium- mediated transformation to develop primary transgenic plants containing the β-carotene biosynthetic genes, phytoene synthase (psy) and phytoene desaturase (crtI), which were engineered for expression and accumulation in the endosperm. Transgenic plants were recovered by selecting for the expression of the phosphomannose isomerase (pmi) gene. Dihaploid plants in addition to haploid and tetraploid plant were generated from anther cultures of these primary transgenic plants. In addition to anatomical features of stomata, pollen of different ploidy-plants, molecular analyses confirmed the stable integration of the genes in the anther culture-derived dihaploid plants, and the yellow color of the polished seeds indicated the accumulation of carotenoids in the endosperm. High performance liquid chromatography (HPLC) analysis of the carotenoid extract further confirmed the levels of β-carotene accumulation in the endosperms of the transgenic dihaploid rice seeds.

The contribution of transgenic plants to better health through improved nutrition: opportunities and constraints

Malnutrition is a prevalent and entrenched global socioeconomic challenge that reflects the combined impact of poverty, poor access to food, inefficient food distribution infrastructure, and an over-reliance on subsistence mono-agriculture. The dependence on staple cereals lacking many essential nutrients means that malnutrition is endemic in developing countries. Most individuals lack diverse diets and are therefore exposed to nutrient deficiencies. Plant biotechnology could play a major role in combating malnutrition through the engineering of nutritionally enhanced crops. In this article, we discuss different approaches that can enhance the nutritional content of staple crops by genetic engineering (GE) as well as the functionality and safety assessments required before nutritionally enhanced GE crops can be deployed in the field. We also consider major constraints that hinder the adoption of GE technology at different levels and suggest policies that could be adopted to accelerate the deployment of nutritionally enhanced GE crops within a multicomponent strategy to combat malnutrition.

Effect of the citrus lycopene β-cyclase transgene on carotenoid metabolism in transgenic tomato fruits

Lycopene β-cyclase (LYCB) is the key enzyme for the synthesis of β-carotene, a valuable component of the human diet. In this study, tomato constitutively express Lycb-1 was engineered. The β-carotene level of transformant increased 4.1 fold, and the total carotenoid content increased by 30% in the fruits. In the transgenic line, the downstream α-branch metabolic fluxes were repressed during the three developmental stages while α-carotene content increased in the ripe stage. Microarray analysis in the ripe stage revealed that the constitutive expression of Lycb-1 affected a number of pathways including the synthesis of fatty acids, flavonoids and phenylpropanoids, the degradation of limonene and pinene, starch and sucrose metabolism and photosynthesis. This study provided insight into the regulatory effect of Lycb-1 gene on plant carotenoid metabolism and fruit transcriptome.

Cloning and characterization of the ζ-carotene desaturase gene from Chlorella protothecoides CS-41

To elucidate the lutein biosynthesis pathway in the lutein-producing alga, Chlorella protothecoides CS-41, the ζ-carotene desaturase gene (zds) was isolated from Chlorella protothecoides using the approach of rapid amplification of cDNA ends. The full-length cDNA sequence was 2031 bp and contained 1755 bp putative open reading frame which encodes a 584 amino acid deduced polypeptide whose computed molecular weight was 63.7 kDa. Sequence homology research indicated that the nucleotide and putative protein had sequence identities of 72.5% and 69.5% with those of the green alga Chlamydomonas reinhardtii, respectively. Phylogenetic analysis demonstrated that the ZDS from C. protothecoides CS-41 had a closer relationship with those of chlorophyta and higher plants than with those of other species. In addition, we also found that the zds gene expression was upregulated in response to light.

Enhancement of carotenoids biosynthesis in Chlamydomonas reinhardtii by nuclear transformation using a phytoene synthase gene isolated from Chlorella zofingiensis

The isolation and characterization of the phytoene synthase gene from the green microalga Chlorella zofingiensis (CzPSY), involved in the first step of the carotenoids biosynthetic pathway, have been performed. CzPSY gene encodes a polypeptide of 420 amino acids. A single copy of CzPSY has been found in C. zofingiensis by Southern blot analysis. Heterologous genetic complementation in Escherichia coli showed the ability of the predicted protein to catalyze the condensation of two molecules of geranylgeranyl pyrophosphate (GGPP) to form phytoene. Phylogenetic analysis has shown that the deduced protein forms a cluster with the rest of the phytoene synthases (PSY) of the chlorophycean microalgae studied, being very closely related to PSY of plants. This new isolated gene has been adequately inserted in a vector and expressed in Chlamydomonas reinhardtii. The overexpression of CzPSY in C. reinhardtii, by nuclear transformation, has led to an increase in the corresponding CzPSY transcript level as well as in the content of the carotenoids violaxanthin and lutein which were 2.0- and 2.2-fold higher than in untransformed cells. This is an example of manipulation of the carotenogenic pathway in eukaryotic microalgae, which can open up the possibility of enhancing the productivity of commercial carotenoids by molecular engineering.

Recent advances in artemisinin production through heterologous expression

Artemisinin the sesquiterpene endoperoxide lactone extracted from the herb Artemisia annua, remains the basis for the current preferred treatment against the malaria parasite Plasmodium falciparum. In addition, artemisinin and its derivatives show additional anti-parasite, anti-cancer, and anti-viral properties. Widespread use of this valuable secondary metabolite has been hampered by low production in vivo and high cost of chemical synthesis in vitro. Novel production methods are required to accommodate the ever-growing need for this important drug. Past work has focused on increasing production through traditional breeding approaches, with limited success, and on engineering cultured plants for high production in bioreactors. New research is focusing on heterologous expression systems for this unique biochemical pathway. Recently discovered genes, including a cytochrome P450 and its associated reductase, have been shown to catalyze multiple steps in the biochemical pathway leading to artemisinin. This has the potential to make a semi-synthetic approach to production both possible and cost effective. Artemisinin precursor production in engineered Saccharomyces cerevisiae is about two orders of magnitude higher than from field-grown A. annua. Efforts to increase flux through engineered pathways are on-going in both E. coli and S. cerevisiae through combinations of engineering precursor pathways and downstream optimization of gene expression. This review will compare older approaches to overproduction of this important drug, and then focus on the results from the newer approaches using heterologous expression systems and how they might meet the demands for treating malaria and other diseases.

Combinatorial genetic transformation generates a library of metabolic phenotypes for the carotenoid pathway in maize

Combinatorial nuclear transformation is a novel method for the rapid production of multiplex-transgenic plants, which we have used to dissect and modify a complex metabolic pathway. To demonstrate the principle, we transferred 5 carotenogenic genes controlled by different endosperm-specific promoters into a white maize variety deficient for endosperm carotenoid synthesis. We recovered a diverse population of transgenic plants expressing different enzyme combinations and showing distinct metabolic phenotypes that allowed us to identify and complement rate-limiting steps in the pathway and to demonstrate competition between beta-carotene hydroxylase and bacterial beta-carotene ketolase for substrates in 4 sequential steps of the extended pathway. Importantly, this process allowed us to generate plants with extraordinary levels of beta-carotene and other carotenoids, including complex mixtures of hydroxycarotenoids and ketocarotenoids. Combinatorial transformation is a versatile approach that could be used to modify any metabolic pathway and pathways controlling other biochemical, physiological, or developmental processes.

Analysis of 90 Mb of the potato genome reveals conservation of gene structures and order with tomato but divergence in repetitive sequence composition

Background

The Solanaceae family contains a number of important crop species including potato (Solanum tuberosum) which is grown for its underground storage organ known as a tuber. Albeit the 4^th most important food crop in the world, other than a collection of ~220,000 Expressed Sequence Tags, limited genomic sequence information is currently available for potato and advances in potato yield and nutrition content would be greatly assisted through access to a complete genome sequence. While morphologically diverse, Solanaceae species such as potato, tomato, pepper, and eggplant share not only genes but also gene order thereby permitting highly informative comparative genomic analyses.

Results

In this study, we report on analysis 89.9 Mb of potato genomic sequence representing 10.2% of the genome generated through end sequencing of a potato bacterial artificial chromosome (BAC) clone library (87 Mb) and sequencing of 22 potato BAC clones (2.9 Mb). The GC content of potato is very similar to Solanum lycopersicon (tomato) and other dicotyledonous species yet distinct from the monocotyledonous grass species, Oryza sativa. Parallel analyses of repetitive sequences in potato and tomato revealed substantial differences in their abundance, 34.2% in potato versus 46.3% in tomato, which is consistent with the increased genome size per haploid genome of these two Solanum species. Specific classes and types of repetitive sequences were also differentially represented between these two species including a telomeric-related repetitive sequence, ribosomal DNA, and a number of unclassified repetitive sequences. Comparative analyses between tomato and potato at the gene level revealed a high level of conservation of gene content, genic feature, and gene order although discordances in synteny were observed.

Conclusion

Genomic level analyses of potato and tomato confirm that gene sequence and gene order are conserved between these solanaceous species and that this conservation can be leveraged in genomic applications including cross-species annotation and genome sequencing initiatives. While tomato and potato share genic features, they differ in their repetitive sequence content and composition suggesting that repetitive sequences may have a more significant role in shaping speciation than previously reported.

Dormancy in potato tuber meristems: chemically induced cessation in dormancy matches the natural process based on transcript profiles

Meristem dormancy in perennial plants is a developmental process that results in repression of metabolism and growth. The cessation of dormancy results in rapid growth and should be associated with the production of nascent transcripts that encode for gene products controlling for cell division and growth. Dormancy cessation was allowed to progress normally or was chemically induced using bromoethane (BE), and microarray analysis was used to demonstrate changes in specific transcripts in response to dormancy cessation before a significant increase in cell division. Comparison of normal dormancy cessation to BE-induced dormancy cessation revealed a commonality in both up and downregulated transcripts. Many transcripts that decrease as dormancy terminates are inducible by abscisic acid particularly in the conserved BURP domain proteins, which include the RD22 class of proteins and in the storage protein patatin. Transcripts that are associated with an increase in expression encoded for proteins in the oxoglutarate-dependent oxygenase family. We conclude that BE-induced cessation of dormancy initiates transcript profiles similar to the natural processes that control dormancy.

Enhancing the carotenoid content of Brassica napus seeds by downregulating lycopene epsilon cyclase

The accumulation of carotenoids in higher plants is regulated by the environment, tissue type and developmental stage. In Brassica napus leaves, beta-carotene and lutein were the main carotenoids present while petals primarily accumulated lutein and violaxanthin. Carotenoid accumulation in seeds was developmentally regulated with the highest levels detected at 35-40 days post anthesis. The carotenoid biosynthesis pathway branches after the formation of lycopene. One branch forms carotenoids with two beta rings such as beta-carotene, zeaxanthin and violaxanthin, while the other introduces both beta- and epsilon-rings in lycopene to form alpha-carotene and lutein. By reducing the expression of lycopene epsilon-cyclase (epsilon-CYC) using RNAi, we investigated altering carotenoid accumulation in seeds of B. napus. Transgenic seeds expressing this construct had increased levels of beta-carotene, zeaxanthin, violaxanthin and, unexpectedly, lutein. The higher total carotenoid content resulting from reduction of epsilon-CYC expression in seeds suggests that this gene is a rate-limiting step in the carotenoid biosynthesis pathway. epsilon-CYC activity and carotenoid production may also be related to fatty acid biosynthesis in seeds as transgenic seeds showed an overall decrease in total fatty acid content and minor changes in the proportions of various fatty acids.

Metabolic engineering of carotenoid accumulation by creating a metabolic sink

Carotenoids are highly beneficial for human nutrition and health because they provide essential nutrients and important antioxidants in our diets. However, many food crops, especially the major staple crops contain only trace to low amounts of carotenoids. Although significant progress has been made in developing food crops rich in carotenoids by altering the expression of carotenoid biosynthetic genes, in many cases it has proved to be difficult to reach the desired levels of carotenoid enrichment. The recent identification and characterization of a novel gene mutation in cauliflower reveals that creating a metabolic sink to sequester carotenoids is an important mechanism to control carotenoid accumulation in plants. The successful demonstration of increased carotenoid accumulation in association with the formation of sink structures in transgenic crops offers a new and alternative approach to increase carotenoid content. Manipulation of the formation of metabolic sink along with the catalytic activity of the pathway may represent a promising strategy for maximally improving the nutritional quality of food crops.

Accumulation of vitamin E in potato (Solanum tuberosum) tubers

Vitamin E (tocopherol) is a powerful antioxidant essential for human health and synthesized only by photosynthetic organisms. The effects of over-expression of tocopherol biosynthetic enzymes have been studied in leaves and seeds, but not in a non-photosynthetic, below-ground plant organ. Genetic and molecular approaches were used to determine if increased levels of tocopherols can be accumulated in potato (Solanum tuberosum L.) tubers through metabolic engineering. Two transgenes were constitutively over-expressed in potato: Arabidopsis thaliana p-hydroxyphenylpyruvate dioxygenase (At-HPPD) and A. thaliana homogentisate phytyltransferase (At-HPT). alpha-Tocopherol levels in the transgenic plants were determined by high-performance liquid chromatography. In potato tubers, over-expression of At-HPPD resulted in a maximum 266% increase in alpha-tocopherol, and over-expression of At-HPT yielded a 106% increase. However, tubers from transgenic plants still accumulated approximately 10- and 100-fold less alpha-tocopherol than leaves or seeds, respectively. The results indicate that physiological and regulatory constraints may be the most limiting factors for tocopherol accumulation in potato tubers. Studying regulation and induction of tocopherol biosynthesis should reveal approaches to more effectively engineer crops with enhanced tocopherol content.

Metabolic engineering of potato carotenoid content through tuber-specific overexpression of a bacterial mini-pathway

Background

Since the creation of “Golden Rice”, biofortification of plant-derived foods is a promising strategy for the alleviation of nutritional deficiencies. Potato is the most important staple food for mankind after the cereals rice, wheat and maize, and is extremely poor in provitamin A carotenoids.

Methodology

We transformed potato with a mini-pathway of bacterial origin, driving the synthesis of beta-carotene (Provitamin A) from geranylgeranyl diphosphate. Three genes, encoding phytoene synthase (CrtB), phytoene desaturase (CrtI) and lycopene beta-cyclase (CrtY) from Erwinia, under tuber-specific or constitutive promoter control, were used. 86 independent transgenic lines, containing six different promoter/gene combinations, were produced and analyzed. Extensive regulatory effects on the expression of endogenous genes for carotenoid biosynthesis are observed in transgenic lines. Constitutive expression of the CrtY and/or CrtI genes interferes with the establishment of transgenosis and with the accumulation of leaf carotenoids. Expression of all three genes, under tuber-specific promoter control, results in tubers with a deep yellow (“golden”) phenotype without any adverse leaf phenotypes. In these tubers, carotenoids increase approx. 20-fold, to 114 mcg/g dry weight and beta-carotene 3600-fold, to 47 mcg/g dry weight.

Conclusions

This is the highest carotenoid and beta-carotene content reported for biofortified potato as well as for any of the four major staple foods (the next best event being “Golden Rice 2”, with 31 mcg/g dry weight beta-carotene). Assuming a beta-carotene to retinol conversion of 6∶1, this is sufficient to provide 50% of the Recommended Daily Allowance of Vitamin A with 250 gms (fresh weight) of “golden” potatoes.

Escherichia coli as a platform for functional expression of plant P450 carotene hydroxylases

Carotenoids and their derivatives are essential for growth, development, and signaling in plants and have an added benefit as nutraceuticals in food crops. Despite the importance of the biosynthetic pathway, there remain open questions regarding some of the later enzymes in the pathway. The CYP97 family of P450 enzymes was predicted to function in carotene ring hydroxylation, to convert provitamin A carotenes to non-provitamin A xanthophylls. However, substrate specificity was difficult to investigate directly in plants, which mask enzyme activities by a complex and dynamic metabolic network. To characterize the enzymes more directly, we amplified cDNAs from a model crop, Oryza sativa, and used functional complementation in Escherichia coli to test activity and specificity of members of Clans A and C. This heterologous system will be valuable for further study of enzyme interactions and substrate utilization needed to understand better the role of CYP97 hydroxylases in plant carotenoid biosynthesis.

Molecular characterization of marker-free transgenic lines of indica rice that accumulate carotenoids in seed endosperm

A single Agrobacterium strain harbouring two binary plasmids was successfully used for the first time to develop a marker-free transgenic rice of improved nutritional value. Sixty-eight T0 co-transformants were obtained in three indica rice cultivars--two popular high-yielding Bangladeshi varieties (BR28 and BR29), and one high-iron rice cultivar (IR68144). Marker-free lines were obtained from 14 out of 24 selected co-transformants screened in the T1 generation. The accumulation of total carotenoids in polished T2 rice seeds of the primary transgenic VPBR29-17-37 reached levels of up to 3.0 microg/g, with the level of beta-carotene reaching 1.8 microg/g. In the cultivars BR28 and IR68144, total carotenoid levels in the transformants reached 2.0 microg/g of polished rice seeds. The levels of lutein and other carotenoids in the seeds were also significantly enhanced. T1 plants obtained from primary transgenics with simple gene-integration patterns tended to have a lower carotenoid content than the original parental lines. This study describes the development of marker-free transgenic rice lines containing high levels of carotenoids, and addresses the relationship between the rearrangement of transgenes and the presence of metabolic end products in transgenic rice.

Agrobacterium-mediated transformation of Solanum phureja

A population of transgenic plants was produced by the transformation of internodal explants of Solanum phureja, DB337/37 (the cultivar Mayan Gold) using an Agrobacterium tumefaciens LBA4404-based vector containing a phytoene synthase gene (crtB). The regeneration strategy utilised a two-step protocol, with a 12-day callus induction stage mediated by 1.07 microM alpha-napthaleneacetic acid (NAA), 7.10 microM zeatin riboside and 0.06 microM gibberellic acid (GA3), followed by a prolonged (up to 90 day) shoot induction stage on medium containing 0.11 microM NAA, 7.10 microM zeatin riboside and 0.06 microM GA3 supplemented with kanamycin at 50 mg l(-1) as the selection agent. Southern analysis of the transgenic population revealed that the transgene copy number varied between one and five in the lines tested. Northern blot analysis showed significant expression of the introduced crtB gene in some lines during tuber development. Cytological analysis of the material showed a high incidence of chromosome doubling in the transgenic population with over 80% of all lines tested having doubled their chromosome complement during the transformation process.