Expression of a feedback insensitive anthranilate synthase gene from tobacco increases free tryptophan in soybean plants

Feedback regulation of plant secondary metabolism: Applications and challenges

Plant secondary metabolites offer resistance to invasion by herbivorous organisms, and are also useful in the chemical, pharmaceutical, cosmetic, and fragrance industries. There are numerous approaches to enhancing secondary metabolite yields. However, a growing number of studies has indicated that feedback regulation may be critical in regulating secondary metabolite biosynthesis. Here, we review examples of feedback regulation in secondary metabolite biosynthesis pathways, phytohormone signal transduction, and complex deposition sites associated with secondary metabolite biosynthesis. We propose a new strategy to enhance secondary metabolite production based on plant feedback regulation. We also discuss challenges in feedback regulation that must be overcome before its application to enhancing secondary metabolite yields. This review discusses recent advances in the field and highlights a strategy to overcome feedback regulation-related obstacles and obtain high secondary metabolite yields.

Biochemical investigation of the tryptophan biosynthetic enzyme anthranilate phosphoribosyltransferase in plants

While mammals require the essential amino acid tryptophan (Trp) in their diet, plants and microorganisms synthesize Trp de novo. The five-step Trp pathway starts with the shikimate pathway product, chorismate. Chorismate is converted to the aromatic compound anthranilate, which is then conjugated to a phosphoribosyl sugar in the second step by anthranilate phosphoribosyltransferase (PAT1). As a single-copy gene in plants, all fixed carbon flux to indole and Trp for protein synthesis, specialized metabolism, and auxin hormone biosynthesis proceeds through PAT1. While bacterial PAT1s have been studied extensively, plant PAT1s have escaped biochemical characterization. Using a structure model, we identified putative active site residues that were variable across plants and kinetically characterized six PAT1s (Arabidopsis thaliana (thale cress), Citrus sinensis (sweet orange), Pistacia vera (pistachio), Juglans regia (English walnut), Selaginella moellendorffii (spike moss), and Physcomitrium patens (spreading earth-moss)). We probed the catalytic efficiency, substrate promiscuity, and regulation of these six enzymes and found that the C. sinensis PAT1 is highly specific for its cognate substrate, anthranilate. Investigations of site-directed mutants of the A. thaliana PAT1 uncovered an active site residue that contributes to promiscuity. While Trp inhibits bacterial PAT1 enzymes, the six plant PAT1s that we tested were not modulated by Trp. Instead, the P. patens PAT1 was inhibited by tyrosine, and the S. moellendorffii PAT1 was inhibited by phenylalanine. This structure-informed biochemical examination identified variations in activity, efficiency, specificity, and enzyme-level regulation across PAT1s from evolutionarily diverse plants.

Molecular mapping and genomics of soybean seed protein: a review and perspective for the future

KEY MESSAGE

Genetic improvement of soybean protein meal is a complex process because of negative correlation with oil, yield, and temperature. This review describes the progress in mapping and genomics, identifies knowledge gaps, and highlights the need of integrated approaches. Meal protein derived from soybean [Glycine max (L) Merr.] seed is the primary source of protein in poultry and livestock feed. Protein is a key factor that determines the nutritional and economical value of soybean. Genetic improvement of soybean seed protein content is highly desirable, and major quantitative trait loci (QTL) for soybean protein have been detected and repeatedly mapped on chromosomes (Chr.) 20 (LG-I), and 15 (LG-E). However, practical breeding progress is challenging because of seed protein content's negative genetic correlation with seed yield, other seed components such as oil and sucrose, and interaction with environmental effects such as temperature during seed development. In this review, we discuss rate-limiting factors related to soybean protein content and nutritional quality, and potential control factors regulating seed storage protein. In addition, we describe advances in next-generation sequencing technologies for precise detection of natural variants and their integration with conventional and high-throughput genotyping technologies. A syntenic analysis of QTL on Chr. 15 and 20 was performed. Finally, we discuss comprehensive approaches for integrating protein and amino acid QTL, genome-wide association studies, whole-genome resequencing, and transcriptome data to accelerate identification of genomic hot spots for allele introgression and soybean meal protein improvement.

Metabolomics of genetically modified crops

Metabolomic-based approaches are increasingly applied to analyse genetically modified organisms (GMOs) making it possible to obtain broader and deeper information on the composition of GMOs compared to that obtained from traditional analytical approaches. The combination in metabolomics of advanced analytical methods and bioinformatics tools provides wide chemical compositional data that contributes to corroborate (or not) the substantial equivalence and occurrence of unintended changes resulting from genetic transformation. This review provides insight into recent progress in metabolomics studies on transgenic crops focusing mainly in papers published in the last decade.

Tryptophan and indole analog mediated plastid transformation

A nonantibiotic/herbicide-resistance selection system for plastid transformation is described here in technical detail. This system is based on the feedback-insensitive anthranilate synthase (AS) α-subunit gene of tobacco (ASA2) as a selective marker and tryptophan (Trp) or indole analogs as selection agents. AS catalyzes the first reaction in the Trp biosynthetic pathway, naturally compartmentalized in the plastids, by converting chorismate to anthranilate and is subjected to feedback inhibition by Trp. In addition to Trp, various Trp analogs and indole compounds that can be converted to Trp analogs can also inhibit AS activity and therefore are toxic to cells. When cells are made to express the feedback-insensitive ASA2, they acquire resistance to these analogs and can be selected for during transformation process. We have demonstrated the feasibility of this selection system in tobacco (Nicotiana tabacum L. cv. Petit Havana). ASA2-expressing transplastomic plants were obtained on medium supplemented with either 7-methyl-DL-tryptophan (7-MT) or 4-methylindole (4-MI). These plants show normal phenotype and fertility and transmit the resistance to the selection agents strictly maternally.

Enhanced accumulation of phytosterol and triterpene in hairy root cultures of Platycodon grandiflorum by overexpression of Panax ginseng 3-hydroxy-3-methylglutaryl-coenzyme A reductase

3-Hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) catalyzes the rate-limiting step in the mevalonate pathway. To elucidate the functions of HMGR in triterpene biosynthesis, Platycodon grandiflorum was transformed with a construct expressing Panax ginseng HMGR (PgHMGR). We used PCR analysis to select transformed hairy root lines and selected six lines for further investigation. Quantitative real-time PCR showed higher expression levels of HMGR and total platycoside levels (1.5-2.5-fold increase) in transgenic lines than in controls. Phytosterols levels were also 1.1-1.6-fold higher in transgenic lines than in controls. Among these lines, line T7 produced the highest level of total platycosides (1.60 ± 0.2 mg g(-1) dry weight) and α-spinasterol (1.78 ± 0.16 mg g(-1) dry weight). These results suggest that metabolic engineering of P. grandiflorum by Agrobacterium-mediated genetic transformation may enhance production of phytosterols and triterpenoids.

Recent advances in soybean transformation and their application to molecular breeding and genomic analysis

Herbicide-resistant transgenic soybean plants hold a leading market share in the USA and other countries, but soybean has been regarded as recalcitrant to transformation for many years. The cumulative and, at times, exponential advances in genetic manipulation have made possible further choices for soybean transformation. The most widely and routinely used transformation systems are cotyledonary node-Agrobacterium-mediated transformation and somatic embryo-particle-bombardment-mediated transformation. These ready systems enable us to improve seed qualities and agronomic characteristics by transgenic approaches. In addition, with the accumulation of soybean genomic resources, convenient or promising approaches will be requisite for the determination and use of gene function in soybean. In this article, we describe recent advances in and problems of soybean transformation, and survey the current transgenic approaches for applied and basic research in Japan.

MS-based analytical methodologies to characterize genetically modified crops

The development of genetically modified crops has had a great impact on the agriculture and food industries. However, the development of any genetically modified organism (GMO) requires the application of analytical procedures to confirm the equivalence of the GMO compared to its isogenic non-transgenic counterpart. Moreover, the use of GMOs in foods and agriculture faces numerous criticisms from consumers and ecological organizations that have led some countries to regulate their production, growth, and commercialization. These regulations have brought about the need of new and more powerful analytical methods to face the complexity of this topic. In this regard, MS-based technologies are increasingly used for GMOs analysis to provide very useful information on GMO composition (e.g., metabolites, proteins). This review focuses on the MS-based analytical methodologies used to characterize genetically modified crops (also called transgenic crops). First, an overview on genetically modified crops development is provided, together with the main difficulties of their analysis. Next, the different MS-based analytical approaches applied to characterize GM crops are critically discussed, and include "-omics" approaches and target-based approaches. These methodologies allow the study of intended and unintended effects that result from the genetic transformation. This information is considered to be essential to corroborate (or not) the equivalence of the GM crop with its isogenic non-transgenic counterpart.

The expression of 1-deoxy-D-xylulose synthase and geraniol-10-hydroxylase or anthranilate synthase increases terpenoid indole alkaloid accumulation in Catharanthus roseus hairy roots

The terpenoid indole alkaloid (TIA) pathway in Catharanthus roseus produces two important anticancer drugs, vinblastine and vincristine, in very low yields. This study focuses on overexpressing several key genes in the upper part of the TIA pathway in order to increase flux toward downstream metabolites within hairy root cultures. Specifically, we constructed hairy root lines with inducible overexpression of 1-deoxy-D-xylulose synthase (DXS) or geraniol-10-hydroxylase (G10H). We also constructed hairy root lines with inducible expression of DXS and anthranilate synthase α subunit (ASA) or DXS and G10H. DXS overexpression resulted in a significant increase in ajmalicine by 67%, serpentine by 26% and lochnericine by 49% and a significant decrease in tabersonine by 66% and hörhammericine by 54%. Co-overexpression of DXS and G10H caused a significant increase in ajmalicine by 16%, lochnericine by 31% and tabersonine by 13%. Likewise, DXS and ASA overexpression displayed a significant increase in hörhammericine by 30%, lochnericine by 27% and tabersonine by 34%. These results point to the need for overexpressing multiple genes within the pathway to increase the flux toward vinblastine and vincristine.

Metabolic profiling to determine the cause of the increased triphenyltetrazolium chloride reduction in mannitol-treated maize callus

When Zea mays callus cultures of two different genotypes were treated with the osmoticum mannitol (0.53M) for 24h their ability to reduce the tetrazolium derivative 2,3,5-triphenyltetrazolium chloride (TTC) to form the insoluble red compound formazan is stimulated. The formazan can be extracted with 95% ethanol for quantitation and this reaction has been used as a measure of viability since only live cells can carry out this reduction. In order to determine the cause of the increased TTC reduction caused by mannitol we carried out metabolic profiling analysis using GC-MS to identify 80 compounds. There were increases in sugar alcohols, hexoses except fructose and in total sugars. The total organic acid pools did not change and nitrogen containing compounds decreased slightly. Principle component analysis showed a large treatment effect due to changes in carbohydrate and nitrogen metabolism. These results indicate that the increased carbohydrate available for the citric acid cycle may be the cause of the increased TTC reduction observed after the mannitol treatment.

New insights into the shikimate and aromatic amino acids biosynthesis pathways in plants

The aromatic amino acids phenylalanine, tyrosine, and tryptophan in plants are not only essential components of protein synthesis, but also serve as precursors for a wide range of secondary metabolites that are important for plant growth as well as for human nutrition and health. The aromatic amino acids are synthesized via the shikimate pathway followed by the branched aromatic amino acids biosynthesis pathway, with chorismate serving as a major intermediate branch point metabolite. Yet, the regulation and coordination of synthesis of these amino acids are still far from being understood. Recent studies on these pathways identified a number of alternative cross-regulated biosynthesis routes with unique evolutionary origins. Although the major route of Phe and Tyr biosynthesis in plants occurs via the intermediate metabolite arogenate, recent studies suggest that plants can also synthesize phenylalanine via the intermediate metabolite phenylpyruvate (PPY), similarly to many microorganisms. Recent studies also identified a number of transcription factors regulating the expression of genes encoding enzymes of the shikimate and aromatic amino acids pathways as well as of multiple secondary metabolites derived from them in Arabidopsis and in other plant species.

Manipulation of amino acid composition in soybean seeds by the combination of deregulated tryptophan biosynthesis and storage protein deficiency

The ability of genetic manipulation to yield greatly increased concentrations of free amino acids (FAAs) in seeds of soybean was evaluated by introduction of a feedback-insensitive mutant enzyme of tryptophan (Trp) biosynthesis into two transformation-competent breeding lines deficient in major seed storage proteins. The storage protein-deficient lines exhibited increased accumulation of certain other seed proteins as well as of FAAs including arginine (Arg) and asparagine in mature seeds. Introduction of the gene for a feedback-insensitive mutant of an alpha subunit of rice anthranilate synthase (OASA1D) into the two high-FAA breeding lines by particle bombardment resulted in a >10-fold increase in the level of free Trp in mature seeds compared with that in nontransgenic seeds. The amount of free Trp in these transgenic seeds was similar to that in OASA1D transgenic seeds of the wild-type cultivar Jack. The composition of total amino acids in seeds of the high-FAA breeding lines remained largely unaffected by the expression of OASA1D with the exception of an increase in the total Trp content. Our results therefore indicate that the extra nitrogen resource originating from storage protein deficiency was used exclusively for the synthesis of inherent alternative nitrogen reservoirs such as free Arg and not for deregulated Trp biosynthesis conferred by OASA1D. The intrinsic null mutations responsible for storage protein deficiency and the OASA1D transgene affecting Trp content were thus successfully combined and showed additive effects on the amino acid composition of soybean seeds.

Recent progress in deciphering the biosynthesis of aspartate-derived amino acids in plants

Plants are either directly or indirectly the source of most of the essential amino acids in animal diets. Four of these essential amino acids-methionine, threonine, isoleucine, and lysine-are all produced from aspartate via a well studied biosynthesis pathway. Given the nutritional interest in essential amino acids, the aspartate-derived amino acid pathway has been the subject of extensive research. Additionally, several pathway enzymes serve as targets for economically important herbicides, and some of the downstream products are biosynthetic precursors for other essential plant metabolites such as ethylene and S-adenosylmethionine. Recent and ongoing research on the aspartate-derived family of amino acids has identified new enzyme activities, regulatory mechanisms, and in vivo metabolic functions. Together, these discoveries will open up new possibilities for plant metabolic engineering.

Tobacco plastid transformation using the feedback-insensitive anthranilate synthase [alpha]-subunit of tobacco (ASA2) as a new selectable marker

Genetic engineering of chloroplasts normally requires the stable introduction of bacterial derived antibiotic or herbicide-resistance genes as selective markers. Ecological and health concerns have been raised due to the presence of such genes within the environment or the food supply. One way to overcome this issue is the use of plant genes able to confer a metabolic or developmental advantage to the transformed cells manipulating the plant's biosynthetic pathways. We explored the feasibility of using, for plastid transformation, the selection system based on the feedback-insensitive anthranilate synthase (AS) alpha-subunit gene of tobacco (ASA2) as a new selective marker and the indole analogue 4-methylindole (4MI) or the tryptophan analogue 7-methyl-DL-tryptophan (7MT) as the selection agents. An expression cassette containing Prrn-ASA2 was effectively integrated into the region between accD and ycf4 of the tobacco plastome by the biolistic process. Plastid transgenic plants were obtained on medium supplemented with 300 microM 7MT or 4MI. Transplastomic plants showed normal phenotype and fertility and the resistance to the selection agents 7MT and 4MI was transmitted maternally. The plastid transformed lines also exhibited a higher level of AS enzyme activity that was less sensitive to Trp-feedback inhibition and, consequently, increased free Trp levels in leaves about 7-fold.

Tissue culture specificity of the tobacco ASA2 promoter driving hpt as a selectable marker for soybean transformation selection

This study was carried out to determine if the tobacco anthranilate synthase ASA2 2.3 kb promoter drives tissue culture specific expression and if it is strong enough to drive hpt (hygromycin phosphotransferase) gene expression at a level sufficient to allow selection of transformed soybean embryogenic culture lines. A number of transformed cell lines were selected showing that the promoter was strong enough. Northern blot analysis of plant tissues did not detect hpt mRNA in the untransformed control or in the ASA2-hpt plants except in developing seeds while hpt mRNA was detected in all tissues of the CaMV35S-hpt positive control line plants. However, when the more sensitive RT-PCR assay was used all tissues of the ASA2-hpt plants except roots and mature seeds were found to contain detectable hpt mRNA. Embryogenic tissue cultures initiated from the ASA2-hpt plants contained hpt mRNA detectable by both northern and RT-PCR analysis and the cultures were hygromycin resistant. Friable callus initiated from leaves of ASA2-hpt plants did in some cases contain hpt mRNA that was only barely detectable by northern hybridization even though the callus was very hygromycin resistant. Thus the ASA2 promoter is strong enough to drive sufficient hpt expression in soybean embryogenic cultures for hygromycin selection and only very low levels of expression were found in most plant tissues with none in mature seeds.

Use of 4-methylindole or 7-methyl-DL-tryptophan in a transformant selection system based on the feedback-insensitive anthranilate synthase alpha-subunit of tobacco (ASA2)

Effective selectable markers are needed for basic research and commercial applications that do not involve antibiotic or herbicide resistance. A novel selection system based on a feedback-insensitive anthranilate synthase alpha-subunit of tobacco (ASA2) as selectable marker using either 4-methylindole (4MI) or 7-methyl-DL-tryptophan (7MT) as the selection agent was developed. We found that these two components were able to discriminate better between ASA2 expressing and untransformed lines than the most commonly used analog 5-methyltryptopan (5MT) in the seedling growth inhibition test. We successfully integrated an expression cassette containing an ASA2 cDNA driven by a cauliflower mosaic virus 35S promoter into tobacco leaf discs by A. tumefaciens and selected transgenic plants on medium supplemented with 300 microM of 7MT or 4MI. Due to the expression of the feedback-insensitive ASA2, the transgenic lines produced showed higher free tryptophan (Trp) concentrations than the untransformed WT control. These results demonstrate the feasibility of the selection system with the ASA2 gene in combination with the use of Trp or indole analogs as selective agent.