An E8 promoter-HSP terminator cassette promotes the high-level accumulation of recombinant protein predominantly in transgenic tomato fruits: a case study of miraculin.

KEY MESSAGE: The E8 promoter-HSP terminator expression cassette is a powerful tool for increasing the accumulation of recombinant protein in a ripening tomato fruit. Strong, tissue-specific transgene expression is a desirable feature in transgenic plants to allow the production of variable recombinant proteins. The expression vector is a key tool to control the expression level and site of transgene and recombinant protein expression in transgenic plants. The combination of the E8 promoter, a fruit-ripening specific promoter, and a heat shock protein (HSP) terminator, derived from heat shock protein 18.2 of Arabidopsis thaliana, produces the strong and fruit-specific accumulation of recombinant miraculin in transgenic tomato. Miraculin gene expression was driven by an E8 promoter and HSP terminator cassette (E8-MIR-HSP) in transgenic tomato plants, and the miraculin concentration was the highest in the ripening fruits, representing 30-630 µg miraculin of the gram fresh weight. The highest level of miraculin concentration among the transgenic tomato plant lines containing the E8-MIR-HSP cassette was approximately four times higher than those observed in a previous study using a constitutive 35S promoter and NOS terminator cassette (Hiwasa-Tanase et al. in Plant Cell Rep 30:113-124, 2011). These results demonstrate that the combination of the E8 promoter and HSP terminator cassette is a useful tool to increase markedly the accumulation of recombinant proteins in a ripening fruit-specific manner.

Identification, Chemical Synthesis, and Sweetness Evaluation of Rhamnose or Xylose Containing Steviol Glycosides of Stevia (Stevia rebaudiana) Leaves.

Steviol glycosides obtained from Stevia rebaudiana leaves are increasingly used in the food industry as natural low-calorie sweeteners. Among them, the sweetness of major glycosides composed of glucose residues (e.g., stevioside and rebaudioside A) has been widely studied. However, the properties of minor natural products containing rhamnose or xylose residues are poorly investigated. In this study, five unreported steviol glycosides containing rhamnose or xylose were extracted from our developing stevia leaves, and their sweetness was evaluated. The highly glycosylated steviol glycosides were identified, and their structures were examined by fragmentation analysis using mass spectrometry. Chemical synthesis of these glycosides confirmed their structures and allowed sensory evaluation of minor steviol glycosides. Our study revealed that a xylose-containing glycoside, rebaudioside FX1, exhibits a well-balanced sweetness, and thus, it is a promising candidate for natural sweeteners used in the food industry.

From miracle fruit to transgenic tomato: mass production of the taste-modifying protein miraculin in transgenic plants.

The utility of plants as biofactories has progressed in recent years. Some recombinant plant-derived pharmaceutical products have already reached the marketplace. However, with the exception of drugs and vaccines, a strong effort has not yet been made to bring recombinant products to market, as cost-effectiveness is critically important for commercialization. Sweet-tasting proteins and taste-modifying proteins have a great deal of potential in industry as substitutes for sugars and as artificial sweeteners. The taste-modifying protein, miraculin, functions to change the perception of a sour taste to a sweet one. This taste-modifying function can potentially be used not only as a low-calorie sweetener but also as a new seasoning that could be the basis of a new dietary lifestyle. However, miraculin is far from inexpensive, and its potential as a marketable product has not yet been fully developed. For the last several years, biotechnological production of this taste-modifying protein has progressed extensively. In this review, the characteristics of miraculin and recent advances in its production using transgenic plants are summarized, focusing on such topics as the suitability of plant species as expression hosts, the cultivation method for transgenic plants, the method of purifying miraculin and future advances required to achieve industrial use.

Novel promoters that induce specific transgene expression during the green to ripening stages of tomato fruit development.

UNLABELLED: Fruit-specific promoters have been used as genetic engineering tools for studies on molecular mechanism of fruit development and advance in fruit quality and additional value by increasing functional component. Especially fruit-ripening specific promoters have been well utilized and studied in tomato; however, few studies have reported the development of promoters that act at fruit developing stages such as immature green and mature green periods. In this study, we report novel promoters for gene expression during the green to ripening stages of tomato fruit development. Genes specifically expressed at tomato fruit were selected using microarray data. Subsequent to confirmation of the expression of the selected 12 genes, upstream DNA fragments of the genes LA22CD07, Les.3122.2.A1_a_at and LesAffx.6852.1.S1_at which specifically expressed at fruit were isolated from tomato genomic DNA as promoter regions. Isolated promoter regions were fused with the GUS gene and the resultant constructs were introduced into tomato by agrobacterium-mediated transformation for evaluation of promoter activity in tomato fruit. The two promoters of LA22CD07, and LesAffx.6852.1.S1_at showed strong activity in the fruit, weak activity in the flower and undetectable activity in other tissues. Unlike well-known fruit-ripening specific promoters, such as the E8 promoter, these promoters exhibited strong activity in green fruit in addition to red-ripening fruit, indicating that the promoters are suitable for transgene expression during green to ripening stages of tomato fruit development. KEY MESSAGE: Novel fruit-specific promoters have been identified and are suitable for transgene expression during green to ripening stages of tomato fruit development.

Uniform accumulation of recombinant miraculin protein in transgenic tomato fruit using a fruit-ripening-specific E8 promoter.

The E8 promoter, a tomato fruit-ripening-specific promoter, and the CaMV 35S promoter, a constitutive promoter, were used to express the miraculin gene encoding the taste-modifying protein in tomato. The accumulation of miraculin protein and mRNA was compared among transgenic tomatoes expressing the miraculin gene driven by these promoters. Recombinant miraculin protein predominantly accumulated in transgenic tomato lines using the E8 promoter (E8-MIR) only at the red fruit stage. The accumulations were almost uniform among all fruit tissues. When the 35S promoter (35S-MIR) was used, miraculin accumulation in the exocarp was much higher than in other tissues, indicating that the miraculin accumulation pattern can be regulated by using different types of promoters. We also discuss the potential of the E8-MIR lines for practical use.

Ubiquitin promoter-terminator cassette promotes genetically stable expression of the taste-modifying protein miraculin in transgenic lettuce.

Lettuce is a commercially important leafy vegetable that is cultivated worldwide, and it is also a target crop for plant factories. In this study, lettuce was selected as an alternative platform for recombinant miraculin production because of its fast growth, agronomic value, and wide availability. The taste-modifying protein miraculin is a glycoprotein extracted from the red berries of the West African native shrub Richadella dulcifica. Because of its limited natural availability, many attempts have been made to produce this protein in suitable alternative hosts. We produced transgenic lettuce with miraculin gene driven either by the ubiquitin promoter/terminator cassette from lettuce or a 35S promoter/nos terminator cassette. Miraculin gene expression and miraculin accumulation in both cassettes were compared by quantitative real-time PCR analysis, Western blotting, and enzyme-linked immunosorbent assay. The expression level of the miraculin gene and protein in transgenic lettuce was higher and more genetically stable in the ubiquitin promoter/terminator cassette than in the 35S promoter/nos terminator cassette. These results demonstrated that the ubiquitin promoter/terminator cassette is an efficient platform for the genetically stable expression of the miraculin protein in lettuce and hence this platform is of benefit for recombinant miraculin production on a commercial scale.

High-level accumulation of recombinant miraculin protein in transgenic tomatoes expressing a synthetic miraculin gene with optimized codon usage terminated by the native miraculin terminator.

In our previous study, a transgenic tomato line that expressed the MIR gene under control of the cauliflower mosaic virus 35S promoter and the nopaline synthase terminator (tNOS) produced the taste-modifying protein miraculin (MIR). However, the concentration of MIR in the tomatoes was lower than that in the MIR gene's native miracle fruit. To increase MIR production, the native MIR terminator (tMIR) was used and a synthetic gene encoding MIR protein (sMIR) was designed to optimize its codon usage for tomato. Four different combinations of these genes and terminators (MIR-tNOS, MIR-tMIR, sMIR-tNOS and sMIR-tMIR) were constructed and used for transformation. The average MIR concentrations in MIR-tNOS, MIR-tMIR, sMIR-tNOS and sMIR-tMIR fruits were 131, 197, 128 and 287 µg/g fresh weight, respectively. The MIR concentrations using tMIR were higher than those using tNOS. The highest MIR accumulation was detected in sMIR-tMIR fruits. On the other hand, the MIR concentration was largely unaffected by sMIR-tNOS. The expression levels of both MIR and sMIR mRNAs terminated by tMIR tended to be higher than those terminated by tNOS. Read-through mRNA transcripts terminated by tNOS were much longer than those terminated by tMIR. These results suggest that tMIR enhances mRNA expression and permits the multiplier effect of optimized codon usage.

Cells reprogramming to stem cells inhibit the reprogramming of adjacent cells in the moss Physcomitrella patens.

Under certain circumstances differentiated cells can be reprogrammed to form stem cells in land plants, but only a portion of the cells reprograms successfully. A long-distance inhibitory signal from reprogrammed cells to surrounding cells has been reported in some ferns. Here we show the existence of anisotropic inhibitory signal to regulate stem cell formation in the moss Physcomitrella patens. When single cells were isolated from a gametophore leaf, over 90% of them were reprogrammed to stem cells with characteristic nuclear expansion. By contrast, when two adjacent cells were isolated, the nuclei of both cells expanded, but successful reprogramming of both cells occurred only in approximately one fifth of the pairs. When three aligned cells were isolated, the reprogramming rate of both edge cells decreased with a living middle cell but did not with a dead middle cell. Furthermore, unequal conversion into stem cells was more prominent in cell pairs aligned parallel to the proximal-distal leaf axis than in those perpendicular to the axis. This study gives an insight into the role of the inhibitory signal in development and evolution as well as the efficient stem cell induction from differentiated cells.

The FT-like gene PehFT in petunia responds to photoperiod and light quality but is not the main gene promoting light quality-associated flowering.

In Arabidopsis, flowering is delayed under red light and induced under far red light and blue light. Studies suggest that the florigen, FLOWERING LOCUS T, is involved in the control of light quality-associated flowering in Arabidopsis. In petunia, similar to Arabidopsis, flowering is delayed under red light and induced under blue light, however its mechanism still remains unknown. Here we isolated a gene which has 75% amino acid sequence similarity with Arabidopsis FT (AtFT), named PehFT. By overexpressing PehFT in Arbidopsis and petunia, we tested its ability to induce flowering. Also, by conducting expression analyses of PehFT under different light quality treatments, we tested its response to light quality. We concluded that PehFT, like AtFT, is a gene which responds to photoperiod and light quality, but unlike AtFT, is not the main gene controlling the light quality-associated flowering.

The HSP terminator of Arabidopsis thaliana induces a high level of miraculin accumulation in transgenic tomatoes.

High-level accumulation of the target recombinant protein is a significant issue in heterologous protein expression using transgenic plants. Miraculin, a taste-modifying protein, was accumulated in transgenic tomatoes using an expression cassette in which the miraculin gene was expressed by the cauliflower mosaic virus (CaMV) 35S promoter and the heat shock protein (HSP) terminator (MIR-HSP). The HSP terminator was derived from heat shock protein 18.2 in Arabidopsis thaliana . Using this HSP-containing cassette, the miraculin concentration in T0 transgenic tomato lines was 1.4-13.9% of the total soluble protein (TSP), and that in the T1 transgenic tomato line homozygous for the miraculin gene reached 17.1% of the TSP. The accumulation level of the target protein was comparable to levels observed with chloroplast transformation. The high-level accumulation of miraculin in T0 transgenic tomato lines achieved by the HSP terminator was maintained in the successive T1 generation, demonstrating the genetic stability of this accumulation system.

A trial of production of the plant-derived high-value protein in a plant factory: photosynthetic photon fluxes affect the accumulation of recombinant miraculin in transgenic tomato fruits.

One of the ultimate goals of plant science is to test a hypothesis obtained by basic science and to apply it to agriculture and industry. A plant factory is one of the ideal systems for this trial. Environmental factors affect both plant yield and the accumulation of recombinant proteins for industrial applications within transgenic plants. However, there have been few reports studying plant productivity for recombinant protein in closed cultivation systems called plant factories. To investigate the effects of photosynthetic photon flux (PPF) on tomato fruit yield and the accumulation of recombinant miraculin, a taste-modifying glycoprotein, in transgenic tomato fruits, plants were cultivated at various PPFs from 100 to 400 (µmol m(-2) s(-)1) in a plant factory. Miraculin production per unit of energy used was highest at PPF100, although miraculin production per unit area was highest at PPF300. The commercial productivity of recombinant miraculin in transgenic tomato fruits largely depended on light conditions in the plant factory. Our trial will be useful to consider the trade-offs between the profits from production of high-value materials in plants and the costs of electricity.

Covering chemical diversity of genetically-modified tomatoes using metabolomics for objective substantial equivalence assessment.

As metabolomics can provide a biochemical snapshot of an organism's phenotype it is a promising approach for charting the unintended effects of genetic modification. A critical obstacle for this application is the inherently limited metabolomic coverage of any single analytical platform. We propose using multiple analytical platforms for the direct acquisition of an interpretable data set of estimable chemical diversity. As an example, we report an application of our multi-platform approach that assesses the substantial equivalence of tomatoes over-expressing the taste-modifying protein miraculin. In combination, the chosen platforms detected compounds that represent 86% of the estimated chemical diversity of the metabolites listed in the LycoCyc database. Following a proof-of-safety approach, we show that % had an acceptable range of variation while simultaneously indicating a reproducible transformation-related metabolic signature. We conclude that multi-platform metabolomics is an approach that is both sensitive and robust and that it constitutes a good starting point for characterizing genetically modified organisms.

The Selaginella genome identifies genetic changes associated with the evolution of vascular plants.

Vascular plants appeared ~410 million years ago, then diverged into several lineages of which only two survive: the euphyllophytes (ferns and seed plants) and the lycophytes. We report here the genome sequence of the lycophyte Selaginella moellendorffii (Selaginella), the first nonseed vascular plant genome reported. By comparing gene content in evolutionarily diverse taxa, we found that the transition from a gametophyte- to a sporophyte-dominated life cycle required far fewer new genes than the transition from a nonseed vascular to a flowering plant, whereas secondary metabolic genes expanded extensively and in parallel in the lycophyte and angiosperm lineages. Selaginella differs in posttranscriptional gene regulation, including small RNA regulation of repetitive elements, an absence of the trans-acting small interfering RNA pathway, and extensive RNA editing of organellar genes.

Spatial and developmental profiling of miraculin accumulation in transgenic tomato fruits expressing the miraculin gene constitutively.

We previously developed a transgenic tomato that expresses the miraculin gene using a constitutive promoter. In this study, we profiled the developmental and spatial accumulation of the miraculin protein and mRNA in transgenic tomato fruits. Miraculin mRNA expression was almost constant up to orange stage, and then the expression increased at red stage. The miraculin protein accumulated gradually during fruit development and reached its highest level at the overripe stage. At the red stage of fruit, miraculin protein was accumulated at the highest level in the exocarp, and similar in other fruit tissues: mesocarp, dissepiment, upper placenta, lower placenta and jelly. Moreover, the pattern of miraculin accumulation in fruit tissues was the same regardless of genetic background and position at which the miraculin gene was inserted in the genome. We also discuss suitable tomato types expressing miraculin for their commercial use.

Production of recombinant miraculin using transgenic tomatoes in a closed cultivation system.

We constructed a cultivation system with a controlled light period, light intensity, temperature, and CO(2) concentration for mass production of the taste-modifying protein miraculin from transgenic tomatoes. The tomato plants exhibited normal growth and produced over 270 g of fresh weight (FW) fruit per plant, with the recombinant miraculin concentration reaching up to 90 microg per g FW of tomatoes. The recombinant miraculin content of transgenic tomatoes was compared to that of plants grown in a netted greenhouse. The recombinant miraculin content of transgenic tomatoes grown in a closed cultivation system was more stable than that of tomatoes grown in a netted greenhouse, suggesting that the closed cultivation system is suitable for the production of recombinant miraculin. We estimate that 45 tFW of tomatoes and 4 kg of recombinant miraculin per 1,000 m(2) of cultivation area can be harvested per year.

Molecular breeding of tomato lines for mass production of miraculin in a plant factory.

A transgenic tomato line (56B, "Moneymaker") that expresses the miraculin gene driven by the CaMV 35S promoter was crossed with a dwarf tomato ("Micro-Tom") for the molecular breeding of cultivars that are suitable for miraculin production in a closed cultivation system. Plant size, miraculin accumulation, and self-pruning growth were used as selection indicators for F2 plants. Two lines were chosen for further analysis, bred to the F6 or F7 generation and cultivated in a closed cultivation system. In 56B and the two crossed lines, the concentrations of miraculin in the pericarp were 140, 367, and 343 microg/g FW, respectively. We also estimated that 26.2, 73.6, and 45.9 kg FW/m2 of tomatoes and 2.2, 16.6, and 9.8 mg/m2 of miraculin in the pericarp, respectively, could be harvested per year. These two crossed lines will be useful for the mass production of miraculin, especially in a closed cultivation system.

Miraculin, a taste-modifying protein is secreted into intercellular spaces in plant cells.

A taste-modifying protein, miraculin, is highly accumulated in ripe fruit of miracle fruit (Richadella dulcifica) and the content can reach up to 10% of the total soluble protein in these fruits. Although speculated for decades that miraculin is secreted into intercellular spaces in miracle fruit, no evidence exists of its cellular localization. To study the cellular localization of miraculin in plant cells, using miracle fruit and transgenic tomato that constitutively express miraculin, immunoelectron microscopy, imaging GFP fusion proteins, and immunological detection of secreted proteins in culture medium of transgenic tomato were carried out. Immunoelectron microscopy showed the specific accumulation of miraculin in the intercellular layers of both miracle fruit and transgenic tomato. Imaging GFP fusion protein demonstrated that the miraculin-GFP fusion protein was accumulated in the intercellular spaces of tomato epidermal cells. Immunological detection of secreted proteins in culture medium of transgenic tomato indicated that miraculin was secreted from the roots of transgenic tomato expressing miraculin. This study firstly showed the evidences of the intercellular localization of miraculin, and provided a new insight of biological roles of miraculin in plants.