Reduced ethylene synthesis by transgenic tomatoes expressing S-adenosylmethionine hydrolase

Ethylene: Management and breeding for postharvest quality in vegetable crops. A review

Ethylene is a two-carbon gaseous plant growth regulator that involved in several important physiological events, including growth, development, ripening and senescence of fruits, vegetables, and ornamental crops. The hormone accelerates ripening of ethylene sensitive fruits, leafy greens and vegetables at micromolar concentrations, and its accumulation can led to fruit decay and waste during the postharvest stage. Several strategies of crops management and techniques of plant breeding have been attempted in the last decades to understand ethylene regulation pathways and ethylene-dependent biochemical and physiological processes, with the final aim to extend the produce shelf-life and improve the postharvest quality of fruits and vegetables. These investigation approaches involve the use of conventional and new breeding techniques, including precise genome-editing. This review paper aims to provide a relevant overview on the state of the art related to the use of modern breeding techniques focused on ethylene and ethylene-related metabolism, as well as on the possible postharvest technological applications for the postharvest management of ethylene-sensitive crops. An updated view and perspective on the implications of new breeding and management strategies to maintain the quality and the marketability of different crops during postharvest are given, with particular focus on: postharvest physiology (ethylene dependent) for mature and immature fruits and vegetables; postharvest quality management of vegetables: fresh and fresh cut products, focusing on the most important ethylene-dependent biochemical pathways; evolution of breeding technologies for facing old and new challenges in postharvest quality of vegetable crops: from conventional breeding and marker assisted selection to new breeding technologies focusing on transgenesis and gene editing. Examples of applied breeding techniques for model plants (tomato, zucchini and brocccoli) are given to elucidate ethylene metabolism, as well as beneficial and detrimental ethylene effects.

The Barley S-Adenosylmethionine Synthetase 3 Gene HvSAMS3 Positively Regulates the Tolerance to Combined Drought and Salinity Stress in Tibetan Wild Barley

Drought and salinity are two of the most frequently co-occurring abiotic stresses. Despite recent advances in the elucidation of the effects of these stresses individually during the vegetative stage of plants, significant gaps exist in our understanding of the combined effects of these two frequently co-occurring stresses. Here, Tibetan wild barley XZ5 (drought tolerant), XZ16 (salt tolerant), and cultivated barley cv. CM72 (salt tolerant) were subjected to drought (D), salinity (S), or a combination of both treatments (D+S). Protein synthesis is one of the primary activities of the green part of the plant. Therefore, leaf tissue is an important parameter to evaluate drought and salinity stress conditions. Sixty differentially expressed proteins were identified by mass spectrometry (MALDI-TOF/TOF) and classified into 9 biological processes based on Gene Ontology annotation. Among them, 21 proteins were found to be expressed under drought or salinity alone; however, under D+S, 7 proteins, including S-adenosylmethionine synthetase 3 (SAMS3), were exclusively upregulated in drought-tolerant XZ5 but not in CM72. HvSAMS3 carries both N-terminal and central domains compared with Arabidopsis and activates the expression of several ethylene (ET)-responsive transcription factors. HvSAMS3 is mainly expressed in the roots and stems, and HvSAMS3 is a secretory protein located in the cell membrane and cytoplasm. Barley stripe mosaic virus-based virus-induced gene silencing (BSMV-VIGS) of HvSAMS3 in XZ5 severely compromised its tolerance to D+S and significantly reduced plant growth and K⁺ uptake. The reduced tolerance to the combined stress was associated with the inhibition of polyamines such as spermidine and spermine, polyamine oxidase, ethylene, biotin, and antioxidant enzyme activities. Furthermore, the exogenous application of ethylene and biotin improved the tolerance to D+S in BSMV-VIGS:HvSAMS3-inoculated plants. Our findings highlight the significance of HvSAMS3 in the tolerance to D+S in XZ5.

Production of Marker-Free Apple Plants Expressing the Supersweet Protein Gene Driven by Plant Promoter

The presence of antibiotic resistance and other marker genes in genetically modified plants causes concern in society because of perceived risks for the environment and human health. The creation of transgenic plants that do not contain foreign genetic material, especially that of bacterial and viral origin, largely alleviates the tension and makes the plants potentially more attractive for consumers. To produce marker-free transgenic apple plants, we used the pMF1 vector, which combines Zygosaccharomyces rouxii recombinaseR and a CodA-nptII bifunctional selectable gene. The thaumatin II gene from the tropical plant Thaumatococcus daniellii, which is under the control of the plant E8 gene (a predominantly fruit-specific promoter) and rbsS3A terminator, was taken as the gene of interest for modification of the fruit taste and enhancing its sweetness. Exploitation of this gene in our laboratory has allowed enhancing the sweetness, as well as improving the taste characteristics, of fruits and vegetables of plants such as strawberry, carrot, tomato and pear. We have obtained three independent transgenic apple lines that have been analyzed by PCR and Southern blot analyses for the presence of T-DNA sequences. Two of them contained a partial sequence of the T-DNA. With one line containing the full insert we then used a delayed strategy for the selection of marker-free plants. After induction of recombinase activity in leaf explants on selective media with 5-fluorocytosine (5-FC) we obtained more than 30 sublines, most of which lost their resistance to kanamycin. Most of the apple sublines showed the expression of the supersweet protein gene in a wide range of levels as detected by RNA accumulation. The plants from the group with the highest transcript level were propagated and grafted onto dwarf rootstocks for early fruit production for future estimates of protein levels and organoleptic analyses. Thus, we developed a protocol that allowed the production of marker-free apple plants expressing the supersweet protein.

Metabolomics for Plant Improvement: Status and Prospects

Post-genomics era has witnessed the development of cutting-edge technologies that have offered cost-efficient and high-throughput ways for molecular characterization of the function of a cell or organism. Large-scale metabolite profiling assays have allowed researchers to access the global data sets of metabolites and the corresponding metabolic pathways in an unprecedented way. Recent efforts in metabolomics have been directed to improve the quality along with a major focus on yield related traits. Importantly, an integration of metabolomics with other approaches such as quantitative genetics, transcriptomics and genetic modification has established its immense relevance to plant improvement. An effective combination of these modern approaches guides researchers to pinpoint the functional gene(s) and the characterization of massive metabolites, in order to prioritize the candidate genes for downstream analyses and ultimately, offering trait specific markers to improve commercially important traits. This in turn will improve the ability of a plant breeder by allowing him to make more informed decisions. Given this, the present review captures the significant leads gained in the past decade in the field of plant metabolomics accompanied by a brief discussion on the current contribution and the future scope of metabolomics to accelerate plant improvement.

Expression of S-adenosylmethionine Hydrolase in Tissues Synthesizing Secondary Cell Walls Alters Specific Methylated Cell Wall Fractions and Improves Biomass Digestibility

Plant biomass is a large source of fermentable sugars for the synthesis of bioproducts using engineered microbes. These sugars are stored as cell wall polymers, mainly cellulose and hemicellulose, and are embedded with lignin, which makes their enzymatic hydrolysis challenging. One of the strategies to reduce cell wall recalcitrance is the modification of lignin content and composition. Lignin is a phenolic polymer of methylated aromatic alcohols and its synthesis in tissues developing secondary cell walls is a significant sink for the consumption of the methyl donor S-adenosylmethionine (AdoMet). In this study, we demonstrate in Arabidopsis stems that targeted expression of AdoMet hydrolase (AdoMetase, E.C. 3.3.1.2) in secondary cell wall synthesizing tissues reduces the AdoMet pool and impacts lignin content and composition. In particular, both NMR analysis and pyrolysis gas chromatography mass spectrometry of lignin in engineered biomass showed relative enrichment of non-methylated p-hydroxycinnamyl (H) units and a reduction of dimethylated syringyl (S) units. This indicates a lower degree of methylation compared to that in wild-type lignin. Quantification of cell wall-bound hydroxycinnamates revealed a reduction of ferulate in AdoMetase transgenic lines. Biomass from transgenic lines, in contrast to that in control plants, exhibits an enrichment of glucose content and a reduction in the degree of hemicellulose glucuronoxylan methylation. We also show that these modifications resulted in a reduction of cell wall recalcitrance, because sugar yield generated by enzymatic biomass saccharification was greater than that of wild-type plants. Considering that transgenic plants show no important diminution of biomass yields, and that heterologous expression of AdoMetase protein can be spatiotemporally optimized, this novel approach provides a valuable option for the improvement of lignocellulosic biomass feedstock.

Tomato Fruits-A Platform for Metabolic Engineering of Terpenes

Terpenoids are a large and diverse class of plant metabolites including mono-, sesqui-, and diterpenes. They have numerous functions in basic physiological processes as well as the interaction of plants with their biotic and abiotic environment. Due to the tight regulation of biosynthetic pathways and the resulting limited natural availability of terpenes, there is a strong interest in increasing their production in plants by metabolic engineering for agricultural, pharmaceutical, and industrial applications. The tomato fruit system was developed as a platform for metabolic engineering of terpenes to overcome detrimental effects on overall plant growth and photosynthesis traits, which are affected when terpenoid engineering is performed in vegetative tissues. Here we describe how the use of fruit-specific promoters for transgene expression can avoid these unwanted effects. In addition, targeting the expression of the introduced terpene biosynthetic gene to fruit tissue can take advantage of the large precursor pool provided by the methylerythritol-phosphate (MEP) pathway, which is highly active during tomato fruit ripening to facilitate the accumulation of carotenoids. We also discuss how the production of high levels of target terpene compounds can be achieved in fruits by the expression of individual or a combination of (i) the MEP or mevalonic acid pathway enzymes, (ii) prenyltransferases, and/or (iii) terpene synthases. Finally, we provide a brief outline of how the emitted as well as internal pools of terpenes can be analyzed in transgenic tomato fruits.

Molecular cloning, characterization and expression analysis of the SAMS gene during adventitious root development in IBA-induced tetraploid black locust

S-Adenosylmethionine synthetase (SAMS) catalyzes the synthesis of S-adenosylmethionine (SAM), a precursor for ethylene and polyamine biosynthesis. Here, we report the isolation of the 1498 bp full-length cDNA sequence encoding tetraploid black locust (Robinia pseudoacacia L.) SAMS (TrbSAMS), which contains an open reading frame of 1179 bp encoding 392 amino acids. The amino acid sequence of TrbSAMS has more than 94% sequence identity to SAMSs from other plants, with a closer phylogenetic relationship to SAMSs from legumes than to SAMS from other plants. The TrbSAMS monomer consists of N-terminal, central, and C-terminal domains. Subcellular localization analysis revealed that the TrbSAMS protein localizes mainly to in the cell membrane and cytoplasm of onion epidermal cells and Arabidopsis mesophyll cell protoplasts. Indole-3-butyric acid (IBA)-treated cuttings showed higher levels of TrbSAMS transcript than untreated control cuttings during root primordium and adventitious root formation. TrbSAMS and its downstream genes showed differential expression in shoots, leaves, bark, and roots, with the highest expression observed in bark. IBA-treated cuttings also showed higher SAMS activity than control cuttings during root primordium and adventitious root formation. These results indicate that TrbSAMS might play an important role in the regulation of IBA-induced adventitious root development in tetraploid black locust cuttings.

Abscisic acid, H2O2 and nitric oxide interactions mediated cold-induced S-adenosylmethionine synthetase in Medicago sativa subsp. falcata that confers cold tolerance through up-regulating polyamine oxidation

S-adenosylmethionine synthetase (SAMS) is the key enzyme catalysing the formation of S-adenosylmethionine (SAM), a precursor of polyamines and ethylene. To investigate the potential role of SAMS in cold tolerance, we isolated MfSAMS1 from the cold-tolerant germplasm Medicago sativa subsp. falcata and analysed the association of SAM-derived polyamines with cold tolerance. The expression of MfSAMS1 in leaves was greatly induced by cold, abscisic acid (ABA), H2O2 and nitric oxide (NO). Our data revealed that ABA, H2O2 and NO interactions mediated the cold-induced MfSAMS1 expression and cold acclimation in falcata. SAM, putrescine, spermidine and spermine levels, ethylene production and polyamine oxidation were sequentially altered in response to cold, indicating that SAMS-derived SAM is preferentially used in polyamine synthesis and homeostasis during cold acclimation. Antioxidant enzyme activities were also induced in response to cold and showed correlation with polyamine oxidation. Overexpression of MfSAMS1 in tobacco resulted in elevated SAM levels, but polyamine levels and ethylene production in the transgenic plants were not significantly changed. Compared to the wild type, transgenic plants had increased levels of apoplastic H2O2, higher transcript levels of genes involved in polyamine synthesis and oxidation, and higher activities of polyamine oxidation and antioxidant enzymes. The results showed that overexpression of MfSAMS1 promoted polyamine synthesis and oxidation, which in turn improved H2 O2 -induced antioxidant protection, as a result enhanced tolerance to freezing and chilling stress in transgenic plants. This is the first report demonstrating that SAMS plays an important role in plant tolerance to cold via up-regulating polyamine oxidation.

Tomato fruits expressing a bacterial feedback-insensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase of the shikimate pathway possess enhanced levels of multiple specialized metabolites and upgraded aroma

Tomato (Solanum lycopersicum) fruit contains significant amounts of bioactive compounds, particularly multiple classes of specialized metabolites. Enhancing the synthesis and accumulation of these substances, specifically in fruits, are central for improving tomato fruit quality (e.g. flavour and aroma) and could aid in elucidate pathways of specialized metabolism. To promote the production of specialized metabolites in tomato fruit, this work expressed under a fruit ripening-specific promoter, E8, a bacterial AroG gene encoding a 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase (DAHPS), which is feedback-insensitive to phenylalanine inhibition. DAHPS, the first enzyme of the shikimate pathway, links between the primary and specialized metabolism derived from aromatic amino acids. AroG expression influenced the levels of number of primary metabolites, such as shikimic acid and aromatic amino acids, as well as multiple volatile and non-volatile phenylpropanoids specialized metabolites and carotenoids. An organoleptic test, performed by trained panellists, suggested that the ripe AroG-expressing tomato fruits had a preferred floral aroma compare with fruits of the wild-type line. These results imply that fruit-specific manipulation of the conversion of primary to specialized metabolism is an attractive approach for improving fruit aroma and flavour qualities as well as discovering novel fruit-specialized metabolites.

Postharvest analysis of lowland transgenic tomato fruits harboring hpRNAi-ACO1 construct

The plant hormone, ethylene, is an important regulator which involved in regulating fruit ripening and flower senescence. In this study, RNA interference (RNAi) technology was employed to silence the genes involved in ethylene biosynthetic pathway. This was achieved by blocking the expression of specific gene encoding the ACC oxidase. Initially, cDNA corresponding to ACO1 of lowland tomato cultivar (MT1), which has high identity with ACO1 of Solanum lycopersicum in GenBank, was cloned through RT-PCR. Using a partial coding region of ACO1, one hpRNAi transformation vector was constructed and expressed ectopically under the 35S promoter. Results showed that transgenic lines harboring the hpRNA-ACO1 construct had lower ethylene production and a longer shelf life of 32 days as compared to 10 days for wild-type fruits. Changes in cell wall degrading enzyme activities were also investigated in cases where the transgenic fruits exhibited reduced rates of firmness loss, which can be associated with a decrease in pectin methylesterase (PME) and polygalacturonase (PG) activities. However, no significant change was detected in both transgenic and wild-type fruits in terms of β-galactosidase (β-Gal) activity and levels of total soluble solid, titratable acid and ascorbic acid.

Role of ethylene receptors during senescence and ripening in horticultural crops

The past two decades have been rewarding in terms of deciphering the ethylene signal transduction and functional validation of the ethylene receptor and downstream genes involved in the cascade. Our knowledge of ethylene receptors and its signal transduction pathway provides us a robust platform where we can think of manipulating and regulating ethylene sensitivity by the use of genetic engineering and making transgenic. This review focuses on ethylene perception, receptor mediated regulation of ethylene biosynthesis, role of ethylene receptors in flower senescence, fruit ripening and other effects induced by ethylene. The expression behavior of the receptor and downstream molecules in climacteric and non climacteric crops is also elaborated upon. Possible strategies and recent advances in altering the ethylene sensitivity of plants using ethylene receptor genes in an attempt to modulate the regulation and sensitivity to ethylene have also been discussed. Not only will these transgenic plants be a boon to post-harvest physiology and crop improvement but, it will also help us in discovering the mechanism of regulation of ethylene sensitivity.

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.

Health considerations regarding horizontal transfer of microbial transgenes present in genetically modified crops

The potential effects of horizontal gene transfer on human health are an important item in the safety assessment of genetically modified organisms. Horizontal gene transfer from genetically modified crops to gut microflora most likely occurs with transgenes of microbial origin. The characteristics of microbial transgenes other than antibiotic-resistance genes in market-approved genetically modified crops are reviewed. These characteristics include the microbial source, natural function, function in genetically modified crops, natural prevalence, geographical distribution, similarity to other microbial genes, known horizontal transfer activity, selective conditions and environments for horizontally transferred genes, and potential contribution to pathogenicity and virulence in humans and animals. The assessment of this set of data for each of the microbial genes reviewed does not give rise to health concerns. We recommend including the above-mentioned items into the premarket safety assessment of genetically modified crops carrying transgenes other than those reviewed in the present study.

Molecular evolution of the E8 promoter in tomato and some of its relative wild species

The E8 gene is related to ethylene biosynthesis in plants. To explore the effect of the expression pattern of the E8 gene on different E8 promoters, the molecular evolution of E8 promoters was investigated. A total of 16 E8 promoters were cloned from 16 accessions of seven tomato species,and were further analysed. The results from 19 E8 promoters including three previously cloned E8 promoters (X13437,DQ317599 and AF515784) showed that the size of the E8 promoters varied from 2101 bp (LA2150) to 2256 bp (LA2192); their sequences shared 69.9% homology and the average A/T content was 74.9%. Slide-window analysis divided E8 promoters into three regions -A,B and C - and the sequence identity in these regions was 72.5%, 41.2% and 70.8%, respectively. By searching the cis -elements of E8 promoters in the PLACE database, mutant nucleotides were found in some functional elements,and deletions or insertions were also found in regions responsible for ethylene biosysnthesis (-1702 to -1274) and the negative effect region (-1253 to -936). Our results indicate that the size of the functional region for ethylene biosynthesis in the E8 promoter could be shortened from 429 bp to 113 bp (-1612 to -1500). The results of molecular evolution analysis showed that the 19 E8 promoters could be classified into four clade groups, which is basically consistent with evolution of the tomato genome. Southern blot analysis results showed that the copy number of E8 promoters in tomato and some other wild species changed from 1 to 4. Taken together, our study provides important information for further elucidating the E8 gene expression pattern in tomato, analysing functional elements in the E8 promoter and reconstructing the potent E8 promoter.

Molecular evolution of the E8 promoter in tomato and some of its relative wild species

The E8 gene is related to ethylene biosynthesis in plants. To explore the effect of the expression pattern of the E8 gene on different E8 promoters, the molecular evolution of E8 promoters was investigated. A total of 16 E8 promoters were cloned from 16 accessions of seven tomato species,and were further analysed. The results from 19 E8 promoters including three previously cloned E8 promoters (X13437,DQ317599 and AF515784) showed that the size of the E8 promoters varied from 2101 bp (LA2150) to 2256 bp (LA2192); their sequences shared 69.9% homology and the average A/T content was 74.9%. Slide-window analysis divided E8 promoters into three regions -A,B and C - and the sequence identity in these regions was 72.5%, 41.2% and 70.8%, respectively. By searching the cis -elements of E8 promoters in the PLACE database, mutant nucleotides were found in some functional elements,and deletions or insertions were also found in regions responsible for ethylene biosysnthesis (-1702 to -1274) and the negative effect region (-1253 to -936). Our results indicate that the size of the functional region for ethylene biosynthesis in the E8 promoter could be shortened from 429 bp to 113 bp (-1612 to -1500). The results of molecular evolution analysis showed that the 19 E8 promoters could be classified into four clade groups, which is basically consistent with evolution of the tomato genome. Southern blot analysis results showed that the copy number of E8 promoters in tomato and some other wild species changed from 1 to 4. Taken together, our study provides important information for further elucidating the E8 gene expression pattern in tomato, analysing functional elements in the E8 promoter and reconstructing the potent E8 promoter.

Assessment of the utility of the tomato fruit-specific E8 promoter for driving vaccine antigen expression

To assess the utility of the tomato fruit-specific E8 gene's promoter for driving vaccine antigen expression in plant, the 2.2 kb and 1.1 kb E8 promoters were isolated and sequenced from Lycopersicon esculentum cv. Jinfeng #1. The 1.1 kb promoter was fused to vaccine antigen HBsAg M gene for the transfer to Nicotiana tabacum, and the CaMV 35S promoter was used for comparison. Cholera toxin B (ctb) gene under the control of the 1.1 kb promoter was transformed into both N. tabacum and L. esculentum. Southern blot hybridization confirmed the stable integration of the target genes into the tomato and tobacco genomes. ELISA assay showed that the expression product of HBsAg M gene under the control of the 1.1 kb E8 promoter could not be detected in transgenic tobacco tissues such as leaves, flowers, and seeds. In contrast, the expression of HBsAg M gene driven by CaMV 35S promoter could be detected in transgenic tobacco. ELISA assay for CTB proved that the 1.1 kb E8 promoter was able to direct the expression of exotic gene in ripe fruits of transgenic tomato, but expression was absent in leaf, flower, and unripe fruit of tomato, and CTB protein was not detected in transgenic tobacco tissues such as leaves, flowers, and seeds when the gene was under the control of the 1.1 kb E8 promoter. The results indicated that the E8 promoter acted not only in an organ-specific, but also in a species-specific fashion in plant transformation.

[Construction of a plant effective expression vector containing the gene of hepatitis B virus surface antigen]

OBJECTIVE

To construct a plant effective expression vector driven by a fruit specific promoter for the expression of hepatitis B virus surface antigen (HBsAg), to further improve the expression of exogenous gene in plant, and to prepare for the development of an effective anti-hepatitis vaccine.

METHODS

Tomato fruit-specific promoters' gene 2A12 and E8 were respectively introduced to pBPFOmega7 to form pB2A12 and pBE8. The DNA fragment containing HBsAg-s gene from plasmid YEP-HBs was inserted respectively into pB2A12 and pBE8 to form pB2A12-HBs and pBE8-HBs. The fragment containing "p35S+2A12+Omega+HBsAg-s+Tnos" of the pB2A12-HBs was sub-cloned into plasmid pCAMBIA1301 to yield the reconstructed plant binary expression plasmid pCAM2A12-HBs, and the fragment containing "p35S+E8+Omega+HBsAg-s+Tnos" of the pBE8-HBs was sub-cloned into plasmid pCAMBIA1301 to yield the plasmid pCAME8-HBs. The inserted gene HBsAg and fruit-specific promoters in the reconstructed plant binary expression vectors were confirmed by sequencing. Then, pCAM2A12-HBs and pCAME8-HBs were directly introduced into Agrobacterium tumefaciens strain EHA105.

RESULTS

Digestion with restriction enzymes proved that all recombinant vectors had the inserts with expected length of the target fragments, and the sequencing results were confirmed correct.

CONCLUSION

In this study, plant expression vector containing HBsAg gene driven by fruit specific promoter and CaMV35s promoter was successfully constructed.

Genetic modulation of ethylene biosynthesis and signaling in plants

With the isolation and characterization of the key enzymes and proteins, and the corresponding genes, involved in ethylene biosynthesis and sensing it has become possible to manipulate plant ethylene levels and thereby alter a wide range of physiological processes. The phytohormone ethylene is an essential signaling molecule that affects a large number of physiological processes; plants deprived of ethylene do not grow and develop normally. In a search for flexible on-off ethylene control, scientists have used inducible organ- and tissue-specific promoters to drive expression of different transgenes. Here, the various strategies that have been used to genetically engineer plants with decreased ethylene biosynthesis and sensitivity are reviewed and discussed.

Genetic engineering for cut-flower improvement

The application of modern biotechnological approaches to cut flowers has clearly become instrumental and rewarding for the floriculture industry. In recent years, several gene-transfer procedures have been developed for some of the major commercial cut flowers. Using Agrobactrium or microprojectile bombardment, several basic protocols are now available. However, despite the great progress and interest in gene transfer to these crops, their transformation is routine in only a limited number of laboratories, and its application is still considered to be an "art form". This review summarizes the reported gene-transfer procedures for the main cut-flower crops, with an emphasis on the unique factors of each method and the recent progress in introducing new traits of horticultural interest into these species.

Transgenic plants with altered ethylene biosynthesis or perception

The plant hormone ethylene is an essential signaling molecule involved in many plant processes including: germination, flower development, fruit ripening and responses to many environmental stimuli. Moreover, large increases in ethylene levels occur during plant stress responses, fruit ripening and flower wilting. Manipulation of ethylene biosynthesis or perception allows us to modulate these processes and thereby create plants with more robust and/or desirable traits, giving us a glimpse into the role of ethylene in the plant. Here, recent and landmark advances in genetic alteration of members of the ethylene pathway in plants and the physiological consequences of these alterations are examined.

Enhanced levels of the aroma and flavor compound S-linalool by metabolic engineering of the terpenoid pathway in tomato fruits

The aromas of fruits, vegetables, and flowers are mixtures of volatile metabolites, often present in parts per billion levels or less. We show here that tomato (Lycopersicon esculentum Mill.) plants transgenic for a heterologous Clarkia breweri S-linalool synthase (LIS) gene, under the control of the tomato late-ripening-specific E8 promoter, synthesize and accumulate S-linalool and 8-hydroxylinalool in ripening fruits. Apart from the difference in volatiles, no other phenotypic alterations were noted, including the levels of other terpenoids such as gamma- and alpha-tocopherols, lycopene, beta-carotene, and lutein. Our studies indicate that it is possible to enhance the levels of monoterpenes in ripening fruits by metabolic engineering.

Temporal and spatial activity of a promoter from a pea enzyme inhibitor gene and its exploitation for seed quality improvement

The promoter from one of the two seed-expressed genes encoding trypsin/chymotrypsin inhibitors (TI) has been isolated and characterised in transgenic pea lines, following its re-introduction by Agrobacterium-mediated transformation, as a TI promoter-beta-glucuronidase (GUS) gene fusion. The promoter from this gene (TI1) directed expression of GUS enzyme at late stages of embryogenesis, comparable to those determined for activity of the homologous native TI genes. GUS expression was detected in roots of plants subjected to drought stress conditions, indicating that the TI1 gene, normally seed-specific in its expression, can be induced under these conditions. A second gene construct utilised the TI1 gene promoter to direct expression of an antisense TI gene. Seed TI activities in some lines transformed with this construct were reduced significantly. A limitation of the pea transformation methodology for antisense manipulations, in particular, is the observed frequency of non-transmission of transgenes from primary transformants (up to 80%).

Plant one-carbon metabolism and its engineering

The metabolism of one-carbon (C1) units is vital to plants. It involves unique enzymes and takes place in four subcellular compartments. Plant C1 biochemistry has remained relatively unexplored, partly because of the low abundance or the lability of many of its enzymes and intermediates. Fortunately, DNA sequence databases now make it easier to characterize known C1 enzymes and to discover new ones, to identify pathways that might carry high C1 fluxes, and to use engineering to redirect C1 fluxes and to understand their control better.

Influence of S-adenosylmethionine pool size on spontaneous mutation, dam methylation, and cell growth of Escherichia coli

Escherichia coli strains that are deficient in the Ada and Ogt DNA repair methyltransferases display an elevated spontaneous G:C-to-A:T transition mutation rate, and this increase has been attributed to mutagenic O(6)-alkylguanine lesions being formed via the alkylation of DNA by endogenous metabolites. Here we test the frequently cited hypothesis that S-adenosylmethionine (SAM) can act as a weak alkylating agent in vivo and that it contributes to endogenous DNA alkylation. By regulating the expression of the rat liver SAM synthetase and the bacteriophage T3 SAM hydrolase proteins in E. coli, a 100-fold range of SAM levels could be achieved. However, neither increasing nor decreasing SAM levels significantly affected spontaneous mutation rates, leading us to conclude that SAM is not a major contributor to the endogenous formation of O(6)-methylguanine lesions in E. coli.

The specific features of methionine biosynthesis and metabolism in plants

Plants, unlike other higher eukaryotes, possess all the necessary enzymatic equipment for de novo synthesis of methionine, an amino acid that supports additional roles than simply serving as a building block for protein synthesis. This is because methionine is the immediate precursor of S-adenosylmethionine (AdoMet), which plays numerous roles of being the major methyl-group donor in transmethylation reactions and an intermediate in the biosynthesis of polyamines and of the phytohormone ethylene. In addition, AdoMet has regulatory function in plants behaving as an allosteric activator of threonine synthase. Among the AdoMet-dependent reactions occurring in plants, methylation of cytosine residues in DNA has raised recent interest because impediment of this function alters plant morphology and induces homeotic alterations in flower organs. Also, AdoMet metabolism seems somehow implicated in plant growth via an as yet fully understood link with plant-growth hormones such as cytokinins and auxin and in plant pathogen interactions. Because of this central role in cellular metabolism, a precise knowledge of the biosynthetic pathways that are responsible for homeostatic regulation of methionine and AdoMet in plants has practical implications, particularly in herbicide design.

Expression of ACC oxidase antisense gene inhibits ripening of cantaloupe melon fruits

The plant hormone ethylene plays a major role in the ripening of climacteric fruit. We have generated transgenic cantaloupe Charentais melons expressing an antisense ACC oxidase gene; ACC oxidase catalyzes the last step of ethylene biosynthesis. Ethylene production of transgenic fruit was < 1% of control untransformed fruit, and the ripening process was blocked both on and off the vine. The antisense phenotype could be reversed by exogenous ethylene treatment. Analysis of antisense ACC oxidase melons indicated that the ripening process includes ethylene-dependent and ethylene-independent pathways. Because the transgenic line we generated displays extended storage life and improved quality, it has a promising potential for commercial development.