Expression of a bifunctional green fluorescent protein (GFP) fusion marker under the control of three constitutive promoters and enhanced derivatives in transgenic grape (Vitis vinifera)

Efficient protocol of de novo shoot organogenesis from somatic embryos for grapevine genetic transformation

Plant genetic transformation is a powerful tool that can facilitate breeding programs for disease tolerance, abiotic stress, fruit production, and quality by preserving the characteristics of fruit tree elite genotypes. However, most grapevine cultivars worldwide are considered recalcitrant, and most available genetic transformation protocols involve regeneration by somatic embryogenesis, which often requires the continuous production of new embryogenic calli. Cotyledons and hypocotyls derived from flower-induced somatic embryos of the Vitis vinifera cultivars Ancellotta and Lambrusco Salamino, in comparison with the model cultivar Thompson Seedless, are here validated for the first time as starting explants for in vitro regeneration and transformation trials. Explants were cultured on two different MS-based culture media, one having a combination of 4.4 µM BAP and 0.49 µM IBA (M1), and the other only supplemented with 13.2 µM BAP (M2). The competence to regenerate adventitious shoots was higher in cotyledons than in hypocotyls on both M1 and M2. M2 medium increased significantly the average number of shoots only in Thompson Seedless somatic embryo-derived explants. This efficient regeneration strategy, that proposes a combination of somatic embryogenesis and organogenesis, has been successfully exploited in genetic engineering experiments. Ancellotta and Lambrusco Salamino cotyledons and hypocotyls produced the highest number of calli expressing eGFP when cultured on M2 medium, while for Thompson Seedless both media tested were highly efficient. The regeneration of independent transgenic lines of Thompson Seedless was observed from cotyledons cultured on both M1 and M2 with a transformation efficiency of 12 and 14%, respectively, and from hypocotyls on M1 and M2 with a transformation efficiency of 6 and 12%, respectively. A single eGFP fluorescent adventitious shoot derived from cotyledons cultured on M2 was obtained for Ancellotta, while Lambrusco Salamino showed no regeneration of transformed shoots. In a second set of experiments, using Thompson Seedless as the model cultivar, we observed that the highest number of transformed shoots was obtained from cotyledons explants, followed by hypocotyls and meristematic bulk slices, confirming the high regeneration/transformation competences of somatic embryo-derived cotyledons. The independent transformed shoots obtained from the cultivars Thompson Seedless and Ancellotta were successfully acclimatized in the greenhouse and showed a true-to-type phenotype. The novel in vitro regeneration and genetic transformation protocols optimized in this study will be useful for the application of new and emerging modern biotechnologies also to other recalcitrant grapevine genotypes.

A bifunctional selectable marker for wheat transformation contributes to the characterization of male-sterile phenotype induced by a synthetic Ms2 gene

KEY MESSAGE

An engineered selectable marker combining herbicide resistance and yellow fluorescence contributes to the characterization of male-sterile phenotype in wheat, the severity of which correlates with expression levels of a synthetic Ms2 gene. Genetic transformation of wheat is conducted using selectable markers, such as herbicide and antibiotic resistance genes. Despite their proven effectiveness, they do not provide visual control of the transformation process and transgene status in progeny, which creates uncertainty and prolongs screening procedures. To overcome this limitation, this study developed a fusion protein by combining gene sequences encoding phosphinothricin acetyltransferase and mCitrine fluorescent protein. The fusion gene, introduced into wheat cells by particle bombardment, enabled herbicide selection, and visual identification of primary transformants along with their progeny. This marker was then used to select transgenic plants containing a synthetic Ms2 gene. Ms2 is a dominant gene whose activation in wheat anthers leads to male sterility, but the relationship between the expression levels and the male-sterile phenotype is unknown. The Ms2 gene was driven either by a truncated Ms2 promoter containing a TRIM element or a rice promoter OsLTP6. The expression of these synthetic genes resulted in complete male sterility or partial fertility, respectively. The low-fertility phenotype was characterized by smaller anthers than the wild type, many defective pollen grains, and low seed sets. The reduction in the size of anthers was observed at earlier and later stages of their development. Consistently, Ms2 transcripts were detected in these organs, but their levels were significantly lower than those in completely sterile Ms2TRIM::Ms2 plants. These results suggested that the severity of the male-sterile phenotype was modulated by Ms2 expression levels and that higher levels may be key to activating total male sterility.

Hairy CRISPR: Genome Editing in Plants Using Hairy Root Transformation

CRISPR/Cas-mediated genome editing is a powerful tool of plant functional genomics. Hairy root transformation is a rapid and convenient approach for obtaining transgenic roots. When combined, these techniques represent a fast and effective means of studying gene function. In this review, we outline the current state of the art reached by the combination of these approaches over seven years. Additionally, we discuss the origins of different Agrobacterium rhizogenes strains that are widely used for hairy root transformation; the components of CRISPR/Cas vectors, such as the promoters that drive Cas or gRNA expression, the types of Cas nuclease, and selectable and screenable markers; and the application of CRISPR/Cas genome editing in hairy roots. The modification of the already known vector pKSE401 with the addition of the rice translational enhancer OsMac3 and the gene encoding the fluorescent protein DsRed1 is also described.

CRISPR/Cas9 Targeted Editing of Genes Associated With Fungal Susceptibility in Vitis vinifera L. cv. Thompson Seedless Using Geminivirus-Derived Replicons

The woody nature of grapevine (Vitis vinifera L.) has hindered the development of efficient gene editing strategies to improve this species. The lack of highly efficient gene transfer techniques, which, furthermore, are applied in multicellular explants such as somatic embryos, are additional technical handicaps to gene editing in the vine. The inclusion of geminivirus-based replicons in regular T-DNA vectors can enhance the expression of clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) elements, thus enabling the use of these multicellular explants as starting materials. In this study, we used Bean yellow dwarf virus (BeYDV)-derived replicon vectors to express the key components of CRISPR/Cas9 system in vivo and evaluate their editing capability in individuals derived from Agrobacterium-mediated gene transfer experiments of 'Thompson Seedless' somatic embryos. Preliminary assays using a BeYDV-derived vector for green fluorescent protein reporter gene expression demonstrated marker visualization in embryos for up to 33 days post-infiltration. A universal BeYDV-based vector (pGMV-U) was assembled to produce all CRISPR/Cas9 components with up to four independent guide RNA (gRNA) expression cassettes. With a focus on fungal tolerance, we used gRNA pairs to address considerably large deletions of putative grape susceptibility genes, including AUXIN INDUCED IN ROOT CULTURE 12 (VviAIR12), SUGARS WILL EVENTUALLY BE EXPORTED TRANSPORTER 4 (VviSWEET4), LESION INITIATION 2 (VviLIN2), and DIMERIZATION PARTNER-E2F-LIKE 1 (VviDEL1). The editing functionality of gRNA pairs in pGMV-U was evaluated by grapevine leaf agroinfiltration assays, thus enabling longer-term embryo transformations. These experiments allowed for the establishment of greenhouse individuals exhibiting a double-cut edited status for all targeted genes under different allele-editing conditions. After approximately 18 months, the edited grapevine plants were preliminary evaluated regarding its resistance to Erysiphe necator and Botrytis cinerea. Assays have shown that a transgene-free VviDEL1 double-cut edited line exhibits over 90% reduction in symptoms triggered by powdery mildew infection. These results point to the use of geminivirus-based replicons for gene editing in grapevine and other relevant fruit species.

Genetic Modification of Grapevine Embryogenic Cultures

Precision breeding is an approach to grapevine genetic improvement that transfers only specific traits among sexually compatible species via the relatively stable mitotic cell division pathway in order to avoid the significant disruption imposed upon conventional breeding by meiosis. Factors enabling precision breeding include the availability of the Vitis genome sequence combined with highly optimized gene insertion and plant regeneration protocols. A protocol for the production of grapevine embryogenic cultures and their genetic transformation is described. Embryogenic cultures are produced from either leaf or floral explants. Somatic embryos at the cotyledonary stage of development are used for Agrobacterium-mediated transformation. Following co-cultivation with Agrobacterium containing the genes of interest, modified embryos are selected on the basis of anthocyanin pigmentation and antibiotic resistance. Somatic embryos are then germinated to produce modified plants that are hardened and transferred to a greenhouse. The presence of the genes of interest is confirmed by PCR.

Expression and Secretion of Cyan Fluorescent Protein (CFP) in B. subtilis using the Chitinase Promoter from Bacillus pumilus SG2

Background:

Improved cyan fluorescent protein (ICFP) is a monochromic, green fluorescent protein (GFP) derivative produced by Aequorea macrodactyla in a process similar to GFP. This protein has strong absorption spectra at wavelengths 426-446 nm. ICFP can be used in cell, organelle or intracellular protein labeling, investigating the protein-protein interactions as well as assessing the promoter activities.

Methods:

In our previous study, the promoters of two chitinases (ChiS and ChiL) from Bacillus pumilus SG2 were assessed in B. subtilis and their regulatory elements were characterized. In the present study, icfp was cloned downstream of several truncated promoters obtained in the former study, and ICFP expression was evaluated in B. subtilis.

Results:

Extracellular expression and secretion of ICFP were analyzed under the control of different truncated versions of ChiSL promoters grown on different media. Results from SDS-PAGE and fluorimetric analyses showed that there were different expression rates of CFP; however, the UPChi-ICFP3 construct exhibited a higher level of expression and secretion in the culture medium.

Conclusion:

Our presented results revealed that inserting this truncated form of Chi promoter upstream of the ICFP, as a reporter gene, in B. subtilis led to an approximately ten fold increase in ICFP expression.

Rice NAC transcription factor ONAC095 plays opposite roles in drought and cold stress tolerance

BACKGROUND

The NAC (NAM, ATAF and CUC) transcriptional factors constitute a large family with more than 150 members in rice and some of them have been demonstrated to play crucial roles in plant abiotic stress response. Here, we report the characterization of a rice stress-responsive NAC gene, ONAC095, and the exploration of its function in drought and cold stress tolerance.

RESULTS

Expression of ONAC095 was up-regulated by drought stress and abscisic acid (ABA) but down-regulated by cold stress. ONAC095 protein had transactivation activity and the C2 domain in C-terminal was found to be critical for transactivation activity. Transgenic rice lines with overexpression of ONAC095 (ONAC095-OE) and dominant chimeric repressor-mediated suppression of ONAC095 (ONAC095-SRDX) were generated. The ONAC095-OE plants showed comparable phenotype to wild type under drought and cold stress conditions. However, the ONAC095-SRDX plants displayed an improved drought tolerance but exhibited an attenuated cold tolerance. The ONAC095-SRDX plants had decreased water loss rate, increased proline and soluble sugar contents, and up-regulated expression of drought-responsive genes under drought condition, whereas the ONAC095-SRDX plants accumulated excess reactive oxygen species, increased malondialdehyde content and down-regulated expression of cold-responsive genes under cold condition. Furthermore, ONAC095-SRDX plants showed an increased ABA sensitivity, contained an elevated ABA level, and displayed altered expression of ABA biosynthetic and metabolic genes as well as some ABA signaling-related genes.

CONCLUSION

Functional analyses through dominant chimeric repressor-mediated suppression of ONAC095 demonstrate that ONAC095 plays opposite roles in drought and cold stress tolerance, acting as a negative regulator of drought response but as a positive regulator of cold response in rice.

Principales promotores utilizados en la transformación genética de plantas

El conocimiento pleno de los promotores determina el éxito en la obtención de nuevos cultivares de plantas a través de técnicas biotecnológicas, ya que dicha secuencia del ADN regula la transcripción de otras regiones adyacentes o cercanas, encontrándose los siguientes promotores: constitutivos, tejido-específicos o estadio-específicos, inducibles y sintéticos. En esta revisión se resume de manera precisa los conceptos, ventajas y limitaciones de los distintos tipos de promotores, con ejemplos claros de ello.Palabras clave: promotor, biotecnología vegetal, transcripción genética.

Transgenic Citrus Expressing an Arabidopsis NPR1 Gene Exhibit Enhanced Resistance against Huanglongbing (HLB; Citrus Greening)

Commercial sweet orange cultivars lack resistance to Huanglongbing (HLB), a serious phloem limited bacterial disease that is usually fatal. In order to develop sustained disease resistance to HLB, transgenic sweet orange cultivars ‘Hamlin’ and ‘Valencia’ expressing an Arabidopsis thaliana NPR1 gene under the control of a constitutive CaMV 35S promoter or a phloem specific Arabidopsis SUC2 (AtSUC2) promoter were produced. Overexpression of AtNPR1 resulted in trees with normal phenotypes that exhibited enhanced resistance to HLB. Phloem specific expression of NPR1 was equally effective for enhancing disease resistance. Transgenic trees exhibited reduced diseased severity and a few lines remained disease-free even after 36 months of planting in a high-disease pressure field site. Expression of the NPR1 gene induced expression of several native genes involved in the plant defense signaling pathways. The AtNPR1 gene being plant derived can serve as a component for the development of an all plant T-DNA derived consumer friendly GM tree.

Overexpression of antimicrobial lytic peptides protects grapevine from Pierce's disease under greenhouse but not field conditions

Pierce's disease (PD) caused by Xylella fastidiosa prevents cultivation of grapevine (Vitis vinifera) and susceptible hybrids in the southeastern United States and poses a major threat to the grape industry of California and Texas. Genetic resistance is the only proven control of X. fastidiosa. Genetic engineering offers an alternative to heretofore ineffective conventional breeding in order to transfer only PD resistance traits into elite cultivars. A synthetic gene encoding lytic peptide LIMA-A was introduced into V. vinifera and a Vitis hybrid to assess in planta inhibition of X. fastidiosa. Over 1050 independent transgenic plant lines were evaluated in the greenhouse, among which nine lines were selected and tested under naturally-inoculated field conditions. These selected plant lines in the greenhouse remain disease-free for 10 years, to date, even with multiple manual pathogen inoculations. However, all these lines in the field, including a grafted transgenic rootstock, succumbed to PD within 7 years. We conclude that in planta production of antimicrobial lytic peptides does not provide durable PD resistance to grapevine under field conditions.

Transient expression assays in grapevine: a step towards genetic improvement

In the past few years, the usefulness of transient expression assays has continuously increased for the characterization of unknown gene function and metabolic pathways. In grapevine (Vitis vinifera L.), one of the most economically important fruit crops in the world, recent systematic sequencing projects produced many gene data sets that require detailed analysis. Due to their rapid nature, transient expression assays are well suited for large-scale genetic studies. Although genes and metabolic pathways of any species can be analysed by transient expression in model plants, a need for homologous systems has emerged to avoid the misinterpretation of results due to a foreign genetic background. Over the last 10 years, various protocols have thus been developed to apply this powerful technology to grapevine. Using cell suspension cultures, somatic embryos, leaves or whole plantlets, transient expression assays enabled the study of the function, regulation and subcellular localization of genes involved in specific metabolic pathways such as the biosynthesis of phenylpropanoids. Disease resistance genes that could be used for marker-assisted selection in conventional breeding or for stable transformation of elite cultivars have also been characterized. Additionally, transient expression assays have proved useful for shaping new tools for grapevine genetic improvement: synthetic promoters, silencing constructs, minimal linear cassettes or viral vectors. This review provides an update on the different tools (DNA constructs, reporter genes, vectors) and methods (Agrobacterium-mediated and direct gene transfer methods) available for transient gene expression in grapevine. The most representative results published thus far are then described.

Precision breeding of grapevine (Vitis vinifera L.) for improved traits

This review provides an overview of recent technological advancements that enable precision breeding to genetically improve elite cultivars of grapevine (Vitis vinifera L.). Precision breeding, previously termed "cisgenic" or "intragenic" genetic improvement, necessitates a better understanding and use of genomic resources now becoming accessible. Although it is now a relatively simple task to identify genetic elements and genes from numerous "omics" databases, the control of major agronomic and enological traits often involves the currently unknown participation of many genes and regulatory machineries. In addition, genetic evolution has left numerous vestigial genes and sequences without tangible functions. Thus, it is critical to functionally test each of these genetic entities to determine their real-world functionality or contribution to trait attributes. Toward this goal, several diverse techniques now are in place, including cell culture systems to allow efficient plant regeneration, advanced gene insertion techniques, and, very recently, resources for genomic analyses. Currently, these techniques are being used for high-throughput expression analysis of a wide range of grapevine-derived promoters and disease-related genes. It is envisioned that future research efforts will be extended to the study of promoters and genes functioning to enhance other important traits, such as fruit quality and vigor.

Genetically engineered Thompson Seedless grapevine plants designed for fungal tolerance: selection and characterization of the best performing individuals in a field trial

The fungi Botrytis cinerea and Erysiphe necator are responsible for gray mold and powdery mildew diseases, respectively, which are among the most devastating diseases of grapes. Two endochitinase (ech42 and ech33) genes and one N-acetyl-β-D-hexosaminidase (nag70) gene from biocontrol agents related to Trichoderma spp. were used to develop a set of 103 genetically modified (GM) 'Thompson Seedless' lines (568 plants) that were established in open field in 2004 and evaluated for fungal tolerance starting in 2006. Statistical analyses were carried out considering transgene, explant origin, and plant response to both fungi in the field and in detached leaf assays. The results allowed for the selection of the 19 consistently most tolerant lines through two consecutive years (2007-2008 and 2008-2009 seasons). Plants from these lines were grafted onto the rootstock Harmony and established in the field in 2009 for further characterization. Transgene status was shown in most of these lines by Southern blot, real-time PCR, ELISA, and immunostrips; the most tolerant candidates expressed the ech42-nag70 double gene construct and the ech33 gene from a local Hypocrea virens isolate. B. cinerea growth assays in Petri dishes supplemented with berry juices extracted from the most tolerant individuals of the selected population was inhibited. These results demonstrate that improved fungal tolerance can be attributed to transgene expression and support the iterative molecular and physiological phenotyping in order to define selected individuals from a population of GM grapevines.

An optimized procedure for plant recovery from somatic embryos significantly facilitates the genetic improvement of Vitis

Plant regeneration from grapevine (Vitis spp.) via somatic embryogenesis typically is poor. Recovery of plants from Vitis rotundifolia Michx. (muscadine grape) is particularly problematic due to extremely low efficiency, including extended culture durations required for embryo-plant conversion. Poor plant recovery is an obstacle to the selection of improved genetically modified lines. Somatic embryos (SEs) of V. rotundifolia cultivar Delicious (Del-HS) and Vitis vinifera L cultivar Thompson Seedless (TS) were used to identify culture media and conditions that promoted embryo differentiation and plant conversion; this resulted in a two-step culture system. In comparative culture experiments, C2D medium containing 6% sucrose was the most effective, among four distinct formulae tested, for inducing precocious SE germination and cell differentiation. This medium, further supplemented with 4 µM 6-benzylaminopurine (C2D4B), was subsequently determined to enhance post-germinative growth of SE. MS medium supplemented with 0.5 µM 1-naphthaleneacetic acid (MSN) was then utilized to stimulate root and shoot growth of germinated SE. An average of 35% and 80% 'Del-HS' and 'TS' SE, respectively, developed into plants. All plants developed robust root and shoot systems and exhibited excellent survival following transfer to soil. Over 150 plants of 'Del-HS' were regenerated and established within 2.5 months, which is a dramatic reduction from the 6- to 12-month time period previously required. Similarly, 88 'TS' plant lines were obtained within the same time period. Subsequently, seven out of eight Vitis cultivars exhibited significantly increased plant conversion percentages, demonstrating broad application of the two-step culture system to produce the large numbers of independent plant lines needed for selection of desired traits.

Large-scale characterization of promoters from grapevine (Vitis spp.) using quantitative anthocyanin and GUS assay systems

Successful implementation of cisgenic/intragenic/ingenic technology for crop improvement necessitates a better understanding of the function of native promoters for driving desired gene expression in host plant. Although the genome of grapevine (Vitis vinifera) has been determined, efforts to explore promoter resources for the development of cisgenics are still lacking. Particularly, there is a shortage of constitutive promoters for marker and/or target gene expression in this species. In this work, we utilized an anthocyanin-based color histogram analysis method to evaluate quantitatively a large number of promoters for their ability to activate transgene expression. Promoter fragments corresponding to known genes were amplified from various genotypes and used to drive the VvMybA1 gene of 'Merlot' for anthocyanin production in non-pigmented somatic embryo (SE) explants to infer transcriptional activity. Results revealed that among 15 tested promoters belonging to seven ubiquitin genes, at least three promoters generated constitutive activities reaching up to 100% value of the d35S promoter. In particular, the high activity levels of VvUb6-1 and VvUb7-2 promoters were verified by transient GUS quantitative assay as well as stable anthocyanin expression in sepal and corolla of transgenic tobacco. Variations in promoter activity of different ubiquitin genes in grapevine did not correlate with the presence and sizes of 5' UTR intron, but seemed to be related positively and negatively to the number of positive cis-acting elements and root-specific elements respectively. In addition, several of the 13 promoters derived from a PR1 gene and a PAL gene produced a higher basal activity as compared to previously reported inducible promoters and might be useful for further identification of strong inducible promoters. Our study contributed invaluable information on transcriptional activity of many previously uncharacterized native promoters that could be used for genetic engineering of grapevine.

Initiation and transformation of grapevine embryogenic cultures

Protocols for the production and transformation of grapevine embryogenic cultures are described. Embryogenic cultures are initiated from leaves or stamens and pistils and transformed with Agrobacterium containing an enhanced green fluorescent protein/neomycin phosphotransferase II (egfp/nptII) fusion gene. Cultures are transferred to induction medium in the dark for callus formation and proliferation. Resulting cultures are transferred to somatic embryo development medium to induce secondary embryogenesis and formation of transgenic somatic embryos. Transgenic embryos identified on the basis on GFP fluorescence and kanamycin resistance are transferred to germination medium to regenerate transgenic plants. The presence of transgenes in independent plant lines is confirmed by PCR.

Use of the VvMybA1 gene for non-destructive quantification of promoter activity via color histogram analysis in grapevine (Vitis vinifera) and tobacco

We report the development of a convenient plant-based reporter system to analyze promoters and facilitate selection of genetically engineered plants. The VvMybA1 gene of grapevine (Vitis vinifera L.) regulates the last metabolic step of anthocyanin biosynthesis and its ectopic expression leads to anthocyanin production in otherwise non-pigmented cells. To develop an anthocyanin-based quantitative reporter system, the VvMybA1 gene was isolated from V. vinifera 'Merlot' and placed under control of three promoters to test its ability to distinguish different activity levels. Promoters included a double enhanced CaMV35S (d35S) promoter, a double enhanced CsVMV (dCsVMV) promoter or a bi-directional dual promoter (BDDP), resulting in transformation vectors DAT, CAT and DEAT, respectively. These vectors were introduced into grapevine and tobacco via Agrobacterium-mediated transformation for transient and stable expression analysis. A linear relationship between the mean red brightness (MRB) and optical density (OD) values with a 0.99 regression coefficient was identified in a dilution series of anthocyanin, thus allowing the use of histogram data for non-destructive and real-time assessment of transcriptional activity. Results of histogram-based analysis of color images from transformed grapevine somatic embryos (SE) and various tissues of transgenic tobacco showed a consistent six to sevenfold promoter activity increase of DEAT over DAT. This expression increase was verified by spectroscopic measurement of anthocyanin concentrations in sepal tissue of transgenic tobacco plants. These results were congruent with previously findings of promoter activity derived from GUS fluorometric assay, thus demonstrating for the first time that the VvMybA1 gene could offer a simple, versatile and reliable plant-based alternative for quantitative promoter analysis in plants.

Spider toxin (Hvt) gene cloned under phloem specific RSs1 and RolC promoters provides resistance against American bollworm (Heliothis armigera)

Spider venoms are neurotoxin proteins that can kill insects. Spider toxin Hvt gene was cloned under two phloem specific RSs1 and RolC promoters, transformed into tobacco plants through Agrobacterium-mediated transformation and tested against Heliothis armigera larvae. Transgenic plants were confirmed through PCR. First instar larvae of H. armigera were released on detached leaves of transformed and non-transformed plants. Insect bioassays showed 93-100% mortality of H. armigera larvae within 72 h on the leaves of transgenic plants while all larvae survived and continued feeding on detached leaves from non-transformed control plants. The Hvt gene expressing under phloem specific RSs1 and RolC promoters could therefore be used for developing H. armigera-resistant, genetically-modified crops.

Vascular-specific expression of GUS and GFP reporter genes in transgenic grapevine (Vitis vinifera L. cv. Albariño) conferred by the EgCCR promoter of Eucalyptus gunnii

In the view of the economic importance of grapevine and the increasing threaten represented by vascular diseases, transgenic grapevine with enhanced tolerance could represent an attractive opportunity. Hitherto, constitutive promoters have been used generally to study the effects of transgene expression in grapevine. Given the fact that constitutive gene expression may be harmful to the host plant, affecting plant growth and development, the use of tissue -specific promoters restricting gene expression to tissues of interest and at given developmental stages could be more appropriate. For this purpose, we decided to study in grapevine the activity of the Eucalyptus gunnii CCR promoter that was previously reported to be vascular-preferential. We transformed grapevine with the "Sonication assisted Agrobacterium-mediated transformation" (SAAT) method and a construct where both GUS and GFP (green fluorescent protein) marker genes were under control of the EgCCR promoter. High GUS and GFP activities were found to be associated with the newly formed vascular tissues in stems, leaves and petioles of transformed grapevine, suggesting a preferential activity of the EgCCR promoter in the vascular tissues of grapevine. These results suggest the tissue-specificity of this promoter from eucalyptus is conserved in grapevine and that it could be used to drive expression of defense genes in order to enhance resistance against vascular pathogens.

Levels and Stability of Expression of Transgenes

It is well known that in a given cell, at a particular time, only a fraction of the entire genome is expressed. Expression of a gene, nuclear, or organellar starts with the onset of transcription and ends in the synthesis of the functional protein. The regulation of gene expression is a complex process that requires the coordinated activity of different proteins and nucleic acids that ultimately determine whether a gene is transcribed, and if transcribed, whether it results in the production of a protein that develops a phenotype. The same also holds true for transgenic crops, which lie at the very core of insert design.

There are multiple checkpoints at which the expression of a gene can be regulated and controlled. Much of the emphasis of studies related to gene expression has been on regulation of gene transcription, and a number of methods are used to effect the control of gene expression. Controlling transgene expression for a commercially valuable trait is necessary to capture its value. Many gene functions are either lethal or produce severe deformity (resulting in loss of value) if over-expressed. Thus, expression of a transgene at a particular site or in response to a particular elicitor is always desirable.

Transcriptional regulation of wheat VER2 promoter in rice in response to abscisic acid, jasmonate, and light

VER2 is a key gene associated with vernalization process in winter wheat. The expression of VER2 can be induced by low temperature treatment. To further understand how the expression of this gene is mediated by various external and internal factors, different lengths of the VER2 promoter region have been transcriptionally fused with a reporter gene, green fluorescence protein (GFP), and transformed into the model plant, rice (Oryza sativa L.). Using confocal and Western blot analyses, we determined several possible response elements in the promoter region, which could sense ABA, JA, and other environmental cues.

Development of grapevine somatic embryogenesis using an air-lift bioreactor as an efficient tool in the generation of transgenic plants

The grapevine genetic transformation programs have relayed on the use of solid media-based somatic embryogenesis. To reach a high throughput of candidate gene evaluation in 'Thompson Seedless', a semi-automatic system allowing viable transformation of explants was designed. An intermediate procedure using liquid media and agitated flasks was first characterized, leading to reduction in the biomass duplication time of pro-embryogenic (PE) cells from 30 d in dishes to 14 d. The oxygen transfer coefficient value in this system was 213h(-1) at 120rpm and 25 degrees C with a 16/8-h (light/darkness) photoperiod. The scaling-up to the air-lift bioreactor decreased the biomass duplication time of PE cells up to 5.3 d post-inoculation (pi) and an average volumetric productivity of 1.6g/(dxL). Although slight browning was seen in the explants during the phase of 8-14 d pi, no losses in their viability and regenerative capability were observed. Cultured cells showed normal elongation in the transition from heart- to the torpedo-shape and finally to advanced developmental stages, with radicle emergence and whole plant generation. Agrobacterium-mediated transformation of cells was efficiently incorporated after this multiplication process by use of conventional procedures in dishes, allowing the generation of transgenic plantlets confirmed by PCR.

Agrobacterium-mediated transformation of embryogenic cultures and plant regeneration in Vitis rotundifolia Michx. (muscadine grape)

A method to produce transgenic plants of Vitis rotundifolia was developed. Embryogenic cultures were initiated from leaves of in vitro grown shoot cultures and used as target tissues for Agrobacterium-mediated genetic transformation. A green fluorescent protein/neomycin phosphotransferase II (gfp/nptII) fusion gene that allowed for simultaneous selection of transgenic cells based on GFP fluorescence and kanamycin resistance was used to optimize parameters influencing genetic transformation. It was determined that both proembryonal masses (PEM) and mid-cotyledonary stage somatic embryos (SE) were suitable target tissues for co-cultivation with Agrobacterium as evidenced by transient GFP expression. Kanamycin at 100 mg l(-1) in the culture medium was effective in suppression of non-transformed tissue and permitting the growth and development of transgenic cells, compared to 50 or 75 mg l(-1), which permitted the proliferation of more non-transformed cells. Transgenic plants of "Alachua" and "Carlos" were recovered after secondary somatic embryogenesis from primary SE explants co-cultivated with Agrobacterium. The presence and stable integration of transgenes in transgenic plants was confirmed by PCR and Southern blot hybridization. Transgenic plants exhibited uniform GFP expression in cells of all plant tissues and organs including leaves, stems, roots, inflorescences and the embryo and endosperm of developing berries.

Proteomic analysis of somatic embryogenesis in Vitis vinifera

Two dimensional gel electrophoresis coupled to mass spectrometry has been used to study the somatic embryogenesis in Vitis vinifera, by comparing embryogenic and non embryogenic calluses of the Thompson seedless cv. More than 1,000 spots were reproducibly resolved in colloidal Coomassie brilliant blue stained gels over a pI nonlinear range of 3-10 in the first dimension and using homogeneous 12.5% polyacrylamide gels in the second dimension. The expression pattern of 35 spots differed significantly between the two samples. These spots were processed by mass spectrometry analysis and the protein identity was assigned by using both the non-redundant protein and EST databases. Several responsive proteins, some already known to be involved in the somatic embryogenesis process while others, for the first time put into relation with this process, have been described. Moreover, they have been subdivided in functional categories, and their putative role is discussed in terms of their relevance in the somatic embryogenesis process.

Transgenic plants from shoot apical meristems of Vitis vinifera L. "Thompson Seedless" via Agrobacterium-mediated transformation

Shoot apical meristem explants of Vitis vinifera "Thompson Seedless" were used for Agrobacterium-mediated genetic transformation. It was determined that the meristems had to be subjected to a dark growth phase then wounded to obtain transgenic plants. Morphological and histological studies illustrated the role of wounding to expose apical meristem cells for transformation. A bifunctional egfp/nptII fusion gene was used to select kanamycin resistant plants that expressed green fluorescent protein (GFP). Kanamycin at a concentration of 16 mg L(-1) in selection medium resulted in recovery of non-chimeric transgenic plants that uniformly expressed GFP, whereas 8 mg L(-1) kanamycin allowed non-transgenic and/or chimeric plants to develop. Polymerase chain reaction (PCR) and Southern blot analyses confirmed the presence of transgenes and their stable integration into the genome of regenerated plants. Up to 1% of shoot tips produced stable transgenic cultures within 6 weeks of treatment, resulting in a total of 18 independent lines.

Genetic diversity and geographical dispersal in grapevine clones revealed by microsatellite markers

Intravarietal genetic diversification associated with geographical dispersal of a vegetatively propagated species was studied using grapevine Vitis vinifera L. ‘Cabernet Sauvignon’ as a model. Fifty-nine clonal samples obtained from 7 countries (France, Chile, Spain, Australia, Hungary, USA, and Italy) were analyzed using 84 microsatellite markers. Eighteen polymorphic microsatellite loci (21.4%) were detected, finding 22 different genotypes in the population analyzed with a genetic similarity of over 97%. The presence of chimeric clones was evidenced at locus VMC5g7 by means of a segregation analysis of descendants by self-pollination of a triallelic Chilean clone and by somatic embryogenesis analysis, showing a mutation in L2 cell layer. Only 2 clones (obtained from France and Australia) presented the ancestral genotype, and the most divergent genotype was exhibited by another French clone, which had accumulated 5 somatic mutations. The 2 largest populations considered (from France and Chile) showed a clear divergency in the polymorphisms detected. These antecedents enabled the tracing of geographical dispersal with a phylogenetic hypothesis supporting France as the center of origin of diversification of Cabernet Sauvignon. The results obtained could help to explain diversification processes in other grapevine cultivars. The possibility that this kind of genetic variability occurs in other vegetatively propagated species is discussed, focusing on possible fingerprinting applications.

The use of green fluorescent protein (GFP) improves Agrobacterium-mediated transformation of 'Spadona' pear (Pyrus communis L.)

An efficient and reproducible system for Agrobacterium-mediated transformation of the pear (Pyrus communis L.) cultivar Spadona was developed. Leaf explants of in vitro propagated plants were cocultivated with the disarmed Agrobacterium strain EHA105 harboring the plasmid pME504, carrying the uidA-intron and nptII genes. Under selective conditions, 5% of the plantlets regenerated and were positively stained for GUS. However, most of the GUS-positive plants re-callused and subsequently died, leaving only 0.3-0.8% of these plantlets to reach maturity. In order to identify transformed shoots at early stages of regeneration, we introduced the green fluorescent protein (GFP) into the pear cultivar Spadona using the plasmid PZP carrying the nuclear-targeted GFP and nptII genes. High expression levels of GFP were detected in transgenic cells as early as 7 days after transformation. GFP marked-callii and transformed plants were observed after 14 and 24 days, respectively. Fluorescence microscopy screening of transformed plant material, under the selection of kanamycin, increased the transformation frequency to 3.0-4.0%. We conclude that the introduction of GFP improves the selection of transformed plants of Spadona pear.

Transgenic plants of Vitis vinifera cv. Seyval blanc

Leaf discs of grapevine cv. Seyval blanc originating from in vitro cultures were transformed with Agrobacterium tumefaciens strain LBA 4404 harbouring the vector pGJ42 carrying genes for chitinase and RIP (ribosome-inactivating protein) in an attempt to improve fungal resistance. The gene for neomycin phosphotransferase II (nptII) was used as the selectable marker gene. The explants were cocultivated for 2 days with recombinant Agrobacteria and then submitted to selection on NN69 medium containing 100 mg/l kanamycin. Successful regeneration and conversion of transgenic plantlets were obtained. Stable integration of foreign DNA was confirmed by PCR and Southern blot analyses, and protein expression was detected by Western blot. The regenerated transgenic plants were adapted to the greenhouse and showed no evidence of phenotypical alterations. The foreign genes introduced into the transformed plants did not effect the expected improvement in fungal disease resistance under field conditions for the major pests Uncinula necator and Plasmopara viticola.

Isolation by improved thermal asymmetric interlaced PCR and characterization of a seed-specific 2S albumin gene and its promoter from grape (Vitis vinifera L.)

A seed-specific 2S albumin gene and its promoter region of grape (Vitis vinifera L.) were isolated using an improved thermal asymmetric interlaced PCR that allowed efficient amplification of target sequence of up to 3 kbp in length directly from genomic DNA. The 2S albumin VvAlb1 (for V. vinifera 2S albumin 1) gene from different grape cultivars encompasses a coding region of 504-540 nucleotides corresponding to a deduced amino acid sequence of 167-179 residues. This deduced protein contains up to 30% glutamine residues and eight cysteine residues arranged in a pattern highly conserved among 2S albumins for disulfide bond formation. DNA sequence alignment revealed that the same VvAlb1 gene among different grape cultivars varied greatly, including an insertion of up to 36 bp near the 3' end of the gene sequence isolated from 'Thompson Seedless'. DNA sequence analysis indicated that several conserved seed-specific regulatory motifs were clustered within a 0.6-kbp region 5' upstream of the transcription start site. To further test promoter activity, the sequence of this region was used to drive a bifunctional EGFP/NPTII fusion gene in Agrobacterium-mediated transformation of grape somatic embryos and leaf discs of grape and tobacco (Nicotiana tabacum L.). A high level of GFP expression, comparable with that derived from an enhanced double CsVMV promoter, was observed in the cotyledonary but not hypocotyl and vegetative tissues of grape and tobacco. These results suggest that the VvAlb1 gene promoter isolated is capable of conferring seed-specific gene expression.

Transformation of fruit trees. Useful breeding tool or continued future prospect?

Regeneration and transformation systems using mature plant material of woody fruit species have to be achieved as a necessary requirement for the introduction of useful genes into specific cultivars and the rapid evaluation of resulting horticultural traits. Although the commercial production of transgenic annual crops is a reality, commercial genetically-engineered fruit trees are still far from common. In most woody fruit species, transformation and regeneration of commercial cultivars are not routine, generally being limited to a few genotypes or to seedlings. The future of genetic transformation as a tool for the breeding of fruit trees requires the development of genotype-independent procedures, based on the transformation of meristematic cells with high regeneration potential and/or the use of regeneration-promoting genes. The public concern with the introduction of antibiotic resistance into food and the restrictions due to new European laws that do not allow deliberate release of plants transformed with antibiotic-resistance genes highlight the development of methods that avoid the use of antibiotic-dependent selection or allow elimination of marker genesfrom the transformed plant as a research priority in coming years.

A comparison of constitutive promoters for expression of transgenes in alfalfa (Medicago sativa)

The activity of constitutive promoters was compared in transgenic alfalfa plants using two marker genes. Three promoters, the 35S promoter from cauliflower mosaic virus (CaMV), the cassava vein mosaic virus (CsVMV) promoter, and the sugarcane bacilliform badnavirus (ScBV) promoter were each fused to the beta-glucuronidase (gusA) gene. The highest GUS enzyme activity was obtained using the CsVMV promoter and all alfalfa cells assayed by in situ staining had high levels of enzyme activity. The 35S promoter was expressed in leaves, roots, and stems at moderate levels, but the promoter was not active in stem pith cells, root cortical cells, or in the symbiotic zones of nodules. The ScBV promoter was active primarily in vascular tissues throughout the plant. In leaves, GUS activity driven by the CsVMV promoter was approximately 24-fold greater than the activity from the 35S promoter and 38-fold greater than the activity from the ScBV promoter. Five promoters, the double 35S promoter, figwort mosaic virus (FMV) promoter, CsVMV promoter, ScBV promoter, and alfalfa small subunit Rubisco (RbcS) promoter were used to control expression of a cDNA from Trichoderma atroviride encoding an endochitinase (ech42). Highest chitinase activity in leaves, roots, and root nodules was obtained in plants containing the CsVMV:ech42 transgene. Plants expressing the endochitinase were challenged with Phoma medicaginis var. medicaginis, the causal agent of spring black stem and leaf spot of alfalfa. Although endochitinase activity in leaves of transgenic plants was 50- to 2650-fold greater than activity in control plants, none of the transgenic plants showed a consistent increase in disease resistance compared to controls. The high constitutive levels of both GUS and endochitinase activity obtained demonstrate that the CsVMV promoter is useful for high-level transgene expression in alfalfa.