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

Establishment of an efficient callus transient transformation system for Vitis vinifera cv. 'Chardonnay'

Grape (Vitis L.), a highly valued fruit crop, poses significant challenges in genetic transformation and functional characterization of genes. Therefore, there is an urgent need for the development of a rapid and effective method for grape transformation and gene function identification. Here, we introduce a streamlined Agrobacterium-mediated transient transformation system for grape calli. Optimal conditions were established with a leaf-derived callus induction medium; chiefly B5 medium supplemented with 0.05 mg/L NAA, 0.5 mg/L 2,4-D, and 2.0 mg/L KT; and a callus proliferation medium (B5 medium supplemented with 0.5 mg/L NAA and 2.0 mg/L 6-BA), respectively. Notably, GUS enzyme activity peaked (352.96 ± 33.95 mol 4-MU/mg/min) by sonication with Agrobacterium tumefaciens EHA105 and 100 μM AS for 4 min, followed by vacuum infection for 5 min, and co-culture at 25 °C in the dark for 1 day using callus as explants at an optical density (OD600) of 0.8. VaCIPK18 gene was transiently transformed into calli, and transcripts of the gene (endogenous and exogenous) were detected at higher levels than in non-transformed calli (endogenous). Moreover, after 10 days of treatment at 4 °C or -4 °C, the callus net weight of transformed callus was significantly higher than that of the untransformed callus, indicating that the VaCIPK18-overexpressing grape callus could improve cold tolerance. Overall, we establish a simple but effective transient transformation approach for grape callus, which could serve as a useful tool for the rapid assessment of gene function in this important crop.

Plasmid-to-plasmid Southern blot analysis validates the presence of nucleotide binding site (nbs) sequences in cloned plasmids

Southern blot analysis is an important molecular biology technique for identifying a specific sequence in DNA samples. Although it is no longer used extensively in recent years, the steps and underlying principles of Southern blot are applicable to modern biology. High sensitivity and limited background are keys to successful Southern blots, whereas obtaining good quality and quantity of genomic DNA as starting materials and detecting a single/low copy target sequence in the genome can be challenging. To ensure student success in performing the technique for the first time, a modified "plasmid-to-plasmid" Southern blot was implemented to confirm the presence of grape nucleotide-binding site (nbs) sequences in cloned plasmids like those described previously. The plasmid DNA and a control plasmid, pSCA7 (T1-T3-W6) containing a known grape nbs sequence, were digested with restriction enzymes, followed by agarose gel electrophoresis. The DNA band corresponding to the nbs sequence of the pSCA7 (T1-T3-W6) was extracted from the gel for PCR digoxigenin (DIG) probe synthesis. At the same time, the cloned plasmid DNA and its digested DNA fragments were blotted from the gel onto nylon membranes to be hybridized with the DIG probe followed by the detection for nbs sequences. Students successfully performed Southern blots to confirm the presence of nbs sequences in their cloned plasmids and wrote up the results following the format of scientific research papers. They learned the principles and applications of Southern blot and gained hands-on experience with associated techniques.

Carbon Nanotube-Mediated Plasmid DNA Delivery in Rice Leaves and Seeds

CRISPR-Cas gene editing technologies offer the potential to modify crops precisely; however, in vitro plant transformation and regeneration techniques present a bottleneck due to the lengthy and genotype-specific tissue culture process. Ideally, in planta transformation can bypass tissue culture and directly lead to transformed plants, but efficient in planta delivery and transformation remains a challenge. This study investigates transformation methods that have the potential to directly alter germline cells, eliminating the challenge of in vitro plant regeneration. Recent studies have demonstrated that carbon nanotubes (CNTs) loaded with plasmid DNA can diffuse through plant cell walls, facilitating transient expression of foreign genetic elements in plant tissues. To test if this approach is a viable technique for in planta transformation, CNT-mediated plasmid DNA delivery into rice tissues was performed using leaf and excised-embryo infiltration with reporter genes. Quantitative and qualitative data indicate that CNTs facilitate plasmid DNA delivery in rice leaf and embryo tissues, resulting in transient GFP, YFP, and GUS expression. Experiments were also initiated with CRISPR-Cas vectors targeting the phytoene desaturase (PDS) gene for CNT delivery into mature embryos to create heritable genetic edits. Overall, the results suggest that CNT-based delivery of plasmid DNA appears promising for in planta transformation, and further optimization can enable high-throughput gene editing to accelerate functional genomics and crop improvement activities.

Physiological, biochemical, and molecular aspects of grafting in fruit trees

Grafting is a widely used practice for asexual propagation of fruit trees. Many physiological, biochemical, and molecular changes occur upon grafting that can influence important horticultural traits. This technology has many advantages, including avoidance of juvenility, modifying the scion architecture, improving productivity, adapting scion cultivars to unfavourable environmental conditions, and developing traits in resistance to insect pests, bacterial and fungal diseases. A limitation of grafting is scion-rootstock incompatibility. It may be caused by many factors, including insufficient genetic proximity, physiological or biochemical factors, lignification at the graft union, poor graft architecture, insufficient cell recognition between union tissues, and metabolic differences in the scion and the rootstock. Plant hormones, like auxin, ethylene (ET), cytokinin (CK), gibberellin (GA), abscisic acid (ABA), and jasmonic acid (JA) orchestrate several crucial physiological and biochemical processes happening at the site of the graft union. Additionally, epigenetic changes at the union affect chromatin architecture by DNA methylation, histone modification, and the action of small RNA molecules. The mechanism triggering these effects likely is affected by hormonal crosstalk, protein and small molecules movement, nutrients uptake, and transport in the grafted trees. This review provides an overview of the basis of physiological, biochemical, and molecular aspects of fruit tree grafting between scion and rootstock.

New Technologies and Strategies for Grapevine Breeding Through Genetic Transformation

Grapevine, as other woody perennials, has been considered a recalcitrant crop to produce transgenic plants. Since the production of transgenic and/or edited plants requires the ability to regenerate plants from transformed tissues, this step is often the biggest bottleneck in the process. The objective of this work is to review the state of the art technologies and strategies for the improvement of grapevine transformation and regeneration, focusing on three aspects: (i) problems associated with grapevine transformation; (ii) genes that promote grapevine regeneration; and (iii) vehicles for gene delivery. Concerning the first aspect, it is well documented that one of the main factors explaining the low success rate in obtaining transgenic plants is the regeneration process. After transgenic integration into receptor cells, tissue culture is required to regenerate transgenic seedlings from transformed cells. This process is time consuming and often requires the addition of environmentally damaging reagents (antibiotics and herbicides) to the culture medium to select transgenic plants. On the other hand, the expression of genes such as the so-called developmental regulators (DR), which induce specific development programs, can be used to avoid traditional tissue culture methods. The ectopic expression of specific combinations of DR in somatic cells has the potential to induce de novo meristems in diverse crops, including grapevine. Successful genome editing by de novo reprogramming of plant meristems in somatic tissues has been reported. Moreover, it has been shown that the expression of certain transcription factors can increase the regeneration efficiency in wheat, citrus, and rice. Finally, recent reports showed the use of nanoparticles, such as carbon dots (CDs), as an attractive alternative to Agrobacterium- and biolistic-mediated plant genetic transformation. In this way, the use of antibiotics in culture media is avoided, overcoming the loss of viability of plant tissues and accelerating the regeneration processes. It has been shown that CDs can act as a vehicle to transport plasmids to plant cells in transient transformation in several crops without negative impacts on photosynthesis or growth. Based on these advances, it is possible to combine these new available strategies and technologies to overcome the regeneration problems of species such as grapevine and other crops considered as recalcitrant.

Developing a rapid and highly efficient cowpea regeneration, transformation and genome editing system using embryonic axis explants

Cowpea (Vigna unguiculata (L.) Walp.) is one of the most important legume crops planted worldwide, but despite decades of effort, cowpea transformation is still challenging due to inefficient Agrobacterium-mediated transfer DNA delivery, transgenic selection and in vitro shoot regeneration. Here, we report a highly efficient transformation system using embryonic axis explants isolated from imbibed mature seeds. We found that removal of the shoot apical meristem from the explants stimulated direct multiple shoot organogenesis from the cotyledonary node tissue. The application of a previously reported ternary transformation vector system provided efficient Agrobacterium-mediated gene delivery, while the utilization of spcN as selectable marker enabled more robust transgenic selection, plant recovery and transgenic plant generation without escapes and chimera formation. Transgenic cowpea plantlets developed exclusively from the cotyledonary nodes at frequencies of 4% to 37% across a wide range of cowpea genotypes. CRISPR/Cas-mediated gene editing was successfully demonstrated. The transformation principles established here could also be applied to other legumes to increase transformation efficiencies.

Embryogenic callus induction and Agrobacterium-mediated genetic transformation of 'Shine Muscat' grape

We established a method for embryogenic callus induction and highly efficient Agrobacterium-mediated genetic transformation of a table grape cultivar 'Shine Muscat' (Vitis labruscana). Embryogenic calli were induced using flower bud filaments from a dormant cane. Agrobacterium strain LBA4404 harboring the binary plasmid pBin19-sgfp, which contains the sgfp and nptII genes, was used to infect embryogenic calli. Infected calli were selected on 1/2 MS medium containing 5% maltose and 2% agar supplemented with 15 mg l^-1 kanamycin. Efficiency of transformation of regenerated plants reached nearly 100% as determined by PCR and Southern blot analyses. The developed method will open a new avenue for genome editing of 'Shine Muscat' and contribute to the advancement of grape breeding.

Robust Genetic Transformation System to Obtain Non-chimeric Transgenic Chickpea

Chickpea transformation is an important component for the genetic improvement of this crop, achieved through modern biotechnological approaches. However, recalcitrant tissue cultures and occasional chimerism, encountered during transformation, hinder the efficient generation of transgenic chickpeas. Two key parameters, namely micro-injury and light emitting diode (LED)-based lighting were used to increase transformation efficiency. Early PCR confirmation of positive in vitro transgenic shoots, together with efficient grafting and an extended acclimatization procedure contributed to the rapid generation of transgenic plants. High intensity LED light facilitate chickpea plants to complete their life cycle within 9 weeks thus enabling up to two generations of stable transgenic chickpea lines within 8 months. The method was validated with several genes from different sources, either as single or multi-gene cassettes. Stable transgenic chickpea lines containing GUS (uidA), stress tolerance (AtBAG4 and TlBAG), as well as Fe-biofortification (OsNAS2 and CaNAS2) genes have successfully been produced.

A plant regeneration platform to apply new breeding techniques for improving disease resistance in grapevine rootstocks and cultivars

Worldwide grapevine cultivation is based on the use of elite cultivars, in many cases strictly linked to local important wine brands. Most of Vitis viniferacultivars have high susceptibility to fungal and viral diseases therefore, new breeding techniques (e.g. Cisgenesis, RNAi and gene editing) offer the possibility to introduce new clones of the main cultivars with increased diseases resistance, in order to reduce environmental impact and improve quality in the intensive wine grape industry. This study is finalized to develop efficient in vitro regeneration and transformation protocols to extend the application of these technologies in wine grape cultivars and rootstocks. With this aim, in vitro regeneration protocols based on the production of meristematic bulks (Mezzetti et al., 2002) were optimized for different grapevine cultivars (Glera, Vermentino, Sangiovese, Thompson Seedless) and rootstocks (1103 Paulsen, and 110 Richter). The meristematic bulks were then used as explants for Agrobacteriummediated genetic transformation protocols, by comparing the use of NPTII and e-GFP as marker genes. Results confirmed the efficiency of meristematic bulks as the regenerating tissue to produce new modified plants in almost all the above genotypes. The highest regeneration efficiency in some genotypes allowed the selection of stable modified lines/calli with only the use of e-GFP marker gene. This protocol can be applied in the use of MYB marker gene for the production of cisgenic lines. Genotypes having the highest regeneration and transformation efficiency were also used for transformation experiments using a hairpin gene construct designed to silence the RNA-dependent RNA polymerase (RpRd) of the GFLV and GLRaV3, which would induce multiple virus resistances, and the Dicer-like protein 1 (Bc-DCL1) and Bc-DCL2 to control B. cinerea infection.

Comparison of regeneration capacity and Agrobacterium-mediated cell transformation efficiency of different cultivars and rootstocks of Vitis spp. via organogenesis

The success of in vitro plant regeneration and the competence of genetic transformation greatly depends on the genotype of the species of interest. In previous work, we developed a method for the efficient Agrobacterium-mediated genetic transformation via organogenesis of V. vinifera cultivar Thompson Seedless, by using meristematic bulk (MB) as starting tissue. In this study, we applied this method for the regeneration and transformation of MBs obtained from the Italian cultivar Ciliegiolo and two of the commonly used Vitis rootstocks, 110 Richter and Kober 5BB, in comparison with Thompson Seedless. The A. tumefaciens strain EHA105, harbouring pK7WG2 binary vector, was used for the transformation trials, which allowed selection through the enhanced-green fluorescent protein (eGFP) and the neomycin phosphotransferase (nptII) gene. Putative transformed tissues and/or shoots were identified by either a screening based on the eGFP expression alone or its use in combination with kanamycin in the medium. MBs obtained from Thompson Seedless showed the highest regeneration and transformation cell competence, which subsequently allowed the recovery of stably transformed plants. Ciliegiolo, 110 Richter, and Kober 5BB, produced actively growing transgenic calli showing eGFP fluorescence, more consistently on selective media, but had no regenerative competence.

Robust Genetic Transformation System to Obtain Non-chimeric Transgenic Chickpea

Chickpea transformation is an important component for the genetic improvement of this crop, achieved through modern biotechnological approaches. However, recalcitrant tissue cultures and occasional chimerism, encountered during transformation, hinder the efficient generation of transgenic chickpeas. Two key parameters, namely micro-injury and light emitting diode (LED)-based lighting were used to increase transformation efficiency. Early PCR confirmation of positive in vitro transgenic shoots, together with efficient grafting and an extended acclimatization procedure contributed to the rapid generation of transgenic plants. High intensity LED light facilitate chickpea plants to complete their life cycle within 9 weeks thus enabling up to two generations of stable transgenic chickpea lines within 8 months. The method was validated with several genes from different sources, either as single or multi-gene cassettes. Stable transgenic chickpea lines containing GUS (uidA), stress tolerance (AtBAG4 and TlBAG), as well as Fe-biofortification (OsNAS2 and CaNAS2) genes have successfully been produced.

Expression of disease resistance in genetically modified grapevines correlates with the contents of viral sequences in the T-DNA and global genome methylation

Increased tolerance to pathogens is an important goal in conventional and biotechnology-assisted grapevine breeding programs worldwide. Fungal and viral pathogens cause direct losses in berry production, but also affect the quality of the final products. Precision breeding strategies allow the introduction of resistance characters in elite cultivars, although the factors determining the plant's overall performance are not fully characterized. Grapevine plants expressing defense proteins, from fungal or plant origins, or of the coat protein gene of grapevine leafroll-associated virus 3 (GLRaV-3) were generated by Agrobacterium-mediated transformation of somatic embryos and shoot apical meristems. The responses of the transformed lines to pathogen challenges were investigated by biochemical, phytopathological and molecular methods. The expression of a Metarhizium anisopliae chitinase gene delayed pathogenesis and disease progression against the necrotrophic pathogen Botrytis cinerea. Modified lines expressing a Solanum nigrum osmotin-like protein also exhibited slower disease progression, but to a smaller extent. Grapevine lines carrying two hairpin-inducing constructs had lower GLRaV-3 titers when challenged by grafting, although disease symptoms and viral multiplication were detected. The levels of global genome methylation were determined for the genetically engineered lines, and correlation analyses demonstrated the association between higher levels of methylated DNA and larger portions of virus-derived sequences. Resistance expression was also negatively correlated with the contents of introduced viral sequences and genome methylation, indicating that the effectiveness of resistance strategies employing sequences of viral origin is subject to epigenetic regulation in grapevine.

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.

Agrobacterium tumefaciens-mediated genetic transformation of Salix matsudana Koidz. using mature seeds

An Agrobacterium tumefaciens-mediated transformation method was developed for Salix matsudana Koidz. using mature seeds as starting material. Multiple shoots were induced directly from embryonic shoot apices of germinating seeds. Although thidiazuron, 6-benzylaminopurine and zeatin induced multiple shoot induction with high frequency, zeatin (4.5 μM) was more effective for elongation of shoots and roots. The binary vector pCAMBIA1303, which contained neomycin phosphotransferase as a selectable marker gene and β-glucuronidase as a reporter gene, was used for transformation. Factors affecting transformation efficiency were examined for optimization of the procedure. Up to 35 of 180 seeds regenerated kanamycin-resistant shoots under optimal transformation conditions as follows: seeds were precultured for 4 days, apices of embryonic shoots were removed and infected with A. tumefaciens strain LBA4404 grown to a cell density equivalent (OD600) of 0.6, and then the infected explants were cultivated at 21 °C for 4 days. Storage of seeds at -20 °C for as long as 3 years had no significant effect on the induction of kanamycin-resistant shoots. Using this method, transgenic plants were obtained within ∼5 months with a transformation frequency of 7.2%. Analysis by polymerase chain reaction (PCR) showed that 36.4-93.8% of plants from all 13 tested kanamycin-resistant lines were PCR positive. Several 'escapes' were eliminated by a second round of selection. PCR, Southern blot and reverse transcriptase-PCR analyses of selected transgenic individuals 2 years after cutting propagation confirmed the successful generation of stable transformants. Our method, which minimizes the duration of axenic culture, may provide an alternative procedure for transformation of other recalcitrant Salix species.

Agrobacterium-mediated transformation of Eucalyptus globulus using explants with shoot apex with introduction of bacterial choline oxidase gene to enhance salt tolerance

Eucalyptus globulus is one of the most economically important plantation hardwoods for paper making. However, its low transformation frequency has prevented genetic engineering of this species with useful genes. We found the hypocotyl section with a shoot apex has the highest regeneration ability among another hypocotyl sections, and have developed an efficient Agrobacterium-mediated transformation method using these materials. We then introduced a salt tolerance gene, namely a bacterial choline oxidase gene (codA) with a GUS reporter gene, into E. globulus. The highest frequency of transgenic shoot regeneration from hypocotyls with shoot apex was 7.4% and the average frequency in four experiments was 4.0%, 12-fold higher than that from hypocotyls without shoot apex. Using about 10,000 explants, over 250 regenerated buds were confirmed as transformants by GUS analysis. Southern blot analysis of 100 elongated shoots confirmed successful generation of stable transformants. Accumulation of glycinebetaine was investigated in 44 selected transgenic lines, which showed 1- to 12-fold higher glycinebetaine levels than non-transgenic controls. Rooting of 16 transgenic lines was successful using a photoautotrophic method under enrichment with 1,000 ppm CO(2). The transgenic whole plantlets were transplanted into potting soil and grown normally in a growth room. They showed salt tolerance to 300 mM NaCl. The points of our system are using explants with shoot apex as materials, inhibiting the elongation of the apex on the selection medium, and regenerating transgenic buds from the side opposite to the apex. This approach may also solve transformation problems in other important plants.

Transient silencing of the grapevine gene VvPGIP1 by agroinfiltration with a construct for RNA interference

Grapevine is an economically important crop, and the recent completion of its genome makes it possible to study the function of specific genes through reverse genetics. However, the analysis of gene function by RNA interference (RNAi) in grapevine is difficult, because the generation of stable transgenic plants has low efficiency and is time consuming. Recently, transient expression of genes in grapevine leaves has been obtained by Agrobacterium tumefaciens infiltration (agroinfiltration). We therefore tested the possibility to silence grapevine genes by agroinfiltration of RNAi constructs. A construct to express a double strand RNA (dsRNA) corresponding to the defense-related gene VvPGIP1, encoding a polygalacturonase-inhibiting protein (PGIP), was obtained and transiently expressed by agroinfiltration in leaves of grapevine plants grown in vitro. Expression of VvPGIP1 and accumulation of PGIP activity were strongly induced by infiltration with control bacteria, but not with bacteria carrying the dsRNA construct, indicating that the gene was efficiently silenced. In contrast, expression of another defense-related gene, VST1, encoding a stilbene synthase, was unaffected by the dsRNA construct. We have therefore demonstrated the possibility of transient down-regulation of grapevine genes by agroinfiltration of constructs for the expression of dsRNA. This system can be employed to evaluate the effectiveness of constructs that can be subsequently used to generate stable RNAi transgenic plants.

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.

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.