Competence of Arabidopsis thaliana genotypes and mutants for Agrobacterium tumefaciens-mediated gene transfer: role of phytohormones

Genetic factors governing bacterial virulence and host plant susceptibility during Agrobacterium infection

Several species of the Agrobacterium genus represent unique bacterial pathogens able to genetically transform plants, by transferring and integrating a segment of their own DNA (T-DNA, transferred DNA) in their host genome. Whereas in nature this process results in uncontrolled growth of the infected plant cells (tumors), this capability of Agrobacterium has been widely used as a crucial tool to generate transgenic plants, for research and biotechnology. The virulence of Agrobacterium relies on a series of virulence genes, mostly encoded on a large plasmid (Ti-plasmid, tumor inducing plasmid), involved in the different steps of the DNA transfer to the host cell genome: activation of bacterial virulence, synthesis and export of the T-DNA and its associated proteins, intracellular trafficking of the T-DNA and effector proteins in the host cell, and integration of the T-DNA in the host genomic DNA. Multiple interactions between these bacterial encoded proteins and host factors occur during the infection process, which determine the outcome of the infection. Here, we review our current knowledge of the mechanisms by which bacterial and plant factors control Agrobacterium virulence and host plant susceptibility.

Agrobacterium strains and strain improvement: Present and outlook

Almost 40 years ago the first transgenic plant was generated through Agrobacterium tumefaciens-mediated transformation, which, until now, remains the method of choice for gene delivery into plants. Ever since, optimized Agrobacterium strains have been developed with additional (genetic) modifications that were mostly aimed at enhancing the transformation efficiency, although an optimized strain also exists that reduces unwanted plasmid recombination. As a result, a collection of very useful strains has been created to transform a wide variety of plant species, but has also led to a confusing Agrobacterium strain nomenclature. The latter is often misleading for choosing the best-suited strain for one's transformation purposes. To overcome this issue, we provide a complete overview of the strain classification. We also indicate different strain modifications and their purposes, as well as the obtained results with regard to the transformation process sensu largo. Furthermore, we propose additional improvements of the Agrobacterium-mediated transformation process and consider several worthwhile modifications, for instance, by circumventing a defense response in planta. In this regard, we will discuss pattern-triggered immunity, pathogen-associated molecular pattern detection, hormone homeostasis and signaling, and reactive oxygen species in relationship to Agrobacterium transformation. We will also explore alterations that increase agrobacterial transformation efficiency, reduce plasmid recombination, and improve biocontainment. Finally, we recommend the use of a modular system to best utilize the available knowledge for successful plant transformation.

A Short History and Perspectives on Plant Genetic Transformation

Plant genetic transformation is an important technological advancement in modern science, which has not only facilitated gaining fundamental insights into plant biology but also started a new era in crop improvement and commercial farming. However, for many crop plants, efficient transformation and regeneration still remain a challenge even after more than 30 years of technical developments in this field. Recently, FokI endonuclease-based genome editing applications in plants offered an exciting avenue for augmenting crop productivity but it is mainly dependent on efficient genetic transformation and regeneration, which is a major roadblock for implementing genome editing technology in plants. In this chapter, we have outlined the major historical developments in plant genetic transformation for developing biotech crops. Overall, this field needs innovations in plant tissue culture methods for simplification of operational steps for enhancing the transformation efficiency. Similarly, discovering genes controlling developmental reprogramming and homologous recombination need considerable attention, followed by understanding their role in enhancing genetic transformation efficiency in plants. Further, there is an urgent need for exploring new and low-cost universal delivery systems for DNA/RNA and protein into plants. The advancements in synthetic biology, novel vector systems for precision genome editing and gene integration could potentially bring revolution in crop-genetic potential enhancement for a sustainable future. Therefore, efficient plant transformation system standardization across species holds the key for translating advances in plant molecular biology to crop improvement.

A robust genetic transformation protocol to obtain transgenic shoots of Solanum tuberosum L. cultivar 'Kufri Chipsona 1'

The genetic transformation of plants is an important biotechnological tool used for crop improvement for many decades. The present study was focussed to investigate various factors affecting genetic transformation of potato cultivar 'Kufri Chipsona 1'. It was observed that explants pre-cultured for 2 days on MS2 medium (MS medium containing 10 µM silver nitrate, 10 µM BA, 15 µM GA₃), injured with a surgical blade and co-cultivated with Agrobacterium tumefaciens strain EHA105 [O.D₆₀₀ (0.6)] for 2 days results in maximum transient β-glucuronidase (GUS) expression. The addition of 100 µM acetosyringone in MS2 medium also increased rate of transient GUS expression in both the explants. Clumps of putative transgenic shoots were regenerated using the optimised culture conditions from leaf and internodal explants. The stable integration of T-DNA was established using histochemical staining for GUS and amplification of DNA fragment specific to nptII and uidA genes. Within the clumps, around 67.85% of shoots showed uniform GUS expression in all the tissues and about 32.15% shoots show intermittent GUS expression establishing chimeric nature. Uniform GUS staining of the tissue was used as initial marker of non-chimeric transgenic shoots. Quantitative expression of nptII transgene was found to be directly proportional to uniformity of GUS staining in transgenic shoots. The present investigation indicated that manipulation of culture conditions and the medium composition may help to get transgenic shoots with uniform expression of transgene in all the tissues of potato cultivar 'Kufri Chipsona 1'.

Highly efficient G-AgarTrap-mediated transformation of the Marchantia polymorpha model strains Tak-1 and Tak-2

The liverwort Marchantia polymorpha L. is an important model species for investigating land plant evolution. Effective genetic transformation techniques are crucial for plant molecular biology and simplified or improved techniques for specific cultivars or strains can accelerate research. Over the past several years, we developed a simple Agrobacterium-mediated transformation technique for M. polymorpha named AgarTrap (Agar-utilized transformation with pouring solutions). AgarTrap is an easy technique that involves pouring the appropriate solutions onto plant materials on a single solid plate of medium. We recently improved AgarTrap using gemmalings (G-AgarTrap) of the M. polymorpha female model strain BC3-38 and achieved a transformation efficiency of nearly 100%. Based on this improved technique, in the current study, we adopted two factors (sealing the Petri dish with Parafilm and dark treatment during co-cultivation) and optimized two factors (Agrobacterium strain and pre-culture period) of the improved G-AgarTrap for other model strains of M. polymorpha, the male strain Takaragaike-1 (Tak-1) and the female strain Takaragaike-2 (Tak-2). After optimization, the transformation efficiency of Tak-1 using G-AgarTrap was as high as 55% compared to approximately 30% using the previous protocol. Furthermore, using Tak-2, we achieved a transformation efficiency of nearly 100%. Our improved G-AgarTrap technique for Tak-1 and Tak-2 represents a promising tool for promoting the study of Marchantia.

Progress in Optimization of Agrobacterium-Mediated Transformation in Sorghum (Sorghum bicolor)

This review archives the achievements made in the last two decades and presents a brief outline of some significant factors influencing the Agrobacterium-mediated transformation of Sorghum bicolor. Recently, progress in successful transformation has been made for this particular monocot crop through direct DNA delivery method and indirect method via Agrobacterium. However, lower transformation rate still proved to be a bottleneck in genetic modification of sorghum. An efficient Agrobacterium transformation system could be attained by optimizing the preliminary assays, comprising of explant source, growth media, antibiotics, Agrobacterium strains and agro-infection response of callus. The selection of competent strains for genetic transformation is also one of the key factors of consideration. Successful transformation is highly dependent on genome configuration of selected cultivar, where non-tannin genotype proved the best suited. Immature embryos from the field source have higher inherent adaptation chances than that of the greenhouse source. A higher concentration of Agrobacterium may damage the explant source. Utilization of anti-necrotic treatments and optimized tissue culture timeframe are the adequate strategies to lower down the effect of phenolic compounds. Appropriate selection of culture media vessels at different stages of tissue culture may also assist in a constructive manner. In conclusion, some aspects such as culture environment with medium composition, explant sources, and genotypes play an indispensable role in successful Agrobacterium-mediated sorghum transformation system.

Agrobacterium-mediated plant transformation: biology and applications

Plant genetic transformation heavily relies on the bacterial pathogen Agrobacterium tumefaciens as a powerful tool to deliver genes of interest into a host plant. Inside the plant nucleus, the transferred DNA is capable of integrating into the plant genome for inheritance to the next generation (i.e. stable transformation). Alternatively, the foreign DNA can transiently remain in the nucleus without integrating into the genome but still be transcribed to produce desirable gene products (i.e. transient transformation). From the discovery of A. tumefaciens to its wide application in plant biotechnology, numerous aspects of the interaction between A. tumefaciens and plants have been elucidated. This article aims to provide a comprehensive review of the biology and the applications of Agrobacterium-mediated plant transformation, which may be useful for both microbiologists and plant biologists who desire a better understanding of plant transformation, protein expression in plants, and plant-microbe interaction.

Mitochondrial Porin Isoform AtVDAC1 Regulates the Competence of Arabidopsis thaliana to Agrobacterium-Mediated Genetic Transformation

The efficiency of Agrobacterium-mediated transformation in plants depends on the virulence of Agrobacterium strains, the plant tissue culture conditions, and the susceptibility of host plants. Understanding the molecular interactions between Agrobacterium and host plant cells is crucial when manipulating the susceptibility of recalcitrant crop plants and protecting orchard trees from crown gall disease. It was discovered that Arabidopsis voltage-dependent anion channel 1 (atvdac1) mutant has drastic effects on Agrobacterium-mediated tumorigenesis and growth developmental phenotypes, and that these effects are dependent on a Ws-0 genetic background. Genetic complementation of Arabidopsis vdac1 mutants and yeast porin1-deficient strain with members of the AtVDAC gene family revealed that AtVDAC1 is required for Agrobacterium-mediated transformation, and there is weak functional redundancy between AtVDAC1 and AtVDAC3, which is independent of porin activity. Furthermore, atvdac1 mutants were deficient in transient and stable transformation by Agrobacterium, suggesting that AtVDAC1 is involved in the early stages of Agrobacterium infection prior to transferred-DNA (T-DNA) integration. Transgenic plants overexpressing AtVDAC1 not only complemented the phenotypes of the atvdac1 mutant, but also showed high efficiency of transient T-DNA gene expression; however, the efficiency of stable transformation was not affected. Moreover, the effect of phytohormone treatment on competence to Agrobacterium was compromised in atvdac1 mutants. These data indicate that AtVDAC1 regulates the competence of Arabidopsis to Agrobacterium infection.

Comparison of Soybean Transformation Efficiency and Plant Factors Affecting Transformation during the Agrobacterium Infection Process

The susceptibility of soybean genotype to Agrobacterium infection is a key factor for the high level of genetic transformation efficiency. The objective of this study is to evaluate the plant factors related to transformation in cotyledonary nodes during the Agrobacterium infection process. This study selected three genotypes (Williams 82, Shennong 9 and Bert) with high transformation efficiency, which presented better susceptibility to Agrobacterium infection, and three low transformation efficiency genotypes (General, Liaodou 16 and Kottman), which showed a relatively weak susceptibility. Gibberellin (GA) levels and soybean GA20ox2 and CYP707A2 transcripts of high-efficiency genotypes increased and were higher than those of low-efficiency genotypes; however, the opposite performance was shown in abscisic acid (ABA). Higher zeatin riboside (ZR) content and DNA quantity, and relatively higher expression of soybean IPT5, CYCD3 and CYCA3 were obtained in high-efficiency genotypes. High-efficiency genotypes had low methyl jasmonate (MeJA) content, polyphenol oxidase (PPO) and peroxidase (POD) activity, and relatively lower expression of soybean OPR3, PPO1 and PRX71. GA and ZR were positive plant factors for Agrobacterium-mediated soybean transformation by facilitating germination and growth, and increasing the number of cells in DNA synthesis cycle, respectively; MeJA, PPO, POD and ABA were negative plant factors by inducing defence reactions and repressing germination and growth, respectively.

Regulation of oncogene expression in T-DNA-transformed host plant cells

Virulent Agrobacterium tumefaciens strains integrate their T-DNA into the plant genome where the encoded agrobacterial oncogenes are expressed and cause crown gall disease. Essential for crown gall development are IaaH (indole-3-acetamide hydrolase), IaaM (tryptophan monooxygenase) and Ipt (isopentenyl transferase), which encode enzymes for the biosynthesis of auxin (IaaH, IaaM) and cytokinin (Ipt). Although these oncogenes are well studied as the tumor-inducing principle, nothing is known about the regulation of oncogene expression in plant cells. Our studies show that the intergenic regions (IGRs) between the coding sequences (CDS) of the three oncogenes function as promoters in plant cells. These promoters possess a eukaryotic sequence organization and cis-regulatory elements for the binding of plant transcription factors. WRKY18, WRKY40, WRKY60 and ARF5 were identified as activators of the Ipt promoter whereas IaaH and IaaM is constitutively expressed and no transcription factor further activates their promoters. Consistent with these results, the wrky triple mutant plants in particular, develops smaller crown galls than wild-type and exhibits a reduced Ipt transcription, despite the presence of an intact ARF5 gene. WRKY40 and WRKY60 gene expression is induced by A. tumefaciens within a few hours whereas the ARF5 gene is transcribed later during crown gall development. The WRKY proteins interact with ARF5 in the plant nucleus, but only WRKY40 together with ARF5 synergistically boosts the activation of the Ipt promoter in an auxin-dependent manner. From our data, we propose that A. tumefaciens initially induces WRKY40 gene expression as a pathogen defense response of the host cell. The WRKY protein is recruited to induce Ipt expression, which initiates cytokinin-dependent host cell division. With increasing auxin levels triggered by ubiquitous expression of IaaH and IaaM, ARF5 is activated and interacts with WRKY40 to potentiate Ipt expression and balance cytokinin and auxin levels for further cell proliferation.

Crown gall development requires the expression of agrobacterial genes in the plant host. These genes are transferred by the T-DNA of the plant pathogen Agrobacterium tumefaciens and include the oncogenes IaaH, IaaM and Ipt, which, according to the tumor-inducing principle, are essential for crown gall development. The oncogenes are involved in auxin and cytokinin production. This results, when at appropriate hormone ratios, in enhanced cell proliferation. The T-DNA transformation process and the encoded oncogene enzymes have been intensively studied, but knowledge of oncogene expression in plant cells and the regulatory host factors is missing. We set out to fill this gap, providing evidence that expression of the Ipt gene is host-cell controlled, whereas the IaaH and IaaM genes are ubiquitously expressed at low levels in T-DNA transformed tissue. This is achieved by A. tumefaciens, which first hijacks transcription factors of the plant pathogen response pathway to activate Ipt oncogene expression and initiates cell proliferation. With increasing auxin levels during the infection process, a transcription factor of the auxin-signaling pathway is recruited, potentiating Ipt gene expression. Thus, for crown gall development, two host-signaling pathways are combined through the interaction of transcription factors that adjust the ratio of cytokinin to auxin.

Plant responses to Agrobacterium tumefaciens and crown gall development

Agrobacterium tumefaciens causes crown gall disease on various plant species by introducing its T-DNA into the genome. Therefore, Agrobacterium has been extensively studied both as a pathogen and an important biotechnological tool. The infection process involves the transfer of T-DNA and virulence proteins into the plant cell. At that time the gene expression patterns of host plants differ depending on the Agrobacterium strain, plant species and cell-type used. Later on, integration of the T-DNA into the plant host genome, expression of the encoded oncogenes, and increase in phytohormone levels induce a fundamental reprogramming of the transformed cells. This results in their proliferation and finally formation of plant tumors. The process of reprogramming is accompanied by altered gene expression, morphology and metabolism. In addition to changes in the transcriptome and metabolome, further genome-wide ("omic") approaches have recently deepened our understanding of the genetic and epigenetic basis of crown gall tumor formation. This review summarizes the current knowledge about plant responses in the course of tumor development. Special emphasis is placed on the connection between epigenetic, transcriptomic, metabolomic, and morphological changes in the developing tumor. These changes not only result in abnormally proliferating host cells with a heterotrophic and transport-dependent metabolism, but also cause differentiation and serve as mechanisms to balance pathogen defense and adapt to abiotic stress conditions, thereby allowing the coexistence of the crown gall and host plant.

Agrobacterium: nature's genetic engineer

Agrobacterium was identified as the agent causing the plant tumor, crown gall over 100 years ago. Since then, studies have resulted in many surprising observations. Armin Braun demonstrated that Agrobacterium infected cells had unusual nutritional properties, and that the bacterium was necessary to start the infection but not for continued tumor development. He developed the concept of a tumor inducing principle (TIP), the factor that actually caused the disease. Thirty years later the TIP was shown to be a piece of a tumor inducing (Ti) plasmid excised by an endonuclease. In the next 20 years, most of the key features of the disease were described. The single-strand DNA (T-DNA) with the endonuclease attached is transferred through a type IV secretion system into the host cell where it is likely coated and protected from nucleases by a bacterial secreted protein to form the T-complex. A nuclear localization signal in the endonuclease guides the transferred strand (T-strand), into the nucleus where it is integrated randomly into the host chromosome. Other secreted proteins likely aid in uncoating the T-complex. The T-DNA encodes enzymes of auxin, cytokinin, and opine synthesis, the latter a food source for Agrobacterium. The genes associated with T-strand formation and transfer (vir) map to the Ti plasmid and are only expressed when the bacteria are in close association with a plant. Plant signals are recognized by a two-component regulatory system which activates vir genes. Chromosomal genes with pleiotropic functions also play important roles in plant transformation. The data now explain Braun's old observations and also explain why Agrobacterium is nature's genetic engineer. Any DNA inserted between the border sequences which define the T-DNA will be transferred and integrated into host cells. Thus, Agrobacterium has become the major vector in plant genetic engineering.

Cytokinins secreted by Agrobacterium promote transformation by repressing a plant myb transcription factor

Agrobacterium-mediated transformation is the most widely used technique for generating transgenic plants. However, many crops remain recalcitrant. We found that an Arabidopsis myb family transcription factor (MTF1) inhibited plant transformation susceptibility. Mutating MTF1 increased attachment of several Agrobacterium strains to roots and increased both stable and transient transformation in both susceptible and transformation-resistant Arabidopsis ecotypes. Cytokinins from Agrobacterium tumefaciens decreased the expression of MTF1 through activation of the cytokinin response regulator ARR3. Mutating AHK3 and AHK4, genes that encode cytokinin-responsive kinases, increased the expression of MTF1 and impaired plant transformation. Mutant mtf1 plants also had increased expression of AT14A, which encodes a putative transmembrane receptor for cell adhesion molecules. Plants overexpressing AT14A exhibited increased susceptibility to transformation, whereas at14a mutant plants exhibited decreased attachment of bacteria to roots and decreased transformation, suggesting that AT14A may serve as an anchor point for Agrobacteria. Thus, by promoting bacterial attachment and transformation of resistant plants and increasing such processes in susceptible plants, treating roots with cytokinins may help engineer crops with improved features or yield.

Adventitious root induction in Arabidopsis thaliana as a model for in vitro root organogenesis

Adventitious root formation, the development of roots on non-root tissue (e.g. leaves, hypocotyls and stems) is a critical step during micropropagation. Although root induction treatments are routinely used for a large number of species micropropagated in vitro as well as for in vivo cuttings, the mechanisms controlling adventitious rooting are still poorly understood. Researchers attempt to gain better insight into the molecular aspects by studying adventitious rooting in Arabidopsis thaliana. The existing assay involves etiolation of seedlings and measurements of de novo formed roots on the elongated hypocotyl. The etiolated hypocotyls express a novel auxin-controlled signal transduction pathway in which auxin response factors (ARFs), microRNAs and environmental conditions that drive adventitious rooting are integrated. An alternative assay makes use of so-called thin cell layers (TCL), excised strips of cells from the inflorescence stem of Arabidopsis thaliana. However, both the etiolated seedling system and the TCL assay are only distantly related to industrial rooting processes in which roots are induced on adult stem tissue. Here, we describe an adventitious root induction system that uses segments of the inflorescence stems of Arabidopsis thaliana, which have a histological structure similar to cuttings or in vitro micropropagated shoots. The system allows multiple treatments with chemicals as well as the evaluation of different environmental conditions on a large number of explants. It is therefore suitable for high throughput chemical screenings and experiments that require numerous data points for statistical analysis. Using this assay, the adventitious root induction capacity of classical auxins was evaluated and a differential response to the different auxins could be demonstrated. NAA, IBA and IAA stimulated adventitious rooting on the stem segment, whereas 2,4-D and picloram did not. Light conditions profoundly influenced the root induction capacity of the auxins. Additionally to the environmental control of adventitious root formation, we also investigated the spatial and temporal aspects of stem-based adventitious root organogenesis. To determine the cells involved in de novo root initiation on the adult stems, we adopted scanning electron microscopy, which allows the visualization of the auxin responsive stem tissue. Using this technique, direct (without callus interface) and indirect (with intermediate callus phase) organogenesis was readily distinguished. The described micro-stem segment system is also suitable for other non-woody species and it is a valuable tool to perform fast evaluations of different treatments to study adventitious root induction.

Recent advances in plant transformation

Plant genetic engineering has become one of the most important molecular tools in the modern molecular breeding of crops. Over the last decade, significant progress has been made in the development of new and efficient transformation methods in plants. Despite a variety of available DNA delivery methods, Agrobacterium- and biolistic-mediated transformation remain the two predominantly employed approaches. In particular, progress in Agrobacterium-mediated transformation of cereals and other recalcitrant dicot species has been quite remarkable. In the meantime, other transgenic-enabling technologies have emerged, including generation of marker-free transgenics, gene targeting, and chromosomal engineering. Although transformation of some plant species or elite germplasm remains a challenge, further advancement in transformation technology is expected because the mechanisms of governing the regeneration and transformation processes are now better understood and are being creatively applied to designing improved transformation methods or to developing new enabling technologies.

Agrobacterium-produced and exogenous cytokinin-modulated Agrobacterium-mediated plant transformation

Agrobacterium tumefaciens is a plant pathogenic bacterium that causes neoplastic growths, called 'crown gall', via the transfer and integration of transferred DNA (T-DNA) from the bacterium into the plant genome. We characterized an acetosyringone (AS)-induced tumour-inducing (Ti) plasmid gene, tzs (trans-zeatin synthesizing), that is responsible for the synthesis of the plant hormone cytokinin in nopaline-type A. tumefaciens strains. The loss of Tzs protein expression and trans-zeatin secretions by the tzs frameshift (tzs-fs) mutant is associated with reduced tumorigenesis efficiency on white radish stems and reduced transformation efficiencies on Arabidopsis roots. Complementation of the tzs-fs mutant with a wild-type tzs gene restored wild-type levels of trans-zeatin secretions and transformation efficiencies. Exogenous application of cytokinin during infection increased the transient transformation efficiency of Arabidopsis roots infected by strains lacking Tzs, which suggests that the lower transformation efficiency resulted from the lack of Agrobacterium-produced cytokinin. Interestingly, although the tzs-fs mutant displayed reduced tumorigenesis efficiency on several tested plants, the loss of Tzs enhanced tumorigenesis efficiencies on green pepper and cowpea. These data strongly suggest that Tzs, by synthesizing trans-zeatin at early stage(s) of the infection process, modulates plant transformation efficiency by A. tumefaciens.

Genetic transformation of carnation (Dianthus caryophylus L.)

This chapter describes a rapid and efficient protocol for explant preparation and genetic transformation of carnation. Node explants from greenhouse-grown plants and leaf explants from in vitro plants are infected with Agrobacterium tumefaciens AGL0 harboring pKT3 plasmid, consisting of GUS and NPTII genes. Explant preparation is an important factor to obtain the transformed plants. The GUS-staining area was located only on the cut end of explants and only explants with a cut end close to the connecting area between node and leaf, produced transformed shoots. The cocultivation medium is also an important factor for the successful genetic transformation of carnation node and leaf explants. High genetic transformation efficiency of node and leaf explants cocultured with Agrobacterium tumefaciens was achieved when the explants were cocultivated on a filter paper soaked with water or water and acetosyringone mixture (AS).

Role of vitronectin-like protein in Agrobacterium attachment and transformation of Arabidopsis cells

The role of plant vitronectin-like protein (Vn) in Agrobacterium-host plant interactions and receptor-specific bacterial attachment is unclear and still open to debate. Using a well-established Agrobacterium-mediated Arabidopsis transformation system, the marker gene beta-glucuronidase (GUS) of Escherichia coli, and biochemical and cytological methods, such as ELISA tests, immunoblots, immunolocalization, and functional in vitro binding assays, we have reassessed the role of Vn in receptor-specific bacterial attachment and transformation. We provide evidence that Vn is present in the host plant cells and anti-human vitronectin antibody cross-reacts with a 65-kDa protein from Arabidopsis cells. The specificity of the immunological cross-reactivity of anti-vitronectin antibodies was further demonstrated by ELISA competition experiments. Immunogold labeling showed that Vn is localized in the plant cell wall, and its level increased considerably after phytohormone treatment of the petiole explants. However, Agrobacterium attachment was unaffected, and no inhibition of petiole cell transformation was detected in the presence of human vitronectin and anti-vitronectin antibodies in the media. Additionally, no correlation between the occurrence of Vn, attachment of bacteria to the cells, and susceptibility to Agrobacterium-mediated transformation was observed. Taken together, our data do not support a functional role of plant Vn as the receptor for site-specific Agrobacterium attachment leading to the transformation of Arabidopsis cells.

A high-throughput Agrobacterium-mediated transformation system for the grass model species Brachypodium distachyon L

In the ongoing process of developing Brachypodium distachyon as a model plant for temperate cereals and forage grasses, we have developed a high-throughput Agrobacterium-mediated transformation system for a diploid accession. Embryogenic callus, derived from immature embryos of the accession BDR018, were transformed with Agrobacterium tumefaciens strain AGL1 carrying two T-DNA plasmids, pDM805 and pWBV-Ds-Ubi-bar-Ds. Transient and stable transformation efficiencies were optimised by varying the pre-cultivation period, which had a strong effect on stable transformation efficiency. On average 55% of 17-day-old calli co-inoculated with Agrobacterium regenerated stable transgenic plants. Stable transformation frequencies of up to 80%, which to our knowledge is the highest transformation efficiency reported in graminaceous species, were observed. In a study of 177 transgenic lines transformed with pDM805, all of the regenerated transgenic lines were resistant to BASTA, while the gusA gene was expressed in 88% of the transgenic lines. Southern blot analysis revealed that 35% of the tested plants had a single T-DNA integration. Segregation analysis performed on progenies of ten selected T(0) plants indicated simple Mendelian inheritance of the two transgenes. Furthermore, the presence of two selection marker genes, bar and hpt, on the T-DNA of pWBV-Ds-Ubi-bar-Ds allowed us to characterize the developed transformation protocol with respect to full-length integration rate. Even when not selected for, full-length integration occurred in 97% of the transformants when using bialaphos as selection agent.

Induction, suppression and requirement of RNA silencing pathways in virulent Agrobacterium tumefaciens infections

Regulation of gene expression through microRNAs (miRNAs) and antiviral defense through small interfering RNAs (siRNAs) are aspects of RNA silencing, a process originally discovered as an unintended consequence of plant transformation by disarmed Agrobacterium tumefaciens strains. Although RNA silencing protects cells against foreign genetic elements, its defensive role against virulent, tumor-inducing bacteria has remained unexplored. Here, we show that siRNAs corresponding to transferred-DNA oncogenes initially accumulate in virulent A. tumefaciens-infected tissues and that RNA interference-deficient plants are hypersusceptible to the pathogen. Successful infection relies on a potent antisilencing state established in tumors whereby siRNA synthesis is specifically inhibited. This inhibition has only modest side effects on the miRNA pathway, shown here to be essential for disease development. The similarities and specificities of the A. tumefaciens RNA silencing interaction are discussed and contrasted with the situation encountered with plant viruses.

Unwounded plants elicit Agrobacterium vir gene induction and T-DNA transfer: transformed plant cells produce opines yet are tumour free

Agrobacterium tumefaciens is well known to cause crown gall tumours at plant wound sites and to benefit from this plant association by obtaining nutrients called opines that are produced by these tumours. Tumourigenesis requires expression of the vir regulon in response to chemical signals that are thought to be released from wound sites. Here, we examine chemical interactions between A. tumefaciens and unwounded plants. To determine whether unwounded plants can release significant amounts of vir gene inducers, we constructed an A. tumefaciens strain carrying a PvirB-gfp fusion. This fusion was strongly induced by co-culture with tobacco seedlings that have been germinated without any intentional wounding. The release of phenolic vir gene inducers was confirmed by GC/MS analysis. We also constructed a strain containing the gfp reporter located on an artificial T-DNA and expressed from a plant promoter. A. tumefaciens efficiently transferred this T-DNA into cells of unwounded plants in the absence of exogenous vir gene inducers. Many cells of seedlings colonized by the bacteria also produced octopine, which was detected using a Pocc-gfp reporter strain. This indicates transfer of the native T-DNA. However, these transformed plant cells did not form tumours. These results suggest that successful colonization of plants by A. tumefaciens, including T-DNA transfer and opine production, does not require wounding and does not necessarily cause cell proliferation. Transformation of plant cells without inciting tumours may represent a colonization strategy for this pathogen that has largely been overlooked.

QTLs controlling the production of transgenic and adventitious roots in Brassica oleracea following treatment with Agrobacterium rhizogenes

Brassica oleracea can be genetically engineered using Agrobacterium rhizogenes. The initial stage of this process is the production of transgenic ('hairy') roots; shoots are subsequently regenerated from these roots. Previous work using gus and gfp reporter genes has shown that genotypes of B. oleracea vary in their performance for transgenic root production. Quantitative trait loci (QTLs) controlling this trait have been located in one mapping population. The current study provides evidence that performance for transgenic root production is associated with performance for adventitious (non-transgenic) root production in B. oleracea across a second mapping population. This is shown by regression analyses between performance for the two traits and the demonstration that QTLs controlling the two traits map to the same positions within the genome. Since the rate of adventitious root production does not differ significantly in the presence and absence of A. rhizogenes, there is no evidence that the expression of Agrobacterium genes induces adventitious root production. It is apparent that genotypes exhibiting high adventitious root production in the absence of A. rhizogenes will also tend to show high transgenic root production, thereby allowing the selection of lines that are more efficiently transformed.

Effect of Wound Position, Auxin, and Agrobacterium vitis Strain F2/5 on Wound Healing and Crown Gall in Grapevine

ABSTRACT Agrobacterium vitis is the causal agent of crown gall disease in grapevine, which can be severe in many regions worldwide. Vitis vinifera cultivars are highly susceptible to freeze injury, providing the wounds necessary for infection by A. vitis. Wound position in relation to the uppermost bud of cuttings was determined to be important in tumor development. Inoculated wounds below buds developed tumors, whereas wounds opposite the bud did not, implying that indole-3-aectic acid flow contributes to tumor formation. If auxin was applied to wounds prior to inoculation with a tumorigenic A. vitis strain, all sites of inoculation developed tumors, accompanied by an increased amount of callus in the cambium. Wounds inoculated with an A. vitis biological control strain F2/5 prior to application of the pathogen did not develop galls. A closer examination of these wounds determined that callus cells formed in the cambium during wound healing are susceptible to transformation by the pathogen. Although the mechanism by which F2/5 prevents transformation is unknown, our observations suggest that F2/5 inhibits normal wound healing by inducing necrosis in the cambium.

Early events in Agrobacterium-mediated genetic transformation of citrus explants

BACKGROUND AND AIMS

Genetic transformation of plants relies on two independent but concurrent processes: integration of foreign DNA into plant cells and regeneration of whole plants from these transformed cells. Cell competence for regeneration and for transformation does not always fall into the same cell type/developmental stage, and this is one of the main causes of the so-called recalcitrance for transformation of certain plant species. In this study, a detailed examination of the first steps of morphogenesis from citrus explants after co-cultivation with Agrobacterium tumefaciens was performed, and an investigation into which cells and tissues are competent for regeneration and transformation was carried out. Moreover, the role of phytohormones in the co-cultivation medium as possible enhancers of gene transfer was also studied.

METHODS

A highly responsive citrus genotype and well-established culture conditions were used to perform a histological analysis of morphogenesis and cell competence for transformation after co-cultivation of citrus epicotyl segments with A. tumefaciens. In addition, the role of phytohormones as transformation enhancers was investigated by flow cytometry.

KEY RESULTS

It is demonstrated that cells competent for transformation are located in the newly formed callus growing from the cambial ring. Conditions conducive to further development of this callus, such as treatment of explants in a medium rich in auxins, resulted in a more pronounced formation of cambial callus and a slower shoot regeneration process, both in Agrobacterium-inoculated and non-inoculated explants. Furthermore, co- cultivation in a medium rich in auxins caused a significant increase in the rate of actively dividing cells in S-phase, the stage in which cells are more prone to integrate foreign DNA.

CONCLUSIONS

Use of proper co-cultivation medium and conditions led to a higher number of stably transformed cells and to an increase in the final number of regenerated transgenic plants.

Transfer of T-DNA and Vir proteins to plant cells by Agrobacterium tumefaciens induces expression of host genes involved in mediating transformation and suppresses host defense gene expression

Agrobacterium tumefaciens is a plant pathogen that incites crown gall tumors by transferring to and expressing a portion of a resident plasmid in plant cells. Currently, little is known about the host response to Agrobacterium infection. Using suppressive subtractive hybridization and DNA macroarrays, we identified numerous plant genes that are differentially expressed during early stages of Agrobacterium-mediated transformation. Expression profiling indicates that Agrobacterium infection induces plant genes necessary for the transformation process while simultaneously repressing host defense response genes, thus indicating successful utilization of existing host cellular machinery for genetic transformation purposes. A comparison of plant responses to different strains of Agrobacterium indicates that transfer of both T-DNA and Vir proteins modulates the expression of host genes during the transformation process.

Expression of the Arabidopsis histone H2A-1 gene correlates with susceptibility to Agrobacterium transformation

Transformation of plant cells by Agrobacterium tumefaciens involves both bacterial virulence proteins and host proteins. We have previously shown that the Arabidopsis thaliana gene H2A-1 (RAT5), which encodes histone H2A-1, is involved in T-DNA integration into the plant genome. Mutation of RAT5 results in a severely decreased frequency of transformation, whereas overexpression of RAT5 enhances the transformation frequency (Mysore et al., 2000b). We show here that the expression pattern of RAT5 correlates with plant root cells most susceptible to transformation. As opposed to a cyclin-GUS fusion gene whose expression is limited to meristematic tissues, the H2A-1 gene is expressed in many non-dividing cells. Under normal circumstances, the H2A-1 gene is expressed in the elongation zone of the root, the region that is most susceptible to Agrobacterium transformation. In addition, when roots are incubated on medium containing phytohormones, a concomitant shift in H2A-1 expression and transformation susceptibility to the root base is observed. Inoculation of root segments with a transfer-competent, but not a transformation-deficient Agrobacterium strain induces H2A-1 expression. Furthermore, pre-treatment of Arabidopsis root segments with phytohormones both induces H2A-1 expression and increases the frequency of Agrobacterium transformation. Our results suggest that the expression of the H2A-1 gene is both a marker for, and a predictor of, plant cells most susceptible to Agrobacterium transformation.