Specific attachment of Agrobacterium tumefaciens to bamboo cells in suspension cultures

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.

Progress of cereal transformation technology mediated by Agrobacterium tumefaciens

Monocotyledonous plants were believed to be not transformable by the soil bacterium Agrobacterium tumefaciens until two decades ago, although convenient protocols for infection of leaf disks and subsequent regeneration of transgenic plants had been well established in a number of dicotyledonous species by then. This belief was reinforced by the fact that monocotyledons are mostly outside the host range of crown gall disease caused by the bacterium and by the failures in trials in monocotyledons to mimic the transformation protocols for dicotyledons. However, a key reason for the failure could have been the lack of active cell divisions at the wound sites in monocotyledons. The complexity and narrow optimal windows of critical factors, such as genotypes of plants, conditions of the plants from which explants are prepared, tissue culture methods and culture media, pre-treatments of explants, strains of A. tumefaciens, inducers of virulence genes, transformation vectors, selection marker genes and selective agents, kept technical hurdles high. Eventually it was demonstrated that rice and maize could be transformed by co-cultivating cells of callus cultures or immature embryos, which are actively dividing or about to divide, with A. tumefaciens. Subsequently, these initial difficulties were resolved one by one by many research groups, and the major cereals are now transformed quite efficiently. As many as 15 independent transgenic events may be regenerated from a single piece of immature embryo of rice. Maize transformation protocols are well established, and almost all transgenic events deregulated for commercialization after 2003 were generated by Agrobacterium-mediated transformation. Wheat, barley, and sorghum are also among those plants that can be efficiently transformed by A. tumefaciens.

Several components of SKP1/Cullin/F-box E3 ubiquitin ligase complex and associated factors play a role in Agrobacterium-mediated plant transformation

• Successful genetic transformation of plants by Agrobacterium tumefaciens requires the import of bacterial T-DNA and virulence proteins into the plant cell that eventually form a complex (T-complex). The essential components of the T-complex include the single stranded T-DNA, bacterial virulence proteins (VirD2, VirE2, VirE3 and VirF) and associated host proteins that facilitate the transfer and integration of T-DNA. The removal of the proteins from the T-complex is likely achieved by targeted proteolysis mediated by VirF and the plant ubiquitin proteasome complex. • We evaluated the involvement of the host SKP1/culin/F-box (SCF)-E3 ligase complex and its role in plant transformation. Gene silencing, mutant screening and gene expression studies suggested that the Arabidopsis homologs of yeast SKP1 (suppressor of kinetochore protein 1) protein, ASK1 and ASK2, are required for Agrobacterium-mediated plant transformation. • We identified the role for SGT1b (suppressor of the G2 allele of SKP1), an accessory protein that associates with SCF-complex, in plant transformation. We also report the differential expression of many genes that encode F-box motif containing SKP1-interacting proteins (SKIP) upon Agrobacterium infection. • We speculate that these SKIP genes could encode the plant specific F-box proteins that target the T-complex associated proteins for polyubiquitination and subsequent degradation by the 26S proteasome.

Reexamining the role of the accessory plasmid pAtC58 in the virulence of Agrobacterium tumefaciens strain C58

Isogenic strains of Agrobacterium tumefaciens carrying pTiC58, pAtC58, or both were constructed and assayed semiquantitatively and quantitatively for virulence and vir gene expression to study the effect of the large 542-kb accessory plasmid, pAtC58, on virulence. Earlier studies indicate that the att (attachment) genes of A. tumefaciens are crucial in the ability of this soil phytopathogen to infect susceptible host plants. Mutations in many att genes, notably attR and attD, rendered the strain avirulent. These genes are located on pAtC58. Previous work also has shown that derivatives of the wild-type strain C58 cured of pAtC58 are virulent as determined by qualitative virulence assays and, hence, pAtC58 was described as nonessential for virulence. We show here that the absence of pAtC58 in pTiC58-containing strains results in reduced virulence but that disruption of the attR gene does not result in avirulence or a reduction in virulence. Our studies indicate that pAtC58 has a positive effect on vir gene induction as revealed by immunoblot analysis of Vir proteins and expression of a PvirB::lacZ fusion.

Agrobacterium-mediated plant transformation: the biology behind the "gene-jockeying" tool

Agrobacterium tumefaciens and related Agrobacterium species have been known as plant pathogens since the beginning of the 20th century. However, only in the past two decades has the ability of Agrobacterium to transfer DNA to plant cells been harnessed for the purposes of plant genetic engineering. Since the initial reports in the early 1980s using Agrobacterium to generate transgenic plants, scientists have attempted to improve this "natural genetic engineer" for biotechnology purposes. Some of these modifications have resulted in extending the host range of the bacterium to economically important crop species. However, in most instances, major improvements involved alterations in plant tissue culture transformation and regeneration conditions rather than manipulation of bacterial or host genes. Agrobacterium-mediated plant transformation is a highly complex and evolved process involving genetic determinants of both the bacterium and the host plant cell. In this article, I review some of the basic biology concerned with Agrobacterium-mediated genetic transformation. Knowledge of fundamental biological principles embracing both the host and the pathogen have been and will continue to be key to extending the utility of Agrobacterium for genetic engineering purposes.

Attachment of Agrobacteria to Grape Cells

The presence of the Ti plasmid favorably influences the attachment of agrobacteria to grape callus cells, especially during the early stages of a 2-h incubation. Agrobacterium strains attached to a similar extent to both the crown gall-resistant cultivar (Catawba), Vitis labruscana , and the crown gall-susceptible cultivar (Chancellor), Vitis sp. Attachment of the virulent strain to grape callus cells is blocked by the avirulent strain HLB-2 in both the tissue culture cell suspension and the seedling root systems.

Characterization of competent cells and early events of Agrobacterium-mediated genetic transformation in Arabidopsis thaliana

The insertion of foreign DNA in plants occurs through a complex interaction between Agrobacteria and host plant cells. The marker gene β-glucuronidase of Escherichia coli and cytological methods were used to characterize competent cells for Agrobacterium-mediated transformation, to study early cellular events of transformation, and to identify the potential host-cell barriers that limit transformation in Arabidopsis thaliana L. Heynh. In cotyledon and leaf explants, competent cells were mesophyll cells that were dedifferentiating, a process induced by wounding and-or phytohormones. The cells were located either at the cut surface or within the explant after phytohormone pretreatment. In root explants, competent cells were present in dedifferentiating pericycle, and were produced only after phytohormone pretreatment. Irrespective of their origin, the competent cells were small, isodiametric with thin primary cell walls, small and multiple vacuoles, prominent nuclei and dense cytoplasm. In both cotyledon and root explants, histological enumeration and β-glucuronidase assays showed that the number of putatively competent cells was increased by preculture treatment, indicating that cell activation and cell division following wounding were insufficient for transformation without phytohormone treatment. Exposure of explants for 48 h to A. tumefaciens produced no characteristic stress response nor any gradual loss of viability nor cell death. However, in the competent cell, association between the polysaccharide of the host cell wall and that of the bacterial filament was frequently observed, indicating that transformation required polysaccharide-to-polysaccharide contact. Flow cytofluorometry and histological analysis showed that abundant transformation required not only cell activation (an early state exhibiting an increase in nuclear protein) but also cell proliferation (which in cotyledon tissue occurred at many ploidy levels). Noncompetent cells could be made competent with the appropriate phytohormone treatments before bacterial infection: this should aid analysis of critical steps in transformation procedures and should facilitate developing new strategies to transform recalcitrant plants.

Factors influencing the efficiency of T-DNA transfer during co-cultivation of Antirrhinum majus with Agrobacterium tumefaciens

The effects of varying the pH of the cocultivation medium, additons of vir-inducing phenolic compounds and the strains of wild-type agrobacteria on transformation rates of a number of different varieties of Antirrhinum majus were studied. In general, optimal transformation was found with strains C58 or A281 and was favoured by low pH and the inclusion of acetosyringone in the co-cultivation medium. However, maximal transformation of the least susceptible variety was achieved at high pH and in the presence of syringaldehyde. This demonstrates the need for the optimization of a wide range of culture conditions when working with new genotypes and offers a rational approach towards the development of Agrobacterium-mediated transformation of new species or varieties.

A nontransformable Triticum monococcum monocotyledonous culture produces the potent Agrobacterium vir-inducing compound ethyl ferulate

Exudates of dicotyledonous plants contain specific phenolic signal molecules, such as acetosyringone, which serve as potent inducers for the expression of the virulence (vir) regulon of the phytopathogen Agrobacterium tumefaciens. This induction activates the Agrobacterium T-DNA transfer process to initiate the genetic transformation of target plant cells. Wounded and metabolically active plant cells are particularly susceptible to Agrobacterium infection, and these cells specifically produce vir-inducing molecules. Most monocotyledonous, as opposed to dicotyledonous, species are resistant to Agrobacterium transformation. One hypothesis for this resistance is that nonsusceptible monocotyledonous cells fail to produce vir signal molecules and, thus, are not recognized by Agrobacterium as transformation targets. Here we demonstrate that monocotyledonous cells make such molecules, and, furthermore, we purify the inducer produced by a Triticum monococcum suspension culture that is resistant to Agrobacterium infection. This molecule is shown to correspond to ethyl ferulate [C12H14O4; 3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid ethyl ester], to be more active for vir induction at low concentrations than acetosyringone, and to be produced in quantities giving significant levels of induction. Thus, at least for the wheat cell line used in this study, monocotyledonous resistance to Agrobacterium transformation must result from a block to a step of the T-DNA transfer process subsequent to vir induction.

Variation in Binding and Virulence of Agrobacterium tumefaciens Chromosomal Virulence (chv) Mutant Bacteria on Different Plant Species

Chromosomal virulence (chv) mutants of Agrobacterium tumefaciens have been reported to be deficient in binding to cells of zinnia, tobacco, and bamboo. The mutants are nonpathogenic on stems of Kalanchoë, sunflower, tomato, Jerusalem artichoke, and tobacco, but they cause tumors on tubers of Solanum tuberosum. We used a root cap cell binding assay to test ability of cells from individual plants of 13 different plant species to bind parent or chv mutant bacteria. The same plants were then inoculated to test for disease response. Cells from nine of the plant species were grossly deficient in their abilities to bind mutant bacteria, and the plants inoculated with mutant bacteria failed to form tumors. In contrast, root cap cells as well as root hairs and root surfaces of S. tuberosum, S. okadae, and S. hougasii bound chv mutant bacteria as well as wild type. Nevertheless, S. tuberosum roots inoculated with mutant bacteria did not develop tumors. Although S. okadae plants inoculated with mutant bacteria formed a few tumors, and S. hougasii developed as many tumors in response to chv mutants as in response to the parent strain, the tumors induced by mutant bacteria were smaller.

Characterization of Agrobacterium tumefaciens virulence proteins induced by the plant factor acetosyringone

The Ti plasmid virulence (vir) loci encode functions essential for the transfer of the T-DNA element from Agrobacterium tumefaciens to plant cells. The expression of these loci is specifically signaled by plant phenolics such as acetosyringone. Here, we characterize the protein products that are induced in Agrobacterium grown in the presence of acetosyringone. More than 10 to 15 proteins are induced in strains harboring different Ti plasmids. Two general classes of acetosyringone-induced proteins are observed, encoded either within or outside the vir region. Synthesis of both classes of proteins requires acetosyringone and the products of the vir regulatory genes A and G. Those proteins encoded outside the vir region define a novel category of proteins, the virulence-related proteins, which are both chromosomally and Ti plasmid-encoded. The molecular weight and subcellular localization of several pTiA6 vir-induced proteins are identified. The most abundant induced protein has a molecular weight of 65,000, and is the single product of the virE locus; this protein distributes into both cell envelope and soluble fractions. Three proteins with molecular weights of approximately 33,000, 80,000 and 25,000 fractionate with the cell envelope and are encoded by genes within the 5' half of the virB locus. The envelope localization of the virB proteins suggests that they play a role in directing T-DNA transfer events that occur at the bacterial surface.

Absence in monocotyledonous plants of the diffusible plant factors inducing T-DNA circularization and vir gene expression in Agrobacterium

T-DNA circularization is one of the molecular events specifically induced in agrobacterial cells upon their infection of dicotyledonous plant cells. We developed a seedling co-cultivation procedure to determine whether or not monocotyledonous plants have the ability to induce T-DNA circularization and vir gene expression. Co-cultivation of Agrobacterium tumefaciens with seedlings of dicotyledonous plants showed that the circularization event takes place efficiently. The exudates and extracts of the seedlings also effectively induced T-DNA circularization and vir gene expression, indicating that dicotyledonous seedlings contain diffusible factors capable of inducing these molecular events. In contrast, neither T-DNA circularization nor vir gene expression was detectable when Agrobacterium was incubated with seedlings of monocotyledonous plants. Supplementing with acetosyringone, a known inducer of vir gene expression and T-DNA circularization, resulted in the induction of circularization during co-cultivation with moncotyledonous seedlings. These results indicate that the seedlings of monocotyledonous plants have no detectable amounts of diffusible inducers, unlike dicotyledonous seedlings. Therefore, it is unlikely that the vir genes are expressed in Agrobacterium inoculated in monocotyledonous plants. This may be one of the blocks in tumorigenesis of monocotyledonous plant by Agrobacterium.

Correlation between binding of Agrobacterium tumefaciens by root cap cells and susceptibility of plants to crown gall

We compared the binding of Agrobacterium tumefaciens by freshly isolated root cap cells with susceptibility of plants to crown gall tumorigenesis. A high binding reaction was strongly correlated with susceptibility to tumorigenesis in a survey of the binding of strain B6 to cells from 48 species in 17 families. In reciprocal experiments with nine virulent A. tumefaciens strains, tumors developed in plant-bacteria combinations that gave a high binding response in the root cap cell assay. Binding was quantified by direct measurement of the number of bacteria bound to the periphery of individual cells. Root cap cells from six susceptible species bound significantly more bacteria than did cells from five resistant species.

Initial interactions of Agrobacterium tumefaciens with plant host cells

Infections of wounded dicotyledonous plants by Agrobacterium tumefaciens result in the formation of crown gall tumors. The initial step in tumor formation is the site-specific attachment of the bacteria to the host cells. The mechanism of recognition and attachment in this interaction has been studied in detail. Current information on the nature of the bacterial binding sites, the nature of the host receptors, the role of bacterial cellulose fibrils, and the genetics of bacterial attachment will be summarized, and a model for the attachment of Agrobacterium to host cells will be presented.