T-DNA of Agrobacterium tumefaciens: 25 years and counting

Repurposing Macromolecule Delivery Tools for Plant Genetic Modification in the Era of Precision Genome Engineering

Efficient delivery of macromolecules into plant cells and tissues is important for both basic research and biotechnology product applications. In transgenic research, the goal is to deliver DNA molecules into regenerable cells and stably integrate them into the genome. Over the past 40 years, many macromolecule delivery methods have been studied. To generate transgenic plants, particle bombardment and Agrobacterium-mediated transformation are the methods of choice for DNA delivery. The rapid advance of genome editing technologies has generated new requirements on large biomolecule delivery and at the same time reinvigorated the development of new transformation technologies. Many of the gene delivery options that have been studied before are now being repurposed for delivering genome editing machinery for various applications. This article reviews the major progress in the development of tools for large biomolecule delivery into plant cells in the new era of precision genome engineering.

Environmental pH sensing: resolving the VirA/VirG two-component system inputs for Agrobacterium pathogenesis

Agrobacterium tumefaciens stands as one of biotechnology's greatest successes, with all plant genetic engineering building on the strategies of this pathogen. By integrating responses to external pHs, phenols, and monosaccharides, this organism mobilizes oncogenic elements to efficiently transform most dicotyledonous plants. We now show that the complex signaling network used to regulate lateral gene transfer can be resolved as individual signaling modules. While pH and sugar perception are coupled through a common pathway, requiring both low pH and sugar for maximal virulence gene expression, various VirA and ChvE alleles can decouple pH and monosaccharide perception. This VirA and ChvE system may represent a common mechanism that underpins external pH perception in prokaryotes, and the use of these simple genetic elements may now be extended to research on specific responses to changes in environmental pH.

Plant transformation by coinoculation with a disarmed Agrobacterium tumefaciens strain and an Escherichia coli strain carrying mobilizable transgenes

Transformation of Nicotiana tabacum leaf explants was attempted with Escherichia coli as a DNA donor either alone or in combination with Agrobacterium tumefaciens. We constructed E. coli donor strains harboring either the promiscuous IncP-type or IncN-type conjugal transfer system and second plasmids containing the respective origins of transfer and plant-selectable markers. Neither of these conjugation systems was able to stably transform plant cells at detectable levels, even when VirE2 was expressed in the donor cells. However, when an E. coli strain expressing the IncN-type conjugation system was coinoculated with a disarmed A. tumefaciens strain, plant tumors arose at high frequencies. This was caused by a two-step process in which the IncN transfer system mobilized the entire shuttle plasmid from E. coli to the disarmed A. tumefaciens strain, which in turn processed the T-DNA and transferred it to recipient plant cells. The mobilizable plasmid does not require a broad-host-range replication origin for this process to occur, thus reducing its size and genetic complexity. Tumorigenesis efficiency was further enhanced by incubation of the bacterial strains on medium optimized for bacterial conjugation prior to inoculation of leaf explants. These techniques circumvent the need to construct A. tumefaciens strains containing binary vectors and could simplify the creation of 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.