VirD5 is required for efficient Agrobacterium infection and interacts with Arabidopsis VIP2

Arabidopsis F-box proteins D5BF1 and D5BF2 negatively regulate Agrobacterium-mediated transformation and tumorigenesis

The pathogen Agrobacterium tumefaciens is known for causing crown gall tumours in plants. However, it has also been harnessed as a valuable tool for plant genetic transformation. Apart from the T-DNA, Agrobacterium also delivers at least five virulence proteins into the host plant cells, which are required for an efficient infection. One of these virulence proteins is VirD5. F-box proteins, encoded in the host plant genome or the Ti plasmid, and the ubiquitin/26S proteasome system (UPS) also play an important role in facilitating Agrobacterium infection. Our study identified two Arabidopsis F-box proteins, D5BF1 and D5BF2, that bind VirD5 and facilitate its degradation via the UPS. Additionally, we found that Agrobacterium partially suppresses the expression of D5BF1 and D5BF2. Lastly, stable transformation and tumorigenesis efficiency assays revealed that D5BF1 and D5BF2 negatively regulate the Agrobacterium infection process, showing that the plant F-box proteins and UPS play a role in defending against Agrobacterium infection.

Genomic consequences associated with Agrobacterium-mediated transformation of plants

Attenuated strains of the naturally occurring plant pathogen Agrobacterium tumefaciens can transfer virtually any DNA sequence of interest to model plants and crops. This has made Agrobacterium-mediated transformation (AMT) one of the most commonly used tools in agricultural biotechnology. Understanding AMT, and its functional consequences, is of fundamental importance given that it sits at the intersection of many fundamental fields of study, including plant-microbe interactions, DNA repair/genome stability, and epigenetic regulation of gene expression. Despite extensive research and use of AMT over the last 40 years, the extent of genomic disruption associated with integrating exogenous DNA into plant genomes using this method remains underappreciated. However, new technologies like long-read sequencing make this disruption more apparent, complementing previous findings from multiple research groups that have tackled this question in the past. In this review, we cover progress on the molecular mechanisms involved in Agrobacterium-mediated DNA integration into plant genomes. We also discuss localized mutations at the site of insertion and describe the structure of these DNA insertions, which can range from single copy insertions to large concatemers, consisting of complex DNA originating from different sources. Finally, we discuss the prevalence of large-scale genomic rearrangements associated with the integration of DNA during AMT with examples. Understanding the intended and unintended effects of AMT on genome stability is critical to all plant researchers who use this methodology to generate new genetic variants.

Diversity and Evolutionary History of Ti Plasmids of "tumorigenes" Clade of Rhizobium spp. and Their Differentiation from Other Ti and Ri Plasmids

Agrobacteria are important plant pathogens responsible for crown/cane gall and hairy root diseases. Crown/cane gall disease is associated with strains carrying tumor-inducing (Ti) plasmids, while hairy root disease is caused by strains harboring root-inducing (Ri) plasmids. In this study, we analyzed the sequences of Ti plasmids of the novel "tumorigenes" clade of the family Rhizobiaceae ("tumorigenes" Ti plasmids), which includes two species, Rhizobium tumorigenes and Rhizobium rhododendri. The sequences of reference Ti/Ri plasmids were also included, which was followed by a comparative analysis of their backbone and accessory regions. The "tumorigenes" Ti plasmids have novel opine signatures compared with other Ti/Ri plasmids characterized so far. The first group exemplified by pTi1078 is associated with production of agrocinopine, nopaline, and ridéopine in plant tumors, while the second group comprising pTi6.2 is responsible for synthesis of leucinopine. Bioinformatic and chemical analyses, including opine utilization assays, indicated that leucinopine associated with pTi6.2 most likely has D,L stereochemistry, unlike the L,L-leucinopine produced in tumors induced by reference strains Chry5 and Bo542. Most of the "tumorigenes" Ti plasmids have conjugative transfer system genes that are unusual for Ti plasmids, composed of avhD4/avhB and traA/mobC/parA regions. Next, our results suggested that "tumorigenes" Ti plasmids have a common origin, but they diverged through large-scale recombination events, through recombination with single or multiple distinct Ti/Ri plasmids. Lastly, we showed that Ti/Ri plasmids could be differentiated based on pairwise Mash or average amino-acid identity distance clustering, and we supply a script to facilitate application of the former approach by other researchers.

The translocated virulence protein VirD5 causes DNA damage and mutation during Agrobacterium-mediated transformation of yeast

The soil bacterium Agrobacterium tumefaciens is a preferred gene vector not only for plants but also for fungi. Agrobacterium delivers a small set of virulence proteins into host cells concomitantly with transferred DNA (T-DNA) to support the transformation process. Here, we find that expression of one of these proteins, called VirD5, in yeast host cells causes replication stress and DNA damage. This can result in both genomic rearrangements and local mutations, especially small deletions. Delivery of VirD5 during cocultivation with Agrobacterium led to mutations in the yeast genome that were unlinked to the integration of T-DNA. This load of mutations can be prevented by using a virD5 mutant for genome engineering, but this leads to a lower transformation frequency.

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-mediated gene transfer: recent advancements and layered immunity in plants

Plant responds to Agrobacterium via three-layered immunity that determines its susceptibility or resistance to Agrobacterium infection. Agrobacterium tumefaciens is a soil-borne Gram-negative bacterium that causes crown gall disease in plants. The remarkable feat of interkingdom gene transfer has been extensively utilised in plant biotechnology to transform plant as well as non-host systems. In the past two decades, the molecular mode of the pathogenesis of A. tumefaciens has been extensively studied. Agrobacterium has also been utilised as a premier model to understand the defence response of plants during plant-Agrobacterium interaction. Nonetheless, the threat of Agrobacterium-mediated crown gall disease persists and is associated with a huge loss of plant vigour in agriculture. Understanding the molecular dialogues between these two interkingdom species might provide a cure for crown gall disease. Plants respond to A. tumefaciens by mounting a three-layered immune response, which is manipulated by Agrobacterium via its virulence effector proteins. Comparative studies on plant defence proteins versus the counter-defence of Agrobacterium have shed light on plant susceptibility and tolerance. It is possible to manipulate a plant's immune system to overcome the crown gall disease and increase its competence via A. tumefaciens-mediated transformation. This review summarises the recent advances in the molecular mode of Agrobacterium pathogenesis as well as the three-layered immune response of plants against Agrobacterium infection.

Bacterial protein secretion systems: Game of types

Protein trafficking across the bacterial envelope is a process that contributes to the organisation and integrity of the cell. It is the foundation for establishing contact and exchange between the environment and the cytosol. It helps cells to communicate with one another, whether they establish symbiotic or competitive behaviours. It is instrumental for pathogenesis and for bacteria to subvert the host immune response. Understanding the formation of envelope conduits and the manifold strategies employed for moving macromolecules across these channels is a fascinating playground. The diversity of the nanomachines involved in this process logically resulted in an attempt to classify them, which is where the protein secretion system types emerged. As our knowledge grew, so did the number of types, and their rightful nomenclature started to be questioned. While this may seem a semantic or philosophical issue, it also reflects scientific rigour when it comes to assimilating findings into textbooks and science history. Here I give an overview on bacterial protein secretion systems, their history, their nomenclature and why it can be misleading for newcomers in the field. Note that I do not try to suggest a new nomenclature. Instead, I explore the reasons why naming could have escaped our control and I try to reiterate basic concepts that underlie protein trafficking cross membranes.

Quorum Sensing Inhibition and Metabolic Intervention of 4-Hydroxycinnamic Acid Against Agrobacterium tumefaciens

The natural product 4-hydroxycinnamic acid (HA) was firstly isolated from the metabolites of Phomopsis liquidambari, one endophytic fungus from Punica granatum leaves. The anti-QS potential of HA was evaluated by β-galactosidase assay and acylated homoserine lactones (AHL) analysis. The MIC of HA was > 1.20 mM. Exposure to HA at sub-MIC concentrations (0.30-0.60 mM) remarkably reduced the β-galactosidase activity and AHL secretion. Transcriptional analysis by qRT-PCR and docking simulation indicated that HA functions as an anti-QS agent by inhibiting the transcriptional levels of traI and traR rather than signal mimicry. The blocked QS lead to suppressed biofilm formation, motilities, and flagella formation after exposure to HA at concentrations ranging from 0.30 to 0.80 mM. The dysfunctional QS also resulted in repressed antioxidant enzymes and intensified oxidative stress. The intensified oxidative stress destroyed membrane integrity, induced energy supply deficiency, resulted in disorder of protein and nuclear acid metabolism, and ultimately weakened pathogenicity of A. tumefaciens. HA may have promising potential for controlling A. tumefaciens.

Maize Transformation: From Plant Material to the Release of Genetically Modified and Edited Varieties

Over the past decades, advances in plant biotechnology have allowed the development of genetically modified maize varieties that have significantly impacted agricultural management and improved the grain yield worldwide. To date, genetically modified varieties represent 30% of the world's maize cultivated area and incorporate traits such as herbicide, insect and disease resistance, abiotic stress tolerance, high yield, and improved nutritional quality. Maize transformation, which is a prerequisite for genetically modified maize development, is no longer a major bottleneck. Protocols using morphogenic regulators have evolved significantly towards increasing transformation frequency and genotype independence. Emerging technologies using either stable or transient expression and tissue culture-independent methods, such as direct genome editing using RNA-guided endonuclease system as an in vivo desired-target mutator, simultaneous double haploid production and editing/haploid-inducer-mediated genome editing, and pollen transformation, are expected to lead significant progress in maize biotechnology. This review summarises the significant advances in maize transformation protocols, technologies, and applications and discusses the current status, including a pipeline for trait development and regulatory issues related to current and future genetically modified and genetically edited maize varieties.

Efficient CRISPR-mediated base editing in Agrobacterium spp

Agrobacterium spp. are important plant pathogens that are the causative agents of crown gall or hairy root disease. Their unique infection strategy depends on the delivery of part of their DNA to plant cells. Thanks to this capacity, these phytopathogens became a powerful and indispensable tool for plant genetic engineering and agricultural biotechnology. Although Agrobacterium spp. are standard tools for plant molecular biologists, current laboratory strains have remained unchanged for decades and functional gene analysis of Agrobacterium has been hampered by time-consuming mutation strategies. Here, we developed clustered regularly interspaced short palindromic repeats (CRISPR)-mediated base editing to enable the efficient introduction of targeted point mutations into the genomes of both Agrobacterium tumefaciens and Agrobacterium rhizogenes As an example, we generated EHA105 strains with loss-of-function mutations in recA, which were fully functional for maize (Zea mays) transformation and confirmed the importance of RolB and RolC for hairy root development by A. rhizogenes K599. Our method is highly effective in 9 of 10 colonies after transformation, with edits in at least 80% of the cells. The genomes of EHA105 and K599 were resequenced, and genome-wide off-target analysis was applied to investigate the edited strains after curing of the base editor plasmid. The off-targets present were characteristic of Cas9-independent off-targeting and point to TC motifs as activity hotspots of the cytidine deaminase used. We anticipate that CRISPR-mediated base editing is the start of "engineering the engineer," leading to improved Agrobacterium strains for more efficient plant transformation and gene editing.

F-Box Gene D5RF Is Regulated by Agrobacterium Virulence Protein VirD5 and Essential for Agrobacterium-Mediated Plant Transformation

We previously reported that the Agrobacterium virulence protein VirD5 possesses transcriptional activation activity, binds to a specific DNA element D5RE, and is required for Agrobacterium-mediated stable transformation, but not for transient transformation. However, direct evidence for a role of VirD5 in plant transcriptional regulation has been lacking. In this study, we found that the Arabidopsis gene D5RF (coding for VirD5 response F-box protein, At3G49480) is regulated by VirD5. D5RF has two alternative transcripts of 930 bp and 1594 bp that encode F-box proteins of 309 and 449 amino acids, designated as D5RF.1 and D5RF.2, respectively. D5RF.2 has a N-terminal extension of 140 amino acids compared to D5RF.1, and both of them are located in the plant cell nucleus. The promoter of the D5RF.1 contains two D5RE elements and can be activated by VirD5. The expression of D5RF is downregulated when the host plant is infected with virD5 deleted Agrobacterium. Similar to VirD5, D5RF also affects the stable but not transient transformation efficiency of Agrobacterium. Some pathogen-responsive genes are downregulated in the d5rf mutant. In conclusion, this study further confirmed Agrobacterium VirD5 as the plant transcription activator and identified Arabidopsis thalianaD5RF.1 as the first target gene of VirD5 in regulation.

Agrobacterium tumefaciens: A Bacterium Primed for Synthetic Biology

Agrobacterium tumefaciens is an important tool in plant biotechnology due to its natural ability to transfer DNA into the genomes of host plants. Genetic manipulations of A. tumefaciens have yielded considerable advances in increasing transformational efficiency in a number of plant species and cultivars. Moreover, there is overwhelming evidence that modulating the expression of various mediators of A. tumefaciens virulence can lead to more successful plant transformation; thus, the application of synthetic biology to enable targeted engineering of the bacterium may enable new opportunities for advancing plant biotechnology. In this review, we highlight engineering targets in both A. tumefaciens and plant hosts that could be exploited more effectively through precision genetic control to generate high-quality transformation events in a wider range of host plants. We then further discuss the current state of A. tumefaciens and plant engineering with regard to plant transformation and describe how future work may incorporate a rigorous synthetic biology approach to tailor strains of A. tumefaciens used in plant transformation.

Overexpression of VIRE2-INTERACTING PROTEIN2 in Arabidopsis regulates genes involved in Agrobacterium-mediated plant transformation and abiotic stresses

Arabidopsis VIRE2-INTERACTING PROTEIN2 (VIP2) was previously described as a protein with a NOT domain, and Arabidopsis vip2 mutants are recalcitrant to Agrobacterium-mediated root transformation. Here we show that VIP2 is a transcription regulator and the C-terminal NOT2 domain of VIP2 interacts with VirE2. Interestingly, AtVIP2 overexpressor lines in Arabidopsis did not show an improvement in Agrobacterium-mediated stable root transformation, but the transcriptome analysis identified 1,634 differentially expressed genes compared to wild-type. These differentially expressed genes belonged to various functional categories such as membrane proteins, circadian rhythm, signaling, response to stimulus, regulation of plant hypersensitive response, sequence-specific DNA binding transcription factor activity and transcription regulatory region binding. In addition to regulating genes involved in Agrobacterium-mediated plant transformation, AtVIP2 overexpressor line showed differential expression of genes involved in abiotic stresses. The majority of the genes involved in abscisic acid (ABA) response pathway, containing the Abscisic Acid Responsive Element (ABRE) element within their promoters, were down-regulated in AtVIP2 overexpressor lines. Consistent with this observation, AtVIP2 overexpressor lines were more susceptible to ABA and other abiotic stresses. Based on the above findings, we hypothesize that VIP2 not only plays a role in Agrobacterium-mediated plant transformation but also acts as a general transcriptional regulator in plants.

OsbZIP81, A Homologue of Arabidopsis VIP1, May Positively Regulate JA Levels by Directly Targetting the Genes in JA Signaling and Metabolism Pathway in Rice

Rice (Oryza sativa L.) is one of the most important food crops in the world. In plants, jasmonic acid (JA) plays essential roles in response to biotic and abiotic stresses. As one of the largest transcription factors (TFs), basic region/leucine zipper motif (bZIP) TFs play pivotal roles through the whole life of plant growth. However, the relationship between JA and bZIP TFs were rarely reported, especially in rice. In this study, we found two rice homologues of Arabidopsis VIP1 (VirE2-interacting protein 1), OsbZIP81, and OsbZIP84. OsbZIP81 has at least two alternative transcripts, OsbZIP81.1 and OsbZIP81.2. OsbZIP81.1 and OsbZIP84 are typical bZIP TFs, while OsbZIP81.2 is not. OsbZIP81.1 can directly bind OsPIOX and activate its expression. In OsbZIP81.1 overexpression transgenic rice plant, JA (Jasmonic Acid) and SA (Salicylic acid) were up-regulated, while ABA (Abscisic acid) was down-regulated. Moreover, Agrobacterium, Methyl Jasmonic Acid (MeJA), and PEG6000 can largely induce OsbZIP81. Based on ChIP-Seq and Random DNA Binding Selection Assay (RDSA), we identified a novel cis-element OVRE (Oryza VIP1 response element). Combining ChIP-Seq and RNA-Seq, we obtained 1332 targeted genes that were categorized in biotic and abiotic responses, including α-linolenic acid metabolism and fatty acid degradation. Together, these results suggest that OsbZIP81 may positively regulate JA levels by directly targeting the genes in JA signaling and metabolism pathway in rice.

The Agrobacterium VirD5 protein hyperactivates the mitotic Aurora kinase in host cells

Aided by translocated virulence proteins, Agrobacterium tumefaciens transforms plant cells with oncogenic T-DNA. In the host cells the virulence protein VirD5 moves to the nucleus, where it becomes localized at the kinetochores, and disturbs faithful chromosome segregation, but the molecular mechanism underlying this remains unknown. To gain more insight, we screened amongst the kinetochore proteins for VirD5 interactors using bimolecular fluorescence complementation assays, and tested chromosome segregation in yeast cells. We found that VirD5 interacts with the conserved mitotic Aurora kinase Ipl1 in yeast and likewise with plant Aurora kinases. In vitro VirD5 was found to stimulate the activity of Ipl1. Phosphorylation of substrates by Ipl1 in vivo is known to result in the detachment between kinetochore and spindle microtubule. This is necessary for error correction, but increased Ipl1/Aurora kinase activity is known to cause spindle instability, explaining enhanced chromosome mis-segregation seen in the presence of VirD5. That activation of the Ipl1/Aurora kinase at least partially underlies the toxicity of VirD5 became apparent by artificial boosting the activity of the specific counteracting phosphatase Glc7 in vivo, which relieved the toxicity. These findings reveal a novel mechanism by which a pathogenic bacterium manipulates host cells.

Agrobacterium-mediated horizontal gene transfer: Mechanism, biotechnological application, potential risk and forestalling strategy

The extraordinary capacity of Agrobacterium to transfer its genetic material to host cell makes it evolve from phytopathogen to a powerful transgenic vector. Agrobacterium-mediated stable transformation is widely used as the preferred method to create transgenic plants for molecular plant biology research and crop breeding. Recent years, both mechanism and application of Agrobacterium-mediated horizontal gene transfer have made significant progresses, especially Agrobacterium-mediated transient transformation was developed for plant biotechnology industry to produce recombinant proteins. Agrobacterium strains are almost used and saved not only by each of microbiology and molecular plant labs, but also by many of plant biotechnology manufacturers. Agrobacterium is able to transfer its genetic material to a broad range of hosts, including plant and non-plant hosts. As a consequence, the concern of environmental risk associated with the accidental release of genetically modified Agrobacterium arises. In this article, we outline the recent progress in the molecular mechanism of Agrobacterium-meditated gene transfer, focus on the application of Agrobacterium-mediated horizontal gene transfer, and review the potential risk associated with Agrobacterium-meditated gene transfer. Based on the comparison between the infecting process of Agrobacterium as a pathogen and the transgenic process of Agrobacterium as a transgenic vector, we realize that chemotaxis is the distinct difference between these two biological processes and thus discuss the possible role of chemotaxis in forestalling the potential risk of Agrobacterium-meditated horizontal gene transfer to non-target plant species.

Application of phiLOV2.1 as a fluorescent marker for visualization of Agrobacterium effector protein translocation

Agrobacterium tumefaciens can genetically transform plants by translocating a piece of oncogenic DNA, called T-DNA, into host cells. Transfer is mediated by a type IV secretion system (T4SS). Besides the T-DNA, which is transferred in a single-stranded form and at its 5' end covalently bound to VirD2, several other effector proteins (VirE2, VirE3, VirD5, and VirF) are translocated into the host cells. The fate and function of the translocated proteins inside the host cell are only partly known. Therefore, several studies were conducted to visualize the translocation of the VirE2 protein. As GFP-tagged effector proteins are unable to pass the T4SS, other approaches like the split GFP system were used, but these require specific transgenic recipient cells expressing the complementary part of GFP. Here, we investigated whether use can be made of the photostable variant of LOV, phiLOV2.1, to visualize effector protein translocation from Agrobacterium to non-transgenic yeast and plant cells. We were able to visualize the translocation of all five effector proteins, both to yeast cells, and to cells in Nicotiana tabacum leaves and Arabidopsis thaliana roots. Clear signals were obtained that are easily distinguishable from the background, even in cases in which by comparison the split GFP system did not generate a signal.

Bases moléculaires de l’infection de plantes parAgrobacterium tumefaciensvia un système de sécrétion de type IV

Agrobacterium tumefaciens is a well studied phytopathogen given its various applications for deciphering host-pathogen interactions, bacterial communication, and capacity to transfer DNA fragments into host cells via a membrane protein system, the type IV secretion system (T4SS). T4SS mechanism is similar to the one responsible for antibiotic resistance gene transmission, and new knowledge gained could be applied to other organisms using such a mechanism. As well, A. tumefaciens is of economic importance in biotechnology due to its capacity to generate genetically modified plants. Agrobacterium tumefaciens harbours a plasmid known as Ti plasmid encoding T4SS function genes used for transferring genetic information and plant colonization. In this review, the authors describe the molecular basis of infection, from detection of host signals, to the description of different regions of Ti plasmid key to infection, ending with substrate transfer through bacterial wall. [Journal translation].