Based on the Vir System: T-DNA Transfer Mechanisms and Engineering Essentials for Plant Genetic Transformation

In plant genetic transformation, Vir system–driven T-DNA transfer is the central molecular basis for stable gene delivery. Representative Agrobacterium-mediated approaches leverage the bacterium’s natural interkingdom DNA transfer capability by disarming the Ti plasmid and engineering the T-DNA region to carry a gene of interest. Upon wound-signal induction in plant tissues, the Vir regulon is activated, enabling processing at the T-DNA border sequences, delivery into plant cells, and subsequent integration into the plant genome, followed by tissue culture–based regeneration of transgenic plants. Owing to its well-established workflow, relatively controllable cost, and stable inheritance of transformation events, this platform has long underpinned dicot genetic improvement and gene function studies, and has been extended to metabolic engineering, early-stage molecular breeding material development, and trait validation.

Keywords: Agrobacterium; plant transformation; Ti plasmid; T-DNA; vir genes; binary vector system; tissue culture; selectable marker; antisense RNA

I. Definition and Technical Positioning

Agrobacterium is a Gram-negative bacterium widely present in soil. It can infect wound sites of many dicotyledonous plants via chemotaxis and induce crown gall tumors or hairy roots. Agrobacterium tumefaciens and Agrobacterium rhizogenes typically carry Ti plasmids and Ri plasmids, respectively; both plasmid types contain a transferable segment, T-DNA. After Agrobacterium infects plant cells, it can deliver T-DNA and insert it into the plant genome; therefore, the Agrobacterium system is regarded as a natural plant genetic transformation system.

In research and breeding applications, by disarming tumorigenicity and loading the gene of interest between T-DNA borders, Agrobacterium’s natural “cross-kingdom DNA transfer” capability is converted into a controllable gene-delivery platform, thereby obtaining stably inherited transformation events and regenerable plants.

II. Development Trajectory and Evolution of Vector Systems

In the 1970s, researchers discovered that the tumor-inducing trait of Agrobacterium is determined by the Ti plasmid, and gradually elucidated the structure and functions of T-DNA. Starting in the 1980s, by deleting the auxin-, cytokinin-, and opine-synthesis genes in the Ti plasmid and adding plant selectable markers and multiple cloning sites, engineered vector systems suitable for plant transformation were constructed. The development of the co-integrative system and the binary vector system significantly simplified recombination operations, promoting Agrobacterium-mediated transformation to become one of the main approaches for genetic improvement of dicotyledonous plants. Subsequently, the introduction of gene regulation strategies such as antisense RNA further expanded its application boundaries in plant gene function research.

2.1 Co-integrative System

(1) Basic concept

First clone the T-DNA into a plasmid that can be manipulated in Escherichia coli. After construction and selection are completed, transfer it into Agrobacterium, where homologous recombination with Ti-plasmid homologous sequences achieves “co-integration,” incorporating the exogenous gene into the Ti-plasmid T-DNA region, which is then used to infect plant cells.

(2) Features

① Reduces the difficulty of performing recombination directly on the large Ti plasmid.

② Requires a homologous recombination step, resulting in a relatively longer construction chain.

③ More sensitive to the strain genetic background and recombination efficiency.

2.2 Binary Vector System

(1) Basic concept

A shuttle plasmid containing T-DNA borders carries the gene of interest and selectable elements; this plasmid can be efficiently cloned and amplified in E. coli and maintained in Agrobacterium. Vir functions are provided by the disarmed Ti plasmid or a helper plasmid in Agrobacterium. After infection, the Vir system processes and excises the T-DNA from the shuttle plasmid and transfers it into plant cells for integration.

(2) Features

① Modular construction, strong generality, and simpler operation.

② Facilitates replacement of elements such as promoters, marker genes, and reporter genes to match different research objectives.

③ Currently the most widely used route for stable transformation.

III. Transformation Method: Gene Delivery Mechanism Driven by the Ti Plasmid and T-DNA

3.1 Ti Plasmid and T-DNA: Molecular Basis of the Natural Tumorigenesis Mechanism

Agrobacterium tumefaciens is a Gram-negative soil bacterium. Many dicotyledonous plants are more susceptible to infection after wounding and can develop crown gall tumors. Its tumorigenic trait is mediated by the Ti plasmid. Wound sites in roots or other tissues release phenolic inducers such as acetosyringone and hydroxyacetosyringone; these signals can induce expression of the vir gene region on the Ti plasmid. Vir gene products perform site-specific processing of T-DNA at border sequences, excising T-DNA and generating a transferable single-stranded T-DNA; meanwhile, products of relevant genes on the bacterial chromosome bind the single-stranded T-DNA to form a complex, promoting transmembrane transport, entry into plant cells, and participation in subsequent integration processes.

Natural T-DNA usually contains three sets of functional genes: two sets participate in auxin and cytokinin synthesis or regulation, inducing uncontrolled proliferation of wound tissue to form crown gall tumors; the third set is related to opine synthesis. Opines mainly include octopine, nopaline, agropine, and succinamopine, among others, and are important nutrient sources that are essential for or utilizable by Agrobacterium.

3.2 Ti-Plasmid Structure and T-DNA Integration Patterns: Implications for Engineering Design

(1) Key points of Ti-plasmid structure

① Size and partitioning: Ti plasmids are typically about 160–240 kb; T-DNA is about 15–30 kb; the vir gene region is about 36 kb; in addition, they include the Con region (conjugation-related region) and the Ori region (origin of replication).

② T-DNA borders: The two ends of T-DNA are the LB and RB border sequences, typically ~25 bp terminal repeat structures, which are the most critical recognition signals during excision and integration.

③ Internal gene clusters within T-DNA:

tms consists of tms1 (iaaM) and tms2 (iaaH);

tmr is usually associated with cytokinins;

tmt consists of several genes and participates in opine synthesis.

④ Ti-plasmid classification: Ti plasmids can be classified based on opine type; different types induce plants to synthesize different opines. Their catabolic enzyme genes are located on the Ti plasmid; degradation products are amino acids and sugars, which can serve as nitrogen and carbon sources for Agrobacterium.

(2) Defined steps of the T-DNA integration mechanism

① Excision and single-strand formation

T-DNA excision is accomplished by specific endonuclease and processing reactions encoded by the vir region; nicks can be generated near LB and RB to form a single-stranded T-DNA intermediate.

② Asymmetric roles of borders

The roles of LB and RB in integration may not be completely symmetric; the RB sequence is usually more relevant to the integration process.

③ Copy number and position effects

Integration can be single-copy or multi-copy, and even arranged in tandem repeats; site specificity of integration is not clear, and expression and phenotypic differences among independent events can be significant.

IV. Engineered Systems: Ti-Plasmid Modification and Selection of Transformation Routes

4.1 Why the Natural Ti Plasmid Cannot Be Directly Used as an Expression Vector

(1) Tumorigenic genes interfere with regeneration

Transformed cells produce large amounts of auxin and cytokinin, which prevents regeneration into whole plants; therefore, genes related to auxin and cytokinin must be removed.

(2) Opine synthesis is unfavorable for culture

Opine synthesis is unrelated to T-DNA transformation and may affect cell growth; it also consumes substrates such as arginine and glutamate, so tmt-related genes are usually removed.

(3) Excessive plasmid size complicates construction

Ti plasmids are about 200 kb; recombination is difficult and it is hard to find a single, convenient restriction site.

(4) Replication system is inconvenient for molecular cloning

Ti plasmids typically cannot replicate in E. coli. To obtain large-scale amplification of recombinant DNA, an E. coli replicon must be added, along with bacterial selectable markers, multiple cloning sites, plant promoters, and polyadenylation signals and other elements.

4.2 Engineering-Point Comparison Between the Co-integrative System and the Binary System

(1) Co-integrative system

Incorporates the T-DNA recombinant module into the Ti plasmid via homologous recombination; suitable for some constructs that need to be “fixed in the Ti-plasmid background,” but the workflow is relatively longer.

(2) Binary system

Uses a shuttle plasmid to carry the T-DNA module; vir functions are provided by a helper plasmid or disarmed Ti plasmid; construction and use are more general and it is the mainstream route.

(3) Common points of attention

Common points of attention include:

① Integrity and orientation of the T-DNA borders.

② Compatibility between selectable markers and the intrinsic antibiotic resistance of the strain.

③ Obtaining multiple independent events to offset position effects and copy-number differences.

V. Improving Plant Traits: Engineering Strategies and Antisense RNA Technology

5.1 Two Types of Strategies: Gene Addition and Gene Knockdown

(1) Gene addition

Alter plant traits by adding one or more genes, such as introducing genes related to stress resistance, disease resistance, quality, or metabolic pathways.

(2) Gene knockdown

Use genetic engineering to inactivate one or more endogenous genes or reduce their expression, to obtain target phenotypes or for functional analysis.

5.2 Antisense RNA (asRNA) Technology: A Classical Route for Endogenous Gene Suppression

(1) Basic principle

Clone a fragment of the target gene into the vector in reverse orientation so that it is transcribed in plants to produce antisense RNA; antisense RNA base-pairs with the normal mRNA to form a sense–antisense hybrid structure, thereby reducing expression of the target gene.

(2) Possible inhibitory pathways

① The double-stranded RNA formed by complementarity may be degraded faster by nucleases.

② Antisense RNA may block ribosome binding to the sense RNA, thereby inhibiting translation.

(3) Significance of combining with stable Agrobacterium transformation

After the antisense expression cassette is loaded into the T-DNA and stably integrated, the suppressive effect can be maintained in regenerated plants and their progeny, providing a heritable material basis for functional studies and trait improvement.

VI. Practical Considerations: From Mechanistic Feasibility to Reproducible Results

6.1 Principles for Matching Materials–Strains–Vectors

(1) Prioritize mature systems

Select plant materials, explant types, regeneration systems, and Agrobacterium strain combinations that have been validated in the literature or by established protocols.

(2) Ensure a clear selection system

Avoid conflicts between the strain’s intrinsic resistance and the vector’s bacterial selectable markers, ensuring that selection can distinguish “vector-bearing” from “non-vector” cells.

(3) Vector design should support verification

It is recommended to configure a reporter gene or a detectable tag for rapid assessment during infection and expression.

6.2 Control of Key Windows: Infection, Co-cultivation, Selection, and Regeneration

(1) Infection and co-cultivation

Control bacterial density, infection time, and co-cultivation duration to balance T-DNA transfer efficiency and the risk of browning/necrosis caused by bacterial overgrowth.

(2) Selection and bacterial suppression

If selection pressure is too low, escapes are likely; if too high, regeneration may be inhibited. An appropriate window should be established between suppressing untransformed tissues and retaining regeneration capacity.

(3) Regeneration and identification

Verify at the DNA level and expression level, screen multiple independent events, and prioritize low-copy, stably expressing materials.

6.3 Quality Control and Traceability Management

(1) Multiple events in parallel

For the same construct, it is recommended to obtain multiple independent transformation events to offset position effects and copy-number differences.

(2) Process records

Standardize and record parameters such as strain batch, infection conditions, co-cultivation time, selection concentration, and medium formulation to improve reproducibility and auditability.

(3) Tiered verification

A tiered verification pathway of “presence–copy number–expression–phenotype” is recommended to avoid drawing conclusions based on a single metric.

VII. Aladdin-Related Products

7.1 Common Agrobacterium Glycerol Stocks for Plant Genetic Transformation

Catalog No.	Product Name	Grade and Purity	Application Scenarios
A743858	Ar.A4 agrobacterium rhizobiaceae glycerol stock for construction of transgenic plant	BioReagent bacteriological grade	Agrobacterium rhizogenes mediated transformation Hairy root induction and root related functional studies
C744347	C58C1 agrobacterium rhizobiaceae glycerol stock for construction of transgenic plant	BioReagent bacteriological grade	Agrobacterium rhizogenes based transformation For hairy root systems and construct validation
E745547	EHA101 agrobacterium tumefaciens glycerol stock for construction of transgenic plant	BioReagent bacteriological grade	Stable transformation via Agrobacterium tumefaciens Commonly used for binary vector systems
G746577	GV3101 agrobacterium tumefaciens glycerol stock for construction of transgenic plant	BioReagent bacteriological grade	Common Agrobacterium strain Suitable for transformation systems such as Arabidopsis
K747256	K599 agrobacterium rhizobiaceae glycerol stock for construction of transgenic plant	BioReagent bacteriological grade	Hairy root induction and root transformation Widely used in root related studies and metabolic engineering
L747592	LBA4404 agrobacterium tumefaciens glycerol stock for construction of transgenic plant	BioReagent bacteriological grade	Stable transformation via Agrobacterium tumefaciens Widely used across dicot species and vector systems

7.2 Common Reagents and Assay Tools for Plant Transformation

Product Category	Product Name	CAS No.	Intended Use
Vir Inducer	Acetosyringone	2478-38-8	Induce vir gene expression and improve T-DNA transfer efficiency
Agrobacterium Suppression	Cefotaxime sodium	64485-93-4	Suppress Agrobacterium after co-cultivation and protect explant regeneration
Agrobacterium Suppression	Carbenicillin disodium salt	4800-94-6	Post co-cultivation bacterial suppression to reduce overgrowth
Bacterial Selection Antibiotic	Rifampicin	13292-46-1	Selection for Agrobacterium strains and maintenance of strain background resistance
Bacterial Selection Antibiotic	Gentamicin sulfate	1405-41-0	Selection for Agrobacterium or binary vectors commonly used in binary systems
Bacterial Selection Antibiotic	Spectinomycin	22189-32-8	Vector selection commonly used for plant transformation plasmid markers
Bacterial Selection Antibiotic	Streptomycin sulfate	3810-74-0	Bacterial selection and contamination control commonly used antibiotic
Plant Selection Agent	Kanamycin sulfate	25389-94-0	Plant selection when nptII is used as the selectable marker
Plant Selection Agent	Hygromycin B	31282-04-9	Plant selection when hpt is used as the selectable marker
Plant Selection Agent	Glufosinate ammonium	77182-82-2	Plant selection when bar or pat is used as the selectable marker
Tissue Culture Hormone	Indole-3-acetic acid	87-51-4	Rooting and growth regulation commonly used during regeneration and acclimation
Tissue Culture Hormone	1-Naphthaleneacetic acid	86-87-3	Root induction and root architecture modulation in rooting media
Tissue Culture Hormone	Indole-3-butyric acid	133-32-4	Rooting promoter often milder and more stable for some plant materials
Tissue Culture Hormone	2,4-Dichlorophenoxyacetic acid	94-75-7	Callus induction and pre-differentiation conditioning in callus-based systems
Tissue Culture Hormone	6-Benzylaminopurine	1214-39-7	Shoot induction and proliferation during regeneration
Tissue Culture Hormone	Kinetin	525-79-1	Cytokinin used as an alternative or in combination with 6-BA for shoot formation
Reporter Substrate	X-Gluc	114162-64-0	GUS staining for rapid assessment of infection and transgene expression
Reporter Substrate	D-Luciferin	2591-17-5	Luciferase reporter detection for live imaging and dynamic expression analysis

Agrobacterium-mediated plant transformation technology is an engineered utilization of the T-DNA transfer mechanism in nature: plant wound signals induce activation of the Vir system; Vir proteins complete processing at the T-DNA border sequences and drive cross-kingdom transport, enabling T-DNA to enter plant cells and integrate into the genome. Through disarming modification and modular vector design (co-integrative system or binary system), this natural process is converted into a stable and controllable gene-delivery platform. For practical applications, establishing standardized workflows around vector construction, strain selection, explants and regeneration systems, selection and identification, and applying parallel screening of multiple events and tiered quality control, are key to obtaining transformation materials with stable inheritance and reliable expression and to supporting high-quality research conclusions.

For more related articles, please see below:

[1] Agrobacterium reforming process, the key technology of plant genetic engineering

[2] Experimental methods of Agrobacterium-mediated transformation of barley

[3] Experiments on efficient Agrobacterium transformation methods in wheat mediated by in situ plant inoculation

[4] Phytohormones commonly used in tissue culture and their functions

[5] Aladdin® Plant Research Related Products

[6] Plant polyploid artificial induction experiment

[7] Wheat Flower Organ Transformation Experiment

Aladdin: https://www.aladdinsci.com/