Over the past 100 years, the importance of Agrobacterium in plant biotechnology has become evident. Since the discovery of the bacterium causing crown gall disease, initially named Bacterium tumefaciens [1], it was subsequently named Agrobacterium tumefaciens [2]. Although some Agrobacterium species are non-pathogenic, the majority can induce tumors in various plants [3]. Initially, it was found that crown gall formation is associated with a large plasmid [4], and it has been observed that Agrobacterium can transfer genetic material to a wide range of plants, including most dicots and some monocots [4]. By this time, scientists had discovered the ability to replace oncogenes in T-DNA with genes of interest, marking the beginning of a new era in plant biotechnology with the tremendous potential for transferring genetic materials from bacteria to plants [5].

The previous study has found that Agrobacterium requires plant wounding or stomatal openings for invasion [6]. It has been further elucidated that plant wounding triggers the secretion of specific chemical compounds, including phenolic compounds, sugars, and certain amino acids, which are known to enhance Agrobacterium's infection and pathogenic capabilities [7]. These compounds serve as signals that Agrobacterium detects, activating its virulence genes [8]. However, it is also important to acknowledge that not all responses to wounding facilitate infection; these plants secrete defensive compounds that can inhibit bacterial growth or infection.

The initiation of this mechanism involves a receptor protein VirA, which, upon detecting phenolic signals, activates VirG, a response regulator, by a phospho-relay system. The activated, phosphorylated VirG then operates as a transcription factor, binding to promoters of other Vir genes [9]. The Chromosomal Virulence (Chv) E protein, a periplasmic receptor, also facilitates sugar and acidic signal mediation with VirA [10, 11]. The ChvG/ChvI two-component system further plays a crucial role in activating additional Vir genes [12, 13], enhancing the Agrobacterium's ability to interact with and transform plant cells. For natural transformation to occur, virulent Agrobacterium strains utilize its Ti-plasmid, crucial for integrating genetic material into the plant genome, to induce plant transformation. This process involves transferring the Ti-plasmid's T-DNA into the plant chromosome, facilitated by numerous Vir proteins [14]. The large Ti-plasmid was engineered into a binary system, comprising a disarmed helper plasmid and a binary vector with a T-DNA [15], facilitating gene cloning and plant biotechnology applications. However, the efficiency of Agrobacterium-mediated transformation varies, with many monocots and some dicots showing resistance [16, 17].

Interestingly, from the view of host plants, bioprocesses of Agrobacterium–plant interactions are an invasion. In amplified fragment length polymorphism (AFLP)-based transcript profiling, plant defense-related genes were identified in Agrobacterium infection [18]. Besides, Agrobacterium-infected plants have been reported to mount the plant innate immune response, such as MAPK activation, pattern-triggered immunity, phytohormone modulation, and reactive oxygen species (ROS) production [19–23]. Thus, the plants can launch a variety of immune responses and limit pathogen infection in Agrobacterium–plant interactions [24]. In nature, phytohormones are essential for the plant development process and pivotal in immune response [25]. Among them, salicylic acid (SA), indole-3-acetic acid (IAA), and ethylene in Arabidopsis thaliana were strongly accumulated after Agrobacterium infection. [26]. Notably, IAA acts as a competitor for inducer of VirA/VirG two-component system [27], and stimulates the synthesis of high amounts of ethylene [28–30]. Ethylene also influences Agrobacterium, and its interaction with plants induces tumor development through vascularization and epidermal disruption [31]. Moreover, previous studies have shown that ethylene is involved in the inhibition of Agrobacterium-mediated transformation in tomato, melon, and bottle gourd [32–35]. In addition to the universal phytohormones, plants contain another negative factor, such as γ-aminobutyric acid (GABA), 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA), and 2-hydroxy-4,7-dimethoxybenzoxazin-3-one (MDIBOA). Despite the presence of virulence signals, DIMBOA and MDIBOA shut down Vir gene expression and limit Agrobacterium growth in maize seedlings [36, 37]. Besides, GABA, non-protein amino acids, accumulates rapidly in response to biotic and abiotic stresses including Agrobacterium invasion and wounding, and plays a role in defense response [38–40]. In Agrobacterium-plant interaction, GABA also activates the attKLM operon, thereby degradation of the quorum-sensing (QS) signal [41]. Because high levels of QS signals are known to trigger a defense response in eukaryotic hosts, it was proposed that suppressing QS during the early stages of infection advantages Agrobacterium virulence [42, 43]. Taken together, Agrobacterium virulence is tightly modulated by phytohormone and plant-derived chemicals.

Agrobacterium's properties for improving transformation are routinely exploited to engineer plants in the scope of fundamental genetic research. With a stepwise strategy to reach this aim, exogenous supplies of Vir gene inducer, such as acetosyringone (AS), sugar, and acidic condition, and suppressor of negative effectors, such as aminoethoxyvinylglycine (AVG), AgNO3, and silver thiosulfate (STS) are used for enhancing the efficiency of Agrobacterium-mediated plant transformation. From another viewpoint, Agrobacterium improvement might also show possibilities for achieving improved transformation efficiency [8, 32, 44]. One of the best ways to enhance transformation efficiencies is to use strains that overexpress specific Vir genes. Indeed, constitutive virG mutants, such as virG^I77V, virG^N54D, and virG^I106L, dramatically improve transformation efficiencies in a variety of plants [45, 46]. Moreover, an increase of VirG wild-type copies increased transient callus transformation of celery (Apium graveolens) and rice [47]. VirA constitutive variants also achieve a high level of Vir gene expression without AS [48]. Although various elements to increase the expression of virulence genes have been studied, issues such as vector complexity and size have made their application difficult. In 2000, the introduction of virG^N54D into a third plasmid overcame these challenges and demonstrated the potential to upgrade Agrobacterium [49]. Recent advancements in deploying ternary vectors enriched with additional Virulence (Vir) genes not only heightened the efficiency of genetic transformation in maize but also significantly boosted the effectiveness of gene editing mediated by CRISPR/Cas9 systems [50–52]. Alternatively, Agrobacterium strains can be engineered to overcome the negative impact of plant-derived molecules. For instance, several versions of 'Super-Agrobacterium have been developed by introducing the GABA transaminase (GabT) or ACC deaminase (AcdS) gene in a ternary vector, so that it could break down GABA or ethylene or both on its own. These engineered strains showed the enhanced ability of T-DNA transfer in Cucumis melo, Erianthus ravennae, Solanum lycopersicum, and Solanum torvum [53–55].

As one of the phytohormones in plant defense, SA plays a regulatory role in response to pathogenic invasion [56–59]. Specifically, SA promotes the expression of attKLM operon, thereby activating the Agrobacterium quorum-quenching system. Furthermore, SA is directly implicated in the suppression of Vir genes through VirA/VirG two-component system [60, 61]. Indeed, SA was accumulated at the infected area in tobacco, and these amounts of SA affected negatively Vir regulon induction [61, 62]. Pseudomonas putida NahG gene was firstly introduced into Arabidopsis and tobacco plants so that SA-degrading NahG plants were shown to enhance susceptibility to diverse bacterial pathogens [63, 64]. As a salicylate hydroxylase, we hypothesize that NahG activity in Agrobacterium infection might play a role in the degradation of plant-secreted SA to enhance susceptibility to Agrobacterium. In this study, we postulated that the Tv system, equipped with an SA modulator, could be utilized to enhance an improved Agrobacterium platform for genetic transformation tailored to the specific characteristics of plants. Consequently, we developed versatile Tv systems to facilitate Agrobacterium-mediated plant transformation. To demonstrate the Tv system as a powerful technology, we applied it using several reporters, a geminiviral replicon system, and even a CRISPR/Cas9 system in tomato, N. benthamiana, A. thaliana, and C. sativa.