Tissue culture and Agrobacterium-mediated genetic transformation of the oil crop sunflower

Sunflower is one of the four major oil crops in the world. 'Zaoaidatou' (ZADT), the main variety of oil sunflower in the northwest of China, has a short growth cycle, high yield, and high resistance to abiotic stress. However, the ability to tolerate adervesity is limited. Therefore, in this study, we used the retention line of backbone parent ZADT as material to establish its tissue culture and genetic transformation system for new variety cultivating to enhance resistance and yields by molecular breeding. The combination of 0.05 mg/L IAA and 2 mg/L KT in MS was more suitable for direct induction of adventitious buds with cotyledon nodes and the addition of 0.9 mg/L IBA to MS was for adventitious rooting. On this basis, an efficient Agrobacterium tumefaciens-mediated genetic transformation system for ZADT was developed by the screening of kanamycin and optimization of transformation conditions. The rate of positive seedlings reached 8.0%, as determined by polymerase chain reaction (PCR), under the condition of 45 mg/L kanamycin, bacterial density of OD600 0.8, infection time of 30 min, and co-cultivation of three days. These efficient regeneration and genetic transformation platforms are very useful for accelerating the molecular breeding process on sunflower.

Molecular responses of agroinfiltrated Nicotiana benthamiana leaves expressing suppressor of silencing P19 and influenza virus-like particles

The production of influenza vaccines in plants is achieved through transient expression of viral hemagglutinins (HAs), a process mediated by the bacterial vector Agrobacterium tumefaciens. HA proteins are then produced and matured through the secretory pathway of plant cells, before being trafficked to the plasma membrane where they induce formation of virus-like particles (VLPs). Production of VLPs unavoidably impacts plant cells, as do viral suppressors of RNA silencing (VSRs) that are co-expressed to increase recombinant protein yields. However, little information is available on host molecular responses to foreign protein expression. This work provides a comprehensive overview of molecular changes occurring in Nicotiana benthamiana leaf cells transiently expressing the VSR P19, or co-expressing P19 and an influenza HA. Our data identifies general responses to Agrobacterium-mediated expression of foreign proteins, including shutdown of chloroplast gene expression, activation of oxidative stress responses and reinforcement of the plant cell wall through lignification. Our results also indicate that P19 expression promotes salicylic acid (SA) signalling, a process dampened by co-expression of the HA protein. While reducing P19 level, HA expression also induces specific signatures, with effects on lipid metabolism, lipid distribution within membranes and oxylipin-related signalling. When producing VLPs, dampening of P19 responses thus likely results from lower expression of the VSR, crosstalk between SA and oxylipin pathways, or a combination of both outcomes. Consistent with the upregulation of oxidative stress responses, we finally show that reduction of oxidative stress damage through exogenous application of ascorbic acid improves plant biomass quality during production of VLPs.

Setting up Agrobacterium tumefaciens-mediated transformation of the tropical legume Aeschynomene evenia, a powerful tool for studying gene function in Nod Factor-independent symbiosis

Most legumes are able to develop a root nodule symbiosis in association with proteobacteria collectively called rhizobia. Among them, the tropical species Aeschynomene evenia has the remarkable property of being nodulated by photosynthetic Rhizobia without the intervention of Nod Factors (NodF). Thereby, A. evenia has emerged as a working model for investigating the NodF-independent symbiosis. Despite the availability of numerous resources and tools to study the molecular basis of this atypical symbiosis, the lack of a transformation system based on Agrobacterium tumefaciens significantly limits the range of functional approaches. In this report, we present the development of a stable genetic transformation procedure for A. evenia. We first assessed its regeneration capability and found that a combination of two growth regulators, NAA (= Naphthalene Acetic Acid) and BAP (= 6-BenzylAminoPurine) allows the induction of budding calli from epicotyls, hypocotyls and cotyledons with a high efficiency in media containing 0,5 μM NAA (up to 100% of calli with continuous stem proliferation). To optimize the generation of transgenic lines, we employed A. tumefaciens strain EHA105 harboring a binary vector carrying the hygromycin resistance gene and the mCherry fluorescent marker. Epicotyls and hypocotyls were used as the starting material for this process. We have found that one growth medium containing a combination of NAA (0,5 μM) and BAP (2,2 μM) was sufficient to induce callogenesis and A. tumefaciens strain EHA105 was sufficiently virulent to yield a high number of transformed calli. This simple and efficient method constitutes a valuable tool that will greatly facilitate the functional studies in NodF-independent symbiosis.

In vitro synthesis of oryzamutaic acid H using recombinant L-lysine ε-dehydrogenase from Agrobacterium tumefaciens

Oryzamutaic acids, possessing a nitrogen-containing heterocyclic skeleton, have been isolated and identified from a rice mutant. Although oryzamutaic acids are expected to be functional ingredients, their functionality is difficult to evaluate, because of their wide variety and presence in trace amounts. Furthermore, how oryzamutaic acid is synthesized in vivo is unclear. Therefore, we developed a simple enzymatic synthesis method for these compounds in vitro. We focused on L-lysine ε-dehydrogenase (LysDH) from Agrobacterium tumefaciens, which synthesizes α-aminoadipate-δ-semialdehyde-a precursor of oryzamutaic acids. LysDH was cloned and expressed in Escherichia coli. Analysis of activity revealed that LysDH catalyzed the synthesis of oryzamutaic acid H at neutral pH in vitro. We synthesized 1.6 mg oryzamutaic acid H from 100 mg L-lysine. The synthesized oryzamutaic acid H exhibited UVA absorption, stability of temperature, and stability at a wide pH range. To our knowledge, this study is the first to report the enzymatic synthesis of oryzamutaic acid H in vitro and provides a basis for understanding the mechanisms of oryzamutaic acid synthesis in vivo.

A simple and efficient in planta transformation method based on the active regeneration capacity of plants

Plant genetic transformation strategies serve as essential tools for the genetic engineering and advanced molecular breeding of plants. However, the complicated operational protocols and low efficiency of current transformation strategies restrict the genetic modification of most plant species. This paper describes the development of the regenerative activity-dependent in planta injection delivery (RAPID) method based on the active regeneration capacity of plants. In this method, Agrobacterium tumefaciens is delivered to plant meristems via injection to induce transfected nascent tissues. Stable transgenic plants can be obtained by subsequent vegetative propagation of the positive nascent tissues. The method was successfully used for transformation of plants with strong regeneration capacity, including different genotypes of sweet potato (Ipomoea batatas), potato (Solanum tuberosum), and bayhops (Ipomoea pes-caprae). Compared with traditional transformation methods, RAPID has a much higher transformation efficiency and shorter duration, and it does not require tissue culture procedures. The RAPID method therefore overcomes the limitations of traditional methods to enable rapid in planta transformation and can be potentially applied to a wide range of plant species that are capable of active regeneration.

In planta transformation in wheat: an improved protocol to develop wheat transformants

BACKGROUND

Lack of efficient transformation protocol continues to be a major bottleneck for successful genome editing or transgenic development in wheat. An in planta transformation method was developed in Indian bread wheat in earlier study (Vasil et al. in Nat Biotechnol 10:667-674, 1992) which was labour-intensive and time-consuming. In the present study, in planta transformation method was improved to make it simple, efficient, less labour-intensive and time-saving.

METHODS AND RESULTS

PCR-based screening for generated transformants at T0 stage was introduced in this method. Shoot apical meristem of two days old wheat seedling was inoculated with the routine active culture of Agrobacterium tumefaciens harboring plasmid pCAMBIA1300-Ubi-GFP having gene GFP under the control of Zea mays ubiquitin promoter. PCR analysis at T0 stage confirmed 27 plants to be transgene positive. These 27 plants were only taken to the next generation (T1) and the rest were discarded. At T1 generation 6 plants were analyzed to be PCR positive. Out of them, 4 plants were confirmed to have stable integration of transgene (GFP). Fluorescent microscopy at T1 stage confirmed the 4 Southern hybridization positive plants to be expressing reporter gene GFP.

CONCLUSIONS

Screening at T0 stage, reduced the load of plants to be taken to T1 generation and their screening thereof at T1 with no overall loss in transformation efficiency. We successfully transformed wheat genotype HD2894 with 3.33% transformation efficiency using a simple, effective method which was less labour-intensive and less time-consuming. This method may be utilized to develop wheat transgenic as well as genome edited lines for desirable traits.

Establishment of an efficient callus transient transformation system for Vitis vinifera cv. 'Chardonnay'

Grape (Vitis L.), a highly valued fruit crop, poses significant challenges in genetic transformation and functional characterization of genes. Therefore, there is an urgent need for the development of a rapid and effective method for grape transformation and gene function identification. Here, we introduce a streamlined Agrobacterium-mediated transient transformation system for grape calli. Optimal conditions were established with a leaf-derived callus induction medium; chiefly B5 medium supplemented with 0.05 mg/L NAA, 0.5 mg/L 2,4-D, and 2.0 mg/L KT; and a callus proliferation medium (B5 medium supplemented with 0.5 mg/L NAA and 2.0 mg/L 6-BA), respectively. Notably, GUS enzyme activity peaked (352.96 ± 33.95 mol 4-MU/mg/min) by sonication with Agrobacterium tumefaciens EHA105 and 100 μM AS for 4 min, followed by vacuum infection for 5 min, and co-culture at 25 °C in the dark for 1 day using callus as explants at an optical density (OD600) of 0.8. VaCIPK18 gene was transiently transformed into calli, and transcripts of the gene (endogenous and exogenous) were detected at higher levels than in non-transformed calli (endogenous). Moreover, after 10 days of treatment at 4 °C or -4 °C, the callus net weight of transformed callus was significantly higher than that of the untransformed callus, indicating that the VaCIPK18-overexpressing grape callus could improve cold tolerance. Overall, we establish a simple but effective transient transformation approach for grape callus, which could serve as a useful tool for the rapid assessment of gene function in this important crop.

Antivirulence effects of cell-free culture supernatant of endophytic bacteria against grapevine crown gall agent, Agrobacterium tumefaciens, and induction of defense responses in plantlets via intact bacterial cells

BACKGROUND

Crown gall disease caused by Agrobacterium tumefaciens is a very destructive affliction that affects grapevines. Endophytic bacteria have been discovered to control plant diseases via the use of several mechanisms. This research examined the potential for controlling crown gall by three endophytic bacteria that were previously isolated from healthy cultivated and wild grapevines including Pseudomonas kilonensis Ba35, Pseudomonas chlororaphis Ba47, and Serratia liquefaciens Ou55.

RESULT

At various degrees, three endophytic bacteria suppressed the populations of A. tumefaciens Gh1 and greatly decreased the symptoms of crown gall. Furthermore, biofilm production and motility behaviors of A. tumefaciens Gh1were greatly inhibited by the Cell-free Culture Supernatant (CFCS) of endophytic bacteria. According to our findings, CFCS may reduce the adhesion of A. tumefaciens Gh1 cells to grapevine cv. Rashe root tissues as well as their chemotaxis motility toward the extract of the roots. When compared to the untreated control, statistical analysis showed that CFCS significantly reduced the swimming, twitching, and swarming motility of A. tumefaciens Gh1. The findings demonstrated that the endophytic bacteria effectively stimulated the production of plant defensive enzymes including superoxide dismutase (SOD), polyphenol oxidase (PPO), peroxidase (POD), phenylalanine ammonia lyase (PAL), and total soluble phenols at different time intervals in grapevine inoculated with A. tumefaciens Gh1. The Ba47 strain markedly increased the expression levels of defense genes associated with plant resistance. The up-regulation of PR1, PR2, VvACO1, and GAD1 genes in grapevine leaves indicates the activation of SA and JA pathways, which play a role in enhancing resistance to pathogen invasion. The results showed that treating grapevine with Ba47 increased antioxidant defense activities and defense-related gene expression, which reduced oxidative damage caused by A. tumefaciens and decreased the incidence of crown gall disease.

CONCLUSION

This is the first study on how A. tumefaciens, the grapevine crown gall agent, is affected by CFCS generated by endophytic bacteria in terms of growth and virulence features. To create safer plant disease management techniques, knowledge of the biocontrol processes mediated by CFCS during microbial interactions is crucial.

An aerotaxis receptor influences invasion of Agrobacterium tumefaciens into its host

Agrobacterium tumefaciens is a soil-borne pathogenic bacterium that causes crown gall disease in many plants. Chemotaxis offers A. tumefaciens the ability to find its host and establish infection. Being an aerobic bacterium, A. tumefaciens possesses one chemotaxis system with multiple potential chemoreceptors. Chemoreceptors play an important role in perceiving and responding to environmental signals. However, the studies of chemoreceptors in A. tumefaciens remain relatively restricted. Here, we characterized a cytoplasmic chemoreceptor of A. tumefaciens C58 that contains an N-terminal globin domain. The chemoreceptor was designated as Atu1027. The deletion of Atu1027 not only eliminated the aerotactic response of A. tumefaciens to atmospheric air but also resulted in a weakened chemotactic response to multiple carbon sources. Subsequent site-directed mutagenesis and phenotypic analysis showed that the conserved residue His100 in Atu1027 is essential for the globin domain's function in both chemotaxis and aerotaxis. Furthermore, deleting Atu1027 impaired the biofilm formation and pathogenicity of A. tumefaciens. Collectively, our findings demonstrated that Atu1027 functions as an aerotaxis receptor that affects agrobacterial chemotaxis and the invasion of A. tumefaciens into its host.

A peptidoglycan N-deacetylase specific for anhydroMurNAc chain termini in Agrobacterium tumefaciens

During growth, bacteria remodel and recycle their peptidoglycan (PG). A key family of PG-degrading enzymes is the lytic transglycosylases, which produce anhydromuropeptides, a modification that caps the PG chains and contributes to bacterial virulence. Previously, it was reported that the polar-growing Gram-negative plant pathogen Agrobacterium tumefaciens lacks anhydromuropeptides. Here, we report the identification of an enzyme, MdaA (MurNAc deacetylase A), which specifically removes the acetyl group from anhydromuropeptide chain termini in A. tumefaciens, resolving this apparent anomaly. A. tumefaciens lacking MdaA accumulates canonical anhydromuropeptides, whereas MdaA was able to deacetylate anhydro-N-acetyl muramic acid in purified sacculi that lack this modification. As for other PG deacetylases, MdaA belongs to the CE4 family of carbohydrate esterases but harbors an unusual Cys residue in its active site. MdaA is conserved in other polar-growing bacteria, suggesting a possible link between PG chain terminus deacetylation and polar growth.

Advances in the Transformation of Cyclamen persicum Mill. Through Direct Regeneration Based on an Optimized Kanamycin Selection Scheme

Gene transfer technology has great value in ornamental plants toward the generation of varieties with new ornate characteristics. In the previous studies through the transformation of cyclamen, hygromycin was mainly used as a selective marker. However, there have been some drawbacks associated with hygromycin usage as a selecting agent. Therefore, in the current study, the optimization of kanamycin concentration in the regeneration media has been considered. Subsequently, the plant transformation using three different in vitro explants from three Cyclamen persicum cultivars using three Agrobacterium tumefaciens strains has been examined. Accordingly, the optimal kanamycin concentrations for regeneration from root and leaf explants were determined as 10 mg/L and for microtuber explants as 30 mg/L. The successful gene transformation in the antibiotic-resistant shoots were examined by PCR and UV-equipped microscopes. The gfp reporter gene transfer resulted in the highest efficiency of transformation (60%) to date, from the leaf explants of cv. Pure White inoculated with Agrobacterium tumefaciens strain LBA4404. In contrast, the lowest gene transfer efficiency (25%) was observed in root explants of cv. Dark Violet and cv. Neon Pink inoculated with strains GV3101 and AGL-1, respectively. The results of the current project are expandable to the subsequent investigations of Cyclamen persicum transformation.

An extensive disulfide bond network prevents tail contraction in Agrobacterium tumefaciens phage Milano

A contractile sheath and rigid tube assembly is a widespread apparatus used by bacteriophages, tailocins, and the bacterial type VI secretion system to penetrate cell membranes. In this mechanism, contraction of an external sheath powers the motion of an inner tube through the membrane. The structure, energetics, and mechanism of the machinery imply rigidity and straightness. The contractile tail of Agrobacterium tumefaciens bacteriophage Milano is flexible and bent to varying degrees, which sets it apart from other contractile tail-like systems. Here, we report structures of the Milano tail including the sheath-tube complex, baseplate, and putative receptor-binding proteins. The flexible-to-rigid transformation of the Milano tail upon contraction can be explained by unique electrostatic properties of the tail tube and sheath. All components of the Milano tail, including sheath subunits, are crosslinked by disulfides, some of which must be reduced for contraction to occur. The putative receptor-binding complex of Milano contains a tailspike, a tail fiber, and at least two small proteins that form a garland around the distal ends of the tailspikes and tail fibers. Despite being flagellotropic, Milano lacks thread-like tail filaments that can wrap around the flagellum, and is thus likely to employ a different binding mechanism.

A highly conserved ligand-binding site for AccA transporters of antibiotic and quorum-sensing regulator in Agrobacterium leads to a different specificity

Plants genetically modified by the pathogenic Agrobacterium strain C58 synthesize agrocinopines A and B, whereas those modified by the pathogenic strain Bo542 produce agrocinopines C and D. The four agrocinopines (A, B, C and D) serve as nutrients by agrobacteria and signaling molecule for the dissemination of virulence genes. They share the uncommon pyranose-2-phosphate motif, represented by the l-arabinopyranose moiety in agrocinopines A/B and the d-glucopyranose moiety in agrocinopines C/D, also found in the antibiotic agrocin 84. They are imported into agrobacterial cytoplasm via the Acc transport system, including the solute-binding protein AccA coupled to an ABC transporter. We have previously shown that unexpectedly, AccA from strain C58 (AccAC58) recognizes the pyranose-2-phosphate motif present in all four agrocinopines and agrocin 84, meaning that strain C58 is able to import agrocinopines C/D, originating from the competitor strain Bo542. Here, using agrocinopine derivatives and combining crystallography, affinity and stability measurements, modeling, molecular dynamics, in vitro and vivo assays, we show that AccABo542 and AccAC58 behave differently despite 75% sequence identity and a nearly identical ligand binding site. Indeed, strain Bo542 imports only compounds containing the d-glucopyranose-2-phosphate moiety, and with a lower affinity compared with strain C58. This difference in import efficiency makes C58 more competitive than Bo542 in culture media. We can now explain why Agrobacterium/Allorhizobium vitis strain S4 is insensitive to agrocin 84, although its genome contains a conserved Acc transport system. Overall, our work highlights AccA proteins as a case study, for which stability and dynamics drive specificity.

Agrobacterium-mediated transfer of the Fusarium graminearum Tri6 gene into barley using mature seed-derived shoot tips as explants

KEY MESSAGE

We transferred the Tri6 gene into the elite barley GemCraft via new transformation method through shoot organogenesis and identified the rearrangements of transgenes and phenotypic variations in the transgenic plants. Despite its agronomic and economic importance, barley transformation is still very challenging for many elite varieties. In this study, we used direct shoot organogenesis to transform the elite barley cultivar GemCraft with the RNAi constructs containing Tri6 gene of Fusarium graminearum, which causes fusarium head blight (FHB). We isolated 4432 shoot tips and co-cultured these explants with Agrobacterium tumefaciens. A total of 25 independent T0 transgenic plants were generated including 15 events for which transgene-specific PCR amplicons were observed. To further determine the presence of transgenes, the T1 progenies of all 15 T0 plants were analyzed, and the expected PCR products were obtained in 10 T1 lines. Droplet digital (dd) PCR analysis revealed various copy numbers of transgenes in the transgenic plants. We determined the insertion site of transgenes using long-read sequencing data and observed the rearrangements of transgenes. We found phenotypic variations in both T1 and T2 generation plants. FHB disease was evaluated under growth chamber conditions, but no significant differences in disease severity or deoxynivalenol accumulation were observed between two Tri6 transgenic lines and the wildtype. Our results demonstrate the feasibility of the shoot tip transformation and may open the door for applying this system for genetic improvement and gene function research in other barley genotypes.

CRISPR/Cas9 system is a suitable gene targeting editing tool to filamentous fungus Monascus pilosus

Monascus pilosus has been used to produce lipid-lowering drugs rich in monacolin K (MK) for a long period. Genome mining reveals there are still many potential genes worth to be explored in this fungus. Thereby, efficient genetic manipulation tools will greatly accelerate this progress. In this study, we firstly developed the protocol to prepare protoplasts for recipient of CRISPR/Cas9 system. Subsequently, the vector and donor DNA were co-transformed into recipients (106 protoplasts/mL) to produce 60-80 transformants for one test. Three genes (mpclr4, mpdot1, and mplig4) related to DNA damage response (DDR) were selected to compare the gene replacement frequencies (GRFs) of Agrobacterium tumefaciens-mediated transformation (ATMT) and CRISPR/Cas9 gene editing system (CGES) in M. pilosus MS-1. The results revealed that GRF of CGES was approximately five times greater than that of ATMT, suggesting that CGES was superior to ATMT as a targeting gene editing tool in M. pilosus MS-1. The inactivation of mpclr4 promoted DDR via the non-homologous end-joining (NHEJ) and increased the tolerances to DNA damaging agents. The inactivation of mpdot1 blocked DDR and led to the reduced tolerances to DNA damaging agents. The inactivation of mplig4 mainly blocked the NHEJ pathway and led to obviously reduced tolerances to DNA damaging agents. The submerged fermentation showed that the ability to produce MK in strain Δmpclr4 was improved by 52.6% compared to the wild type. This study provides an idea for more effective exploration of gene functions in Monascus strains. KEY POINTS: • A protocol of high-quality protoplasts for CGES has been developed in M. pilosus. • The GRF of CGES was about five times that of ATMT in M. pilosus. • The yield of MK for Δmpclr4 was enhanced by 52.6% compared with the wild type.

Agroinoculation of Geminiviral Infectious Clones into Plants

Most geminiviruses are not transmitted by mechanical inoculation. Therefore, pathogenicity and plant-pathogen interaction studies rely on agroinoculation using infectious clones, which involves cloning the geminiviral genome in a binary vector (see previous chapter for details). A suspension containing the infectious clone inserted into Agrobacterium tumefaciens cells is then inoculated into plants, i.e., agroinoculated. Below is a simple protocol for agroinoculation of an infectious geminivirus clone into plants.

Coffee Cell Suspensions as a Platform for Transient Gene Expression Analysis

Coffea arabica L. is a crucial crop globally, but its genetic homogeneity leads to its susceptibility to diseases and pests like the coffee berry borer (CBB). Chemical and cultural control methods are difficult due to the majority of the CBB life cycle taking place inside coffee beans. One potential solution is the use of the gene cyt1Aa from Bacillus thuringiensis as a biological insecticide. To validate candidate genes against CBB, a simple, rapid, and efficient transient expression system is necessary. This study uses cell suspensions as a platform for expressing the cyt1Aa gene in the coffee genome (C. arabica L. var. Catuaí) to control CBB. The Agrobacterium tumefaciens strain GV3101::pMP90 containing the bar and cyt1Aa genes are used to genetically transform embryogenic cell suspensions. PCR amplification of the cyt1Aa gene is observed 2, 5, and 7 weeks after infection. This chapter describes a protocol that can be used for the development of resistant varieties against biotic and abiotic stresses and CRISPR/Cas9-mediated genome editing.

CRISPR/Cas9-Mediated Genome Editing in Indica Rice (Oryza sativa L. subsp. indica var. CR-5272)

Tissue culture optimization protocols limit indica rice breeding. Such a challenge is vital because emergent techniques still rely on tissue culture methods and could allow the breeding of new varieties with higher production and toleration of adverse environmental effects caused by climate change. Genome editing technology, using CRISPR/Cas9, is a fast and precise method for accelerated plant breeding. It limited its use in indica subspecies because of the recalcitrant response to in vitro culture methods. This chapter describes a protocol for CRISPR/Cas9 editing in indica subspecies, specifically in the CR-5272 variety derived from parental lines IR-822, using Agrobacterium tumefaciens and biolistic transformation.

Genetic Transformation of Cryptococcus Species with Agrobacterium Transfer DNA

Transformation of foreign DNA into Cryptococcus species is a powerful tool for exploring gene functions in these human pathogens. Agrobacterium tumefaciens-mediated transformation (AtMT) has been used for the stable introduction of exogenous DNA into Cryptococcus for over two decades, being particularly impactful for insertional mutagenesis screens to discover new genes involved in fungal biology. A detailed protocol to conduct this transformation method is provided in the chapter. Scope for modifications and the benefits and disadvantages of using AtMT in Cryptococcus species are also presented.

Transmission and Management of Pathogenic Agrobacterium tumefaciens and Rhodococcus fascians in Select Ornamentals

Pathogenic Agrobacterium tumefaciens and Rhodococcus fascians are phytobacteria that induce crown gall and leafy gall disease, respectively, resulting in undesirable growth abnormalities. When present in nurseries, plants infected by either bacterium are destroyed, resulting in substantial losses for growers, especially those producing plants valued for their ornamental attributes. There are many unanswered questions regarding pathogen transmission on tools used to take cuttings for propagation and whether products used for bacterial disease control are effective. We investigated the ability to transmit pathogenic A. tumefaciens and R. fascians on secateurs and the efficacy of registered control products against both bacteria in vitro and in vivo. Experimental plants used were Rosa × hybrida, Leucanthemum × superbum, and Chrysanthemum × grandiflorum for A. tumefaciens and Petunia × hybrida and Oenothera 'Siskiyou' with R. fascians. In separate experiments, we found secateurs could convey both bacteria in numbers sufficient to initiate disease in a host-dependent manner and that bacteria could be recovered from secateurs after a single cut through an infected stem. In in vivo assays, none of six products tested against A. tumefaciens prevented crown gall disease, although several products appeared promising in in vitro trials. Likewise, four compounds trialed against R. fascians failed to prevent disease. Sanitation and clean planting material remain the primary means of disease management.

The LysR-type transcription factor LsrB regulates beta-lactam resistance in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a plant pathogen, broadly known as the causal agent of the crown gall disease. The soil bacterium is naturally resistant to beta-lactam antibiotics by utilizing the inducible beta-lactamase AmpC. Our picture on the condition-dependent regulation of ampC expression is incomplete. A known regulator is AmpR controlling the transcription of ampC in response to unrecycled muropeptides as a signal for cell wall stress. In our study, we uncovered the global transcriptional regulator LsrB as a critical player acting upstream of AmpR. Deletion of lsrB led to severe ampicillin and penicillin sensitivity, which could be restored to wild-type levels by lsrB complementation. By transcriptome profiling via RNA-Seq and qRT-PCR and by electrophoretic mobility shift assays, we show that ampD coding for an anhydroamidase involved in peptidoglycan recycling is under direct negative control by LsrB. Controlling AmpD levels by the LysR-type regulator in turn impacts the cytoplasmic concentration of cell wall degradation products and thereby the AmpR-mediated regulation of ampC. Our results substantially expand the existing model of inducible beta-lactam resistance in A. tumefaciens by establishing LsrB as higher-level transcriptional regulator.

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.

Agroinfiltration for Enhanced Transgene Expression in Coffee Leaves (Coffea arabica L.)

The Coffea spp. plant is a significant crop in Latin America, Africa, and Asia, and recent advances in genomics and transcriptomics have opened possibilities for studying candidate genes and introducing new desirable traits through genetic engineering. While stable transformation of coffee plants has been reported using various techniques, it is a time-consuming and laborious process. To overcome this, transient transformation methods have been developed, which avoid the limitations of stable transformation. This chapter describes an ex vitro protocol for transient expression using A. tumefaciens-mediated infiltration of coffee leaves, which could be used to produce coffee plants expressing desirable traits against biotic and abiotic stresses, genes controlling biochemical and physiological traits, as well as for gene editing through CRISPR/Cas9.

An efficient targeted gene deletion approach for Cochliobolus heterostrophus using Agrobacterium tumefaciens-mediated transformation

Cochliobolus heterostrophus is a plant pathogenic fungus of southern corn leaf blight, which has been regarded as a model necrotrophic plant pathogen. Many methods have been developed to knock out targeted genes in C. heterostrophus, of which the most widely-used one is protoplast-mediated transformation. However, there are several problems of this method associated with protoplast preparation, DNA product, time consumption, or high cost. In this study, a highly efficient target gene deletion approach in C. heterostrophus was established and optimized, based on Agrobacterium tumefaciens-mediated transformation (ATMT); the transformation efficiency of this approach was 85-88 transformants per 105 conidia, and the homologous recombination efficiency was approximately 68.3%. Furthermore, six gene knockout mutants of C. heterostrophus were obtained using this ATMT method. The phenotypes of this fungus altered in the mutant strains, and the virulence of the mutants significantly reduced compared to of the wild type strain. Taken together, this ATMT system established in this study can be used as a genetic manipulation tool for C. heterostrophus, to better understand the functions of genes and its relation to virulence.

Structural and biochemical characterisation of the N-carbamoyl-β-alanine amidohydrolase from Rhizobium radiobacter MDC 8606

N-carbamoyl-β-alanine amidohydrolase (CβAA) constitutes one of the most important groups of industrially relevant enzymes used in the production of optically pure amino acids and derivatives. In this study, a CβAA-encoding gene from Rhizobium radiobacter strain MDC 8606 was cloned and overexpressed in Escherichia coli. The purified recombinant enzyme (RrCβAA) showed a specific activity of 14 U·mg-1 using N-carbamoyl-β-alanine as a substrate with an optimum activity at 55 °C and pH 8.0. In this work, we report also the first prokaryotic CβAA structure at a resolution of 2.0 Å. A discontinuous catalytic domain and a dimerisation domain attached through a flexible hinge region at the domain interface have been revealed. We identify key ligand binding residues, including a conserved glutamic acid (Glu131), histidine (H385) and arginine (Arg291). Our results allowed us to explain the preference of the enzyme for linear carbamoyl substrates, as large and branched carbamoyl substrates cannot fit in the active site of the enzyme. This work envisages the use of RrCβAA from R. radiobacter MDC 8606 for the industrial production of L-α-, L-β- and L-γ-amino acids. The structural analysis provides new insights on enzyme-substrate interaction, which shed light on engineering of CβAAs for high catalytic activity and broad substrate specificity.

Agrobacterium tumefaciens-mediated transformation of Nigrospora sp. isolated from switchgrass leaves and antagonistic toward plant pathogens

Nigrospora is a diverse genus of fungi colonizing plants through endophytic, pathogenic, or saprobic interactions. Endophytic isolates can improve growth and development of host plants, as well as their resistance to microbial pathogens, but exactly how they do so remains poorly understood. Developing a reliable transformation method is crucial to investigate these mechanisms, in particular to identify pivotal genes for specific functions that correlate with specific traits. In this study, we identified eight isolates of Nigrospora sp. internally colonizing the leaves of switchgrass plants cultivated in North Carolina. Using an Agrobacterium tumefaciens-mediated transformation approach with control and GFP-expressing vectors, we report the first successful transformation of two Nigrospora isolates. Finally, we demonstrate that wild-type and transgenic isolates both negatively impact the growth of two plant pathogens in co-culture conditions, Bipolaris maydis and Parastagonospora nodorum, responsible for the Southern Leaf Blight and Septoria Nodorum Blotch diseases, respectively. The GFP-transformed strains developed here can therefore serve as accurate reporters of spatial interactions in future studies of Nigrospora and pathogens in the plant. Finally, the transformation method we describe lays the foundation for further genetic research on the Nigrospora genus to expand our mechanistic understanding of plant-endophyte interactions.

Vacuum-Forced Agroinfiltration for In planta Transformation of Recalcitrant Plants: Cacao as a Case Study

Transient in planta transformation is a fast and cost-effective alternative for plant genetic transformation. Most protocols for in planta transformation rely on the use of Agrobacterium-mediated transformation. However, the protocols currently in use are standardized for small-sized plants due to the physical and economic constraints of submitting large-sized plants to a vacuum treatment. This work presents an effective protocol for localized vacuum-based agroinfiltration customized for large-sized plants. To assess the efficacy of the proposed method, we tested its use in cacao plants, a tropical plant species recalcitrant to genetic transformation. Our protocol allowed applying up to 0.07 MPa vacuum, with repetitions, to a localized aerial part of cacao leaves, making it possible to force the infiltration of Agrobacterium into the intercellular spaces of attached leaves. As a result, we achieved the Agrobacterium-mediated transient in planta transformation of attached cacao leaves expressing for the RUBY reporter system. This is also the first Agrobacterium-mediated in planta transient transformation of cacao. This protocol would allow the application of the vacuum-based agroinfiltration method to other plant species with similar size constraints and open the door for the in planta characterization of genes in recalcitrant woody, large-size species.

Hidden history of the tobacco BY-2 cell line

For almost 50 years, tobacco (Nicotiana tabacum) BY-2 cells have been widely recognized as an important cell line for plant biology. The cell line grows rapidly, can be synchronized to a high degree, and is excellent for imaging; over the years, these features have led to many high-impact discoveries. However, certain other uses of this cell line are virtually unknown. In the early days, I was involved in distributing the cells to laboratories around the world. Many of these scientists wanted to study the cell cycle; however, I also distributed the cells to scientists who were elucidating the mechanism of plant transformation by Agrobacterium tumefaciens. In fact, BY-2 cells played an essential role in the identification and analysis of Vir genes on the Ti plasmid; likewise, the cells were important for discovering the factor that induces the expression of Vir genes. Thus, BY-2 cells were crucial for the development of modern plant biotechnology. Here, I recount the story of how this came to pass and explain why the use of BY-2 cells in this work was never recognized.

The redox-sensing mechanism of Agrobacterium tumefaciens NieR as a thiol-based oxidation sensor for hypochlorite stress

NieR is a TetR family transcriptional repressor previously shown to regulate the NaOCl-inducible efflux pump NieAB in Agrobacterium tumefaciens. NieR is an ortholog of Escherichia coli NemR that specifically senses hypochlorite through the redox switch of a reversible sulfenamide bond between C106 and K175. The amino acid sequence of NieR contains only one cysteine. NieR has C104 and R166, which correspond to C106 and K175 of NemR, respectively. The aim of this study was to investigate the redox-sensing mechanism of NieR under NaOCl stress. C104 and R166 were subjected to mutagenesis to determine their roles. Although the substitution of R166 by alanine slightly reduced its DNA-binding activity, NieR retained its repressor function. By contrast, the DNA-binding and repression activities of NieR were completely lost when C104 was replaced by alanine. C104 substitution with serine only partially impaired the repressor function. Mass spectrometry analysis revealed an intermolecular disulfide bond between the C104 residues of NieR monomers. This study demonstrates the engagement of C104 in the mechanism of NaOCl sensing. C104 oxidation induced the formation of a disulfide-linked dimer that was likely to alter conformation, thus abolishing the DNA-binding ability of NieR and derepressing the target genes.

Agrobacterium-mediated in planta transformation of cut coleoptile: a new, simplified, and tissue culture-independent method to deliver the CRISPR/Cas9 system in rice

BACKGROUND

Agrobacterium-mediated transformation and particle bombardment are the two common approaches for genome editing in plant species using CRISPR/Cas9 system. Both methods require careful manipulations of undifferentiated cells and tissue culture to regenerate the potentially edited plants. However, tissue culture techniques are laborious and time-consuming.

METHODS AND RESULTS

In this study, we have developed a simplified, tissue culture-independent protocol to deliver the CRISPR/Cas9 system through in planta transformation in Malaysian rice (Oryza sativa L. subsp. indica cv. MR 219). Sprouting seeds with cut coleoptile were used as the target for the infiltration by Agrobacterium tumefaciens and we achieved 9% transformation efficiency. In brief, the dehusked seeds were surface-sterilised and imbibed, and the coleoptile was cut to expose the apical meristem. Subsequently, the cut coleoptile was inoculated with A. tumefaciens strain EHA105 harbouring CRISPR/Cas9 expression vector. The co-cultivation was conducted for five to six days in a dark room (25 ± 2 °C) followed by rooting, acclimatisation, and growing phases. Two-month-old plant leaves were then subjected to a hygromycin selection, and hygromycin-resistant plants were identified as putative transformants. Further validation through the polymerase chain reaction verified the integration of the Cas9 gene in four putative T0 lines. During the fruiting stage, it was confirmed that the Cas9 gene was still present in three randomly selected tillers from two 4-month-old transformed plants.

CONCLUSION

This protocol provides a rapid method for editing the rice genome, bypassing the need for tissue culture. This article is the first to report the delivery of the CRISPR/Cas9 system for in planta transformation in rice.

Agrobacterium tumefaciens-mediated transformation of the white-rot fungus Dichomitus squalens

Dichomitus squalens is an efficient white-rot fungus that generates a wide range of extracellular enzymes to degrade lignocellulose in nature. Although a protoplast-mediated transformation method for D. squalens has been developed, the transformation efficiency remains low. Here, we established a highly efficient Agrobacterium tumefaciens-mediated transformation (ATMT) procedure for D. squalens by transferring a binary vector harboring the neomycin phosphotransferase II (nptII) resistance gene fused with DsRed-Express2, under the control of the native glyceraldehyde-3-phosphate dehydrogenase (GPD) gene promoter. Key factors affecting the efficiency of transformation were tested. A. tumefaciens EHA105 strain with a cell density of 0.4 OD600nm and 96 h co-cultivation resulted in the highest transformation efficiency, with an average of 98 ± 11 transformants per co-cultivation plate. Besides, the strong expression of DsRed-Express2 indicates the effectiveness of the DsGPD promoter in driving gene expression in D. squalens. This ATMT system of D. squalens would be beneficial for its molecular genetic studies.

Agrobacterium tumefaciens-Mediated Genetic Engineering of Green Microalgae, Chlorella vulgaris

Agrobacterium tumefaciens-mediated transformation (AMT) serves as a widely employed tool for manipulating plant genomes. However, A. tumefaciens exhibit the capacity for gene transfer to a diverse array of species. Numerous microalgae species lack well-established methods for reliably integrating genes of interest into their nuclear genome. To harness the potential benefits of microalgal biotechnology, simple and efficient genome manipulation tools are crucial. Herein, an optimized AMT protocol is presented for the industrial microalgae species Chlorella vulgaris, utilizing the reporter green fluorescent protein (mGFP5) and the antibiotic resistance marker for Hygromycin B. Mutants are selected through plating on Tris-Acetate-Phosphate (TAP) media containing Hygromycin B and cefotaxime. Expression of mGFP5 is quantified via fluorescence after over ten generations of subculturing, indicating the stable transformation of the T-DNA cassette. This protocol allows for the reliable generation of multiple transgenic C. vulgaris colonies in under two weeks, employing the commercially available pCAMBIA1302 plant expression vector.

The ChvG-ChvI Regulatory Network: A Conserved Global Regulatory Circuit Among the Alphaproteobacteria with Pervasive Impacts on Host Interactions and Diverse Cellular Processes

The ChvG-ChvI two-component system is conserved among multiple Alphaproteobacteria. ChvG is a canonical two-component system sensor kinase with a single large periplasmic loop. Active ChvG directs phosphotransfer to its cognate response regulator ChvI, which controls transcription of target genes. In many alphaproteobacteria, ChvG is regulated by a third component, a periplasmic protein called ExoR, that maintains ChvG in an inactive state through direct interaction. Acidic pH stimulates proteolysis of ExoR, unfettering ChvG-ChvI to control its regulatory targets. Activated ChvI among different alphaproteobacteria controls a broad range of cellular processes, including symbiosis and virulence, exopolysaccharide production, biofilm formation, motility, type VI secretion, cellular metabolism, envelope composition, and growth. Low pH is a virulence signal in Agrobacterium tumefaciens, but in other systems, conditions that cause envelope stress may also generally activate ChvG-ChvI. There is mounting evidence that these regulators influence diverse aspects of bacterial physiology, including but not limited to host interactions.

Highly efficient gene knockout system in the maize pathogen Colletotrichum graminicola using Agrobacterium tumefaciens-mediated transformation (ATMT)

Colletotrichum graminicola, a hemibiotrophic pathogenic fungus, is the causal agent of anthracnose of maize, which causes significant yield losses worldwide, especially in warm and humid maize production regions. An efficient targeted genes knockout protocol is crucial to explore molecular mechanisms of fungal virulence to the host. In this study, we established a gene knockout transformation system by employing Agrobacterium tumefaciens-mediated transformation to knockout genes in M 1.001 strain of C. graminicola. The conidia germination status, induction medium type, and ratio of Agrobacterium cell and conidia suspension were optimized for the knockout of CgBRN1(OR352905), a gene relating to the fungal melanin biosynthesis pathway. Additionally, CgPKS18 (OR352906) and CgCDC25 (OR352903) were knocked out to test the applicability of the gene knockout transformation system. In this established system, transformation efficiency was 176 transformants per 1 × 105 conidia and the homologous recombination efficiency was 53.3 to 75%. Furthermore, disease index, lesion number and lesion size caused by the three above-mentioned mutant strains were found to be reduced significantly compared to the wild-type strain, which indicated reduction in fungal virulence due to the lack of those genes.

A critical inter-subunit interaction for the transmission of the allosteric signal in the Agrobacterium tumefaciens ADP-glucose pyrophosphorylase

ADP-glucose pyrophosphorylase is a key regulatory enzyme involved in starch and glycogen synthesis in plants and bacteria, respectively. It has been hypothesized that inter-subunit communications are important for the allosteric effect in this enzyme. However, no specific interactions have been identified as part of the regulatory signal. The enzyme from Agrobacterium tumefaciens is a homotetramer allosterically regulated by fructose 6-phosphate and pyruvate. Three pairs of distinct subunit-subunit interfaces are present. Here we focus on an interface that features two symmetrical interactions between Arg11 and Asp141 from one subunit with residues Asp141 and Arg11 of the neighbor subunit, respectively. Previously, scanning mutagenesis showed that a mutation at the Arg11 position disrupted the activation of the enzyme. Considering the distance of these residues from the allosteric and catalytic sites, we hypothesized that the interaction between Arg11 and Asp141 is critical for allosteric signaling rather than effector binding. To prove our hypothesis, we mutated those two sites (D141A, D141E, D141N, D141R, R11D, and R11K) and performed kinetic and binding analysis. Mutations that altered the charge affected the regulation the most. To prove that the interaction per se (rather than the presence of specific residues) is critical, we partially rescued the R11D protein by introducing a second mutation (R11D/D141R). This could not restore the activator effect on kcat , but it did rescue the effect on substrate affinity. Our results indicate the critical functional role of Arg11 and Asp141 to relay the allosteric signal in this subunit interface.

Establishment of RNA Interference Genetic Transformation System and Functional Analysis of FlbA Gene in Leptographium qinlingensis

Leptographium qinlingensis is a pathogenic fungus of Pinus armandii that is epidemic in the Qinling Mountains. However, an effective gene interference strategy is needed to characterize the pathogenic genes in this fungus on a functional level. Using the RNA silencing vector pSilent-1 as a template, we established an RNA interference genetic transformation system mediated by Agrobacterium tumefaciens GV3101, which is suitable for the gene study for Leptographium qinlingensis by homologous recombination and strain interference system screening. The LqFlbA gene was silenced using the RNA interference approach described above, and the resulting transformants displayed various levels of silencing with a gene silencing effectiveness ranging from 41.8% to 91.4%. The LqFlbA-RNAi mutant displayed altered colony morphology, sluggish mycelium growth, and diminished pathogenicity toward the host P. armandii in comparison to the wild type. The results indicate that this method provides a useful reverse genetic system for studying the gene function of L. qinlingensis, and that LqFlbA plays a crucial role in the growth, development, and pathogenicity of L. qinlingensis.

Multiplicity of the Agrobacterium Infection of Nicotiana benthamiana for Transient DNA Delivery

Biological DNA transfer into plant cells mediated by Agrobacterium represents one of the most powerful tools for the engineering and study of plant systems. Transient expression of transfer DNA (T-DNA) in particular enables rapid testing of gene products and has been harnessed for facile combinatorial expression of multiple genes. In analogous mammalian cell-based gene expression systems, a clear sense of the multiplicity of infection (MOI) allows users to predict and control viral transfection frequencies for applications requiring single versus multiple transfection events per cell. Despite the value of Agrobacterium-mediated transient transformation of plants, MOI has not been quantified. Here, we analyze the Poisson probability distribution of the T-DNA transfer in leaf pavement cells to determine the MOI for the widely used model system Agrobacterium GV3101/Nicotiana benthamiana. These data delineate the relationship between an individual Agrobacterium strain infiltration OD600, plant cell perimeter, and leaf age, as well as plant cell coinfection rates. Our analysis establishes experimental regimes where the probability of near-simultaneous delivery of >20 unique T-DNAs to a given plant cell remains high throughout the leaf at infiltration OD600 above ∼0.2 for individual strains. In contrast, single-strain T-DNA delivery can be achieved at low strain infiltration OD600: at OD600 0.02, we observe that ∼40% of plant cells are infected, with 80% of those infected cells containing T-DNA product from just a single strain. We anticipate that these data will enable users to develop new approaches to in-leaf library development using Agrobacterium transient expression and reliable combinatorial assaying of multiple heterologous proteins in a single plant cell.

Development of Agrobacterium-mediated in planta transformation protocol through coleoptile in rice

Genetic modification of rice is mainly carried out by Agrobacterium-mediated transformation of callus accompanied by tissue culture. It is time consuming, laborious and unapplicable for cultivars unable to induce callus. In this study, we have reported a novel gene transfer protocol that involves pulling out primary leaf from coleoptile and injection of Agrobacterium culture into the empty channel. Out of 25 plants survived after injection of Agrobacterium tumefaciens EHA105 culture harboring pCAMBIA1301-RD29A-AtDREB1A, 8 T0 plants revealed the expected size of around 811 bp corresponding to AtDREB1A gene and Southern blotting analysis on 18 T1 plants suggested introgression of AtDREB1A. 3 T2 lines (7-9, 12-3, 18-6) exhibited accumulation of free proline and soluble sugars, yet increase of chlorophyll content, but decrease of electrolyte leakage and methane dicarboxylic aldehyde under cold stress condition at the vegetative growth stage. Yield components investigation on T2 lines showed earlier heading date and no yield loss compared to wild type plants grown under normal condition. GUS expression analysis and integrated transgene detection in T0 and T1 plants followed by evaluation of cold stress tolerance in T2 lines suggest the advantage of this in planta transformation protocol to obtain transgenic rice.

CRISPR/Cas9 and Agrobacterium tumefaciens virulence proteins synergistically increase efficiency of precise genome editing via homology directed repair in plants

CRISPR/Cas9 genome editing and Agrobacterium tumefaciens-mediated genetic transformation are widely-used plant biotechnology tools derived from bacterial immunity-related systems, each involving DNA modification. The Cas9 endonuclease introduces DNA double-strand breaks (DSBs), and the A. tumefaciens T-DNA is released by the VirD2 endonuclease assisted by VirDl and attached by VirE2, transferred to the plant nucleus and integrated into the genome. Here, we explored the potential for synergy between the two systems and found that Cas9 and three virulence (Vir) proteins achieve precise genome editing via the homology directed repair (HDR) pathway in tobacco and rice plants. Compared with Cas9T (Cas9, VirD1, VirE2) and CvD (Cas9-VirD2) systems, the HDR frequencies of a foreign GFPm gene in the CvDT system (Cas9-VirD2, VirD1, VirE2) increased 52-fold and 22-fold, respectively. Further optimization of the CvDT process with a donor linker (CvDTL) achieved a remarkable increase in the efficiency of HDR-mediated genome editing. Additionally, the HDR efficiency of the three rice endogenous genes ACETOLACTATE SYNTHASE (ALS), PHYTOENE DESATURASE (PDS), and NITROGEN TRANSPORTER 1.1 B (NRT1.1B) increased 24-, 32- and 16-fold, respectively, in the CvDTL system, compared with corresponding Cas9TL (Cas9T process with a donor linker). Our results suggest that collaboration between CRISPR/Cas9 and Agrobacterium-mediated genetic transformation can make great progress towards highly efficient and precise genome editing via the HDR pathway.

Agrobacterium tumefaciens: a Transformative Agent for Fundamental Insights into Host-Microbe Interactions, Genome Biology, Chemical Signaling, and Cell Biology

Agrobacterium tumefaciens incites the formation of readily visible macroscopic structures known as crown galls on plant tissues that it infects. Records from biologists as early as the 17th century noted these unusual plant growths and began examining the basis for their formation. These studies eventually led to isolation of the infectious agent, A. tumefaciens, and decades of study revealed the remarkable mechanisms by which A. tumefaciens causes crown gall through stable horizontal genetic transfer to plants. This fundamental discovery generated a barrage of applications in the genetic manipulation of plants that is still under way. As a consequence of the intense study of A. tumefaciens and its role in plant disease, this pathogen was developed as a model for the study of critical processes that are shared by many bacteria, including host perception during pathogenesis, DNA transfer and toxin secretion, bacterial cell-cell communication, plasmid biology, and more recently, asymmetric cell biology and composite genome coordination and evolution. As such, studies of A. tumefaciens have had an outsized impact on diverse areas within microbiology and plant biology that extend far beyond its remarkable agricultural applications. In this review, we attempt to highlight the colorful history of A. tumefaciens as a study system, as well as current areas that are actively demonstrating its value and utility as a model microorganism.

Cryo-EM structure of the Agrobacterium tumefaciens T4SS-associated T-pilus reveals stoichiometric protein-phospholipid assembly

Agrobacterium tumefaciens causes crown gall disease in plants by the horizontal transfer of oncogenic DNA. The conjugation is mediated by the VirB/D4 type 4 secretion system (T4SS) that assembles an extracellular filament, the T-pilus, and is involved in mating pair formation between A. tumefaciens and the recipient plant cell. Here, we present a 3 Å cryoelectron microscopy (cryo-EM) structure of the T-pilus solved by helical reconstruction. Our structure reveals that the T-pilus is a stoichiometric assembly of the VirB2 major pilin and phosphatidylglycerol (PG) phospholipid with 5-start helical symmetry. We show that PG head groups and the positively charged Arg 91 residues of VirB2 protomers form extensive electrostatic interactions in the lumen of the T-pilus. Mutagenesis of Arg 91 abolished pilus formation. While our T-pilus structure is architecturally similar to previously published conjugative pili structures, the T-pilus lumen is narrower and positively charged, raising questions of whether the T-pilus is a conduit for ssDNA transfer.

<em>Agrobacterium tumefaciens</em>-Mediated Genetic Transformation of Narrowleaf Plantain

Species in the genus Plantago have several unique traits that have led to them being adapted as model plants in various fields of study. However, the lack of a genetic manipulation system prevents in-depth investigation of gene function, limiting the versatility of this genus as a model. Here, a transformation protocol is presented for Plantago lanceolata, the most commonly studied Plantago species. Using Agrobacterium tumefaciens-mediated transformation, 3 week-old roots of aseptically grown P. lanceolata plants were infected with bacteria, incubated for 2-3 days, and then transferred to a shoot induction medium with appropriate antibiotic selection. Shoots typically emerged from the medium after 1 month, and roots developed 1-4 weeks after the shoots were transferred to the root induction medium. The plants were then acclimated to a soil environment and tested for the presence of a transgene using the β-glucuronidase (GUS) reporter assay. The transformation efficiency of the current method is ~20%, with two transgenic plants emerging per 10 root tissues transformed. Establishing a transformation protocol for narrowleaf plantain will facilitate the adoption of this plant as a new model species in various areas.

Engineered Agrobacterium improves transformation by mitigating plant immunity detection

Agrobacterium tumefaciens microbe-associated molecular pattern elongation factor Tu (EF-Tu) is perceived by orthologs of the Arabidopsis immune receptor EFR activating pattern-triggered immunity (PTI) that causes reduced T-DNA-mediated transient expression. We altered EF-Tu in A. tumefaciens to reduce PTI and improved transformation efficiency. A robust computational pipeline was established to detect EF-Tu protein variation in a large set of plant bacterial species and identified EF-Tu variants from bacterial pathogen Pseudomonas syringae pv. tomato DC3000 that allow the pathogen to escape EFR perception. Agrobacterium tumefaciens strains were engineered to substitute EF-Tu with DC3000 variants and examined their transformation efficiency in plants. Elongation factor Tu variants with rarely occurred amino acid residues were identified within DC3000 EF-Tu that mitigates recognition by EFR. Agrobacterium tumefaciens strains were engineered by expressing DC3000 EF-Tu instead of native agrobacterial EF-Tu and resulted in decreased plant immunity detection. These engineered A. tumefaciens strains displayed an increased efficiency in transient expression in both Arabidopsis thaliana and Camelina sativa. The results support the potential application of these strains as improved vehicles to introduce transgenic alleles into members of the Brassicaceae family.

Improving Agrobacterium tumefaciens−Mediated Genetic Transformation for Gene Function Studies and Mutagenesis in Cucumber (Cucumis sativus L.)

In the post−genomics era, Agrobacterium tumefaciens−mediated genetic transformation is becoming an increasingly indispensable tool for characterization of gene functions and crop improvement in cucumber (Cucumis sativus L.). However, cucumber transformation efficiency is still low. In this study, we evaluated the effects of several key factors affecting the shoot−regeneration rate and overall transformation efficiency in cucumber including genotypes, the age and sources of explants, Agrobacterium strains, infection/co−cultivation conditions, and selective agents. We showed that in general, North China cucumbers exhibited higher shoot−regeneration rate than US pickling or slicing cucumbers. The subapical ground meristematic regions from cotyledons or the hypocotyl had a similar shoot−regeneration efficiency that was also affected by the age of the explants. Transformation with the Agrobacterium strain AGL1 yielded a higher frequency of positive transformants than with GV3101. The antibiotic kanamycin was effective in selection against non−transformants or chimeras. Optimization of various factors was exemplified with the development of transgenic plants overexpressing the LittleLeaf (LL) gene or RNAi of the APRR2 gene in three cucumber lines. The streamlined protocol was also tested in transgenic studies in three additional genes. The overall transformation efficiency defined by the number of verified transgenic plants out of the number of seeds across multiple experiments was 0.2–1.7%. Screening among T1 OE transgenic plants identified novel, inheritable mutants for leaf or fruit color or size/shape, suggesting T−DNA insertion as a potential source of mutagenesis. The Agrobacterium−mediated transformation protocol from this study could be used as the baseline for further improvements in cucumber transformation.

Co-immunoprecipitation for Assessing Protein-Protein Interactions in Agrobacterium-Mediated Transient Expression System in Nicotiana benthamiana

The characterization of protein-protein interactions (PPI) often provides functional information about a target protein. Yeast-two-hybrid (Y2H) and luminescence/fluorescence-based detections, therefore, have been widely utilized for assessing PPI. In addition, a co-immunoprecipitation (co-IP) method has also been adopted with transient protein expression in Nicotiana benthamiana (N. benthamiana) infiltrated with Agrobacterium tumefaciens. Herein, we describe a co-IP procedure in which structural maintenance of chromosome 1 (SMC1), identified from a Y2H screening, was verified as an interacting partner for microchidia 1 (MORC1), a protein well known for its function in plant immunity and epigenetics. SMC1 and MORC1 were transiently expressed in N. benthamiana when infiltrated by Agrobacterium with the respective genes. From this approach, we identified a region of SMC1 responsible for interacting with MORC1. The co-IP method, of which outputs are mainly from immunoblot analysis, provided information about target protein expression as well, which is often useful for troubleshooting. Using this feature, we showcased a PPI confirmation from our SMC1-MORC1 study in which a full-length SMC1 protein was not detectable, and, therefore, a subsequent truncated mutant analysis had to be employed for PPI verification.

Construction of Full-Length Infectious cDNA Clones of Citrus Mosaic Virus RNA1 and RNA2 and Infection of Citrus Seedlings by Agrobacterium-Mediated Vacuum-Infiltration

The development of full-length infectious cDNA clones for plant RNA viruses is important for studying their molecular biological characteristics, functional genomics, pathogenesis, and vectorization applications. Citrus mosaic virus (CiMV), a member of the genus Sadwavirus, is of economic importance to the citrus industry and comprises a bipartite, positive-sense, single-stranded RNA genome encapsidated in icosahedral virions. In the present study, full-length cDNA clones of CiMV RNA1 and RNA2 were constructed based on a ternary yeast-Escherichia coli-Agrobacterium tumefaciens shuttle vector, pTY, using transformation-associated recombination (TAR) strategy. Infectivity of cDNA clones of CiMV RNA1 and RNA2 was examined in multiple citrus varieties via Agrobacterium-mediated vacuum-infiltration (AVI) through symptom observation, RT-PCR, and virion detection with an electron microscope. Furthermore, the genome-sized RT-PCR fragments of RNA1 and RNA2 were obtained from symptomatic Jinchengyou (Citrus grandis) plants infected by the cloned virus (CiMV211). In addition, CiMV211 produced typical symptoms of wild-type CiMV in cowpea (Vigna angularis) plants inoculated by Agrobacterium-mediated injection. This is the first report of infectious cDNA clones of CiMV, which may lay the foundation for research on the pathogenesis and vectorization of the virus.

Activation of ChvG-ChvI regulon by cell wall stress confers resistance to β-lactam antibiotics and initiates surface spreading in Agrobacterium tumefaciens

A core component of nearly all bacteria, the cell wall is an ideal target for broad spectrum antibiotics. Many bacteria have evolved strategies to sense and respond to antibiotics targeting cell wall synthesis, especially in the soil where antibiotic-producing bacteria compete with one another. Here we show that cell wall stress caused by both chemical and genetic inhibition of the essential, bifunctional penicillin-binding protein PBP1a prevents microcolony formation and activates the canonical host-invasion two-component system ChvG-ChvI in Agrobacterium tumefaciens. Using RNA-seq, we show that depletion of PBP1a for 6 hours results in a downregulation in transcription of flagellum-dependent motility genes and an upregulation in transcription of type VI secretion and succinoglycan biosynthesis genes, a hallmark of the ChvG-ChvI regulon. Depletion of PBP1a for 16 hours, results in differential expression of many additional genes and may promote a stress response, resembling those of sigma factors in other bacteria. Remarkably, the overproduction of succinoglycan causes cell spreading and deletion of the succinoglycan biosynthesis gene exoA restores microcolony formation. Treatment with cefsulodin phenocopies depletion of PBP1a and we correspondingly find that chvG and chvI mutants are hypersensitive to cefsulodin. This hypersensitivity only occurs in response to treatment with β-lactam antibiotics, suggesting that the ChvG-ChvI pathway may play a key role in resistance to antibiotics targeting cell wall synthesis. Finally, we provide evidence that ChvG-ChvI likely has a conserved role in conferring resistance to cell wall stress within the Alphaproteobacteria that is independent of the ChvG-ChvI repressor ExoR.

Insights into the molecular mechanisms of CRISPR/Cas9-mediated gene targeting at multiple loci in Arabidopsis

Homologous recombination-mediated gene targeting (GT) enables precise sequence knockin or sequence replacement, and thus is a powerful tool for heritable precision genome engineering. We recently established a clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR/Cas9)-mediated approach for heritable GT in Arabidopsis (Arabidopsis thaliana), but its broad utility was not tested, and the underlying molecular mechanism was unclear. Here, we achieved precise GT at 14 out of 27 tested endogenous target loci using the sequential transformation approach and obtained vector-free GT plants by backcrossing. Thus, the sequential transformation GT method provides a broadly applicable technology for precise genome manipulation. We show that our approach generates heritable GT in the egg cell or early embryo of T1 Arabidopsis plants. Analysis of imprecise GT events suggested that single-stranded transfer DNA (T-DNA)/VirD2 complexes produced during the Agrobacterium (Agrobacterium tumefaciens) transformation process may serve as the donor templates for homologous recombination-mediated repair in the GT process. This study provides new insights into the molecular mechanisms of CRISPR/Cas9-mediated GT in Arabidopsis.

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.

T-LOC: A comprehensive tool to localize and characterize T-DNA integration sites

Scientists have developed many approaches based on PCR or next-generation sequencing to localize and characterize integrated T-DNAs in transgenic plants generated by Agrobacterium tumefaciens-mediated T-DNA transfer. However, none of these methods has the robust ability to handle all transgenic plants with diversified T-DNA patterns. Utilizing the valuable information in the whole-genome sequencing data of transgenic plants, we have developed a comprehensive approach (T-LOC) to localize and characterize T-DNA integration sites (TISs). We evaluated the performance of T-LOC on genome sequencing data from 48 transgenic rice (Oryza sativa) plants that provide real and unbiased resources of T-DNA integration patterns. T-LOC discovered 75 full TISs and reported a diversified pattern of T-DNA integration: the ideal single-copy T-DNA between two borders, multiple-copy of T-DNAs in tandem or inverted repeats, truncated partial T-DNAs with or without the selection hygromycin gene, the inclusion of T-DNA backbone, the integration at the genome repeat region, and the concatenation of multiple ideal or partial T-DNAs. In addition, we reported that DNA fragments from the two A. tumefaciens plasmids can be fused with T-DNA and integrated into the plant genome. Besides, T-LOC characterizes the genomic changes at TISs, including deletion, duplication, accurate repair, and chromosomal rearrangement. Moreover, we validated the robustness of T-LOC using PCR, Sanger sequencing, and Nanopore sequencing. In summary, T-LOC is a robust approach to studying the TISs independent of the integration pattern and can recover all types of TISs in transgenic plants.