Agrobacterium-Mediated Stable Genetic Transformation of Populus angustifolia and Populus balsamifera

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.

Simplified Method for Agrobacterium-Mediated Genetic Transformation of Populus x berolinensis K. Koch

The rapid advancement of genetic technologies has made it possible to modify various plants through both genetic transformation and gene editing techniques. Poplar, with its rapid in vitro growth and regeneration enabling high rates of micropropagation, has emerged as a model system for the genetic transformation of woody plants. In this study, Populus × berolinensis K. Koch. (Berlin poplar) was chosen as the model organism due to its narrow leaves and spindle-shaped crown, which make it highly suitable for in vitro manipulations. Various protocols for the Agrobacterium-mediated transformation of poplar species have been developed to date. However, the genetic transformation procedures are often constrained by the complexity of the nutrient media used for plant regeneration and growth, which could potentially be simplified. Our study presents a cheaper, simplified, and relatively fast protocol for the Agrobacterium-mediated transformation of Berlin poplar. The protocol involved using internode sections without axillary buds as explants, which were co-cultivated in 10 µL droplets of bacterial suspension directly on the surface of a solid agar-based medium without rinsing and sterile paper drying after inoculation. We used only one regeneration Murashige and Skoogbased medium supplemented with BA (0.2 mg·L-1), TDZ (0.02 mg·L-1), and NAA (0.01 mg·L-1). Acetosyringone was not used as an induction agent for vir genes during the genetic transformation. Applying our protocol and using the binary plasmid pBI121 carrying the nptII selective and uidA reporter genes, we obtained the six transgenic lines of poplar. Transgenesis was confirmed through a PCR-based screening of kanamycin-selected regenerants for the presence of both mentioned genes, Sanger sequencing, and tests for detecting the maintained activity of both genes. The transformation efficiency, considering the 100 explants taken originally, was 6%.

Optimization of High-Efficiency Tissue Culture Regeneration Systems in Gray Poplar

A series of tissue culture regeneration protocols were conducted on gray poplar (P. tremula × P. alba) to select the most efficient callus induction medium, adventitious shoot differentiation medium, shoot elongation medium and rooting medium, which laid the foundation for the optimization of genetic transformation technology for gray poplar. The results showed that the Woody Plant Medium (WPM) supplemented with 0.10 mg L-1 kinetin (KT) and 1.00 mg L-1 2,4-dichlorophenoxyacetic acid (2,4-D) was the most suitable medium for callus induction. The callus induction rates of different tissues were greater than 85.7%. The optimal adventitious shoot differentiation medium was the WPM supplemented with 0.02 mg L-1 thidiazuron (TDZ), and the adventitious shoot differentiation rates of young tissues were 22.2-41.9%. The optimal direct differentiation medium was the Murashige and Skoog (MS) medium supplemented with 0.20 mg L-1 6-benzylaminopurine (6-BA), 0.10 mg L-1 indole butyric acid (IBA) and 0.001 mg L-1 TDZ, and the differentiation rate of adventitious shoots was greater than 94%. The best shoot elongation medium for adventitious shoots was the MS medium with 0.10 mg L-1 naphthylacetic acid (NAA). After 45 days of cultivation in the MS medium with 0.10 mg L-1 NAA, the average plant height was 1.8 cm, and the average number of elongated adventitious shoots was 11 per explant. The 1/2 MS medium with 0.10 mg L-1 NAA showed the best performance for rooting, and later, shoot growth. The direct shoot induction pathway can induce adventitious shoots much faster than the indirect adventitious shoot induction pathway can, and the time cost via the direct adventitious shoot induction pathway can be shortened by 2-6 weeks compared to that of the indirect shoot induction pathway.

Agrobacterium-mediated genetic transformation of the most widely cultivated superior clone Eucalyptus urophylla × E. grandis DH32-29 in Southern China

Eucalyptus, as an economically important species for wood and paper industries, remains a challenge to genetic improvement by transgenic technology owing to the deficiency of a highly efficient and stable genetic transformation system, especially in cultivated superior clones. Eucalyptus urophylla × E. grandis clone DH32-29 is most widely planted in southern China, but it is relatively recalcitrant to adventitious bud regeneration, which blocks the establishment of a genetic transformation system. Here, an efficient adventitious bud regeneration and transformation system of Eucalyptus was established using E. urophylla × E. grandis DH32-29 as material. The in vitro leaves from microshoots that were subcultured for 20-25 days were immersed into liquid Woody Plant Medium supplemented with 0.02 mg·L^-1 α-naphthaleneacetic acid (NAA) and 0.24 mg·L^-1 forchlorfenuron [callus-inducing medium (CIM)]. After 15 days, explants were transferred to a medium containing 0.10 mg·L^-1 NAA and 0.50 mg·L^-1 6-benzyladenine (shoot-inducing medium, SIM) for adventitious bud induction. The highest regeneration efficiency of adventitious buds was 76.5%. Moreover, an Agrobacterium tumefaciens-mediated genetic transformation system was optimized. The leaves were precultured for 7 days and infected for 30 min with A. tumefaciens strain EHA105 grown to a bacterial density of 0.3 (OD₆₀₀). After 72 h of cocultivation in the dark, leaves were transferred to CIM supplemented with 100 mg·L^-1 cefotaxime (Cef), 100 mg·L^-1 timentin, and 15 mg·L^-1 kanamycin (Kan) for 15 days to induce calluses. Then, the explants were transferred to SIM supplemented with the same concentration of antibiotics, and the fresh medium was replaced every 15 days until resistant adventitious buds appeared. After inducing roots in root-inducing medium supplemented with 200 mg·L^-1 Cef and 75 mg·L^-1 Kan, completely transgenic plants were obtained. Using the aforementioned method, the transformation frequency can reach 1.9%. This provides a powerful approach for genetic improvement of E. urophylla × E. grandis DH32-29 and gene function analysis in Eucalyptus.

Two high hierarchical regulators, PuMYB40 and PuWRKY75, control the low phosphorus driven adventitious root formation in Populus ussuriensis

Adventitious rooting is an essential biological process in the vegetative propagation of economically important horticultural and forest tree species. It enables utilization of the elite genotypes in breeding programmes and production. Promotion of adventitious root (AR) formation has been associated with starvation of inorganic phosphate and some factors involved in low phosphorus (LP) signalling. However, the regulatory mechanism underlying LP-mediated AR formation remains largely elusive. We established an efficient experimental system that guaranteed AR formation through short-term LP treatment in Populus ussuriensis. We then generated a time-course RNA-seq data set to recognize key regulatory genes and regulatory cascades positively regulating AR formation through data analysis and gene network construction, which were followed by experimental validation and characterization. We constructed a multilayered hierarchical gene regulatory network, from which PuMYB40, a typical R2R3-type MYB transcription factor (TF), and its interactive partner, PuWRKY75, as well as their direct targets, PuLRP1 and PuERF003, were identified to function upstream of the known adventitious rooting genes. These regulatory genes were functionally characterized and proved their roles in promoting AR formation in P. ussuriensis. In conclusion, our study unveiled a new hierarchical regulatory network that promoted AR formation in P. ussuriensis, which was activated by short-term LP stimulus and primarily governed by PuMYB40 and PuWRKY75.

An Efficient Agrobacterium-Mediated Transformation Method for Hybrid Poplar 84K (Populus alba × P. glandulosa) Using Calli as Explants

A highly efficient Agrobacterium-mediated transformation method is needed for the molecular study of model tree species such as hybrid poplar 84K (Populus alba × P. glandulosa cv. ‘84K’). In this study, we report a callus-based transformation method that exhibits high efficiency and reproducibility. The optimized callus induction medium (CIM1) induced the development of calli from leaves with high efficiency, and multiple shoots were induced from calli growing on the optimized shoot induction medium (SIM1). Factors affecting the transformation frequency of calli were optimized as follows: Agrobacterium concentration sets at an OD₆₀₀ of 0.6, Agrobacterium infective suspension with an acetosyringone (AS) concentration of 100 µM, infection time of 15 min, cocultivation duration of 2 days and precultivation duration of 6 days. Using this method, transgenic plants are obtained within approximately 2 months with a transformation frequency greater than 50%. Polymerase chain reaction (PCR), reverse transcription-PCR (RT-PCR) and β-galactosidase (GUS) histochemical staining analyses confirmed the successful generation of stable transformants. Additionally, the calli from leaves were subcultured and used to obtain new explants; the high transformation efficiency was still maintained in subcultured calli after 6 cycles. This method provides a reference for developing effective transformation protocols for other poplar species.

Laying it on thick: a study in secondary growth

The development of secondary vascular tissue enhances the transport capacity and mechanical strength of plant bodies, while contributing a huge proportion of the world's biomass in the form of wood. Cell divisions in the cambium, which constitutes the vascular meristem, provide progenitors from which conductive xylem and phloem are derived. The cambium is a somewhat unusual stem cell population in two respects, making it an interesting subject for developmental research. Firstly, it arises post-germination, and thus represents a model for understanding stem cell initiation beyond embryogenesis. Secondly, xylem and phloem differentiate on opposing sides of cambial stem cells, making them bifacial in nature. Recent discoveries in Arabidopsis thaliana have provided insight into the molecular mechanisms that regulate the initiation, patterning, and maintenance of the cambium. In this review, the roles of intercellular signalling via mobile transcription factors, peptide-receptor modules, and phytohormones are described. Crosstalk between these regulatory pathways is becoming increasingly apparent, yet the underlying mechanisms are not fully understood. Future study of the interaction between multiple independently identified regulators, as well as the functions of their orthologues in trees, will deepen our understanding of radial growth in plants.

Efficient Transformation of Catalpa bungei Shows Crystal Genes Conferring Resistance to the Shoot Borer Omphisa plagialis

Although Catalpa bungei is a forest plant with considerable economic and ornamental value in China, its wood and decorative qualities are constrained by insect pests such as the shoot borer Omphisa plagialis (Lepidoptera). Overexpressing insect resistance genes such as crystal genes to develop an insect-resistant variety of C. bungei is an environmental and ecological approach. However, genotype limitations and low regeneration rates of embryogenic calli (EC) inhibit the development of transformation and the insect-resistant gene expression system in C. bungei. Here, we first established embryogenic callus induction and regeneration systems of five genotypes using mature seed and stem segment explants; the highest induction and regeneration rates of EC were 39.89 and 100%, respectively. Next, an efficient and stable Agrobacterium-mediated genetic transformation system was developed from EC and its positive frequency was up to 92.31%. Finally, using the transformation system, 15 and 22 transgenic C. bungei lines that expressed Cry2A and Cry9Aa-like were generated, respectively. These transgenic lines that exhibited significantly higher resistance to O. plagialis in the laboratory and field have great promise for meeting the challenge of future pest management under changing climatic conditions. Additionally, this efficient, fast, and stable transformation system could be a potential tool for gene function analysis and forest tree genetic improvement.

An efficient system for Agrobacterium-mediated transient transformation in Pinus tabuliformis

BACKGROUND

Functional genomic studies using genetics approaches of conifers are hampered by the complex and enormous genome, long vegetative growth period, and exertion in genetic transformation. Thus, the research carried out on gene function in Pinus tabuliformis is typically performed by heterologous expression based on the model plant Arabidopsis. However, due to the evolutionary and vast diversification from non-flowering (gymnosperms) to flowering (angiosperms) plants, several key differences may alter the underlying genetic concerns and the analysis of variants. Therefore, it is essential to develop an efficient genetic transformation and gene function identification protocol for P. tabuliformis.

RESULTS

In the present study we established a highly efficient transgene Agrobacterium-mediated transient expression system for P. tabuliformis. Using a β-glucuronidase gene (GUS) as a reporter gene expression, the highest transformation efficiency (70.1%) was obtained by co-cultivation with Agrobacterium strain GV3101 at an optical density at 600 nm of 0.8, with 150 μM acetosyringone for 30 min followed by 3 days in the dark at 23 ± 1 °C. This protocol would be applied to other conifers; GUS staining was observed 24 h post-infection.

CONCLUSIONS

We report a simple, fast, and resilient system for transient Agrobacterium-mediated transformation high-level expression of target genes in P. tabuliformis, which will also improve transformation efficiency in other conifer species.

Choice of Explant for Plant Genetic Transformation

Particle bombardment or biolistic transformation is an efficient, versatile method. This method does not need any vector for the gene transfer and is not dependent on the cell type, species, and genotype. The success of any transformation technique depends on the starting experimental materials or the explants. Here, we describe the factors that have influenced the choice of explants in biolistic transformation. Many general factors in the selection of explants in the development of transgenic plants are presented here. Therefore, this chapter provides extensive guidelines regarding the choice of explants for researchers working on various plant genetic transformation techniques.

Xylan in the Middle: Understanding Xylan Biosynthesis and Its Metabolic Dependencies Toward Improving Wood Fiber for Industrial Processing

Lignocellulosic biomass, encompassing cellulose, lignin and hemicellulose in plant secondary cell walls (SCWs), is the most abundant source of renewable materials on earth. Currently, fast-growing woody dicots such as Eucalyptus and Populus trees are major lignocellulosic (wood fiber) feedstocks for bioproducts such as pulp, paper, cellulose, textiles, bioplastics and other biomaterials. Processing wood for these products entails separating the biomass into its three main components as efficiently as possible without compromising yield. Glucuronoxylan (xylan), the main hemicellulose present in the SCWs of hardwood trees carries chemical modifications that are associated with SCW composition and ultrastructure, and affect the recalcitrance of woody biomass to industrial processing. In this review we highlight the importance of xylan properties for industrial wood fiber processing and how gaining a greater understanding of xylan biosynthesis, specifically xylan modification, could yield novel biotechnology approaches to reduce recalcitrance or introduce novel processing traits. Altering xylan modification patterns has recently become a focus of plant SCW studies due to early findings that altered modification patterns can yield beneficial biomass processing traits. Additionally, it has been noted that plants with altered xylan composition display metabolic differences linked to changes in precursor usage. We explore the possibility of using systems biology and systems genetics approaches to gain insight into the coordination of SCW formation with other interdependent biological processes. Acetyl-CoA, s-adenosylmethionine and nucleotide sugars are precursors needed for xylan modification, however, the pathways which produce metabolic pools during different stages of fiber cell wall formation still have to be identified and their co-regulation during SCW formation elucidated. The crucial dependence on precursor metabolism provides an opportunity to alter xylan modification patterns through metabolic engineering of one or more of these interdependent pathways. The complexity of xylan biosynthesis and modification is currently a stumbling point, but it may provide new avenues for woody biomass engineering that are not possible for other biopolymers.

Poplar Transformation

First publications of successful Agrobacterium-mediated transformation of tobacco were published more than 30 years ago. Protocols for Agrobacterium-based transformation as well as biolistic bombardment and PEG transformation of protoplasts are available for more than 150 plant species from various plant families. Also for many Populus species and hybrids, adapted transformation protocols have been published. The standard protocol for Agrobacterium-mediated transformation of different Populus genotypes is the leaf-disc method. Here, we first describe the transfer of genes into poplar by using the Agrobacterium-based leaf disc methods. In addition, alternative basic transformation methods, namely, biolistic bombardment and PEG transformation of protoplasts, are also described. Further, we present improved poplar transformation protocols by simplifying the transformation procedure and optimizing tissue preparation and plant regeneration.

Agrobacterium tumefaciens-mediated transformation of Dendrobium lasianthera J.J.Sm: An important medicinal orchid

A protocol for genetic transformation mediated by Agrobacterium tumefaciens and production of transgenic Dendrobium lasianthera has been developed for the first time. The 8-week-old protocorm explants were used as target of transformation with Agrobacterium tumefaciens strain LBA4404 carrying plasmid pG35SKNAT1. Several parameters such as infection period, Agrobacterium density, concentration of acetosyringone, and co-cultivation period were evaluated for the transformation efficiency. The data were analyzed using one-way analysis of variance (ANOVA) and Duncan's Multiple Range Test (DMRT) with p < 0.05. Subsequently, KNAT1 gene expression was confirmed by polymerase chain reaction (PCR) analysis. The highest efficiency of transformation (70%) obtained from protocorm explants infected with Agrobacterium culture was at the OD₆₀₀ concentration of 0.6 for 30 min, and co-cultivated with acetosyringone 100 µM for 5 days. The results of confirmation by PCR analysis show that the KNAT1 gene has been integrated and expressed in the genome of Dendrobium lasianthera transgenic.

Simple, rapid and efficient transformation of genotype Nisqually-1: a basic tool for the first sequenced model tree

Genotype Nisqually-1 is the first model woody plant with an available well-annotated genome. Nevertheless, a simple and rapid transformation of Nisqually-1 remains to be established. Here, we developed a novel shoot regeneration method for Nisqually-1 using leaf petiole and stem segment explants. Numerous shoots formed in the incision of explants within two weeks. The optimized shoot regeneration medium (SRM) contained 0.03 mg l-1 6-benzylaminopurine, 0.02 mg l-1 indole-3-butyric acid and 0.0008 mg l-1 thidiazuron. Based on this, Agrobacterium-mediated genetic transformation of stem explants was examined using the vector pBI121 that contains the β-glucuronidase (GUS) as a reporter gene. Consequently, factors affecting transformation frequency of GUS-positive shoots were optimized as follows: Agrobacteria cell suspension with an OD600 of 0.4, 20 min infection time, 2 days of co-cultivation duration and the addition of 80 µM acetosyringone into Agrobacteria infective suspension and co-cultivation SRM. Using this optimized method, transgenic plantlets of Nisqually-1 - with an average transformation frequency of 26.7% - were obtained with 2 months. Southern blot and GUS activity staining confirmed the integration of the foreign GUS gene into Nisqually-1. This novel transformation system for Nisqually-1 was rapid, efficient, and simple to operate and will improve more genetic applications in this model tree.