Agrobacterium-mediated genetic transformation and plant regeneration of the hardwood tree species Fraxinus profunda

Establishment and validation of a callus tissue transformation system for German chamomile (Matricaria chamomilla L.)

BACKGROUND

German chamomile (Matricaria chamomilla L.) is an important medicinal plant, and the essential oils in the flowers have various biological activities. Genetic transformation systems are important for plant quality improvement and molecular research. To the best of our knowledge, a genetic transformation system has not yet been reported for German chamomile.

RESULTS

In this study, we developed Agrobacterium-mediated transformation protocols for German chamomile callus tissues. This involved optimizing key parameters, such as hygromycin and cefotaxime concentrations, bacterial density, and infection and co-culture durations. We also performed gas chromatography-mass spectrometry analysis to identify volatile compounds in non-transgenic and transgenic callus and hairy root tissues. Furthermore, to compare and verify the callus transformation system of German chamomile, we transferred McFPS to the hairy roots of German chamomile. The results showed that the optimal conditions for Agrobacterium-mediated callus tissue transformation were as follows: explant, petiole; cefotaxime concentration, 300 mg/L; hygromycin concentration, 10 mg/L; and bacterial solution concentration, OD600 = 0.6; callus transformation efficiency was the highest when the co-culture time was 3 days.

CONCLUSIONS

Establishment of a high-efficiency callus transformation system will lay the foundation for gene function identification in German chamomile.

Development of a New Genetic Transformation System for White and Green Ash Using Embryogenic Cultures

All North American ash (Fraxinus spp.) species are threatened by the emerald ash borer (EAB; Agrilus planipennis), an exotic beetle which has already destroyed millions of ash trees in the U.S. and Canada. Although both chemical insecticides and biological control can be effective, and host resistance appears possible, the speed of the invasion has defied traditional management approaches. One potential, innovative approach to managing this destructive insect is to develop a host tree-induced gene silencing strategy using RNA interference (RNAi) constructs targeting EAB-specific genes. An important requirement for applying RNAi technology is a reliable transformation/regeneration system for the host tree species. We developed an Agrobacterium-mediated gene transfer system for white ash (F. americana) and green ash (F. pennsylvanica) using the embryogenic cultures of these species as target material. Embryogenic suspension cultures of multiple genotypes of both species were plated and inoculated with A. tumefaciens carrying the pFHI-GUSi expression vector, which carries the nptII selectable marker and intron-GUS reporter genes, followed by selection on a semi-solid medium containing geneticin. Putative transgenic events showed expression of the GUS gene at all tested developmental stages from callus to plantlets, and transgene presence in the leaves of regenerated plants was confirmed using PCR. The overall average transformation efficiency achieved was 14.5 transgenic events per gram of tissue. Transgenic somatic seedlings of two white ash and three green ash genotypes were produced and acclimated to greenhouse conditions.

Transformation of European Ash (Fraxinus excelsior L.) Callus as a Starting Point for Understanding the Molecular Basis of Ash Dieback

The population of European ash (Fraxinus excelsior L.) is currently facing the risk of collapse, mainly due to ash dieback, a disease caused by a pathogenic fungus, Hymenoscyphus fraxineus. To facilitate studies into the molecular basis of ash dieback and design breeding strategies for a generation of resistant trees, it is necessary to develop tools enabling the study of gene function in F. excelsior. Despite this, a method for the genetic engineering of F. excelsior is still missing. Here, we report the first successful genetic transformation of F. excelsior callus and a selection process enabling the formation of stable transgenic callus lines. The protocol relies on the use of Agrobacterium tumefaciens to transform callus tissue derived from embryos of F. excelsior. In our experiments, we used the β-glucuronidase (GUS) reporter system to demonstrate the transformation of callus cells and performed RT-PCR experiments to confirm the stable expression of the transgene. Since ash dieback threatens the long-term stability of many native F. excelsior populations, we hope that the transformation techniques described in this manuscript will facilitate rapid progress in uncovering the molecular basis of the disease and the validation of gene targets previously proposed to be linked to the resistance of trees to H. fraxineus pathogenicity.

Agrobacterium tumefaciens-mediated transformation of Coniella granati

Pomegranate fruit rot caused by Coniella granati is among the most devastating diseases threatening pomegranate production. The pathogenic mechanism of this pathogen remains largely unknown due to lack of genetic transformation method. Herein, we developed an approach to the Agrobacterium tumefaciens-mediated transformation (ATMT) of C. granati using a plasmid vector encoding the green fluorescent protein (GFP) and hygromycin resistance (Hyg) genes. This approach yielded C. granati transformants that exhibited uniform, stable green fluorescence. We further optimized this ATMT protocol, enabling us to achieve a transformation efficiency of up to 300 transformants per 0.5 cm² mycelial plug. Together, we thus provide the first report of the stable transformation of C. granati, laying a foundation for future functional studies characterizing this economically important fungal pathogen.

High frequency plant regeneration from leaf culture of Neolamarckia cadamba

Neolamarckia cadamba is a miracle tree species with considerable economic potential uses as a timber wood, woody forage and traditional medicine resource. The present study aimed to establish a highly efficient and robust protocol of plant regeneration for N. cadamba. Greenish callus was induced from very young leaf explants of sterile in vitro plantlets cultured on Murashige and Skoog's (MS) medium supplemented with 3 mg l^-1 thidiazuron (TDZ), 0.1 mg l^-1 2,4-dichlorophenoxyacetic acid (2,4-D) and 0.05 mg l^-1 α-naphthaleneacetic acid (NAA). The callus could differentiate into nodular embryogenic structures or adventitious shoots, and these two regeneration pathways often occurred in the same callus clumps. The micro-shoots developed roots in MS supplemented with 0.05 mg l^-1 NAA and 0.05 mg l^-1 indole-3-butyric acid (IBA), while the nodular embryogenic structures germinated directly and developed into plantlets on induction medium contained with 0.5 mg l^-1 (or 1 mg l^-1) 6-benzyladenine (6-BA) and 0.05 mg l^-1 NAA. The rooted plantlets could be successfully acclimatized to a greenhouse with more than 92.0% survival. This regeneration protocol can be used in large scale cultivation needs and may be useful for future genetic modifications of N. cadamba.

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.