Transient plant gene expression: a simple and reproducible method based on flowing particle gun

Resequencing of cv CRI-12 family reveals haplotype block inheritance and recombination of agronomically important genes in artificial selection

Although efforts have been taken to exploit diversity for yield and quality improvements, limited progress on using beneficial alleles in domesticated and undomesticated cotton varieties is limited. Given the complexity and limited amount of genomic information since the completion of four cotton genomes, characterizing significant variations and haplotype block inheritance under artificial selection has been challenging. Here we sequenced Gossypium hirsutum L. cv CRI-12 (the cotton variety with the largest acreage in China), its parental cultivars, and progeny cultivars, which were bred by the different institutes in China. In total, 3.3 million SNPs were identified and 118, 126 and 176 genes were remarkably correlated with Verticillium wilt, salinity and drought tolerance in CRI-12, respectively. Transcriptome-wide analyses of gene expression, and functional annotations, have provided support for the identification of genes tied to these tolerances. We totally discovered 58 116 haplotype blocks, among which 23 752 may be inherited and 1029 may be recombined under artificial selection. This survey of genetic diversity identified loci that may have been subject to artificial selection and documented the haplotype block inheritance and recombination, shedding light on the genetic mechanism of artificial selection and guiding breeding efforts for the genetic improvement of cotton.

Approaches to achieve high-level heterologous protein production in plants

Plants offer an alternative to microbial fermentation and animal cell cultures for the production of recombinant proteins. For protein pharmaceuticals, plant systems are inherently safer than native and even recombinant animal sources. In addition, post-translational modifications, such as glycosylation, which cannot be achieved with bacterial fermentation, can be accomplished using plants. The main advantage foreseen for plant systems is reduced production costs. Plants should have a particular advantage for proteins produced in bulk, such as industrial enzymes, for which product pricing is low. In addition, edible plant tissues are well suited to the expression of vaccine antigens and pharmaceuticals for oral delivery. Three approaches have been followed to express recombinant proteins in plants: expression from the plant nuclear genome; expression from the plastid genome; and expression from plant tissues carrying recombinant plant viral sequences. The most important factor in moving plant-produced heterologous proteins from developmental research to commercial products is to ensure competitive production costs, and the best way to achieve this is to boost expression. Thus, considerable research effort has been made to increase the amount of recombinant protein produced in plants. This research includes molecular technologies to increase replication, to boost transcription, to direct transcription in tissues suited for protein accumulation, to stabilize transcripts, to optimize translation, to target proteins to subcellular locations optimal for their accumulation, and to engineer proteins to stabilize them. Other methods include plant breeding to increase transgene copy number and to utilize germplasm suited to protein accumulation. Large-scale commercialization of plant-produced recombinant proteins will require a combination of these technologies.

Investigation of the endosperm-specific sucrose synthase promoter from rice using transient expression of reporter genes in guar seed tissue

We report the investigation of an endosperm-specific promoter from the rsus3 gene from rice (Oryza sativa). The promoter was characterized by deletion analysis and transient expression in guar (Cyamopsis tetragonoloba) seed-tissue. Transient expression was monitored by histochemical GUS assay, and quantitative dual reporter assays comprising firefly luciferase as a test reporter, and Renilla luciferase and GUS as reference reporters. These revealed high expression levels of the reporter genes directed by the rsus3 promoter in guar endosperm. Specificity for this tissue in seeds was apparent from a virtual absence of reporter activity in guar cotyledons. Removal of a putative intron region only slightly raised the expression level, whereas duplication of the minimal promoter region, in a tandem-repeat rsus3 promoter construct, retained endosperm specificity in guar, and displayed three times the reporter activity observed with the single copy construct.

A fast and flexible PEG-mediated transient expression system in plants for high level expression of secreted recombinant proteins

Plant expression systems offer a valuable alternative to traditional systems for the production of recombinant biopharmaceuticals. A highly efficient polyethyleneglycol (PEG)-mediated transient expression system for secreted recombinant proteins in plants has been developed. The human vascular endothelial growth factor 121 (rhVEGF) has been successfully expressed and efficiently secreted into the culture medium by transiently transformed moss protoplasts. In order to obtain secretion efficiency data, different expressed signal peptides were analysed and time course studies were performed with expression constructs containing different promoters. The transformation procedure was optimised for high level expression (up to 10 microg/ml) and successfully performed even with a transgenic glyco-engineered strain lacking plant-specific immunogenic sugar residues in N-glycans. The amount of rhVEGF was produced in such quantity that it allowed for the analysis of biological activity, silver-staining and Western blotting, revealing the correct formation and processing of the homodimer. This fast and flexible transient expression system enables feasibility studies and construct optimisation to be concluded within a few days, thus avoiding the time consuming step of having to generate stably transformed lines.

Electric pulse-mediated gene delivery to various animal tissues

Electroporation designates the use of electric pulses to transiently permeabilize the cell membrane. It has been shown that DNA can be transferred to cells through a combined effect of electric pulses causing (1) permeabilization of the cell membrane and (2) an electrophoretic effect on DNA, leading the polyanionic molecule to move toward or across the destabilized membrane. This process is now referred to as DNA electrotransfer or electro gene transfer (EGT). Several studies have shown that EGT can be highly efficient, with low variability both in vitro and in vivo. Furthermore, the area transfected is restricted by the placement of the electrodes, and is thus highly controllable. This has led to an increasing use of the technology to transfer reporter or therapeutic genes to various tissues, as evidenced from the large amount of data accumulated on this new approach for non-viral gene therapy, termed electrogenetherapy (EGT as well). By transfecting cells with a long lifetime, such as muscle fibers, a very long-term expression of genes can be obtained. A great variety of tissues have been transfected successfully, from muscle as the most extensively used, to both soft (e.g., spleen) and hard tissue (e.g., cartilage). It has been shown that therapeutic levels of systemically circulating proteins can be obtained, opening possibilities for using EGT therapeutically. This chapter describes the various aspects of in vivo gene delivery by means of electric pulses, from important issues in methodology to updated results concerning the electrotransfer of reporter and therapeutic genes to different tissues.

DNA electrotransfer: its principles and an updated review of its therapeutic applications

The use of electric pulses to transfect all types of cells is well known and regularly used in vitro for bacteria and eukaryotic cells transformation. Electric pulses can also be delivered in vivo either transcutaneously or with electrodes in direct contact with the tissues. After injection of naked DNA in a tissue, appropriate local electric pulses can result in a very high expression of the transferred genes. This manuscript describes the evolution in the concepts and the various optimization steps that have led to the use of combinations of pulses that fit with the known roles of the electric pulses in DNA electrotransfer, namely cell electropermeabilization and DNA electrophoresis. A summary of the main applications published until now is also reported, restricted to the in vivo preclinical trials using therapeutic genes.

Analysis of the alternative oxidase promoters from soybean

Alternative oxidase (Aox) is a nuclear-encoded mitochondrial protein. In soybean (Glycine max), the three members of the gene family have been shown to be differentially expressed during normal plant development and in response to stresses. To examine the function of the Aox promoters, genomic fragments were obtained for all three soybean genes: Aox1, Aox2a, and Aox2b. The regions of these fragments immediately upstream of the coding regions were used to drive beta-glucuronidase (GUS) expression during transient transformation of soybean suspension culture cells and stable transformation of Arabidopsis. The expression patterns of the GUS reporter genes in soybean cells were in agreement with the presence or absence of the various endogenous Aox proteins, determined by immunoblotting. Deletion of different portions of the upstream regions identified sequences responsible for both positive and negative regulation of Aox gene expression in soybean cells. Reporter gene analysis in Arabidopsis plants showed differential tissue expression patterns driven by the three upstream regions, similar to those reported for the endogenous proteins in soybean. The expression profiles of all five members of the Arabidopsis Aox gene family were examined also, to compare with GUS expression driven by the soybean upstream fragments. Even though the promoter activity of the upstream fragments from soybean Aox2a and Aox2b displayed the same tissue specificity in Arabidopsis as they do in soybean, the most prominently expressed endogenous genes in all tissues of Arabidopsis were of the Aox1 type. Thus although regulation of Aox expression generally appears to involve the same signals in different species, different orthologs of Aox may respond variously to these signals. A comparison of upstream sequences between soybean Aox genes and similarly expressed Arabidopsis Aox genes identified common motifs.

Graft transmission of induced and spontaneous post-transcriptional silencing of chitinase genes

Sense and antisense tobacco chitinase (CHN) transgenes, Luciferase-CHN transcriptional fusions, and promoterless CHN cDNAs were introduced biolistically into CHN transformants of tobacco that never exhibit spontaneous gene silencing. All of the constructs tested induced systemic silencing of the resident CHN transgene and endogenes. Nuclear run-on transcription assays showed that local introduction of additional gene copies triggers systemic post-transcriptional gene silencing (PTGS). Together, this provides evidence that additional transgene copies need not be either highly transcribed or produce sense transcripts to evoke production of systemic PTGS signals. CHN PTGS was transmitted by top grafting, but not by reciprocal grafting of mature stems or the exchange of tissue plugs. Thus, the commonly encountered difficulties in achieving graft-transmission could reflect the method used. Silencing in sense but not antisense transformants was transmitted by grafting to a high-expressing sense CHN scion suggesting that the elaboration of mobile signals may not be an essential feature of antisense-mediated gene silencing.

Interactions and nuclear import of the N and P proteins of sonchus yellow net virus, a plant nucleorhabdovirus

We have characterized the interaction and nuclear localization of the nucleocapsid (N) protein and phosphoprotein (P) of sonchus yellow net nucleorhabdovirus. Expression studies with plant and yeast cells revealed that both N and P are capable of independent nuclear import. Site-specific mutagenesis and deletion analyses demonstrated that N contains a carboxy-terminal bipartite nuclear localization signal (NLS) located between amino acids 465 and 481 and that P contains a karyophillic region between amino acids 40 and 124. The N NLS was fully capable of functioning outside of the context of the N protein and was able to direct the nuclear import of a synthetic protein fusion consisting of green fluorescent protein fused to glutathione S-transferase (GST). Expression and mapping studies suggested that the karyophillic domain in P is located within the N-binding domain. Coexpression of N and P drastically affected their localization patterns relative to those of individually expressed proteins and resulted in a shift of both proteins to a subnuclear region. Yeast two-hybrid and GST pulldown experiments verified the N-P and P-P interactions, and deletion analyses have identified the N and P interacting domains. N NLS mutants were not transported to the nucleus by import-competent P, presumably because N binding masks the P NLS. Taken together, our results support a model for independent entry of N and P into the nucleus followed by associations that mediate subnuclear localization.

Subcellular localization determines the availability of non-targeted proteins to plasmodesmatal transport

BACKGROUND

Individual plant cells are encased in a cell wall. To enable cell-to-cell communication, plants have evolved channels, termed plasmodesmata, to span thick walls and interconnect the cytoplasm between adjacent cells. How macromolecules pass through these channels is now beginning to be understood.

RESULTS

Using two green fluorescent protein (GFP) reporters and a non-invasive transfection system, we assayed for intercellular macromolecular traffic in leaf epidermal cells. Plasmodesmata were found in different states of dilation. We could distinguish two forms of protein movement across plasmodesmata, non-targeted and targeted. Although leaves have generally been considered closed to non-specific transport of macromolecules, we found that 23% of the cells had plasmodesmatal channels in a dilated state, allowing GFP that was not targeted to plasmodesmata to move into neighboring cells. GFP fusions that were targeted to the cytoskeleton or to the endoplasmic reticulum did not move between cells, whereas those that were localized to the cytoplasm or nucleus diffused to neighboring cells in a size-dependent manner. Superimposed upon this non-specific exchange, proteins that were targeted to the plasmodesmata could transit efficiently between 62% of transfected cells.

CONCLUSIONS

A significant population of leaf cells contain plasmodesmata in a dilated state, allowing macromolecular transport between cells. Protein movement potential is regulated by subcellular address and size. These parameters of protein movement illustrate how gradients of signaling macromolecules could be formed and regulated, and suggest that non-cell-autonomous development in plants may be more significant than previously assumed.

In vivo gene gun-mediated DNA delivery into rodent brain tissue

Various types of gene transfer into live tissues have been tried. However, in vivo gene transfer into brain tissue or neuronal cells without virus vector has required a great effort. Particle-mediated gene transfer into live brain tissue was thought to be impossible because of its fragility and the mechanical problem of a previous type of gene gun. In addition, particle-mediated DNA transfer into monolayer-cultured cells without mechanical damage has been difficult. We successfully transferred DNA into rodent live brain tissue and also into monolayer-cultured cells without mechanical damage by using a new type of gene gun and also confirmed gene expression in the brain. This new method represents another variation of gene transfer into the brain.

Activation of systemic acquired silencing by localised introduction of DNA

BACKGROUND

In plants, post-transcriptional gene silencing results in RNA degradation after transcription. Among tobacco transformants carrying a nitrate reductase (Nia) construct under the control of the cauliflower mosaic virus 35S promoter (35S-Nia2), one class of transformants spontaneously triggers Nia post-transcriptional gene silencing (class II) whereas another class does not (class I). Non-silenced plants of both classes become silenced when grafted onto silenced stocks, indicating the existence of a systemic silencing signal. Graft-transmitted silencing is maintained in class II but not in class I plants when removed from silenced stocks, indicating similar requirements for spontaneous triggering and maintenance.

RESULTS

Introduction of 35S-Nia2 DNA by the gene transfer method called biolistics led to localised acquired silencing (LAS) in bombarded leaves of wild-type, class I and class II plants, and to systemic acquired silencing (SAS) in class II plants. SAS occurred even if the targeted leaf was removed 2 days after bombardment, indicating that the systemic signal is produced, transmitted and amplified rapidly. SAS was activated by sense, antisense and promoterless Nia2 DNA constructs, indicating that transcription is not required although it does stimulate SAS.

CONCLUSIONS

SAS was activated by biolistic introduction of promoterless constructs, indicating that the DNA itself is a potent activator of post-transcriptional gene silencing. The systemic silencing signal invaded the whole plant by cell-to-cell and long-distance propagation, and reamplification of the signal.

The wheat wcs120 promoter is cold-inducible in both monocotyledonous and dicotyledonous species

The wcs120 gene is specifically induced by low temperature (LT) and encodes a protein that is thought to play an important role in the cold acclimation process in wheat. To identify the regulatory elements involved in its LT responsiveness, the transient expression activity of different promoter regions was determined using the luciferase reporter gene. The data indicate the involvement of putative enhancer elements, negative and positive regulatory regions in the transcriptional regulation of this gene. The promoter was found to be cold-inducible in different freezing-tolerant and -sensitive monocot and dicot species, suggesting that universal transcription factors responsive to LT may be present in all plants. This promoter could be used to drive the genes needed for LT tolerance in sensitive species.