Specificity of Agrobacterium-mediated delivery of maize streak virus DNA to members of the Gramineae

Geminivirus-Derived Vectors as Tools for Functional Genomics

A persistent issue in the agricultural sector worldwide is the intensive damage caused to crops by the geminivirus family of viruses. The diverse types of viruses, rapid virus evolution rate, and broad host range make this group of viruses one of the most devastating in nature, leading to millions of dollars' worth of crop damage. Geminiviruses have a small genome and can be either monopartite or bipartite, with or without satellites. Their ability to independently replicate within the plant without integration into the host genome and the relatively easy handling make them excellent candidates for plant bioengineering. This aspect is of great importance as geminiviruses can act as natural nanoparticles in plants which can be utilized for a plethora of functions ranging from vaccine development systems to geminivirus-induced gene silencing (GIGS), through deconstructed viral vectors. Thus, the investigation of these plant viruses is pertinent to understanding their crucial roles in nature and subsequently utilizing them as beneficial tools in functional genomics. This review, therefore, highlights some of the characteristics of these viruses that can be deemed significant and the subsequent successful case studies for exploitation of these potentially significant pathogens for role mining in functional biology.

Symptom evolution following the emergence of maize streak virus

For pathogens infecting single host species evolutionary trade-offs have previously been demonstrated between pathogen-induced mortality rates and transmission rates. It remains unclear, however, how such trade-offs impact sub-lethal pathogen-inflicted damage, and whether these trade-offs even occur in broad host-range pathogens. Here, we examine changes over the past 110 years in symptoms induced in maize by the broad host-range pathogen, maize streak virus (MSV). Specifically, we use the quantified symptom intensities of cloned MSV isolates in differentially resistant maize genotypes to phylogenetically infer ancestral symptom intensities and check for phylogenetic signal associated with these symptom intensities. We show that whereas symptoms reflecting harm to the host have remained constant or decreased, there has been an increase in how extensively MSV colonizes the cells upon which transmission vectors feed. This demonstrates an evolutionary trade-off between amounts of pathogen-inflicted harm and how effectively viruses position themselves within plants to enable onward transmission.

Characterization of an Australian isolate of taro bacilliform virus and development of an infectious clone

The badnavirus taro bacilliform virus (TaBV) has been reported to infect taro (Colocasia esculenta L.) and other edible aroids in several South Pacific island countries, but there are no published reports from Australia. Using PCR and RCA, we identified and characterized an Australian TaBV isolate. A terminally redundant cloned copy of the TaBV genome was generated and shown to be infectious in taro following agro-inoculation. This is the first report of TaBV from Australia and also the first report of an infectious clone for this virus.

Development of an efficient vector system for gene knock-out and near in-cis gene complementation in the sugarcane smut fungus

Sporisorium scitamineum is the causative agent responsible for smut disease of sugarcane worldwide. However, lack of efficient gene manipulation system makes this fungus much behind the type model of the smut fungi in molecular biology. Here, we report the development of a CRISPR/Cas9 and T-DNA based dual vector system that allowed efficient knock-out or knock-in of a gene of interest in the S. scitamineum in a site-specific manner. By using Mfa2, a key player in the mating event in S. scitamineum as a tester gene, site-specific insertions of the introduced fragments were achieved both for Mfa2 knockout and complementation. Of particular advantage of this system is the simplicity of selection and identification for the desired transformants by using drug resistance coupled with PCR. This system greatly facilitates the gene function study in S. scitamineum, and could potentially be used for other basidiomycete fungi.

Replication of Plant Viruses

Viruses replicate using both their own genetic information and host cell components and machinery. The different genome types have different replication pathways which contain controls on linking the process with translation and movement around the cell as well as not compromising the infected cell. This chapter discusses the replication mechanisms, faults in replication and replication of viruses co-infecting cells.

Viruses replicate using both their own genetic information and host cell components and machinery. The different genome types have different replication pathways which contain controls on linking the process with translation and movement around the cell as well as not compromising the infected cell. This chapter discusses the replication mechanisms, faults in replication and replication of viruses coinfecting cells.

Agrobacterium: nature's genetic engineer

Agrobacterium was identified as the agent causing the plant tumor, crown gall over 100 years ago. Since then, studies have resulted in many surprising observations. Armin Braun demonstrated that Agrobacterium infected cells had unusual nutritional properties, and that the bacterium was necessary to start the infection but not for continued tumor development. He developed the concept of a tumor inducing principle (TIP), the factor that actually caused the disease. Thirty years later the TIP was shown to be a piece of a tumor inducing (Ti) plasmid excised by an endonuclease. In the next 20 years, most of the key features of the disease were described. The single-strand DNA (T-DNA) with the endonuclease attached is transferred through a type IV secretion system into the host cell where it is likely coated and protected from nucleases by a bacterial secreted protein to form the T-complex. A nuclear localization signal in the endonuclease guides the transferred strand (T-strand), into the nucleus where it is integrated randomly into the host chromosome. Other secreted proteins likely aid in uncoating the T-complex. The T-DNA encodes enzymes of auxin, cytokinin, and opine synthesis, the latter a food source for Agrobacterium. The genes associated with T-strand formation and transfer (vir) map to the Ti plasmid and are only expressed when the bacteria are in close association with a plant. Plant signals are recognized by a two-component regulatory system which activates vir genes. Chromosomal genes with pleiotropic functions also play important roles in plant transformation. The data now explain Braun's old observations and also explain why Agrobacterium is nature's genetic engineer. Any DNA inserted between the border sequences which define the T-DNA will be transferred and integrated into host cells. Thus, Agrobacterium has become the major vector in plant genetic engineering.

Adaptive evolution by recombination is not associated with increased mutation rates in Maize streak virus

BACKGROUND

Single-stranded (ss) DNA viruses in the family Geminiviridae are proving to be very useful in real-time evolution studies. The high mutation rate of geminiviruses and other ssDNA viruses is somewhat mysterious in that their DNA genomes are replicated in host nuclei by high fidelity host polymerases. Although strand specific mutation biases observed in virus species from the geminivirus genus Mastrevirus indicate that the high mutation rates in viruses in this genus may be due to mutational processes that operate specifically on ssDNA, it is currently unknown whether viruses from other genera display similar strand specific mutation biases. Also, geminivirus genomes frequently recombine with one another and an alternative cause of their high mutation rates could be that the recombination process is either directly mutagenic or produces a selective environment in which the survival of mutants is favoured. To investigate whether there is an association between recombination and increased basal mutation rates or increased degrees of selection favoring the survival of mutations, we compared the mutation dynamics of the MSV-MatA and MSV-VW field isolates of Maize streak virus (MSV; Mastrevirus), with both a laboratory constructed MSV recombinant, and MSV recombinants closely resembling MSV-MatA. To determine whether strand specific mutation biases are a general characteristic of geminivirus evolution we compared mutation spectra arising during these MSV experiments with those arising during similar experiments involving the geminivirus Tomato yellow leaf curl virus (Begomovirus genus).

RESULTS

Although both the genomic distribution of mutations and the occurrence of various convergent mutations at specific genomic sites indicated that either mutation hotspots or selection for adaptive mutations might elevate observed mutation rates in MSV, we found no association between recombination and mutation rates. Importantly, when comparing the mutation spectra of MSV and TYLCV we observed similar strand specific mutation biases arising predominantly from imbalances in the complementary mutations G → T: C → A.

CONCLUSIONS

While our results suggest that recombination does not strongly influence mutation rates in MSV, they indicate that high geminivirus mutation rates are at least partially attributable to increased susceptibility of all geminivirus genomes to oxidative damage while in a single stranded state.

Recombination hotspots and host susceptibility modulate the adaptive value of recombination during maize streak virus evolution

BACKGROUND

Maize streak virus -strain A (MSV-A; Genus Mastrevirus, Family Geminiviridae), the maize-adapted strain of MSV that causes maize streak disease throughout sub-Saharan Africa, probably arose between 100 and 200 years ago via homologous recombination between two MSV strains adapted to wild grasses. MSV recombination experiments and analyses of natural MSV recombination patterns have revealed that this recombination event entailed the exchange of the movement protein - coat protein gene cassette, bounded by the two genomic regions most prone to recombination in mastrevirus genomes; the first surrounding the virion-strand origin of replication, and the second around the interface between the coat protein gene and the short intergenic region. Therefore, aside from the likely adaptive advantages presented by a modular exchange of this cassette, these specific breakpoints may have been largely predetermined by the underlying mechanisms of mastrevirus recombination. To investigate this hypothesis, we constructed artificial, low-fitness, reciprocal chimaeric MSV genomes using alternating genomic segments from two MSV strains; a grass-adapted MSV-B, and a maize-adapted MSV-A. Between them, each pair of reciprocal chimaeric genomes represented all of the genetic material required to reconstruct - via recombination - the highly maize-adapted MSV-A genotype, MSV-MatA. We then co-infected a selection of differentially MSV-resistant maize genotypes with pairs of reciprocal chimaeras to determine the efficiency with which recombination would give rise to high-fitness progeny genomes resembling MSV-MatA.

RESULTS

Recombinants resembling MSV-MatA invariably arose in all of our experiments. However, the accuracy and efficiency with which the MSV-MatA genotype was recovered across all replicates of each experiment depended on the MSV susceptibility of the maize genotypes used and the precise positions - in relation to known recombination hotspots - of the breakpoints required to re-create MSV-MatA. Although the MSV-sensitive maize genotype gave rise to the greatest variety of recombinants, the measured fitness of each of these recombinants correlated with their similarity to MSV-MatA.

CONCLUSIONS

The mechanistic predispositions of different MSV genomic regions to recombination can strongly influence the accessibility of high-fitness MSV recombinants. The frequency with which the fittest recombinant MSV genomes arise also correlates directly with the escalating selection pressures imposed by increasingly MSV-resistant maize hosts.

Reconstructing the history of maize streak virus strain a dispersal to reveal diversification hot spots and its origin in southern Africa

Maize streak virus strain A (MSV-A), the causal agent of maize streak disease, is today one of the most serious biotic threats to African food security. Determining where MSV-A originated and how it spread transcontinentally could yield valuable insights into its historical emergence as a crop pathogen. Similarly, determining where the major extant MSV-A lineages arose could identify geographical hot spots of MSV evolution. Here, we use model-based phylogeographic analyses of 353 fully sequenced MSV-A isolates to reconstruct a plausible history of MSV-A movements over the past 150 years. We show that since the probable emergence of MSV-A in southern Africa around 1863, the virus spread transcontinentally at an average rate of 32.5 km/year (95% highest probability density interval, 15.6 to 51.6 km/year). Using distinctive patterns of nucleotide variation caused by 20 unique intra-MSV-A recombination events, we tentatively classified the MSV-A isolates into 24 easily discernible lineages. Despite many of these lineages displaying distinct geographical distributions, it is apparent that almost all have emerged within the past 4 decades from either southern or east-central Africa. Collectively, our results suggest that regular analysis of MSV-A genomes within these diversification hot spots could be used to monitor the emergence of future MSV-A lineages that could affect maize cultivation in Africa.

Construction of infectious clones for DNA viruses: mastreviruses

To characterize a virus at the molecular and biological levels, it is necessary to produce an infectious clone. For most of the Geminiviridae, cloning of the genome is relatively easy because of their small genomes and the presence of the virus double-stranded (replicative) DNA form in infected plants. Indeed, the presence of conserved sequences between species in the genera Begomovirus, Curtovirus, and Topocuvirus allows the PCR amplification of most genomes using degenerate "universal" primers. Unlike the other genera, no universal primers are reported that are suitable for all mastreviruses and alternative, more time-consuming methods must be used. This chapter describes a method that has proven successful for the preparation and testing of infectious clones for a wide range of mastreviruses. It has been designed to ensure its applicability for laboratories throughout the world. Methods are presented for the isolation of total plant DNA and the purification of the replicative (cccDNA) form of the virus using a commercially available plasmid purification kit. Restriction enzyme digestion of the purified DNA using a restriction enzyme with a unique site in the viral genome allows the cloning of a full-length copy of the genome into a high copy number vector, thereby providing a template for sequence analysis and further cloning. The only efficient method for confirming infectivity of mastrevirus clones is using agroinoculation (also termed agroinfection). This requires the production of a multimeric copy of the genome in a T-DNA binary vector, transformation of specific Agrobacterium strains with the binary vector clone, and inoculation of specific regions of seedlings, or seeds, of the appropriate host species. These specific requirements are described and discussed.

An intact RBR-binding motif is not required for infectivity of Maize streak virus in cereals, but is required for invasion of mesophyll cells

The replication-associated protein (RepA) of Maize streak virus interacts in yeast with retinoblastoma-related protein (RBR), the negative regulator of cell-cycle progression. This may allow geminiviruses to subvert cell-cycle control to provide an environment that is suitable for viral DNA replication. To determine the importance of this interaction for MSV infection, the RBR-binding motif, LxCxE, was mutated to IxCxE or LxCxK. Whilst RBR binding in yeast could not be detected for the LxCxK mutant, the IxCxE protein retained limited binding activity. Both mutants were able to replicate in maize cultures and to infect maize plants. However, whereas the wild-type virus invaded mesophyll cells of mature leaves, the LxCxK mutant was restricted to the vasculature, which is invaded prior to leaf maturity. Mature leaves contain high levels of RBR and it is suggested that the MSV RepA-RBR interaction is essential only in tissues with high levels of active RBR.

Transmission of Maize streak virus by vascular puncture inoculation with unit-length genomic DNA

The infectivity of cloned unit-length genomes of Maize streak virus (MSV) was tested using vascular puncture inoculation (VPI). VPI of kernels with plasmid DNA (pUC19) carrying a tandem repeat of the MSV genome produced 33+/-8% infection. Similar plasmids carrying the unit-length MSV genome were not infectious. If the MSV genome was released from the plasmid prior to VPI, 16+/-4% of plants became infected. Ligation of the free linear MSV genome did not increase infectivity. The three infective inocula produced symptoms of similar severity in maize. Bioassay of systemically infected leaves indicated the virus was equally infectious regardless of inoculum. In Southern blots of bioassay plants, no differences in MSV genome restriction endonuclease sites were observed. Thus, inoculation with the free linear or circularized MSV unit-length genome produced infections similar to those with plasmids carrying tandemly repeated genomes. The infectivity of free linear MSV unit-length genomes will facilitate molecular analysis of MSV, because cloning steps are minimized.

Mungbean yellow mosaic virus-Vi Agroinfection by Codelivery of DNA A and DNA B From One Agrobacterium Strain

Agroinfection of bipartite geminiviruses is routinely done by mixing two Agrobacterium strains that independently harbor partial tandem repeats of DNA A and DNA B. We report here an improved agroinfection method for bipartite geminiviruses that utilizes one strain of Agrobacterium that harbors DNA A and DNA B partial tandem repeats on two compatible replicons. A cointegrate vector, pGV2260∷pGV1.3A, with the partial tandem repeat of Mungbean yellow mosaic virus-Vi (MYMV-Vi) DNA A and a binary vector, pGA1.9B, with the partial tandem repeat of MYMV-Vi DNA B gave an agroinfection efficiency of 24% when harbored in two Agrobacterium strains and an efficiency of 61% when harbored in one Agrobacterium strain. A combination of binary vectors, pGA1.9A with MYMV-Vi DNA A partial tandem repeat and pGA1.9B with DNA B partial tandem repeat, gave an agroinfection efficiency of 74% when harbored in two strains. But pGA1.9A and pPZP1.9B (a partial tandem repeat of DNA B), when present in the same Agrobacterium strain, gave 100% agroinfection. Accumulation of viral DNA was shown by Southern blotting. The single-strain method using two compatible replicons consistently gave 100% agroinfection efficiency.

A single amino acid change in the coat protein of Maize streak virus abolishes systemic infection, but not interaction with viral DNA or movement protein

Summary Functional coat protein (CP) is important for host plant infection by monopartite geminiviruses. We identified a proline-cysteine-lysine (PCK) motif at amino acids 180-182 of the maize streak virus (MSV) CP that is conserved in most of the cereal-infecting Mastreviruses. Substitution of the lysine (K) with a valine (V) in the CP of MSV to produce mutant MSVCP182V abolished systemic infection in maize plants, although the mutant replicated around the inoculation site and, unlike other MSV CP mutants, enabled single-stranded (ss) DNA accumulation in suspension cells. The stability of the mutant protein, CP182V, in infected cells was confirmed by immunoblotting, but virions could not be detected. Like the wild-type (wt) CP, CP182V localized to the nucleus when expressed in insect and tobacco cells, and the Escherichia coli-expressed protein bound both ss and double-stranded DNA and interacted with movement protein in vitro. Taken together, these data suggest that mutation of amino acid 182 affects virion formation of MSV, either by affecting encapsidation per se or by affecting particle stability, and that virions are necessary for the long-distance movement of MSV in maize plants.

Interaction of the movement and coat proteins of Maize streak virus: implications for the transport of viral DNA

We have shown previously that the movement protein (MP) and coat protein (CP) of Maize streak virus (MSV) are both required for systemic infection. Towards understanding the roles of these two proteins in virus movement, each was expressed in E: coli and interactions of the MP with viral DNA or CP were investigated using south-western, gel overlay and immunoprecipitation assays. Unlike the CP, the MP did not bind to viral DNA but it interacted with the CP in vitro and an MP-CP complex was detected in extracts from MSV-infected maize, indicating the potential for an interaction in vivo. Microinjection showed that the MP could prevent the nuclear transport of an MSV CP-DNA complex in maize and tobacco cells. These results are consistent with a model in which the MP diverts a CP-DNA complex from the nucleus (where viral DNA replication takes place) to the cell periphery, and in co-operation with the CP, mediates the cell-to-cell movement of the viral DNA. In this respect, the MSV MP and CP have functional analogy with the BC1 and BV1 proteins, respectively, of the BEGOMOVIRUS: genus of the GEMINIVIRIDAE:

Improved Efficiency of Zea mays Agroinoculation with Maize streak virus

Agroinoculation is a technique permitting the transmission of geminivirus genomes cloned in Agrobacterium tumefaciens into a wide variety of mono- and dicotyledonous host plants. Most geminiviruses are obligately transmitted by insect vector species under natural conditions; therefore, agroinoculation has greatly simplified the study of this group of viruses. In many cases, agroinoculation has replaced insect transmission, and has been used to compare virulence characteristics among viruses. Here we report on the discovery that, in agroinfectious Maize streak virus constructs, the orientation of cloned viral genomes relative to the Cauliflower mosaic virus 35S (CaMV35S) promoter of the binary cloning vector pBI121 can significantly affect agroinfectivity of the constructs. Rates at which plants became symptomatic were significantly higher when agroinoculating maize seedlings with constructs containing the CaMV35S promoter upstream of the viral replication-associated protein (Rep) gene than when the same viruses were cloned either in the opposite orientation or into a vector without a strong eukaryotic promoter sequence. Plants infected using the construct with Rep cloned downstream of the CaMV35S promoter also displayed more stunting and, in the early stages of the infection, more severe chlorotic streak symptoms.

Discrete regions of the sensor protein virA determine the strain-specific ability of Agrobacterium to agroinfect maize

The ability of Agrobacterium strains to infect transformation-recalcitrant maize plants has been shown to be determined mainly by the virA locus, implicating vir gene induction as the major factor influencing maize infection. In this report, we further explore the roles of vir induction-associated bacterial factors in maize infection using the technique of agroinfection. The Ti plasmid and virA source are shown to be important in determining the ability of a strain to infect maize, and the monosaccharide binding protein ChvE is absolutely required for maize agroinfection. The linker domain of VirAC58 from an agroinfection-competent strain, C58, is sufficient to convert VirAA6 of a nonagroinfecting strain, A348,to agroinfection competence. The periplasmic domain of VirAC58 is also able to confer a moderate level of agroinfection competence to VirAA6. In addition, the VirAA6 protein from A348 is agroinfection competent when removed from its cognate Ti plasmid background and placed in a pTiC58 background. The presence of a pTiA6-encoded, VirAA6-specific inhibitor is hypothesized and examined.

Infectivity and complete nucleotide sequence of the genome of a genetically distinct strain of maize streak virus from Reunion Island

A complete infectious genome of an isolate of maize streak subgroup 1 geminivirus from Reunion Island (MSV-R) was cloned and sequenced. Using an Agrobacterium tumefaciens Ti plasmid delivery system, the cloned 2.7 kb circular DNA was shown to be infectious in maize. The agroinfected virus could be transmitted by Cicadulina mbila, the most common vector species of MSV in Reunion. Analysis of open reading frames (ORFs) revealed seven potential coding regions including the 4 ORFs conserved in all geminiviruses infecting monocotyledonous plants, the 2 on the viral "+" strand (MP, CP), and the 2 on the complementary "-" strand (RepA, RepB). The nucleotide sequence of MSV-R was compared to previously determined sequence of three African clones from Nigeria (MSV-N), Kenya (MSV-K), and South Africa (MSV-S). More similarity was found between the African clones (97.0-97.3%) than between these and MSV-R (94.4-95.3%). Nucleotide substitutions were frequent in the large intergenic region, particularly in and around the most likely TATA box for the complementary sense genes, and in the 5' end of ORF V1. The comparison of the predicted peptide sequences of the proteins encoded by ORFs MP, RepA and RepB confirmed the higher similarity between the African clones (97.8-99.3%) than between these and MSV-R (95.1-97.1%). However the amino acid sequences of the protein encoded by ORF CP (capsid protein) were very conserved among all the 4 clones, suggesting a high selection pressure on this ORF.

Isolation of monoclonal antibodies reacting with the core component of lipopolysaccharide from Rhizobium leguminosarum strain 3841 and mutant derivatives

Monoclonal antibodies reacting with the core oligosaccharide or lipid A component of Rhizobium lipopolysaccharide (LPS) could be useful for the elucidation of the structure and biosynthesis of this group of macromolecules. Mutant derivatives of Rhizobium leguminosarum 3841 with LPS structures lacking the major O-antigen moiety were used as immunogens, and eight antibodies were selected for further study. All the antibodies reacted with the fast-migrating species known as LPS-2 following gel electrophoresis of Rhizobium cell extracts. For four of these antibodies, reactivity with affinity-purified LPS was lost after mild acid hydrolysis, indicating that they probably recognized the core oligosaccharide component. The four other antibodies still reacted with acid-treated LPS and may recognize the lipid A moiety, which is stable to mild acid hydrolysis. The pattern of antibody staining after gel electrophoresis revealed differences in LPS-2 epitope structure between each of the mutants and the wild type. Furthermore, for each of the mutants the antibodies crossreacted with a minor band that migrated more slowly than LPS-2; we have termed this more slowly migrating form LPS-3. The majority of the antibodies also reacted with LPS from strain CE109, a derivative of Rhizobium etli CE3, confirming that the LPS core antigens can be relatively conserved between strains of different Rhizobium species. One of the antibodies isolated in this study (JIM 32) was unusual because it appeared to react with all forms of LPS from strain 3841 (namely, LPS-1, LPS-2, and LPS-3). Furthermore, JIM 32 reacted positively with the LPS from many strains of Rhizobium tested (excluding the Rhizobium meliloti subgroup). JIM 32 did not react with representative strains from Bradyrhizobium, Azorhizobium or other related bacterial species.

Construction of Agrobacterium strains by electroporation of genomic DNA and its utility in analysis of chromosomal virulence mutations

We have extended the technique of electroporation as a genetic tool for manipulating the Agrobacterium tumefaciens chromosome. We used this technique to introduce chromosomal DNA into recipient A. tumefaciens strains by electroporation and constructed isogenic chvE mutants that share the same chromosomal background but differ in their types of pTi (octopine or nopaline). Both nopaline and octopine pTi-carrying chvE mutants were deficient in vir regulon induction and exhibited similar reductions in host range.

Host recognition by the VirA, VirG two-component regulatory proteins of agrobacterium tumefaciens

Agrobacterium tumefaciens contains about 25 vir genes localized on a 200-kb tumour-inducing (Ti) plasmid that direct a conjugation-like transfer of tumorigenic DNA from the bacterium to the nuclei of infected plant cells. These genes are strongly and coordinately induced during infection in response to three different classes of stimuli which are thought to be key chemical features of a typical wound site. These stimuli are (i) guaiacol and syringol derivatives such as acetosyringone, (ii) sugars such as glucose and glucuronic acid, and (iii) acidic pH. The sensing of these compounds is carried out by the VirA, VirG and ChvE proteins. VirA is a four-domain histidine protein kinase, while VirG is a transcriptional activator which is activated by VirA-mediated phosphorylation. ChvE is a chromosomally encoded periplasmic sugar binding protein which is required for sensing sugars but dispensable for sensing the other two stimuli. Here we will review the nature of these chemical stimuli, the structure and function of the three regulatory proteins, their similarity to sensors found in human and animal pathogens, the factors influencing their pool size, and their role in the host range of different strains of A. tumefaciens.

Geminiviruses: plant viral vectors

Geminiviruses are being used as convenient autonomously replicating vectors for foreign gene amplification in plants. Using tissue culture techniques, they have been adapted for the analysis of the regulation of gene expression in a wide range of hosts, including both mono- and dicotyledonous species. In monocotyledonous plants that are particularly recalcitrant to transformation, geminivirus symptom-induction has been used as a sensitive marker for DNA uptake.

An Agrobacterium two-component regulatory system for the detection of chemicals released from plant wounds

Crown gall tumorigenesis by Agrobacterium tumefaciens requires the co-ordinate transcriptional induction of a set of pathogenesis genes. At least three classes of environmental stimuli act synergistically to induce these genes: (i) monocyclic aromatic hydrocarbons such as acetosyringone, coniferyl alcohol, and vanillin, (ii) neutral or acidic monosaccharides such as glucose and glucuronic acid, and (iii) acidic pH. Three proteins are required to sense and respond to these stimuli: (i) VirA, a transmembrane sensory protein and histidine protein kinase, (ii) VirG, a transcriptional activator which is phosphorylated by phosphoryl VirA, and (iii) ChvE, a periplasmic sugar-binding protein. VirA and VirG are members of the so-called two-component family of regulatory proteins. This regulatory system continues to offer new discoveries in the areas of signal transduction, host-microbe interactions, and host range.

Mutational analysis of the small intergenic region of maize streak virus

Maize streak virus (MSV) is a leafhopper-transmitted geminivirus containing one molecule of circular single-stranded DNA of about 2.7 kb. We have tested the infectivity of MSV mutants. A deletion of 29 bases in the small intergenic region (SIR) did not affect the infectivity of MSV. Mutants containing insertions of oligonucleotides of up to 32 bases at the AsnI site in SIR were also infectious. However, the infection efficiency of the insertion mutants decreased as the size of the oligonucleotides increased. This reduced virulence may be due to a decreased efficiency of 3'-end formation of the complementary sense mRNA(s). The stability of MSV mutants in infected maize plants was analyzed by polymerase chain reaction (PCR). Some oligonucleotide-insertion mutants were completely stable while others obtained deletions. Infectivity of the mutants, however, did not require deletion formation. Deletion rearrangement was shown to be dependent on the sequence of the inserted oligonucleotide. The AsnI site is the only site known to permit insertion of nonviral DNA without abolishing MSV infectivity. The system described may allow the use of MSV as a delivery vector to introduce at least small segments of DNA into maize.

Point substitution in a promoter-like region and the V1 gene affect the host range and symptoms of maize streak virus

The nucleotide sequences of full-length infectious clones of two symptomatic and host range variants (MSV-Ns and MSV-Nm) of the Nigerian strain of maize streak virus (MSV) have been determined and shown to differ by only three nucleotides. MSV-Ns produced symptoms in infected maize plants sooner and the streaks were wider and more chlorotic than those of MSV-Nm; variant MSV-Ns also had a wider host range within the Gramineae. None of the three nucleotide differences resulted in amino acid changes. Site-directed mutagenesis showed that a substitution at nucleotide (nt) 40 in the V1 gene affected streak width, while severity of chlorosis, length of streaks, latency, and host range was determined by a single base change at nt 2473 in the large intergenic region. The nt 2473 change altered a potential promoter sequence (TATA box) in MSV-Ns 101 nucleotides upstream of the initiation codon of the C1 gene. Mutagenesis of TATA sequences located downstream of TATA -101 showed that TATA -101 alone was sufficient to confer a wide host range phenotype on MSV-Ns and suggested that it might function as a promoter for the expression of complementary-sense open reading frames. When compared with an updated promoter consensus derived from genes of the Gramineae, the promoter context around TATA -101 in MSV-Ns was not more favorable than those found at -57 and -62 in MSV-Nm.

Inheritance and expression of chimeric genes in the progeny of transgenic maize plants

We obtained transgenic maize plants by using high-velocity microprojectiles to transfer genes into embryongenic cells. Two selectable genes were used to confer resistance to either chlorsulfuron or phosphinothricin, and genes encoding either E. coli beta-glucuronidase or firefly luciferase were used as markers to provide convenient assays for transformation. When regenerated without selection, only two of the eight transformed embryogenic calli obtained produced transgenic maize plants. With selection, transgenic plants were obtained from three of the other eight calli. One of the two initial lines produced 15 fertile transgenic plants. The progeny of these plants contained and expressed the foreign genes. Luciferase expression could be visualized, in the presence of added luciferin, by overlaying leaf sections with color film.

Host range and symptoms are determined by specific domains of the maize streak virus genome

We have cloned two distinct symptomatic variants of the geminivirus streak virus from maize plants infected with the Nigerian strain (MSV-N). Following "agroinoculation" to maize plants MSV-Nm produces narrow, mildly chlorotic discontinuous streaks, whereas MSV-Ns-infected tissue has wide, severely chlorotic streaks. Symptom appearance is delayed following MSV-Nm inoculation. MSV-Nm has a narrow host range within the Gramineae comprising a fraction of that of the wide host range isolate MSV-Ns. The two isolates are highly homologous and have identical restriction enzyme maps. In order to localize the determinants of pathogenicity we constructed, in vitro, hybrid genomes by restriction enzyme fragment exchange. The determinants of host range, severity of chlorosis, streak length, and timing of symptom appearance map to a fragment which includes the large intergenic region and the 5' terminus of the complementary sense C1 gene. Streak width is determined by the virion-sense portion of the genome, which is consistent with the observation that the virion-sense gene products (V1 and V2) are required for spread of the virus.

Agrobacterium-mediated plant transformation by novel mini-T vectors in conjunction with a high-copy vir region helper plasmid

A new binary vector system for Agrobacterium-mediated plant transformation was developed. A set of four mini-T vectors comprised of T-DNA border sequences from nopaline-type Ti-plasmid pTiC58 flanking a chimaeric hygromycin-resistance gene for selection of transformants and up to eight unique restriction sites for cloning foreign DNA was constructed on a broad-host replicon containing the oriV of plasmid pSa. In two of the constructs these multiple cloning sites are flanked by a strong promoter to activate transcription of inserted DNA in planta. High-efficiency transformation was prompted by a high-copy, stable virulence helper plasmid pUCD2614, which contains a cloned virulence region of pTiC58 and tandem copies of the par locus of plasmid pTAR. Southern blot hybridization and genetic analyses of the progeny of transformed plants showed that the hygromycin resistance gene was stably inherited.

Agrobacterium - and microprojectile - mediated viral DNA delivery into barley microspore-derived cultures

Anther cultures of barley (Hordeum vulgare L. var. "Igri") were used as targets for Agrobacterium-mediated DNA transfer and direct DNA uptake by particle bombardment. A wheat dwarf virus construct which can replicate to a high copy number in cereal cells provided a sensitive marker for successful DNA delivery. Although DNA delivery was achieved using both procedures, particle bombardment gave more reproducible and higher levels of infection. The ability to deliver DNA into cereal cells which have a high regeneration capacity may provide a route for stable transformation.