Agrobacterium-mediated inoculation of plants with tomato golden mosaic virus DNAs

Strategies for expression of foreign genes in plants. Potential use of engineered viruses

Advances in gene transfer techniques for higher plants have already permitted important achievements towards crop protection and improvement using recombinant DNA technology. Besides plant genetic engineering, the possible use of plant viruses to express foreign genes could be of considerable interest to plant biotechnology. However, insuring containment of engineered viruses for environmental use is an important safety issue that must be addressed.

Infectivity and complete nucleotide sequence of the cloned genomic components of a bipartite squash leaf curl geminivirus with a broad host range phenotype

Through cloning and molecular analysis we have identified two highly homologous bipartite geminiviruses as causing squash leaf curl disease. Mechanical and Agrobacterium-mediated inoculation of plants with cloned viral DNA components identified the two genomic components of SqLCV-E, a squash leaf curl virus with an unexpectedly broad host range for a whitefly-transmitted geminivirus. Nucleotide sequence analysis of the genome of this virus showed it to have the same bipartite component organization characteristic of other whitefly-transmitted geminiviruses. Sequence comparison with the genomic components of tomato golden mosaic virus and bean golden mosaic virus revealed a close evolutionary relationship with these two bipartite geminiviruses, with which SqLCV-E shares common hosts. These studies provide clear molecular evidence for the assignment of SqLCV to the subfamily of bipartite geminiviruses.

Molecular characterization of two bipartite geminiviruses causing squash leaf curl disease: role of viral replication and movement functions in determining host range

The genomes of two distinct, but highly homologous, bipartite geminiviruses have been identified in and cloned from extracts of squash leaf curl diseased field squash. These two squash leaf curl viruses (SqLCVs) have covalently closed, circular single-stranded DNA genomes with the same bipartite component organization characteristic of other whitefly-transmitted geminiviruses. Infectivity studies using virus preparations or cloned viral genomic components on different potential host plants demonstrated that these two SqLCVs have different host range phenotypes which can be explained by specific interactions among the different viral genomic components that act to influence viral replication and systemic movement in the plant. Analysis of Agrobacterium-inoculated leaf discs demonstrated that replication of the restricted virus was rescued in trans by the nonrestricted virus, providing an explanation for the mixtures of viral DNA components often found in particular hosts in the field. Sequence analysis of the common regions of these two SqLCVs identified a 13-base deletion in the restricted virus as compared to the nonrestricted virus, suggesting a potential sequence alteration likely to be involved in their host range phenotypic differences and strengthening the conclusion based on hybridization studies of their close evolutionary relationship. Also identified in the original field squash was a defective viral component which appeared to interfere with movement of the restricted SqLCV in its normally permissive hosts and accounted for another aspect of host range variation observed for this virus.

Transactivation in a geminivirus: AL2 gene product is needed for coat protein expression

The beta-glucuronidase (GUS) reporter gene was used to replace the coat protein gene (open reading frame AR1) of tomato golden mosaic virus (TGMV) and transiently expressed in tobacco protoplasts. While these TGMV/GUS genomes gave a high level of GUS activity, genomes which also contained a mutation in the AL2 open reading frame (TGMV/GUS/AL2-) did not express GUS. GUS activity could be restored by cotransfecting protoplasts with the TGMV/GUS/AL2- genome and a wild-type TGMV genome. Thus, the AL2 gene product transactivates expression of TGMV coat protein gene.

Genetic analysis of tomato golden mosaic virus: ORF AL2 is required for coat protein accumulation while ORF AL3 is necessary for efficient DNA replication

Tomato golden mosaic virus (TGMV) is a geminivirus whose genome is divided between two DNA components, designated A and B. The TGMV genome contains six open reading frames (ORFs) which can encode proteins of greater than 10 kDa. We have used a protoplast transfection system to determine the effects of viral proteins, as defined by these ORFs, on the accumulation of viral DNA in infected cells. The accumulation of cost protein was also examined in leaf discs. Our results indicate that mutations in ORFs AR1 and AL2 do not affect viral double-stranded DNA (dsDNA) levels, although AR1 and AL2 mutants accumulate only small amounts of single-stranded viral DNA (ssDNA). In contrast, a large reduction in both ss- and dsDNA levels is observed when a mutation is introduced into ORF AL3. Mutations within either of the two DNA B ORFs do not affect DNA replication. The AL3, BR1, and BL1 mutants are capable of synthesizing coat protein; however, coat protein is not detected in leaf discs inoculated with AR1 or AL2 mutants. Testable models are proposed to explain the influence of AL2 protein on coat protein accumulation and to account for the stimulation of viral DNA synthesis mediated by the AL3 gene product.

Localized transient expression of GUS in leaf discs following cocultivation with Agrobacterium

A chimaeric gene has been constructed that expresses beta-D-glucuronidase (GUS) in transformed plant tissues, but not in bacterial cells. This gene has proved extremely useful for monitoring transformation during the period immediately following gene transfer from Agrobacterium tumefaciens. GUS expression was detectable 2 days after inoculation, peaked at 3-4 days and then declined; if selection was imposed expression increased again after 10-14 days. The extent of transient expression after 4 days correlated well with stable integration as measured by kanamycin resistance, hormone independence, and gall formation. Histochemical staining of inoculated leaf discs confirmed the transient peak of GUS expression 3-4 days after inoculation. The most surprising result was that the blue staining was concentrated in localized zones on the circumference of the disc; within these zones, essentially all the cells appeared to be expressing GUS. We suggest that the frequency of gene transfer from Agrobacterium is extremely high within localized regions of leaf explants, but that the frequency of stable integration is several orders of magnitude lower.

Transcription map of the B genome component of tomato golden mosaic virus and comparison with A component transcripts

In a previous study, the bipartite genome of tomato golden mosaic virus (TGMV) was shown to be transcribed into at least six polyadenylated RNAs (G. Sunter, W.E. Gardiner, and D. M. Bisaro, 1989, Virology 170, 243-250). Two of these, a 1.3-kb complementary sense and a 0.9-kb viral sense transcript, were mapped to the B genome component of this geminivirus. The results of more detailed primer extension and S1 nuclease protection experiments presented here define the limits of the single transcription unit corresponding to the 0.9-kb RNA which spans the BR1 open reading frame (ORF). The data also demonstrate that complementary sense TGMV RNAs are more complex than indicated by our earlier studies. Analysis of the 1.3-kb BL1-specific RNA indicates that it is actually a family of distinct transcripts with different start sites. Three transcripts have 5' ends that map near the common region of DNA B and all of these start sites lie upstream of the BL1 ORF. Similar analysis of the 1.6-kb complementary sense AL1 RNA indicates that a complex set of transcripts also map to the analogous region of genome component A. Four transcripts have 5' ends that map near the common region but only one of these start sites is upstream of the initiation codon for the AL1 open reading frame (ORF). None of the transcripts appear to be processed. The possible significance of multiple transcripts in these regions of the TGMV genome is discussed, and the common region-proximal transcription units of the A and B genome components are compared.

The coat protein of beet curly top virus is essential for infectivity

We have applied the procedure of Agrobacterium-mediated inoculation to develop a simple, efficient, and reproducible assay for the infectivity of the leafhopper-transmitted geminivirus, beet curly top virus (BCTV). This assay system was used to show that a coat protein mutant of BCTV is not infectious, but could be complemented by coagroinoculation with a second mutant bearing a lethal mutation in the complementary-sense open reading frame, C1. Furthermore, the coat protein mutant retained the ability to replicate and to produce both ssDNA and dsDNA when electroporated into Nicotiana tabacum protoplasts. We conclude that the coat protein of BCTV is essential for spread of the virus. The results are discussed in the light of results with coat protein mutants of other geminiviruses.

DNA transfer from Agrobacterium to Zea mays or Brassica by agroinfection is dependent on bacterial virulence functions

DNA transfer from Agrobacterium tumefaciens, a soil bacterium, to the non-host graminaceous monocotyle-donous plant Zea mays, was analysed using the recently developed technique of agroinfection. Agroinfection of Z. mays with maize streak virus using strains of A. tumefaciens carrying mutations in the pTiC58 virulence region showed an almost absolute dependence on the products of the bacterial virC genes. In contrast, agroinfection of the control host Brassica rapa with cauliflower mosaic virus was less dependent on the virC gene products. In other respects, the basic mechanism of the plant-bacterium interaction was found to be similar. While intact virA, B, D and G functions were absolutely necessary, mutants in virE were attenuated. Agroinfection of maize was effective in the absence of an exogenously supplied vir gene inducer, and indeed wounded Z. mays tissues were found to produce substance(s) which induced the expression of A. tumefaciens vir genes. These findings are discussed in the light of current knowledge about the function of Agrobacterium vir genes.

Identification of tomato golden mosaic virus-specific RNAs in infected plants

The bipartite genome of the geminivirus tomato golden mosaic virus (TGMV) contains at least six open reading frames (ORFs) with the potential to code for proteins of greater than 100 amino acids. In order to investigate the expression of these coding regions, RNA preparations from plants infected with TGMV have been examined for the presence of viral transcripts. We have identified six polyadenylated, virus-specific RNAs which correspond in size, polarity and map location to the six ORFs. Primer extension and S1 nuclease analysis of an RNA which maps to the viral coat protein gene (ORF AR1) has shown that this transcription unit begins at nucleotide 319 or 320 and ends in the vicinity of nucleotide 1090 of the TGMV A sequence, in agreement with a previous report (I.T.D. Petty, R.H.A. Coutts, and K.W. Buck, 1988, J. Gen. Virol. 69, 1359-1365). The data presented here confirm the bidirectional transcription strategy implied by the arrangement of ORFs on both strands of double-stranded TGMV DNA intermediates and lay the ground-work for further studies of viral transcription and its control.

Geminivirus genes and vectors

The geminiviruses are very small plant viruses with circular single-stranded DNA genomes. Recent advances have identified genes involved in replication, spread of virus or DNA in the plant, and insect transmission. Gene replacement experiments suggest that useful plant gene expression vectors can be constructed from these viruses.

Cassava latent virus infections mediated by the Ti plasmid of Agrobacterium tumefaciens containing either monomeric or dimeric viral DNA

Plant infections with cassava latent virus (CLV) were mediated by the Ti plasmid of Agrobacterium tumefaciens containing either monomeric or dimeric copies of the virus genome. The CLV DNAs caused typical symptoms when they were inoculated in Agrobacterium strains C58, LBA4404 and a virE mutant A1026, but not other Agrobacterium strains with mutations in other vir loci or an E. coli polA strain. Virus-specific DNA forms characteristic of normal CLV infections were found after such infection. Characterization of progeny CLV DNA from selected plants identified several infectious mutants. These were found to be small insertions and/or deletions in the coat protein gene of DNA 1 and in the intergenic region of DNA 2.