Virus resistant plants having antisense RNA

Abstract: The present invention discloses the making of plant cells which con­ tain antisense RNA (aRNA) complementary to the mRNA of a target plant virus. Construction of aRNA genes and transformation of aRNA genes into plant cells is also taught. Such cells are relatively resistant to infec­ tion by the target virus when compared with cells not containing aRNA. Furthermore, methods and DNA molecules useful for producing plant cells containing aRNA are also disclosed.

Description: FIELD The present invention is in the fields of genetic engineering and plant husbandry, and especially provides a means for producing a virus resistant plant by transforming a plant to contain a plant-expressible foreign gene directing synthesis of RNA complementary to a viral mRNA. Also provided are plant-transforming prokaryotic plasmid vectors carrying such plant-expressible RNA and plant cells transformed by such a vector. BACKGROUND Overview of Agrobacterium Virulent strains of the gram-negative genus Agrobacterium harbor large plasmids known as Ti (tumor- or transformation-inducing) plasmids (pTi) in A . tumefaciens and Ri (root-inducing) plasmids (pRi) in A . rhizogenes , often classified by the opine which they catabolize or cause to be synthesized. Ti and Ri plasmids both contain DNA sequences, known as T-DNA (transferred-DNA), which in tumors are found to be integrated into the genome of the host plant. Several T-DNA genes are under control of T-DNA promoters which resemble canonical eukaryotic promoters in structure. These plasmids also carry genes outside the T-DNA region. Ti and Ri plasmids are for many purposes functionally equivalent. Reviews of Agrobacterium -caused disease, plant transformation, genetic engineering, and gene expression include those by, or found in, Merlo DJ (1982) Adv. Plant Pathol. 1 :139-178; Ream LW and Gordon MP (1982) Science 218 :854-859; Bevan MW and Chilton M-D (1982) Ann. Rev. Genet. 16 :357-384; Kahl G and Schell J (1982) Molecular Biology of Plant Tumors ; Barton KA and Chilton M-D (1983) Meth. Enzymol. 101 :527-539; Depicker A et al . (1983) in Genetic Engineering of Plants: an Agricultural Perspective , eds.: Kosuge T et al ., pp. 143-176; Caplan A et al . (1983) Science 222 :815-821; Hall TC et al ., European Patent application 126,546; Binns AN (1984) Oxford Surveys Plant Mol. Cell Biol. 1 :130-160; Hall TC (1985) Oxford Surveys Plant Mol. Biol. 2 :329-338; Hooykaas PJJ and Schilperoort RA (1985) Trends Biochem. Sci. 10 :307-309; Thomas TL and Hall TC (1985) Bioassays 3 :149-153; Weissbach A and Weissbach H, eds. (1986) Meth. Enzymol. 118 (see especially Rogers SG et al ., pp. 627-640); Pühler A, ed, (1983) Molecular Genetics of the Bacteria-Plant Interaction ; and Schilperoort RA (1984) in Efficiency in Plant Breeding (Proc. 10th Congr. Eur. Assoc. Res. Plant Breeding), eds.: Lange W et al ., pp. 251-285. Transformation of Plants by Agrobacterium Plant cells can be transformed by Agrobacterium by several methods well known in the art. For a review of recent work, see Syono K (1984) Oxford Surveys Plant Mol. Cell Biol. 1 :217-219. The infection of plant tissue by Agrobacterium is a simple technique well known to those skilled in the art. Typically after being wounded, a plant is inoculated with a suspension of the bacteria. Alternatively, tissue pieces are innoculated, e.g. leaf disks (Horsch RB et al . (1985) Science 227 :1229-1231). After induction with wild-type Agrobacterium , the tumors are capable of phytohormone-independent growth. Traditional inoculation and culture techniques may be modified for use of disarmed T-DNA vectors incapable of hormone-independent growth (e.g. see Zambryski P et al . (1984) in Genetic Engineering, Principles, and Methods , 6 , eds.: Hollaender A and Setlow J, pp. 253-278). Agrobacterium is also capable of infecting isolated cells, cells grown in culture, callus cells, and isolated protoplasts (e.g. Fraley RT et al . (1984) Plant Mol, Biol. 3 :371-378; Fraley RT and Horsch RB (1983) in Genetic Engineering of Plants: an Agricultural Perspective , eds.: Kosuge T et al ., pp. 177-194; Muller A et al . (1983) Biochem, Biophys. Res. Comm. 123 :458-462). The transformation frequency of inoculated callus pieces can be increased by addition of an opine or opine precursors (Cello LM and Olsen WL, U.S. Patent 4,459,355). The host range of crown gall pathogenesis may be influenced by T-DNA-encoded functions such as onc genes (Hoekema A et al . (1984) J. Bacteriol. 158 :383-385; Hoekema A et al . (1984) EMBO J. 3 :3043-3047; Buchholz WC and Thomasshow MF (1984) 160 :327-332; Yanofsky M (1985) Mol. Gen. Genet. 201 :237-246). Vir genes also affect host range (Yanofsky, supra ). Ausich RL, European Patent Application 108,580, reports transfer of T-DNA from A . tumefaciens to green algal cells, and expression therein of ocs and Tn 5 kanamycin resistance genes. Hooykaas-van Slogteren GMS et al . (1984) Nature 311 :763-764, and Hernalsteens J-P et al . (1984) EMBO J. 3 :3039-3041, have demonstrated transformation of monocot cells by Agrobacterium without the customary tumorigenesis. T-DNA, disarmed T-DNA, and functional foreign genes of transformed plants are usually transmitted through meiosis to progeny seemingly unaltered in a dominant, closely-linked, Mendelian fashion (e.g. see Horsch RB et al . (1984) Science 223 :496-498; Tepfer D (1984) Cell 37 :959-967; DeBlock M et al . (1984) EMBO J. 3 :1681-1689; Wöstemeyer A et al . (1984) Mol. Gen. Genet. 194 :500-507; Wallroth M et al . (1986) Mol. Gen. Genet. 202 :6-15). Two unlinked T-DNAs can transform a single cell and, after plant regeneration, segregate in the F1 generation (de Framond AJ et al . (1986) Mol. Gen. Genet. 202 :125-131). Ti Plasmid DNA T-DNA is often integrated (i.e. inserted) into host DNA at multiple sites in the nucleus. Flanking plant DNA may be either repeated or low copy number sequences. Integrated T-DNA can be found in either direct or inverted tandem arrays and can be separated by spacers. T-DNA can also transform chloroplasts (De Block M et al . (1985) EMBO J. 4 :1367-1372; see review by Flavell RB (1985) Bioessays 3 :177-178). The complete sequence of the T-DNA of an octopine-type plasmid found in ATCC 15955, pTi15955, has been reported (Barker RF et al . (1983) Plant Mol. Biol. 2 :335-350) as has the T L region of pTiAch5 (Gielen J et al . (1984) EMBO J. 3 :835-846). Published T-DNA genes do not contain introns. Sequences resembling canonical eukaryotic promoter elements and polyadenylation sites can be recognized. Octopine Ti plasmids carry an ocs gene which encodes octopine synthase (lysopine dehydrogenase). Koncz C et al . (1983) EMBO J. 2 : 1597-1603 provides a functional analysis of ocs . Dhaese P et al . (1983) EMBO J. 2 :419-426, reported the utilization of various polyadenylation sites by "transcript 7" (ORF3 of Barker R et al ., supra ) and ocs . The presence of the enzyme octopine synthase within a tissue can protect that tissue from the toxic effect of various amino acid analogs, e.g. aminoethyl cysteine (Dahl GA and TempeJ (1983) Theor. Appl. Genet. 66 :233-239; Koziel MG et al . (1984) J. Mol. Appl. Genet. 2 :549-562). Nopaline Ti plasmids encode the nopaline synthase gene ( nos ) (sequenced by Depicker A et al . (1982) J. Mol. Appl. Genet. 1 :561-573). Shaw CH et al . (1984) Nucl. Acids Res. 12 :7831-7846, provides a functional analysis of nos . Genes equivalent to tms and tmr have been identified on a nopaline-type plasmid (Willmitzer L et al . (1983) Cell 32 :1045-1056). Ti and Ri plasmid genes outside of the T-DNA region include the vir genes, which when mutated result in an avirulent Ti plasmid. The vir genes function in trans , being capable of causing the transformation of plant cells with T-DNA of a different plasmid type and physically located on another plasmid. Such arrangements are known as binary systems and the T-DNA bearing plasmids are generally known as micro-Ti plasmids. Disclosed binary systems and micro-Ti plasmids include the following: Hoekema A et al . (1985) Plant Mol. Biol. 5 :85-89; Deblaere R et al . (1985) Nucl. Acids Res. 13 :4777-4788; van den Elzen P et al (1985).Plant Mol. Biol. 5 :149-154; Anderson DM, U.S. Patent 4,536,475; de Framond AJ et al . (1983) Biotechnol. 1 :262-269; Hoekema A et al . (1983) Nature 303 :179-180; Hille J et al , (1984) J. Bacteriol. 158 :754-756; Hoekema A et al . (1984) J. Bacteriol. 158 :383-385; An G et al . (1985) EMBO J. 4 :277-284; Anderson DM, U.S. Patent 4,536,475; Klee HJ et al . (1985) Biotechnol. 3 :637-642); de Framond AJ et al . (1986) Mol. Gen. Genet. 202 :125-131; Dahl GA et al ., European Patent application 140,556; and Bevan M (1984) Nucl. Acids Res. 12 :8711-8721. T-DNA need not be on a plasmid to transform a plant cell; chromosomally located T-DNA is functional (Hoekema A et al . (1984) EMBO J. 3 :2485-2490). T-DNA has direct repeats of about 25 base pairs associated with the left and right borders, i.e. with the T-DNA/plant DNA junctions, which may be involved in either transfer from Agrobacterium or integration into the host genome. Ti plasmid-determined characteristics have been reviewed by Merlo, supra (see especially Table II), and Ream and Gordon, supra . Foreign Gene Expression A gene encoding bean phaseolin has been transferred into and expressed in sunflower tumors (Murai N et al . (1983) Science 222 :476-482). The phaseolin gene was expressed at a high level in developing tobacco seeds (Sengupta-Gopalan C et al . (1985) Proc. Natl. Acad. Sci. USA 82 :3320-3324). Similar results have been observed with a homologous gene, soybean beta -conglycinin (Beachy RN et al . (1985) EMBO J. 4 :3047-3053). A gene for the endosperm protein zein, from the monocot Zea mays , is transcribed in sunflower callus (Matzke MA et al . (1984) EMBO J. 3 : 1525-1531). Expression of a pea RuBP-case small subunit gene is light-regulated in transformed petunia cells; the pea small subunit protein produced is correctly processed and sequestered within chloroplasts (Broglie R et al . (1984) Science 224 :838-843). Sequences involved in this light-inducibility and those needed for maximal expression have been identified (Morelli G et al . (1985) Nature 315 :200-204; Nagy F et al . (1985) EMBO J. 4 :3063-3068; Timko MP et al . (1985) Nature 318 :579-582). Expression of a wheat chlorophyll a / b binding protein gene is light-regulated and organ-specific in transformed tobacco plants (Lamppa G et al . (1985) Nature 316 :750-752). A soybean heat shock gene is thermoinducable in sunflower tissue (Schöffl F and Baumann G (1985) EMBO J. 4 :1119-1124). (A Drosophila melanogaster heat shock promoter is similarly functional in tobacco tissue (Spena A et al . (1985) EMBO J. 4 :2739-2743).) Chimeric Genes Having T-DNA Promoters The nos promoter can drive expression of drug resistance structural genes useful for selection of transformed plant cells. Resistance genes have been created for kanamycin (Bevan MW et al . (1983) Nature 304 :184-187; Fraley RT et al . (1983) Proc. Natl. Acad. Sci. USA 80 :4803-4807; Herrera-Estrella L et al . (1983) EMBO J. 2 :987-995), methotrexate (Herrera-Estrella et al ., supra ), chloramphenicol (Herrera-Estrella L et al . (1983) Nature 303 :209-213), hygromycin B (van den Elzen PJM et al . (1985) Plant Mol. Biol. 5 :299-302). Helmer G et al . (1984) Biotechnol. 2 :520-527, have created a fusion gene having the promoter and 5′-end of the structural gene of nos fused to E . coli beta-galactosidase ( lacZ ) sequences. Plant tissues transformed with this screenable marker may be recognized by a characteristic color when grown on the appropriate chromogenic substrate. Fusion protein genes between the ocs structural gene, which also provided promoters, and structural genes for hygromycin B resistance and phaseolin have been created and are functional (Waldron C et al . (1985) Plant Mol. Biol. 5 :103-108; Murai N et al . (1983) Science 222 :476-482). An ocs driven glyphosate gene has been constructed (Comai L et al . (1985) Nature 317 :741-744). Promoters for octopine T L genes ORF24 and ORF25 could also drive structural gene expression (Velten J et al . (1984) EMBO J. 3 :2723-2730; Velten J and Schell J (1985) Nucl. Acids Res. 13 :6981-6998; Gelvin SB et al . (1985) Mol. Gen. Genet. 199 :240-248; Comai L et al . (1985) Nature 317 :741-744). Chimeric Genes Having Plant Promoters A Chimeric RuBP-Case small subunit/kanamycin resistance protein was translocated into the chloroplast (Van den Broeck G et al . (1985) Nature 313 :358-363). That gene's promoter confers light-inducible expression in callus to chloramphenicol and kanamycin resistance genes (Herrera-Estrella L et al . (1984) Nature 310 :115-120; Facciotti D et al .1985) Biotechnol. 3 :241-246). A chalcone synthase promoter also drove light-inducible expression of a kanamycin resistance gene (Kaulen H et al . (1986) EMBO J. 5 :1-8). Chlorophyll a / b binding protein promoters have been used to drive expression of ocs and kanamycin resistance structural gene (Jones JDG et al . (1985) EMBO J. 4 :2411-2418; Simpson J et al . (1985) EMBO J. 4 :2723-2729). Chimeric Genes Having Viral Promoters A kanamycin resistance gene under control of a cauliflower mosaic virus (CaMV) promoter was expressed in plant cells transformed by T-DNA (Koziel MG et al . (1984) J. Mol. Appl. Genet. 2 :549-562). A methotrexate resistance gene behind the CaMV 35S promoter conferred methotrexate resistance (Brisson N et al . (1984) Nature 310 :511-514). Tobacco mosaic virus coat protein has been expressed in transformed tobacco tissue under control of a CaMV promoter (Bevan MW et al . (1985) EMBO J. 4 :1921-1926). Odell JT et al . (1985) Nature 313 :810-812, have mapped sequences of the CaMV 35S promoter needed for transcription. Transformation of Plants without Agrobacterium Direct transfer of DNA into plant cells has been recently reviewed (Jones MGK (1985) Nature 317 :579-580; Potrykus I et al . (1985) Plant Mol. Biol. Reptr. 3 :117-128; Howe C (1985) Trends Genet. 1 :38-39; Paszkowski J and Saul MW (1986) Meth. Enzymol. 118 :659-668; Power JB et al . (1986) Meth. Enzymol. 118 :578-594). Both dicot and monocot cells can be directly transformed by kanamycin-selectable markers under control of either a nos or CaMV promoter (Paszkowski J et al . (1984) EMBO J. 3 :2717-2722; Gardner RC et al . (1984) Plant Mol. Biol. Reporter 2 :3-8; Hain R et al . (1985) Mol. Gen. Genet. 199 :161-168; Potrykus I et al . (1985) Mol. Gen. Genet. 199 :183-188; Lörz H et al . (1985) Mol. Gen. Genet. 199 :178-182; Shillito RD et al . (1985) Biotechnol. 3 :1099-1103; Meyer P. et al . (1985) Mol. Gen. Genet. 201 :513-518). Distinct DNA molecules can be cotransformed into a plant cell; it is advantageous that one DNA in the mixture carries a selectable marker (Peerbolte R et al . (1985) Plant Mol. Biol. 5 :235-246). Descendants of plants regenerated from such transformed cells inherit the transformed hybrid gene as a single, dominant, Mendelian trait (Potrykus et al ., Mol, Gen. Genet., supra ). CaMV has proven useful as a plant transformation vector (Brisson N et al . (1984) Nature 310 :511-514; Brisson N and Hohn T (1986) Meth. Enzymol. 118 :659-668). Bromegrass mosaic virus (BMV), an RNA virus, can also be used in plants to express foreign structural genes (French R et al . (1986) Science 231 :1294-1297). Electroporation has proven useful for introducing chimeric genes into plant cells in a transient expression system (Fromm M et al . (1985) Proc. Natl. Acad. Sci. USA 82 :5824-5828) and for stable transformation of maize cells (Fromm ME et al . (1986) Nature 319 :791-793). Cells can take up DNA surrounded by membranes. DNA, including PTi DNA, may be introduced via liposomes (e.g. Deshayes A et al . (1985) EMBO J. 4 :2731-2737) or by fusion of plant and bacterial cells after removal of their respective cell walls (e.g. Hain R et al . (1984) Plant Cell Rept. 3 :60-64). Plant protoplasts can take up cell wall delimited Agrobacterium cells. T-DNA can be transmitted to tissue regenerated from fused protoplasts. DNA can be stably integrated into a plant genome after microinjection (Crossway A et al . (1986) Mol. Gen. Genet. 202 :179-185). Introduction of DNA into plant cells during fertilization or pollination has been reported for corn and cotton by Ohta Y (1986) Proc. Natl. Acad. Sci. USA 83 :715-719, and Zhou G-Y et al . (1983) Meth. Enzymol. 101 :433-481, respectively. Overview of AMV Alfalfa mosaic virus (AMV) is one of a class of plant viruses having a tripartite, single-stranded, plus-stranded RNA genome. The genome (excluding the subgenomic RNA molecules) is segmented into three RNA molecules. This class includes: the alfalfa mosaic virus (AMV) group, the ilarviruses, the bromoviruses, the cucumoviruses, and the hordeiviruses (van Vloten-Doting L et al . (1981) Intervirol. 15 :198-203; Matthews REF (1982) Classification and Nomenclature of Viruses ). The genome segments are separately encapsidated in bacilliform particles of different lengths. Besides the three genomic RNA components (RNA1, RNA2, and RNA3), a fourth subgenomic RNA (RNA4) is found in virus preparations. A mixture of the three genome RNAs together with a small amount of coat protein or its messenger, RNA4 (Bol JF et al . 1971) Virol. 46 :73-85), is required to initiate infection. The complete sequence of AMV RNA4 has been disclosed (Brederode FT et al . (1980) Nucl. Acids Res. 8 :2213-2223). RNA4 is 881 nucleotides in length. The coding region is 660 nucleotides (not including the initiation and termination codon) flanked by a 5′-untranslated region of 39 nucleotides and a 3′-untranslated region of 182 nucleotides. The sequence of RNA4 is present in and located at the 3′-end of RNA3 (Gould AR and Symons RH (1978) Eur. J. Biochem. 91 :269-278). The complete nucleotide sequence of AMV RNA3 has been disclosed (Barker RF et al . (1983) Nucl. Acids Res. 11 :2881-2891). A 240 base 5′-noncoding region preceeds a 903 nucleotide open reading frame (ORF). This ORF is followed by a 49 base intercistronic region a 666 nucleotide ORF, this latter ORF encoding AMV coat protein. The coat protein gene is followed by a 179 base 3′-untranslated sequence. AMV RNA4 is identical to and determined by the 3′-end of RNA3, having 36 nucleotides of the intercistronic region, the coat protein gene, and the 3′-untranslated sequence. The complete nucleotide sequence of AMV RNA1 has been obtained (Cornelissen BJC et al . (1983) Nucl. Acids Res. 11 :1253-1265). RNA1 is 3645 nucleotides in length and it contains a long ORF for a protein of M r 125,685 flanked by a 5′-untranslated sequence of 99 nucleotides and a 3'-untranslated region of 163 nucleotides. Comparison of the 3′-terminal sequences of all four AMV RNAs reveal extensive homology between the 3′-terminal 140 to 150 nucleotides (Houwing CJ and Jaspars EMJ (1978) Biochem. 17 :2927-2933). There are about 20 base substitutions in the 3′-terminal 145 nucleotides of the AMV RNAs; these are either located in the loops of base-paired structures or convert A-U base pairs to G-C base pairs in the stems of the secondary structure hairpins (Koper-Zwarthoff EC et al . (1979) Nucl. Acids Res. 7 :1887-1900). Antisense RNA Sequeria L (1984) Trends Biotechnol. 2 :25-29, reviews cross protection and induced resistance. Stebbing N (1979) Pharmac. Ther. 6 :291-332 reviews the design of antiviral agents based on viral RNA sequences and antiviral effect of single-stranded nucleic acids. Weintraub H et al . (1985) Trends Genet. 1 :22-25, reviews published and unpublished results using antisense RNA for genetic analysis. Travers A (1984) Nature 311 :410, and Laporte DC (1984) Trends Biochem. Sci. 9 :463, review regulatory roles of antisense RNA (aRNA). In nature, antisense or complementary RNA can function in a regulatory manner. In E. coli , a 174 b ompC RNA regulates ompF expression. Antisense RNA, which would most likely bind to mRNA regions likely to come in contact with ribosomes, seemed most effective in decreasing gene expression (Mizuno T et al . (1984) Proc. Natl. Acad. Sci. USA 81 :1966-1970; Coleman J et al . (1984) Cell 37 :429-436). Complementary RNA can inhibit DNA replication (Tomizawa J et al . (1981) Proc. Natl. Acad. Sci. USA 78 :1421-1425; Tomizawa J and Itoh T (1982) Cell 31 :575-583. In vitro , an oligonucleotide complementary to as little as five nucleotides of the 3′-terminal region of E. coli 16S ribosomal RNA can inhibit translational initiation of bacterial virus mRNAs (Eckhardt H and Lührmann R (1979) J. Biol. Chem. 254 :11185-11188; Jayaraman K et al . (1981) Proc. Natl. Acad. Sci. USA 78 :1537-1541; Taniguchi T and Weissmann C (1978) Nature 275 :770-772). In vitro translation of mRNA can be inhibited if the mRNA is mixed with a complementary DNA (cDNA) and subjected to nucleic acid annealing conditions; an unannealed mixture of mRNA and cDNA can be translated (Paterson BM et al . (1977) Proc. Natl. Acad. Sci. USA 74 :4370-4374). In vitro replication of plant viral RNA fragments can be inhibited if the RNA fragments are mixed with a cDNA and subjected to annealing conditions; an unannealed mixture of the RNA fragments and cDNA can be replicated (Ahlquist P et al . (1984) Plant Mol. Biol. 3 :37-44.) Rous sarcoma virus replication and cell transformation is inhibited by addition of an oligonucleotide complementary to 13 nucleotides of a 5′ and 3′ terminal repeat (Zamecnik PC and Stephenson ML (1978) Proc. Natl. Acad. Sci. USA 75 :280-284) Globin aRNA inhibited translation of globin mRNA when both were injected into the cytoplasm of frog oocytes. The aRNA inhibited translation when injected with or before mRNA. aRNA:mRNA hybrids appeared to form within the oocytes, though the hybrids were much shorter than expected for full-length duplexes (Melton DA (1985) Proc. Natl. Acad. Sci. USA 82 :144-148). Results suggest that secondary structure could limit the regions of nucleic acids which formed hybrids. Production of viral antisense RNA sequences has been shown to partially protect E . coli cells against infection by the corresponding bacteriophage. Sequence complementary to ribosome binding sites were more effective inhibitors than a sequence complementary to the 3′-end of a structural gene and the 3′-untranslated region (Coleman J et al . (1985) Nature 315 :601-603). Antisense RNA can reduce gene expression in transient expression system in mouse cells. When a herpes simple virus (HSV) thymidine kinase (TK) gene and a 100-fold excess of a modification of that gene, having the protein-encoding sequence flipped so that a TK aRNA was encoded, were coinjected into TK⁻ cells, TK activity was observed to drop 4-fold (Izant JG and Weintraub H (1984) Cell 36 :1007-1015). Lac gene expression in mouse cells was observed to be reduced 10-fold when cotransformed with an aRNA gene at a 1:1 or higher gene:aRNA gene ratio (Rubenstein JLR et al . (1984) C. R. Acad. Sci., Paris 299 :271-274). SUMMARY OF THE INVENTION This invention relates to the occurence of viral infections in plants and to the efforts of horticulturists and agronomists to combat these infections in economically significant plant species. Virus infections occur in every known plant species and cause significant reductions in the yield and quality of all agricultural and horticultural plant species. The plant industry in no country in the world is exempt from such virally caused damage and no consistent treatment is known to treat or prevent such viral infections. For example, 90% of the cassava plants in Kenya are infected by cassava mosaic virus resulting in an estimated 75% reduction in yield. As another example, in a recent viral epidemic in Ghana, more than one hundred million cacao trees were lost by infection with swollen shoot virus. Many other examples could be given making it evident that viral epidemics and infections have a vast economic significance. The reduction in yield from food crops is also relevant to the steadily increasing human population of the world and to the chronic malnutrition that already exists. Therefore, it is clear that both the means for creating virus-resistant plant genotypes and the result plants themselves would be very useful for increasing the world's ability to feed itself. In particular, alfalfa mosaic has been shown to cause serious decreases in crop yield. AMV infects alfalfa and other annual legume crops. This is economically important; in the United States alone approximately 30 million acres are planted in alfalfa. Alfalfa mosaic virus also causes economically important diseases in crop plants such as peppers, potatoes, celery, peas, and beans. Alfalfa can be an overwintering host from which aphids carry the virus. The disease is also spread from alfalfa to other species of crop plants following a build-up of aphid infestation. In many cases, plants infected by AMV show no symptoms making it difficult to detect the occurrence and spread of the disease. In other cases, the mosaic disease is evident but by that time the virus has almost certainly spread through a large area of plants in a field. Apart from the removal of infected plants and the use of insecticides to kill off the aphids which transmit the virus, there are no practical methods developed for preventing the spread of AMV. Therefore, it is clear that both the means for creating AMV-resistant genotypes and the resulting plants would be very useful for increasing agricultural productivity of a number of crops. Therefore, it is an object of the present invention to provide plants having novel virus resistance genes and, in particular, to provide plants resistant to AMV infection. Towards this goal, methods for creating viral resistance genes, in particular, antisense RNA genes inhibitory of viral replication, are provided. Also are plants, plant cells, plant tissues, and plant seeds containing aRNA genes which are responsible for those plant materials having a viral resistance phenotype. Furthermore, DNA molecules useful for creating such virus resistant plants are also described. The present invention is exemplified by putting an aRNA gene complementary to alfalfa mosaic virus coat protein messenger RNA into tobacco. Antisense RNA has never been shown to affect gene expression in plants, though it has been shown to do so in some nonplant systems. There are a number of reasons why the presense of viral aRNA might not protect a plant from a viral infection. Viral aRNA is an RNA that is not naturally found in the nucleus; viral aRNA might not be stable in that environment. The level of accumulation of aRNA might be too low to inhibit infection. The aRNA may not accumulate in a part of a cell where viral infections are initiated or maintained. Though viral aRNA can be stable in plant cytoplasm, viral aRNA might not be stable in a plant cytoplasm in the absense of a viral infection. A viral infection might become established faster than aRNA could anneal to viral RNA. Accumulation of viral aRNA molecules at levels sufficient to interfere with a virus's life-cycle might be toxic to plant cells. Indeed, the presence of an aRNA gene might be detrimental to plants. DETAILED DESCRIPTION OF THE INVENTION The following definitions are provided, in order to remove ambiguities to the intent or scope of their usage in the specification and claims. Promoter: Refers to sequences at the 5'-end of a structural gene involved in initiation of transcription. A plant-expressible promoter is any promoter capable of driving transcription in at least one type of plant cell in at least one developmental stage. Eukaryotic promoter sequences are commonly recognized by the presence of DNA sequences homologous to the canonical form 5'...TATAA...3' about 10-30 base pairs (bp) 5'-to the location of the 5'-end of the mRNA (cap site). About 30 bp 5'-to the TATAA, another promoter sequence is often found which is recognized by the presence of DNA sequences homologous to the canonical form 5'...CCAAT...3'. Transcript Terminator: Refers herein to any nucleic acid sequence capable of determining the position of the 3'-end of a transcript. The transcript terminator DNA segment may itself be a composite of segments derived from a plurality of sources, naturally occuring or synthetic, prokaryotic, or eukaryotic, and may be from a genomic DNA or an mRNA-derived cDNA (mRNA: messenger RNA). Transcript termination sites include polyadenylation sites and sites determining the 3'-end of ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), and nonpolyadenylated mRNAs (e.g. histone mRNA: Krieg PA and Melton DA (1984) Nature 308 :203-206). A polyadenylation site is a nucleic acid sequence correlated with polyadenylation of mRNA in eukaryotes, i.e. after transcriptional termination polyadenylic acid "tails" are added to the 3'-end of mRNA precursors. Polyadenylation sites are commonly recognized by the presence of homology to the canonical form 5'...AATAAA...3', although variations of distance 5' to the 3'-end of the transcript, partial "read-thru", and multiple tandem canonical sequences are not uncommon. DNA sequences between 20 and 35 bp downstream from the transcript's 3′-end seem to be necessary (McDevitt MA et al . (1984) Cell 37 :993-999). It should be recognized that a canonical "polyadenylation site" may actually determine the location of the 3′-end of the mRNA and not polyadenylation per se (Proudfoot N (1984) Nature 307 :412-413; Birnstiel ML et al . (1985) Cell 41 :349-359). Transcription Controlling Sequences : Refers to a promotor/transcript terminator site combination flanking a structural gene. The promoter and terminator DNA sequences flanking a particular foreign structural gene need not be derived from the same source genes (e.g. pairing two different T-DNA transcripts) or the same taxonomic source (e.g. pairing sequences from T-DNA with sequences from non-T-DNA sources such as plants, animals, fungi, yeasts, eukaryotic viruses, bacteria, and synthetic sequences). Antisense RNA (aRNA) : Refers herein to an RNA that is in part or in whole complementary to at least one target viral mRNA (sense) sequence. A target virus is a virus to which one wants a resistance. The target viral sequence comprised in complement by the aRNA may consist of a full-length target viral mRNA or a portion thereof, the portion being from a coding region, a translational initiation site and/or a 5′, 3′, or internal untranslated region. An aRNA may include sequences complementary to a plurality of distinct viruses, thereby conferring resistance to each of the plurality of viruses. The aRNA itself may or may not function as an mRNA. Antisense RNA Gene (aRNA gene) : Refers herein to a promoter, a DNA sequence determining an aRNA (viral cDNA), and a transcript terminator, the promoter, the viral cDNA, and the terminator having such position and orientation with respect to each other that, when in a plant cell, an aRNA may be transcribed under control of the promoter. In other words, an aRNA gene expresses an RNA comprising an RNA complimentary to a viral mRNA. An aRNA gene may be a composite of segments derived from a plurality of sources, naturally occurring or synthetic. An aRNA gene transcript may include sequences derived in whole or in part from prokaryotic DNA, eukaryotic DNA, episomal DNA, plasmid DNA, plastid DNA, genomic DNA, cDNA, viral DNA, viral cDNA, chemically synthesized DNA, or the like. It is further contemplated that an aRNA gene may contain one or more modifications in either the transcription control sequences, transcribed sequences, or viral cDNA, which could affect the biological activity or chemical structure of the aRNA, the rate of expression, or the manner of expression control. Such modifications include, but are not limited to, mutations, insertions, deletions, and substitutions of one or more nucleotides, and modifications that do not alter aRNA function but which affect intercellular localization, transport, or stability of the aRNA. DNA encoding an aRNA may determine an uninterrupted aRNA sequence or it may include one or more introns, bounded by the appropriate plant-functional splice junctions, which may be obtained from synthetic or a naturally occurring source. cDNA (Complementary DNA) : Though this term is well understood in the art, it has two meanings. (1) A cDNA can be a single-stranded DNA complementary to an RNA (e.g. a viral mRNA). (2) A cDNA can also be a double-stranded DNA segment, one strand of which is complementary to an RNA, the other strand having a sequence equivalent to that RNA (substituting T for U). Generally, a double-stranded cDNA is derived from a single-stranded cDNA. However, as defined herein, a double-stranded DNA encoding mRNA sequences, e.g. the DNA of a structural gene, is included within the term cDNA. Within the Claims, cDNA always refers to the double-stranded form (meaning (2)). Elsewhere in this Specification, the meaning of cDNA is defined by context and will be well understood by those in the art. Plus-Stranded : Refers to viruses having genomes having sequences equivalent to that of the viruses' mRNA; i.e. RNA found in virus particles is not complementary to viral mRNA. Often the viral RNA of plus-stranded viruses can serve as an mRNA. AMV is an example of a plus-stranded virus; each of the four RNAs found in AMV virions is capable of serving as an mRNA. Tripartite RNA Genome : Refers to organization of a virus's genetic material. "Genome" refers to the total genetic material of the virus. "RNA genome" states that as present in virions (virus particles), the genome is in RNA form. "Tripartite" indicates that the genome is divided among three separate RNA molecules. An example of a virus with a tripartite RNA genome is AMV. The genome of AMV is carried by AMV RNAs 1, 2, and 3. The sequence of RNA4 is totally contained by RNA3 and RNA4 is not replicated; therefore, RNA4 is referred to as a subgenomic RNA and is not counted as one of the genomic RNAs. Translational Initiation Site : Refers herein to the 5′AUG3′ translational start codon at the 5′-end of a structural gene, the nucleotide following the AUG, and the 3 nucleotides preceeding the AUG (see Kozak M (1983) Microbiol. Rev. 47 :1-45, and Kozak M (1984) Nucl. Acids Res. 12 :857-872). 5′-Untranslated Sequence : Refers herein to the part of an mRNA between its 5′-end, or "cap site", and the translation start codon. 3′-Conserved Sequence : Refers herein to a sequence at the 3′-end of a multipartite, nonpolyadenylated RNA genome that is the same for all genome components. The AMV 3′-conserved sequence extends about 145 nucleotides from the 3′-end of all 4 AMV RNAs. Essentially Full-Length cDNA : Refers herein to a cDNA that is complementary to an entire mRNA, possibly excepting a few (e.g. five) nucleotides at either end of that mRNA sequence. Plant-Expressible Selectable or Screenable Marker : Refers herein to a genetic marker functional in a plant cell. A selectable marker (e.g. a kanamycin resistance gene) allows cells containing and expressing that marker to grow under conditions unfavorable to growth of cells not expressing that marker. A screenable marker (e.g. a beta -galactosidase gene) facilitates identification of cells which express that marker. Transforming : Refers to the act of causing a cell to contain a nucleic molecule or sequence not originally part of that cell. Plant Tissue : Includes differentiated and undifferentiated tissues of plants including but not limited to roots, shoots, pollen, seeds, tumor tissue, such as crown galls, and various forms of aggregations of plant cells in culture, such as embryos and calluses. The plant tissue may be in planta or in organ, tissue, or cell culture. Plant Cell : As used herein includes plant cells in planta and plant cells and protoplasts in culture. The following terms are well known in the art and are not specifically or specially defined herein: single-stranded, genome, alfalfa mosaic virus group (see Matthews REF (1982) Classification and Nomenclature of Viruses , p. 177), Tobamovirus (see Matthews, supra , pp. 158-159), CaMV 19S promoter (see Hohn T et al . (1982) Curr. Top. Microbiol. Immunol. 96 :193-236), octopine-type T-DNA (positions, orientations, and open reading frames (ORFs) are defined as designated by Barker RF et al . (1983) Plant Mol. Biol. 2 :335-350), T-DNA border repeat, transcription under control of a promoter, ligating, descended, and structural gene. Production of a genetically modified cell expressing an aRNA gene combines the specific teachings of the present disclosure with a variety of techniques and expedients known in the art. In most instances, alternative expedients exist for each stage of the overall process. The choice of expedients depends on variables such as the choice of the particular virus to which resistance is desired, the basic vector system for the introduction and stable maintenance of the aRNA gene, the plant species to be modified, and the desired regeneration strategy, the particular transcriptional control sequences used, the particular viral sequences comprised by the aRNA gene transcript, and the like, all of which present alternative process steps which those of ordinary skill are able to select and use to achieve a desired result. As novel means are developed for the stable insertion and transcription of foreign DNA in plant cells, those of ordinary skill in the art will be able to select among those alternate process steps to achieve a desired result. The fundamental aspects of the invention are the nature of the aRNA gene and its use to confer resistance to viral infections of plants transformed therewith. Other aspects include the nature and structure of the aRNA sequence and its means of insertion and expression in a plant genome. The remaining steps of the preferred embodiment for obtaining a genetically modified plant include inserting the aRNA gene into T-DNA, transferring the modified T-DNA to a plant cell wherein the modified T-DNA becomes stably integrated as part of the plant cell genome, techniques for in vitro culture and eventual regeneration into whole plants, which may include steps for selecting and detecting transformed plant cells and steps of transferring the introduced aRNA gene from the originally transformed strain into commercially acceptable cultivars, and monitoring expression in transformed plants. A starting point for construction of an aRNA gene is obtaining DNA clones of viral sequences. If the virus is a DNA virus, the DNA clones are obtainable using methods well known in the art of recombinant DNA. If the virus is an RNA virus, a cDNA clone must be made from the viral sequence desired. A number of methods for making cDNA clones are known in the art of recombinant DNA; choice of methods will depend on variables such as polyadenylation, RNA length, prior knowledge of RNA sequence, prior preparations within the particular laboratory for other cDNA cloning experiments, and the like. Cloned viral sequences are a necessary component of an aRNA gene. A principal feature of the present invention in its preferred embodiment is the construction of a T-DNA derivative having an inserted gene under control of plant-expressible transcription controlling sequences, i.e., between a promoter and a transcript terminator, as these terms have been defined, supra . The aRNA-encoding DNA must be inserted in correct position and orientation with respect to the promoter. Position relates to which side of the promoter the aRNA-encoding DNA is inserted. It is known that the majority of promoters control initiation of transcription and translation in one direction only along the DNA. The region of DNA lying under promoter control is said to lie "downstream" or alternatively "behind" or "3′ to" the promoter. Therefore, to be controlled by the promoter, the correct position of an aRNA-encoding DNA insertion must be "downstream" from the promoter. Orientation refers to the directionality of the structural gene. That portion of an aRNA-encoding DNA which is complementary to viral mRNA encoding the amino terminus of a viral protein is termed the 3′-end of the aRNA-encoding DNA, while that end which is complementary to viral mRNA encoding amino acids near the carboxyl end of the protein is termed the 5′-end of the aRNA-encoding DNA. In other words, the 5′-end and the 3′-end of the aRNA-encoding DNA respectively encoded the 3′-end and the 5′-end of viral mRNA. Correct orientation of the aRNA-encoding DNA is with the 5′-end thereof proximal to the promoter. Similarly to the promoter region, the transcript terminator must be located in correct position and orientation relative to the aRNA, being proximal to the 3′-end of the aRNA. Differences in rates of aRNA gene expression or developmental control may be observed depending on the particular aRNA components, promoters, transcript terminators, flanking DNA sequences, or sites of insertion into the transformed plant's genome. aRNA accumulation may also be greatly influenced by the details of the aRNA secondary structure, especially stem-loop structures. Different properties, including, but not limited to, such properties as stability, intracellular localization, posttranscriptional processing, and other functional properties of the expressed aRNA itself may be observed when aRNA gene components are varied. All of these variations present numerous opportunities to manipulate and control the functional properties of the viral resistance phenotype, depending upon the desired physiological properties within the plant cell, plant tissue, and whole plant. The fundamental principle of the present invention is that the presence of an antisense RNA sequence in a plant cell is capable of conferring at least some level of viral resistance to that cell. The requirements for which viral sequence segments are to be included in an aRNA are best couched in functional terms. The presence of aRNA in a cell confers viral resistance by interfering with one or more viral functions. Such functions might include translation of viral proteins, replication of viral RNAs, encapsidation of viral nucleic acid, etc. aRNA presumably interfers by annealing to a viral plus-strand segment or by binding at least one protein involved in the viral life-cycle. The present invention is not limited by the mode or mechanism of interference. The aRNA need not comprise a full-length viral sequence; any sequence segment complementary to a viral plus-strand segment and capable of interfering with a viral function is sufficient. In other words, the presence of at least one viral sequence capable of interfering with viral functions is required in an aRNA and any viral sequence segment incapable of such interference may be dispensed with, subject to considerations of RNA stability, cellular localization, and the like. Viral sequences from several distinct viruses, each sequence being capable of functioning as an aRNA, may be combined to form an aRNA having antiviral activity against each of the several viruses. As defined, an aRNA may include nonviral sequences. Even in cases where a full-length viral sequence is included, an aRNA may include nonviral sequences. Usually these nonviral sequences will be at the 5′- and 3′-ends of the aRNA. Often these nonviral components will be derived from promoter or transcript terminator DNA segments. Inclusion of various nonviral sequences may affect RNA stability, cellular localization of aRNA, posttranscriptional processing, and the like. It is known to the art that RNA stability is affected by terminal structures such as 5′-capping and 3′-polyadenylation and by the extent of internal structure, i.e. intramolecular basepairing. An aRNA may be localized within a cell, e.g. an aRNA having a translatable sequence encoding a signal peptide-containing polypeptide will tend to be bound to rough endoplasmic reticulum. An intron may be included in an aRNA, provided that, if the splice sites are derived from two different genes, the intron splice sites be compatable. An aRNA must be designed with several biological factors in mind. RNA secondary structure may affect antiviral activity as well as stability and processing. In particular, intramolecular base-pairing involving sequences which interfere with viral functions must not be so strong that annealing to viral mRNA is precluded. An aRNA transcript should be incapable of being converted by the viral replication machinery into a functional sense viral component capable of increasing the severity of the viral disease. Production of sense-strand may be blocked by inclusion in the aRNA of nonviral sequences, especially at one or both aRNA ends, which interfere with replicase function. Should plus-strand production not be blocked, other steps may be taken. The deletion of a protein-encoding sequence segment or introduction of an appropriately positioned frameshift mutation can cause the viral protein templated by the sense-strand to be nonfunctional. Frameshift mutations are conveniently introduced by opening a viral cDNA carrying plasmid at a unique, sticky-end generating restriction site, removing the sticky-ends, and religation. Depending on the enzyme and methods used, this will delete or insert two or four basepairs, thereby generating a frameshift (and also eliminating that restriction site). An important embodiment of the present invention is inhibition of viral propagation caused by binding to a viral mRNA of an essentially (e.g. to within five nucleotides of either end of the mRNA sequence) full-length cDNA. A full-length cDNA has four important regions: a 5′-untranslated sequence, a translational initiation site, structural gene, and a 3′-conserved sequence. Binding of an aRNA to a 5′-untranslated sequence may interfere with translational initiation by interfering with preinitiation scanning of the mRNA for translational initiation sites by 40S ribosomal subunits (see Kozak M (1983) Microbiol. Rev. 47 :1-45). Binding to a translational initiation site may interfer with translational initiation. Binding to the structural gene sequence, i.e. the protein-encoding sequence, may lower translational efficiency. Binding to the 3′-conserved sequence may interfere with initiation of replication. Combining of DNA segments, including viral, nonviral, promoter, and transcript terminator sequences, to form an aRNA gene is accomplished by means known and understood by those of ordinary skill in the art of recombinant DNA technology. Choice of promoter depends on the developmental regulation desired. Use of developmentally regulated promoters for gene expression in plants is described in the Background. T-DNA or CaMV promoters are advantageous as they are constitutive. The RuBP-Case small subunit promoter may be useful for expression in the green tissues of an aRNA gene-transformed plant. In the preferred embodiments, the transcript terminator is a polyadenylation site. The plant gene source of the polyadenylation site is not crucial provided that the polyadenylation site, the promoter and the aRNA are compatable for transcription and posttranscriptional processing. As will be apparent to those of ordinary skill in the art, the aRNA gene combination may be placed between any restriction sites convenient for removing the combination from the plasmid it is carried on and convenient for insertion into the plant transformation vector of choice. For example, location of the aRNA gene insertion site within T-DNA is not critical as long as the transfer function of sequences immediately surrounding the T-DNA borders are not disrupted, since in prior art studies these regions appear to be essential for insertion of the modified T-DNA into the plant genome. The combination is inserted into the plant transformation vector by standard techniques well known to those skilled in the art. The orientation of the assembled aRNA gene, with respect to the direction of transcription and translation of endogenous vector genes is not usually critical; generally, either of the two possible orientations is functional. As reviewed in the Background (Ti Plasmid DNA), T-DNA of micro-Ti plasmids can be transferred from an Agrobacterium strain to a plant cell provided the Agrobacterium strain contains certain trans -acting genes whose function is to promote the transfer of T-DNA to a plant cell. Micro-Ti plasmids are advantageous in that they are small and relatively easy to manipulate directly, eliminating the need to transfer the gene to T-DNA from a shuttle vector by homologous recombination. After the desired aRNA gene has been inserted, they can easily be introduced directly into an Agrobacterium cell containing trans -acting vir genes, the vir genes usually being on a "helper plasmid", that promotes T-DNA transfer. Introduction into an Agrobacterium strain is conveniently accomplished either by transformation of the Agrobacterium strain or by conjungal transfer from a donor bacterial cell, the techniques for which are well known to those of ordinary skill. For purposes of introduction of novel DNA sequences into a plant genome, Ti plasmids, Ri plasmids, micro-Ti plasmids, and T-DNA integrated into chromosomes should be considered functionally equivalent. T-DNA having an aRNA gene can be transferred to plant cells by any technique known in the art. For example, this transfer is most conveniently accomplished by cocultivation of the Agrobacterium strain with plant cells or with plant tissues. Using these methods, a certain proportion of the plant cells are transformed, that is to say have T-DNA transferred therein and inserted in the plant cell genome. In either case, the transformed cells must be selected or screened to distinguish them from untransformed cells. Selection is most readily accomplished by providing a selectable marker incorporated into the T-DNA in addition to the aRNA gene. Examples of artificial markers include those reviewed in the Background (see the sections on Chimeric Genes). In addition, the T-DNA provides endogenous markers such as gene(s) controlling abnormal morphology of Ri-induced tumor roots and gene(s) that control resistance to toxic compounds such as amino acid analogs, such resistance being provided by an opine synthesizing enzyme (e.g. ocs ). Screening methods well known to those skilled in the art include, but are not limited to, assays for opine production, specific hybridization to characteristic nucleic acid sequences (e.g. aRNA or T-DNA) or immunological assays for specific proteins. Additionally, a phenotype of an expressed aRNA gene (e.g. resistance to a target virus) can be used to identify transformed tissue. Assay of viral resistance may be done by one of a number of methods well known in the art of plant virology, including detection of decrease production of a virus component (e.g. a viral RNA or a viral protein), decrease infectivity as assayed by protoplast infection or local lession assays, decrease production of progeny virus, and the like. Although the preferred embodiments involve use of micro-Ti plasmids, other T-DNA-based vector systems known to the art may readily be substituted. Furthermore, though the preferred embodiment of this invention incorporates a T-DNA-based Agrobacterium -mediated system for incorporation of the aRNA gene into the genome of the plant which is to be transformed, other means for transferring and incorporating the aRNA gene are also included within the scope of this invention. Other means for the stable incorporation of the aRNA gene into a plant genome additionally include, but are not limited to, use of vectors based upon viral genomes, minichromosomes, transposons, and homologous or nonhomologous recombination into plant chromosomes. Alternate forms of delivery of these vectors into a plant cell additionally include, but are not limited to, fusion with vector-containing liposomes or bacterial spheroplasts, microinjection, encapsidation in viral coat protein followed by an infection-like process, and direct uptake of DNA, possibly after induction of plasmalemma permeability by an electric pulse, a laser, or a chemical agent. Means for transient incorporation and/or expression are also included within the scope of this invention. Systems based on Agrobacterium cells and T-DNAs can be used to transform angiosperms, including dicots and monocots, by transfer of DNA from a bacterium to a plant cell; systems based on alternate vectors or means for vector delivery may be used to transform gymnosperms and angiosperms. Regeneration of transformed cells and tissues is accomplished by resort to known techniques. An object of the regeneration step is to obtain a whole plant that grows and reproduces normally but which retains integrated T-DNA. The techniques of regeneration vary somewhat according to principles known in the art, and may depend upon the plant transformation vector and the species of the transformed plant. Regeneration of transformed tobacco plants, petunia plants, and plants of related species is well known to the art. As means for regeneration of other plant species are developed, the art will understand, without undue experimentation, how to adapt these newly discovered means for regeneration of plants from transformed plant cells and transformed plant tissues. The genotype of the plant tissue transformed is often chosen for the ease with which its cells can be grown and regenerated in in vitro culture and for susceptibility to the selective agent to be used. Should a cultivar of agronomic interest be unsuitable for these manipulations, a more amenable variety is first transformed. After regeneration, the newly introduced aRNA gene may be readily transferred to the desired agronomic cultivar by techniques well known to those skilled in the arts of plant breeding and plant genetics. Sexual crosses of transformed plants with the agronomic cultivars yield initial hybrids. These hybrids can then be back-crossed with plants of the desired genetic background. Progeny are continuously screened and/or selected for the continued presence of integrated aRNA gene DNA, T-DNA, or for a new phenotype resulting from expression of the aRNA gene or other genes carried by the inserted DNA. In this manner, after a number of rounds of back-crossing and selection, plants can be produced having a genotype essentially identical to the agronomically desired parents with the addition of inserted DNA sequences. An alternative to incorporation of an aRNA gene into a plant genome for making an aRNA-containing plant cell is to infect a plant with a vector viral RNA capable of being maintained in that plant, the viral RNA having aRNA sequences to a distinct target virus (i.e. a virus to which one wants protection). Typically, double-stranded cDNA sequences of the vector virus and the target virus are manipulated using recombinant DNA technology. After assembly of a DNA sequence corresponding to that of the desired vector RNA/target aRNA combination, the plant viral vector cDNA/aRNA cDNA combination may be placed behind a promoter that can drive in vitro transcription. Descriptions of such vectors and conditions for their use include Melton DA et al . (1984) Nucl. Acids Res. 12 :7035-7056, Krieg PA and Melton DA (1984) Nucl. Acids Res. 12 :7057-7070, Ahlquist P and Janda M (1984) Mol. Cell. Biol. 4 :2876-2882, and French R et al . (1986) Science 231 :1294-1297. After such a viral vector/aRNA/ in vitro transcription vector combination is assembled, a viral vector RNA/target aRNA combination may be produced by in vitro transcription and mixed with any other viral RNA components necessary for maintenance of the viral vector in a plant cell. Infection by a vector/aRNA combination of a plant cell may then be effected by known methods and, after inoculation with the target virus or target virus RNA, inhibition of infection or production of a viral component may be assayed by methods well known in the art. Similarly, plant viral DNA vectors may be used to introduce an aRNA gene into a plant cell. The utility of such vectors has been demonstrated by Brisson N et al . (1984) Nature 310 :511-514. The means for creating functional aRNA genes as taught by the present invention can be combined with use of plant DNA virus-based vectors by those of ordinary skill in the art. After infection of an appropriate plant host cell, inhibition of target, virus infection may be assayed as described above. EXAMPLES The following Examples are presented for the purpose of illustrating specific embodiments within the scope of the present invention without limiting the scope, the scope being defined by the Claims. Numerous variations will be readily apparent to those of ordinary skill in the art. The Examples utilize many techniques well known and accessible to those skilled in the arts of molecular biology and manipulation of T-DNA and Agrobacterium ; such methods are fully described in one or more of the cited references if not described in detail herein. All references cited in this Specification are hereby incorporated by reference. Enzymes are obtained from commercial sources and are used according to the vendors' recommendations and other variations known to the art. Reagents, buffers, and culture conditions are also known to those in the art. Reference works containing such standard techniques include the following: Wu R, ed. (1979) Meth. Enzymol. 68 ; Wu R et al ., eds. (1983) Meth. Enzymol. 100 and 101 ; Grossman L and Moldave K, eds. (1980) Meth. Enzymol. 65 ; Weissbach A and Weissbach H, eds. (1986) Meth. Enzymol. 118 (see especially Rogers SG et al ., pp. 627-640); Miller JH (1972) Experiments in Molecular Genetics ; Davis R et al . (1980) Advanced Bacterial Genetics ; Schleif RF and Wensink PC (1982) Practical Methods in Molecular Biology ; Walker JM and Gaastra W, eds. (1983) Techniques in Molecular Biology ; and Maniatis T et al . (1982) Molecular Cloning . Additionally, Lathe RF et al . (1983) Genet. Engin. 4 :1-56, make useful comments on DNA manipulations. Textual use of the name of a restriction endonuclease in isolation, e.g. Bcl I", refers to use of that enzyme in an enzymatic digestion, except in a diagram where it can refer to the site of a sequence susceptible to action of that enzyme, e.g. a restriction site. In the text, restriction sites are indicated by the additional use of the work "site", e.g. " Bcl I site". The additional use of the work "fragment", e.g. " Bcl I fragment", indicates a linear double-stranded DNA molecule having ends generated by action of the named enzyme (e.g. a restriction fragment). A phrase such as " Bcl I/ Sma I fragment" indicates that the restriction fragment was generated by the action of two different enzymes, here Bcl I and Sma I, the two ends resulting from the action of different enzymes. Plasmids, and only plasmids, are prefaced with a "p", e.g., pTi15955 or PH400, and strain designations parenthetically indicate a plasmid harbored within, e.g., A . tumefaciens (pTi15955) or E . coli H802 (pH400). The following strains are on deposit and will be made available to the art upon grant of letters patent: E . coli MC1061 (pH400A4I) NRRL B-18062 E . coli K802 (pH4-1) NRRL B-18009 A . tumefaciens (pTi15955) ATCC 15955 E . coli CSH52 (pSUP106) NRRL B-15486 E . coli SM10 NRRL B-15481 E . coli S17-1 NRLL B-15483 (ATCC: American Type Culture Collectio, 12301 Parklawn Dr., Rockville, MD 20852 USA; NRRL; ARS Patent Collection, Northern Regional Research Center, 1815 N. University St., Peoria, IL 61614 USA.) Other plasmids and strains are widely available and accessible to those in the art. Example 1: Preparation of AMV RNA4 cDNA pSP65A4 (Loesch-Fries LS et al . (1985) Virol. 146 :177-187) carries a full-length cDNA copy of AMV RNA4. pSP65A4 DNA digested with Eco RI and Sma I was subjected to the agarose gel electrophoresis and a 0.89 kbp fragment was eluted from the gel. This fragment was mixed with and ligated to the linearized pSP64 DNA and transformed into MC1061. Ampicillin-resistant transformants were screened by hybidization to an AMV RNA4 probe. A colony was identified which harbored a plasmid, designated pSP64A4I, carrying a full-length AMV RNA4 cDNA. pSP64 and pSP65 are designed for in vitro transcription under control of a bacteriophage SP6 promoter (Melton DA et al . (1984) Nucl. Acids Res. 12 :7035-7056; Krieg PA and Melton DA (1984) Nucl. Acids Res. 12 :7057-7070). pSP65A4 and pSP64A4I respectively direct synthesis of AMV RNA4 plus-strand sequences and AMV RNA4 aRNA sequences. When the two plasmids are cut respectively by Sma I and Eco RI in vitro run-off transcripts are essentially full-length coat protein mRNA sequence and the complement thereto. AMV cDNA Eco RI/ Sma I fragments of pSP65A4 and pSP64A4I are identical. Example 2: In vitro tests of aRNA inhibition of AMV infection Alfalfa and tobacco protoplasts were infected in vitro with RNAs essentially as described by Samac DA et al . (1983) Virol. 131 :455-462; the major modification being that RNA was added in a small volume (e.g. 10 µl) to the protoplast pellet, which was then resuspended in residual supernatant before addition of polyethylene glycol (PEG). This modification to Samac et al .'s method of adding RNA to the PEG before combination with the protoplasts lead to an approximately fifty-fold reduction in the amount of RNA needed for an infection. Conditions of inoculation are believed to be unfavorable for formation of sense-strand:antisense-strand duplexes outside of a cell. Synthetic RNAs were made essentially as described by Melton et al ., supra , and by protocol of the SP6 polymerase vendor. 5′-capped RNAs were synthesized by inclusion of 0.5 m M ⁷mG 5′ ppp 5′ G and reduction of GTP concentration from 0.5 m M to 0.025 m M . Synthetic AMV RNA4 was templated by Sma I-linearized pSP65A4 and synthetic AMV RNA4 aRNA was templated by Eco RI-linearized pSP64A4I. In a typical experiment using either alfalfa or tobacco protoplasts, natural AMV RNA1, RNA2, and RNA3 mixed with synthetic capped RNA4 resulted in infections of about two-thirds of the inoculated protoplasts. Inclusion of an equal amount, relative to the synthetic RNA4, of capped RNA4 aRNA led to at least a hundred-fold decrease in infection levels, and often to no detectable infection. Example 3: Preparation of CaMV transcription controlling sequences pDOB512, carrying cauliflower mosaic virus (CaMV) transcription controlling sequences (obtained from Dr. Ken Richards, Centre National de la Recherche Scientifique, Institute de Biologie Moleculaire et Cellulaire, 15, rue Descartes, F-67084 Strasbourg, France) was constructed as follows: (For a review of CaMV, see Hohn T et al . (1982) Curr. Top. Microbiol. Immunol. 96 :193-236.) A Hin dIII fragment carrying the CaMV 19S RNA promoter region (CaMV nucleotides 5376-5851) was inserted into pBR322 and was trimmed back to within one base pair of the 19S transcript cap site. A Hin cII fragment carrying the CaMV 19S transcript terminator (CaMV nucleotides 7018-7794) to which Bam HI linkers had been added was then inserted behind the 19S promoter; the resulting plasmid is designated pDOB412. pDOB412 DNA was digested with Bgl II and Sal I, filled in by the Klenow fragment of E . coli DNA polymerase I, and religated, thereby deleting DNA, which includes Bam HI and Hin dIII sites, between the CaMV position 7644 Bgl II site and the pBR322 position 650 Sal I site and regenerating a Bgl II site. The resultant plasmid was designated pDOB512. The sticky-ends of Hin dIII linearized pDOB512 DNA were filled in by Klenow fragment (or alternatively by T4 DNA polymerase). The blunt-ended pDOB512 DNA was mixed with and ligated to commercially available Bgl II linkers which were then trimmed by digestion with Bgl II and religated. The ligation mix was transformed into E . coli K802 and an ampicillin-resistant transformant was isolated which harbored a plasmid, designated pDOB513. pDOB513 has CaMV 19S transcription controlling sequences on a Bgl II fragment. Sma I and Bam HI sites are found between the DNA segments having the promoter and the polyadenylation site in both pDOB412, pDOB512, and pDOB513, thereby providing a convenient location for insertion of foreign DNA that is to be a template for a transcript. Example 4: Placement of AMV cDNA behind the CaMV promoter pSP65A4 DNA was digested with Eco RI and Sma I and the 0.89 kb AMV cDNA was purified by elution from an agarose gel after electrophoretic separation. The Eco RI sticky-end was converted to a blunt-end by incubation with the Klenow fragment of E . coli DNA polymerase I. After Sma I-linearized pDOB513 DNA was mixed with and ligated to the blunt-ended cDNA, the ligation products were transformed into MC1061. Plasmid DNAs isolated from ampicillin-resistant transformants were screened by hybridization of colony blots to an AMV RNA4 probe. A colony was identified which harbored a plasmid, designated pDOBA4I, having AMV cDNA inserted between CaMV 19S transcription controlling sequences oriented so that when transcribed, an aRNA would be synthesized; i.e. so that the Eco RI end and an internal Bam HI site are distal to the promoter and proximal to the transcript terminator. A CaMV transcription controlling sequences/AMV aRNA combination may be removed from pDOBA4I on a 1.92 kbp Bgl II fragment. Example 5: Construction of pH400, a micro-Ti Plasmid pH4-1 is a micro-Ti plasmid harbored by E . coli K802 (pH4-1), which is on deposit as NRRL B-18009. pH4-1 is disclosed by Sutton DW et al ., U.S. Patent application ser. no. 778,384, which is hereby incorporated by reference, and by Merlo D et al . (1985) Abstracts, 1st Int. Cong. Plant Mol. Biol., ed.: Galau GA. pH4-1 contains two T-DNA fragments, a Hin dIII fragment spanning positions 602 and 2,290 (as defined by Barker RF et al . (1983) Plant Mol. Biol. 2 :335-350) carrying the left border of T L and promoter sequences associated with ORF1, and a Sma I/ Bcl I fragment spanning positions 11,207 and 14,711, having a 3′-deleted tml , an intact ocs , and the right border of T L . Between the position 3,390 Hin dIII site and the position 11,207 Sma I site (this Sma I site having been converted to a Bgl III site insertion of Bgl II linkers) of these two fragments is a plant-expressible selectable marker. This marker has a CaMV 19S promoter, a Tn 5 kanamycin resistance structural gene encoding neomycin phosphotransferase II, and two polyadenylation sites, one from CaMV and another from T-DNA ORF26, donated by a T-DNA fragment spanning Hin cII sites at positions 21,727 and 22,440. The kanamycin resistance gene is oriented parallel to ocs and tml and antiparallel to the ORF1 promoter. The T-DNA/selectable marker combination is inserted into the Hin dIII site of pSUP106, an 11 kbp wide host-range plasmid capable of maintenance in both E . coli and Agrobacterium (Priefer UB et al . (1985) J. Bacteriol. 163 :324-330; E . coli CSH52 (pSUP106) is on deposit as NRRL B-15486). The T-DNA is oriented within pSUP106 so that the T L right border is proximal to the pSUP106 Eco RI site, which is present within pSUP106 chloramphenicol resistance gene. pH4-1 has two Bgl II sites, both of which flank the kan selectable marker. One of the Bgl II sites was removed as follows, thereby leaving a unique Bgl II site useful for insertion of extraneous coding DNA. pH4-1 DNA was linearized by being partially digested with Bgl II and full-length, linear DNA was electrophoretically isolated. The Bgl II sticky-ends were then removed by incubation with the Klenow fragment of E . coli DNA polymerase I. The resulting blunt-ended DNA was ligated to itself and transformed into E . coli . Plasmid DNAs isolated from transformants resistant to chloramphenicol were screened by restriction analysis and a colony was identified which harbored a plasmid designated pH400. pH400 was identical to pH4-1 except for the absense of the Bgl II site between the kan gene and the ORF1 promoter, the unique pH400 Bgl II site being located between the kan gene and the ocs gene. Example 6: Insertion of an AMV4 aRNA gene into pH400 pDOBA4I DNA was digested with Bgl II and then mixed with and ligated to Bgl II-linearized pH400 DNA. After transformation into MC1061 and selection for chloramphenicol resistance, colonies were blotted and hybridized with an AMV RNA4 probe. A colony was identified which harbored a plasmid, designated pH400A4I, having an aRNA gene inserted into the pH400 Bgl II site. pH400A4I has an aRNA gene having full-length AMV RNA4 sequences, the gene being oriented parallel to the 3′-deleted tml gene, ocs , and the kanamycin selectable marker. Example 7: Plant transformation pH400A4I was transferred into A . tumefaciens LBA4404 (Ooms G et al . (1981) Gene 14 :33-50), a vir gene-harboring, micro-Ti-mobilizing strain, by the triparental mating technique. (Ruvkun GB and Ausubel FM (1981) Nature 289 :85-88), which is well known in the art, or by mating from a mobilizing strain of E . coli , e.g. SM10 (NRRL B-15481) or S17-1 (NRRL B-15483) (Simon R et al . (1982) Biotechnol. 1 :784-791). Tobacco leaf tissue was obtained from 4- or 5-week old Xanthi NC plants grown axenically in Magenta boxes. Inoculation was by a modification of the method Horsch RB et al . (1985) Science 227 :1229-1231. Inocula were prepared by placing two loopfuls of agrobacteria in 10 ml of L-broth. After suspension by forceful pipetting with a Pasteur pipet, inocula could be used immediately. Leaves were excised and midribs were removed; cutting surfaces were wetted with L-Broth to help keep the leaves wet. Leaf pieces were about 2-4 mm wide and about 7-10 mm long. Leaf pieces were dipped in the inoculum for 5-10 min, though in some experiments, leaf pieces were just dipped into the inoculum or were infiltrated with the inoculum in a vacumn flask. Pieces were then blotted dry on sterile filter paper and placed upside down on feeder plates prepared from a Xanthi suspension culture. The feeder plates had a SMPi medium (SMPi: MX⁻ supplemented with 0.1 mg/l p-chlorophenoxyacetic acid (pCPA) and 7.5 mg/l 6-(8,8-dimethylallylamino)purine (2iP); MX⁻: 1.65 g/l NH₄NO₃, 1.9 g/l KNO₃, 440 mg/l CaCl₂·2H₂O, 370 mg/l MgSO₄·7H₂O, 170 mg/l KH₂PO₄, 0.83 mg/l KI, 6.2 mg/l H₃BO₃, 22.3 mg/l MnSO₄·4H₂O, 8.6 mg/l ZnSO₄·7H₂O, 0.25 mg/l Na₂MoO₄·2H₂O, 0.025 mg/l CuSO₄·5H₂O, 0.025 mg/l CoCl₂·6H₂O, 1 g/l inositol, 50 mg/l nicotinic acid, 50 mg/l pyroxidine·HCl, 50 mg/l thiamine·Hcl, 30 g/l sucrose, pH 5.8 solidified with 8 g/l agar). Leaf pieces were removed from feeder plates after 4-6 days and placed on SMPi medium supplemented with 500 mg/l carbenicillin, 50 mg/l cloxacillin, and 100-300 mg/l kanamycin (200 mg/l optimum). The resulting shoots were excised and placed on MX⁻ medium supplemented with 100-300 mg/l kanamycin (200 mg/l optimum). Example 8: Expression in Plants Regenerated tobacco plants descended from cells transformed by A . tumefaciens LBA4404 (pH400A4I) were self-fertilized. The resulting seeds were germinated on MX⁻ supplemented with 100-300 mg/l kanamycin (200 mg/i optimum) to select plants containing the aRNA gene-bearing T-DNA. Presence of pH400A4I T-DNA was confirmed by Southern blot analysis. Presence of aRNA was confirmed by Northern blot analysis. Untransformed control tobacco plants and aRNA gene-containing plants are challenged by being inoculated with all four AMV RNAs after abraision of leaves with carborundum, a method well known to the art. Under these conditions, translation of AMV RNA4 molecules present in the inoculum is necessary for establishment of an AMV infection. When compared to control plants, plants having an aRNA gene are observed to be resistant to AMV infection or to have reduced or delayed symptoms, depending on the degree of aRNA expression in tissues of a particular plant.

Claims: A virus-resistant plant cell, plant tissue, plant seed or plant that is uninfected by a virus and that comprises a stably integrated DNA molecule comprising a promoter, a cDNA encoding anti-sense RNA to at least a portion of a RNA genome of a plant virus having a single-stranded, plus-stranded, tripartite RNA genome, and a transcript terminator, wherein said cDNA encodes anti-sense RNA capable of conferring viral resistance to a plant, wherein the promoter, the cDNA, and the transcript terminator have such position and orientation with respect to each other that the anti-sense RNA may be transcribed in a plant cell under control of the promoter and terminator. A virus-resistant plant cell, plant tissue, plant seed or plant that is uninfected by a virus and that comprises a stably integrated DNA molecule comprising a promoter, a cDNA encoding anti-sense RNA to RNA4 from a plant virus having a single-stranded, plus-stranded, tripartite RNA genome, and a transcript terminator, wherein said cDNA encodes anti-sense RNA capable of conferring viral resistance to a plant, wherein the promoter, the cDNA, and the transcript terminator have such position and orientation with respect to each other that the anti-sense RNA may be transcribed in a plant cell under control of the promoter and terminator. A virus-resistant plant cell, plant tissue, plant seed or plant that is uninfected by a virus and that comprises a stably integrated DNA molecule comprising a promoter, a cDNA encoding anti-sense RNA to RNA1, RNA2 or RNA3 from a plant virus having a single-stranded, plus-stranded, tripartite RNA genome, and a transcript terminator, wherein said cDNA encodes anti-sense RNA capable of conferring viral resistance to a plant, wherein the promoter, the cDNA, and the transcript terminator have such position and orientation with respect to each other that the anti-sense RNA may be transcribed in a plant cell under control of the promoter and terminator. The plant, plant tissue, plant seed, or plant of any of claims 1, 2 or 3, wherein the virus is of the alfalfa mosaic virus group. The plant cell, plant tissue, plant seed, or plant of claim 4, wherein the plant virus is AMV. The plant cell, plant tissue, plant seed, or plant of claim 4, wherein the plant virus is AMV strain 425. The plant cell, plant tissue, plant seed, or plant of any one of claims 1, 2 or 3, wherein the plant virus is selected from the group consisting of an ilarvirus, a bromovirus, a cucumovirus and a hordeivirus. The plant cell, plant tissue, plant seed, or plant according to any preceding claim, wherein the anti-sense RNA is complementary to a viral 5'-untranslated sequence. The plant cell, plant tissue, plant seed, or plant according to any preceding claim, wherein the anti-sense RNA is complementary to a viral translation initiation site. The plant cell, plant tissue, plant seed, or plant according to any preceding claim, wherein the anti-sense RNA is complementary to a viral 3'-conserved sequence. The plant cell, plant tissue, plant seed, or plant according to any one of claims 1-7, wherein the cDNA is essentially full-length cDNA. The plant cell, plant tissue, plant seed, or plant according to any preceding claim, wherein the transcript terminator is a polyadenylation site. The plant cell, plant tissue, plant seed, or plant according to claim 12, wherein the polyadenylation site is a CaMV polyadenylation site. The plant cell, plant tissue, plant seed, or plant according to any preceding claim, said DNA molecule further comprising a plant-expressible selectable or screenable marker. The plant cell, plant tissue, plant seed, or plant according to claim 14, wherein the selectable marker encodes neomycin phosphotransferase. The plant cell, plant tissue, plant seed, or plant according to any preceding claim, said DNA molecule further comprising an ocs gene. The plant cell, plant tissue, plant seed, or plant according to any preceding claim, wherein the DNA molecule further comprises a T-DNA border repeat. The plant cell, plant tissue, plant seed, or plant according to claim 17, wherein the T-DNA is the T-DNA of pH400, wherein pH400 is obtainable by removing the BglII site between the Kan gene and the ORF1 promoter by partial digestion of pH4-1 and treating linear full-length pH4-1 with the Klenow-fragment of E. coli DNA polymerase I and subsequent blunt-end self ligation. The plant cell, plant tissue, plant seed, or plant according to claim 18, wherein the T-DNA is the T-DNA of pH400A4I (NRRL B-18062). The use of a DNA molecule as recited in any one of claims 1-19 to confer viral resistance to a plant cell, plant tissue, plant seed or plant. The use of a DNA molecule according to claim 20, wherein the viral resistance is to the AMV virus. The use of a DNA molecule according to claim 20, wherein the viral resistance is to a virus selected from the group consisting of an ilarvirus, a bromovirus, a cucumovirus and a hordeivirus. A method for producing a plant resistant to a plant virus comprising transforming a plant cell with a DNA molecule as recited in any one of claims 1-19.

Inventors: JARVIS NANCY P, LOESCH-FRIES SUE L, MERLO DONALD J, JARVIS, NANCY P., LOESCH-FRIES, SUE L., MERLO, DONALD J.
Applicants: PIONEER HI BRED INT, PIONEER HI-BRED INTERNATIONAL, INC.
Application year: 1987
Main classification: C12N15
IPC codes: C12N1, A01H1, A01H5, C12N15, C12N5, C12R1