Abstract:
The present invention relates to nucleic acid molecules found in the genome of the Citrus Leprosis Virus (CiLV), which is associated to Citrus Leprosis (CiL) disease. The cloned CiLV nucleic acid molecules can be used as probes or can be used to design oligonucleotide primers useful in assays, such as a polymerase chain reaction, for detecting the presence of CiLV in biological samples, particularly leaves, roots and other tissues or organs of plants, such as plants from the genera  Citrus  and  Poncirus.  The invention comprises introducing the mentioned nucleic acid molecules in cloning vectors and cloning the recombinant nucleic acid molecules in cells, such as prokaryotes (e.g., bacteria like  E. coli ), and eukaryotes (e.g., yeast, COS, CHO, and other cells). The cloned CiLV nucleic acid molecules are expressed in cells to provide immunogenic proteins which can be used to raise antibodies against the CiLV, which can then be used to detect the presence of the CiLV virus in biological samples. It also comprises the nucleic acid molecules represented in SEQ ID Nos. 5 and 8, in whole or part, as well as transgenic plants, such as monocots and dicots, containing the CiLV nucleic acid molecules, in any kind of combination, so that expression increases resistance to CiL disease.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority of Application Serial No. 60/651,828, filed on Feb. 10, 2005, Application Serial No. 60/641,335, filed on Jan. 4, 2005, Application Serial No. 60/633,921, filed on Dec. 6, 2004, Application Serial No. 60/629,866, filed on Nov. 19, 2004, and Application Serial No. 60/620,169, filed on Oct. 18, 2004, all of which are incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to the fields of molecular biology, biochemistry, plant pathology, and agriculture. More particularly, the invention relates to nucleic acid molecules and proteins from a phytopathogenic virus, which are suitable for disease diagnosis, treatment of plant pathologies and generation of virus resistant transgenic plants. 
       BACKGROUND AND PRIOR ART 
       [0003]      Citrus  diseases caused by viruses lead to significant economical loss worldwide (Derrick, K. S. and Timmer, L. W., Annu. Rev. Phytopathol., 38:181-205 (2000)). One example of a disease causing virus is Citrus Tristeza Virus (CTV), a member of the  Closterovirus  group, which induces serious disease syndromes in citrus, including quick decline resulting in death of trees on sour orange rootstock, and stem pitting of scion cultivars regardless of the rootstock used (Bar-Joseph et al., Annu. Rev. Phytopathol. 27:291-316 (1989)). 
         [0004]    In 1937, millions of sweet orange trees grafted on to sour orange rootstocks were lost in Brazil, due to citrus tristeza. The exchange of sour orange stock by Rangpur lime rootstock solved this problem (Gimenes-Femandes, N. and Bassanezi, R. B. Summa, Phytopathologica., 27:93 (2001)). 
         [0005]    In addition to CTV, other disease causing viruses are economically important. For example, CSDV causes citrus tree (sweet orange) death a few months after symptom detection (Gimenes-Fernandes, N. and Bassanezi, R. B. Summa Phytopathologica., 27:93 (2001)). The disease is associated with the presence of a  Tymovirus  in symptomatic trees and it is believed to be carried by an insect vector (Maccheroni Jr. et al, Journal of Virology, 79(5):3028-37 (2005)). 
         [0006]    Citrus leprosis (CiL) is another economically relevant virus related disease, especially in the State of São Paulo, the largest citrus producing area in Brazil, where the disease is endemic. CiL is also starting to move to Central American countries, including Guatemala, Honduras, Costa Rica and Panama (Dominguez, F. S.; Bandel, A.; Childers, C; Kitajima, E. W. Plant Disease, 85:228 (2001)). CiL is vectored by mites from the  Brevipalpus  genus which transmit a virus herein designated Citrus leprosis virus (CiLV), and usually affects sweet orange (Rodriguez, J. C.; Kitajima, E. W.; Childers, C. C.; Chagas, C. M. Exp and Appl Acarol, 30:161-79 (2003)). The symptoms related to this disease include circular clorotic lesions on both sides of the leaf. Eventually, the lesions become necrotic, assuming a central brownish color, leading to defoliation. Lesions are also present on branches and fruits, causing severe fruit damage and drop, leading to serious tree decline. Apart from sweet orange trees, which are very susceptible to CiL, the CiL symptoms can affect other varieties such as tangerine, but the susceptibility to the virus can vary from resistant to tolerant depending on the citrus variety. Mechanical transmission of CiLV can be achieved successfully from citrus to citrus, and from citrus to herbaceous plants (e.g.,  Chenopodium amaranticolor, C. quinoa  and  Gomphrena globosa ) using lesion extracts (Colariccio, A.; Lovisolo, O.; Chagas, C. M.; Galetti, S. R.; Rossetti, V.; Kitajima, E. W. Fitopatol. Bras., 20:208-213 (1995)). 
         [0007]    CiLV has been found in two different forms. Under electron microscopy, CiLV can be seen as rod shaped, enveloped particules, 30-40 nm×110-130 nm in size, present in the cytoplasm of leaf tissues (Kitajima, E. W.; Mulller, G. W.; Costa, A. S.; Yuri, W. Virology, 50:254-258 (1972)). In another report, similarly sized and shaped particles were found in the nucleus; however these were not surrounded by an envelope (Colariccio, A.; Lovisolo, O.; Chagas, C. M.; Galetti, S. R.; Rossetti, V.; Kitajima, E. W. Fitopatol. Bras., 20:208-213 (1995)). Based on the morphology and occurrence of the two forms, CiLV has been classified as a  Rhabdovirus.    
         [0008]    Recently, Locali et al. (Plant Disease, 87:1317-1321 (2003)) were able to isolate some CiLV sequences using RT-PCR from RNA samples prepared from symptomatic leaves. 
         [0009]    The control of CiL is currently carried out with acaricides, which exterminate the vector,  Brevipalpus phaenicis;  however, expenditures directed to exterminating this mite cost over 100 million dollars every year. (Rodriguez, J. C.; Kitajima, E. W.; Childers, C. C.; Chagas, C. M. Exp and Appl Acarol, 30:161-79 (2003)). This figure represents 80% of the total expenses relating to defenses against the pest. Therefore, there is the need to improve the means to control this disease that is both cost effective and efficient. Also, there is a worldwide concern to reduce the amount of toxic products, such as acaricides, which polute the environment. 
         [0010]    In order to explore the possibility of using alternative ways to control CiL that are economically sound and do not involve environmentally toxic compounds, a program to sequence the genome of CiLV was established. Several approaches were used to sequence the genome of this virus. Initially, short specific sequences from the Replicase (402 bp) and Movement Protein (MP) (339 bp) genes were obtained from symptomatic leaves of sweet orange, using diagnostic RT-PCR and gene specific primers MP-F (5′ GCGTATTGGCGTTGGATTTCTGAC 3′) (SEQ ID NO: 1) and MP-R (5′ TGTATACCAAGCCGCCTGTGAACT 3′) (SEQ ID NO: 2) for the Movement Protein and REP-F (5′ GATACGGGACGCATAACA 3′) (SEQ ID NO: 3) and REP-R (5′ TTCTGGCTCAACATCTGG 3′) (SEQ ID NO: 4) for the Replicase (Locali, E. C.; Freitas-Astua, J.; Takita, M. A.; Astua-Monge, G.; Antoniolli, R.; Kitajima, E. W.; Machado, M. A. Plant Disease, 87:1317-1321 (2003)). These sequences were used as templates to design primers to extend these regions and obtain more virus sequences toward the 5′ and 3′ regions of the two genes. Another approach was the construction of a subtractive library. In this technique, polyA RNA from symptomatic and asymptomatic leaves were extracted and converted into cDNA, separately. These cDNAs were hybridized to each other at specific temperature and salt concentrations and amplified by PCR in such a way that only the sequences specific to the symptomatic leaves were amplified. The remaining cDNA sequences that are common to both cDNA populations were not amplified and therefore lost. The subtracted PCR amplified sequences were cloned and sequenced to search for virus sequences. Also, the subtractive library was enriched for viral sequences using primers based on the viral sequences obtained by the RT-PCR in the first experiment described above. The combination of these 2 approaches generated the complete sequence of the CiLV genome, which is a bipartite virus, i.e., one that presents two RNA segments. The nucleotide sequence of RNA1 is 8730 bp long (SEQ ID NO: 5). One large ORF of 7539 bp, starting with an AUG codon at position 108-110 and terminating with an UAA codon at 7644-7646, was detected. This ORF encodes a 2512 amino acid replicase protein (with an estimated molecular weight of 286.4 KDa) (SEQ ID NO: 6) with sequence similarities to other replicases from plant viruses, especially the  Furovirus.  The following domains are present in the polyprotein: i) a methyl transferase, from residues 128 to 325; ii) a putative protease comprising amino acids 683 to 803; iii) a helicase, from amino acid 1521 to amino acid 1697 and iv) a RNA-dependent RNA polymerase domain comprising amino acid 2221 to 2458. All numbering refers to SEQ. ID. NO: 6. There is a second ORF (792 bp), in RNA1, starting at position 7709-7711 (AUG) and terminating at 8498-8500 (UAG). It encodes a putative 263 amino acid polypeptide (with an estimated molecular weight of 29.1 KDa) (SEQ ID NO: 7), herein called p29. This polypeptide has 32% identity to  Sindbis  virus capsid protein. RNA1 has 107 nucleotide long 5′ UTR (1 to 107) and a 230 nucleotide 3′ UTR (position 8501 to 8730), excluding its polyA tail. 
         [0011]    The sequence of RNA2 is 4975 bps long (SEQ ID NO: 8), and contains 4 putative ORFs. The first one, closest to the 5′ end, “p15”, is 393 nucleotide long (AUG at position 67 and UAA at 459), encoding a 130 amino acid polypeptide (SEQ ID NO: 9). This polypeptide has 21% identity to an envelope glycoprotein from Human Immunodeficiency Virus-1 (Yamaguchi, Y., Delehouzee, S., Handa, H., Microbes Infect. 4, 1169-75 (2002)). The second ORF, “p61”, comprises 1614 nucleotides, starting at 1590 (AUG) and terminating at 3203 (UAA), encoding a 537 amino acid polypeptide (SEQ ID NO: 10). This polypeptide has 23% identity to a  Saccharomyces cerevisiae  mannosyltransferase KTR4. The third ORF (p32) is 894 nucleotide long, with its start codon at position 3228 (AUG) and stop codon at 4121 (UAA). It encodes a 297 amino acid polypeptide with an estimated molecular weight of 32.5 KDa (SEQ ID NO: 11). Similarity searches at the NCBI GenBank showed significant homology to movement proteins of plant viruses (E value=7e-09), especially those from  Furovirus  and  Bromovirus.  The last ORF in RNA2 (p24) is 645 nucleotide long. It is transcribed in a different frame than p61 and p32, and overlaps with the terminal part of the movement protein by 29 nucleotides. Its start codon is located at position 4093 and its stop codon is at position 4737 (UGA). It encodes a protein of 214 amino acids length (SEQ ID NO: 12). This polypeptide has similarity to a glycoprotein precursor (CD47-like protein) from Sheeppox Virus (Tulman, E. R., Afonso, C. L., Lu, Z, Zsak L., Sur, J. H., Sandybaev, N. T., Kerembekova, U. Z., Zaitsev, V. L., Kutish, G. F., Rock, D. L. J Virol. 76, 6054-61 (2002)). All nucleotide references for RNA2 refer to SEQ. ID. NO: 8. 
         [0012]    The sequence of CiLV permitted design of primers to amplify all parts of its genome in overlapping fragments, by RT-PCR ( FIG. 1A ). Analysis of symptomatic and asymptomatic leaves by RT-PCR ( FIG. 1B ) showed that only symptomatic leaves contained viral sequences. Northern blots prepared with total RNA extracted from symptomatic and asymptomatic leaves and hybridized to radioactive probes made with DNA fragments from RNA1 and RNA2 showed that only the symptomatic leaves have both 8730 and 4975 bp bands associated to CiL ( FIG. 2 ). Also, RT-PCR made with RNA extracted from viruliferous mites and gene specific primers amplified a 1.7 kb band ( FIG. 3 ). These results associate CiLV genomic sequence to CiL symptoms and the presence of the virus in the vector  B. phoenicis.  Based on these data the new virus is considered to be associated with CiL disease. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention relates to isolated nucleic acid molecules of and from the genome of a virus that is associated with CiL disease. It is an object of the invention to provide nucleic acid molecules which encode infectious CiLV, and the proteins contained therein. Such nucleic acid molecules are referred to throughout the application as “CiLV nucleic acid molecules”. 
         [0014]    For purposes of this application, “nucleic acid molecules” refers to RNA, DNA, cDNA or any variant thereof with functions equivalent to RNA, gDNA, and cDNA, such as the synthesis of CiLV polypeptides. 
         [0015]    The invention also relates to the use of the CiLV nucleic acid molecules to produce polypeptides, and the resulting polypeptides. “Nucleic acid molecules of the invention” refers to, e.g., CiLV nucleic acid molecules, mutations of CiLV nucleic acid molecules, chimeric nucleic acid molecules and so forth. In one embodiment, polypeptides are produced by cells transformed or transfected with nucleic acid molecules of the invention. In another embodiment, the polypeptide or polypeptides are produced recombinantly from a fragment or portion of the nucleic acid molecules of the invention. In yet another embodiment, the polypeptides are chemically synthesized. 
         [0016]    The polypeptides of the invention can serve, e.g., as immunogens to develop diagnostic assays for detecting the presence of CiLV in biological samples, as they provoke antibody production, and the antibodies can then be used in assays. 
         [0017]    The invention also relates to the use of CiLV nucleic acid molecules for diagnostic purposes, in which oligonucleotide primers containing from 20 to 100, preferably 30-100, and more preferably, 50-100, and most preferably 30-80 nucleotides presenting from 90 to 100% identity with the CiLV nucleic acid sequences can be used in, e.g., RT-PCR reactions, so that parts of the CiLV nucleic acid molecules can be amplified and detected, e.g., on ordinary agarose gels, thus serving as a diagnostic method for determining the presence of the virus or lack thereof. 
         [0018]    The invention also relates to methods of transforming plants, such as monocots or dicots, with constructs containing the CiLV nucleic acid molecules, to produce plants that are resistant to CiLV. Such methods include the introduction of constructs containing at least one CiLV nucleic acid molecule into plant parts, such as scions, rootstock cultivars, and so forth, as well as into citrus germplasm and breeding lines. Transformed CiLV-resistant germplasm and breeding lines can be used in conventional breeding programs, to create new cultivars that carry and express the resistance genes. 
         [0019]    Accordingly, the invention features (i) isolated and/or purified CiLV nucleic acid molecules that encode polypeptides that have at least 80% sequence identity with the amino acid sequences of SEQ ID NOS: 6, 7, 9, 10, 11 or 12; (ii) the nucleotide sequences set forth at SEQ ID NO: 5 or 8 or sequences complementary to nucleotides, whose complement hybridizes under highly stringent conditions to one of the nucleotide sequences of SEQ ID NOS: 5 or 8. 
         [0020]    “Highly stringent conditions”, as used herein, refers to parameters with which the art is familiar, such as hybridization in 3.5×SSC, 1×Denhardt&#39;s solution, 25 mM sodium phosphate buffer (pH 7.0), 0.5% SDS, and 2 mM EDTA for 18 hours at 65° C., followed by 4 washes of the filter at 65° C. for 20 minutes, in 2×SSC, 0.1% SDS, and a final wash for up to 20 minutes in 0.5×SSC, 0.1% SDS, or 0.3×SSC and 0.1% SDS for greater stringency, and 0.1×SSC, 0.1% SDS for even greater stringency. Other conditions may be substituted, as long as the degree of stringency is equal to that provided herein, using a 0.5×SSC final wash. 
         [0021]    The invention also features expression vectors or constructs in which the CiLV nucleic acid molecules are operably linked to one or more expression control sequences or promoters. 
         [0022]    The invention further features a transgenic plant, such as one of the genera  Citrus  and  Poncirus,  into which a CiLV nucleic acid molecule has been introduced. Exemplary are citrus scions and rootstock cultivars (e.g., from sour orange [ Citrus aurantium ], Rangpur lime [ Citrus limonia ], rough lemon [ Citrus limonia  and  Citrus jambhiri ], mandarin “Cleopatra” [ Citrus reshni ], Sunki [ Citrus sunki ], Volkamerian lemon [ Citrus volkameriana ], “Caipira” orange [Citrus sinensis]) and intrageneric hybrids (e.g., tangelos [ Citrus paradisi×Citrus reticulata ], tangors [ Citrus reticulata×Citrus sinensis],  citrumelo [ Poncirus trifoliata×Citrus maxima ], and citrange [ Citrus sinensis×Poncirus trifoliate ]). The plant can be a breeding line. It can also be one from  Fortunella  and  Citrofortunella  species, including calamondin and kumquat. The nucleic acid molecule in the plant includes a selectable marker such as an herbicide resistance gene. 
         [0023]    The invention relates to nucleic acids, and the polypeptides they encode, which were identified using sequencing of viral genes from citrus plants with or without symptoms of CiL. These sequences come from a newly identified virus which, when it infects a plant, such as a citrus plant, causes CiL disease, leading to the death of the plant. Methods of genetic transformation to produce plants that are resistant to CiLV strains are within the scope of the invention. Such methods include the introduction of constructs containing the CiLV sequences into scions and/or rootstocks, so that commercial varieties or any other germplasm useful for breeding programs can be produced and used to create new cultivars resistant to CiLV. 
         [0024]    Accordingly, the invention features purified nucleic acid molecules of the genome sequence of CiLV, isolated from citrus plants manifesting symptoms of CiL, which, when used in appropriate constructs, have the ability to confer resistance to pathologies caused by CiLV infection in plants infected with CiLV. The isolated nucleic acid molecule can be a portion of a gene, e.g., a nucleic acid molecule whose nucleotide sequence encodes a protein that has at least 80% sequence identity to the amino acid sequence of SEQ ID NOS: 6, 7, 9, 10, 11 or 12. The nucleotide sequences can be that of SEQ ID NOS: 5 or 8 or one that defines polynucleotides whose complement hybridizes under high stringency conditions to the nucleotide sequences of SEQ ID NOS: 5 or 8. 
         [0025]    The invention also features expression vectors, isolated recombinant cells, and plants and plant parts containing the nucleic acid molecule of the invention, e.g., one of the nucleic acid molecules described above. The nucleic acid molecule in the vector, cell, seed or plant can be operably linked to one or more expression control sequences or promoters. 
         [0026]    In addition, the invention features the isolated proteins encoded by the open reading frames (ORFs) of genes present in the sequence of CiLV. The proteins include amino acid sequences that (a) share at least 80% sequence identity with SEQ ID NOS: 6, 7, 9, 10, 11 or 12; or (b) comprise the amino acid sequences of SEQ ID NOS: 6, 7, 9, 10, 11 or 12. Proteins of the invention can be expressed in bacteria either as “neat” proteins, or as heterologous polypeptides. 
         [0027]    The invention also features antibodies raised against the proteins of the invention, e.g., those described above. The antibodies can further include a detectable label. 
         [0028]    The invention also features a plant transfected with one of the nucleic acid molecules of the invention, e.g., any purified CiLV sequences. The plant may be of the genera  Citrus  and  Poncirus  (e.g., sweet orange [ Citrus sinensis ], mandarin [ Citrus reticulata ], sour orange [ Citrus aurantium ], Rangpur lime [ Citrus limonia ], rough lemon [ Citrus limonia  and  Citrus jambhiri ], mandarin “Cleopatra” [ Citrus reshni ], Sunki [ Citrus sunki ], Volkamerian lemon [ Citrus volkameriana ], “Caipira” orange [ Citrus sinensis ]) and intrageneric hybrids (e.g., tangelos [ Citrus paradisi×Citrus reticulata ], tangors [ Citrus reticulata×Citrus sinensis ], citrumelo [ Poncirus trifoliata×Citrus maxima ], and citrange [ Citrus sinensis×Poncirus trifoliate ]). The plant can be a breeding line. It can also be one from  Fortunella  and  Citrofortunella  species, including calamondin and kumquat. 
         [0029]    The invention features a method of producing a disease resistant plant by introducing constructs containing purified nucleic acid molecules under the control of a suitable plant promoter, into the plant, transforming the plant with  Agrobacterium  strains or microprojectile bombardment. 
         [0030]    All technical terms used herein are terms commonly used in biochemistry, molecular biology, phytopathology and agriculture, and will be understood by one of ordinary skill in the art to which this invention belongs. Those technical terms can be found in, e.g., Molecular Cloning: A Laboratory Manual, 3rd ed., vol. 1-3, ed. Sambrook and Russel, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 2001; Current Protocols in Molecular Biology, ed. Ausubel et al., Greene Publishing Associates and Wiley-Interscience, New York, 1988 (with periodic updates); Short Protocols in Molecular Biology: A Compendium of Methods from Current Protocols in Molecular Biology, 5 th  ed., vol. 1-2, ed. Ausubel et al., John Wiley &amp; Sons, Inc., 2002; Genome Analysis: A Laboratory Manual, vol. 1-2, ed. Green et al., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1997. Methods involving plant biology techniques are described herein and are described in detail in methodology treatises such as Methods in Plant Molecular Biology: A Laboratory Course Manual, ed. Maliga et al., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1995. Various techniques using PCR are described, e.g., in Innis et al., PCR Protocols: A Guide to Methods and Applications, Academic Press, San Diego, 1990 and in Dieffenbach and Dveksler, PCR Primer: A Laboratory Manual, 2 nd  ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 2003. PCR-primer pairs can be derived from known sequences by known techniques such as using computer programs intended for that purpose (e.g., Primer, Version 0.5, 1991, Whitehead Institute for Biomedical Research, Cambridge, Mass.). Methods for chemical synthesis of nucleic acids are discussed, for example, in Beaucage and Caruthers (1981) Tetra. Letts. 22:1859-1862 and Matteucci and Caruthers (1981) J. Am. Chem. Soc. 103:3185. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0031]    The invention can be more readily understood by reference to the accompanying drawings, wherein: 
           [0032]      FIG. 1A  shows the localization of the primers used in RT-PCR along the two RNAs of the CiLV genome.  FIG. 1B  shows the results of diagnostic RT-PCR for CiL performed on total RNA isolated from symptomatic leaves collected from 4 different regions in the State of São Paulo, Brazil, and asymptomatic leaves from Piracicaba, State of São Paulo, Brazil. 
           [0033]      FIG. 2  shows the Northern blot for RNA1 and RNA2 from symptomatic and asymptomatic leaves. 
           [0034]      FIG. 3  shows the amplification of CiLV sequence by RT-PCR from total RNA extracted from viruliferous mites and leaves. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0035]    Nucleic acid molecules from the genome of a virus that causes Citrus Leprosis (CiL) have been cloned and sequenced. Such nucleic acid molecules are referred to throughout the application as “CiLV nucleic acid molecules”. Polypeptides encoded by CiLV nucleic acid molecules have been analyzed using software programs including BLAST, and have been shown to encode, inter alia, a Replicase involved in virus replication and a Movement Protein involved in the translocation of the virus throughout the plant. 
         [0036]    The molecular cloning of CiLV nucleic acid molecules provides the means to develop diagnostic methods to detect the presence of CiLV in biological samples, including, but not being limited to, tissues, cells and organs of plants, such as plants of the genus  Citrus.  The molecular cloning of CiLV nucleic acid molecules also provides the means to create CiL-resistant plants, such as those of the genus  Citrus  through genetic transformation. Genetic transformation of plants can be secured via  Agrobacterium  transformation methods. Such methods include cloning constructs containing CiLV nucleic acid molecules operably ligated to promoter and enhancer regions, initiation and termination sequences. These constructs can also contain genes for selectable markers, such as herbicide resistance. These constructs may be cloned in the Ti plasmid of  Agrobacterium.  Plasmid vector-containing constructs are used to transform commonly used  Agrobacterium  strains, which are subsequently used to transform plants, such as those of the genus  Citrus.  Plasmid vector-containing constructs may be also introduced into plants by microprojectile bombardment. The constructs containing the CiLV nucleic acid molecules are useful for creating CiL resistant plants such as all common types of citrus fruits, including, but not limited to, sweet oranges, grapefruit, mandarins, tangerines, pummelos, lemons, limes, citrons, bergamots, limequats, meyer lemons, silver limes, key limes, kaffir limes, lavender gems, blood oranges, satsumas, oroblancos, melogolds, intrageneric hybrids such as tangelos and tangors, and citrus-type fruit such as calamondins and kumquats ( Fortunella  spp.). For example, resistance gene molecules can be introduced into commercially utilized rootstock cultivars, including, but not being limited to, Rangpur lime, sour orange, rough lemon, various mandarins, and citrus intrageneric and intergeneric hybrids. CiL resistant citrus plants, composed of genetic modified scions and rootstocks, can then be used by citrus growers to counter CiL disease, and to avoid decreasing productivity and/or tree death and replanting costs. 
         [0037]    Methods involving conventional molecular biology techniques can be found in the following references: Molecular Cloning: A Laboratory Manual, 3rd ed., vol. 1-3, ed. Sambrook and Russel, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 2001; Current Protocols in Molecular Biology, ed. Ausubel et al., Greene Publishing Associates and Wiley-Interscience, New York, 1988 (with periodic updates); Short Protocols in Molecular Biology: A Compendium of Methods from Current Protocols in Molecular Biology, 5 th  ed., vol. 1-2, ed. Ausubel et al., John Wiley &amp; Sons, Inc., 2002; Genome Analysis: A Laboratory Manual, vol. 1-2, ed. Green et al., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1997. Methods involving plant biology techniques are described herein and are described in detail in methodology treatises such as Methods in Plant Molecular Biology: A Laboratory Course Manual, ed. Maliga et al., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1995. Various techniques using PCR are described, e.g., in Innis et al., PCR Protocols: A Guide to Methods and Applications, Academic Press, San Diego, 1990 and in Dieffenbach and Dveksler, PCR Primer: A Laboratory Manual, 2 nd  ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 2003. PCR-primer pairs can be derived from known sequences by known techniques such as using computer programs intended for that purpose (e.g., Primer, Version 0.5, 1991, Whitehead Institute for Biomedical Research, Cambridge, Mass.). Methods for chemical synthesis of nucleic acids are discussed, for example, in Beaucage and Caruthers (1981) Tetra. Letts. 22:1859-1862 and Matteucci and Caruthers (1981) J. Am. Chem. Soc. 103:3185. Chemical synthesis of nucleic acids can be performed, for example, on commercial automated oligonucleotide synthesizers. 
         [0038]    The invention provides purified nucleic acid molecules (polynucleotides) that encode polypeptides having an amino acid sequence selected from the group consisting of SEQ ID NOS: 6, 7, 9, 10, 11 and 12. 
         [0039]    The CiLV nucleic acid molecules of the present invention can be obtained from CiLV infected plants. The molecules of the present invention may be in the form of RNA or DNA, preferably in the form of cDNA. The cDNA may be double- or single-stranded, and, if single-stranded, may be the coding (sense) strand or noncoding (anti-sense) strand. The sequence may be identical to a nucleotide sequence consisting of SEQ ID NOS: 5 or 8. It may also be a different coding sequence which, as a result of the redundancy or degeneracy of the genetic code, encodes the same polypeptide as the sequences of SEQ ID NOS: 5 or 8. Other nucleic acid molecules within the invention are variants of CiLV nucleic acid molecules such as those that encode fragments, analogs and derivatives of native CiLV nucleic acid molecules. Such variants may be, e.g., naturally occurring polymorphic variants of native CiLV nucleic acid molecules, or a non-naturally occurring variant of native CiLV nucleic acid molecules. For example, the nucleotide sequence of such variants can feature a deletion, addition, or substitution of one or more nucleotides of native CiLV nucleic acid molecules. 
         [0040]    Naturally occurring variants of native CiLV nucleic acid molecules within the invention are nucleic acids isolated from CiLV infected plants that have at least 65% sequence identity with native CiLV nucleic acid molecules, and encode polypeptides having at least one functional activity in common with native CiLV nucleic acid molecules encoding proteins. 
         [0041]    Shorter oligonucleotides (e.g., those of 6-200, preferably 12-150, more preferably 30-125, and even more preferably 50-100 base pairs in length) that match perfectly to the CiLV nucleic acid molecules or hybridize with CiLV nucleic acid molecules at stringent conditions as defined herein can be used as probes, primers, or antisense molecules. 
         [0042]    Longer polynucleotides (e.g., those of 300 to 800 base pairs) that encode or hybridize with CiLV nucleic acid molecules can be used in place of a native CiLV nucleic acid molecule in applications where it is desired to modulate the functional activity of a native CiLV nucleic acid molecule. 
         [0043]    Nucleic acids molecules that hybridize under stringent conditions as defined herein to a nucleic acid selected from the group consisting of SEQ ID NOS: 5 and 8 or to the complement of a sequence selected from the group consisting of SEQ ID NOS: 5 and 8 are also within the invention. For example, such nucleic acids can be those that hybridize to a sequence selected from the group consisting of SEQ ID NOS: 5 and 8 or to the complement of a sequence selected from the group consisting of SEQ ID NOS: 5 and 8 under low stringency conditions, moderate stringency conditions, or high stringency conditions are within the invention. Preferred nucleic acids molecules are those having a nucleotide sequence that is the complement of all or a portion of a sequence selected from the group consisting of SEQ ID NOS: 5 and 8. Other variants of CiLV nucleic acid molecules within the invention are polynucleotides that share at least 65% sequence identity to a sequence selected from the group consisting of SEQ ID NOS: 5 and 8 or to the complement of a sequence selected from the group consisting of SEQ ID NOS: 5 and 8. 
         [0044]    CiLV nucleic acid molecules encoding polypeptides are also within the invention. Such polypeptides can be made by preparing a construct (e.g., an expression vector) that expresses CiLV nucleic acid molecules encoding polypeptides, when introduced into a suitable host. Variant CiLV nucleic acid molecules-encoding polypeptides can be produced by those skilled in molecular biology procedures using standard nucleic acid mutagenesis techniques or chemical synthesis. 
         [0045]    Another aspect of the invention relates to the use of purified antisense nucleic acid molecules to inhibit expression of CiLV nucleic acid molecules. Antisense nucleic acid molecules within the invention are those that specifically hybridize under cellular conditions to cellular mRNA and/or genomic RNA of CiLV in a manner that inhibits expression of the genes encoded by the CiLV genome. 
         [0046]    The antisense nucleic acid molecules should be delivered into cells that express CiLV genes. For instance, constructs expressing antisense molecules under the control of a strong promoter can be introduced into citrus plants by genetic transformation using  Agrobacterium  or microprojectile bombardment (Ghorbel et al. Tree Physiology. 20. 1183-1189 (2000); Bespalhok et al., Crop Breed. Appl. Biotech. 1. 27-34 (2001); Bespalhok et al., Braz. Arch. Biol. Technol. 46. 1. 1-6 (2003); Molinari et al., Scientia Horticulturae. 99. 3-4. 379-385 (2004); Jia-Long et al., Plant Science. 113. 2. 175-183 (1996)). 
         [0047]    The expression of CiLV nucleic acid molecules can be modulated by RNA interference (RNAi) (Lee et al. Nature Biotech. 19. 500-505 (2002); Voinnet, O. Trends Genet. 17. 449-459 (2001)) by which a construct driving the synthesis of sequence-specific double-stranded RNA (dsRNA) is introduced into an organism or cell in order to silence the targeted gene (Hannon, Nature. 418. 244-251 (2002)). Selected sequences corresponding to CiLV nucleic acid molecules can be used to create, after expression, a sequence-specific dsRNA that can interfere with accumulation of endogenous RNA encoded by the CiLV nucleic acid molecules. 
         [0048]    The present invention is further illustrated by the following specific examples. The examples are provided for illustration only and are not to be construed as limiting the scope or content of the invention in any way. 
       EXAMPLE 1  
       [0049]    This example describes the identification and cloning of nucleic acid molecules from CiLV by RT-PCR. Total RNA was extracted from symptomatic and asymptomatic leaves. First strand cDNA was synthesized using 4 μg of total RNA as template, 250 ηg of oligonucleotides, and was denatured at 65° C. for 5 minutes. The solution was then incubated on ice while adding 1 μl of 10 mM dNTP mix and 3 μl of First Strand buffer (250 mM Tris-HCl, pH 8.3, 375 mM KCl, 15 mM MgCl 2 ) to the tube. The mixture was incubated at room temperature for 2 min, 1 μl (200 U) of reverse transcriptase was added and the solution was further incubated at room temperature for 10 min. followed by 60 min. at 42° C. For PCR, 2 μl of the first strand cDNA was used as template, 0.125 mM each dNTP, 2 mM MgCl 2 , 1×reaction buffer (20 Mm Tris-HCl, pH 8.4, 50 mM KCl), 1 U of Taq polymerase and 0.4 μM of each of primer pair: REP13 5′ ACAGACTACGTGAAATATACC 3′ (SEQ ID NO. 13) and REP14 5′ CGTGAAACTCCGAATCCATT 3′ (SEQ ID NO 14); REP15 5′ GGAAATTGCTTCCTCACTTG 3′ (SEQ ID NO 15) and REP16 5′ GGTGTTGTGGTACCACCACC 3′ (SEQ ID NO 16); REP19 5′ GTACCGCACTTGAGCCTATC 3′ (SEQ ID NO 17) and REP18 5′ AACCTCGCCCAGCTGACAAC 3′ (SEQ ID NO 18); MP21 5′ GGTTGCTTTACGTTCGAGTGTGA 3′ (SEQ ID NO 19) and MP13 5′ CGTTGTGGAGACCCAGAGCA 3′ (SEQ ID NO 20); MP 15 5′GGTAGTGATATCATGTCATGTG 3′ (SEQ ID NO 21) and MP16 5′ GGTACACCACCAGCTAGAAG 3′ (SEQ ID NO 22). Citrus chalcone synthase primers were used as internal controls. The reaction was heated for 5 min. at 94° C. and subjected to 30 amplification cycles (30 s at 94° C., 30 s at 55° C., 1 min at 72° C.). DNA fragments were separated in a 1% agarose gel, stained with 100 ng/ml ethidium bromide and analyzed under UV light ( FIG. 1B ). The bands generated by RT-PCR were cloned in pGEM-T and sequenced using an ABI 3700 sequencer. The BLASTX and BLASTN against NR databases showed sequence similarity to the Movement protein of Sorghum Chlorotic Spot Virus (BAA94804) and the replicase protein of Barley Strip Mosaic Protein (NP — 604474). 
       EXAMPLE 2  
       [0050]    In this experiment Northern blotting was carried out on RNA samples extracted from symptomatic and asymptomatic leaves. DNA fragments from the movement protein and replicase partial genes CiLV were used as probes.  FIG. 2  shows the band pattern in symptomatic plants and positive controls with both probes, while no bands were identified in the asymptomatic material with both probes. 
         [0051]    Total RNA from bark tissue of symptomatic and asymptomatic plants was extracted using the Trizol Reagent (Invitrogen) according to the manufacturer&#39;s protocol. Ten micrograms of total RNA from each sample were separated by denaturing electrophoresis on a 1% agarose gel containing 1×MOPS and 0.6M formaldehyde, according to Sambrook and Russell (2001), supra. The gel was subsequently transferred to a Hybond-N+ nylon membrane by capillary transfer in 10×SSC (1×SSC is 0.15M NaCl; 0.015M sodium citrate) for 16 hours. The membrane was baked at 80° C. for 2 hours, prehybridized in hybridization buffer for 2 hours at 65° C., followed by hybridization in a fresh aliquot of solution containing 20 ng/ml of probe (2×10 7  cpm/ml) for 16 hours at 65° C. The probes consisted of a 339 bp fragment of the CiLV Movement protein and 402 bp fragment from the Replicase gene. The probe was radioactively labeled by random-priming with [α-32P]dCTP (6000 ci/mmol) using commercially available products. After hybridization, the membrane was washed at room temperature in 2×SSC, 0.05% SDS for 4×10 min, and then twice at 55° C. for 20 min each in 0.1×SSC; 0.1% SDS. The blot was exposed for 1 hour and analyzed by phosphoimaging. 
       EXAMPLE 3  
       [0052]    This example describes the identification and cloning of nucleic acid molecules from CiLV-C by RT-PCR from viruliferous mites. First strand cDNA was synthesized using 4 μg of total RNA as template, 250 ηg of oligonucleotides and denatured at 65° C. for 5 minutes. The solution was then incubated on ice while adding 1 μl of 10 mM dNTP mix and 3 μl of First Strand buffer (250 mM Tris-HCl, pH 8.3, 375 mM KCl, 15 mM MgCl 2 ) to the tube. The mixture was incubated at room temperature for 2 min, 1 μl (200 U) of reverse transcriptase was added and the solution was further incubated at room temperature for 10 min. followed by 60 min. at 42° C. For PCR, 2 μl of the first strand cDNA was used as template, 0.125 mM each dNTP, 2 mM MgCl 2 , 1×reaction buffer (20 mM Tris-HCl, pH 8.4, 50 mM KCl), 1 U of Taq polymerase and 0.4 μM primers: MP21 5′ GGTTGCTTTACGTTCGAGTGTGA 3′ (SEQ ID NO 19) and MP13 5′ CGTTGTGGAGACCCAGAGCA 3′ (SEQ ID NO 20). The reaction was heated for 5 min. at 94° C. and subjected to 30 amplification cycles (30 s at 94° C., 30 s at 55° C., 1 min at 72° C.). DNA fragments were separated in a 1% agarose gel, stained with 100 ng/ml ethidium bromide and analyzed under UV light ( FIG. 1B ). The bands generated by RT-PCR were cloned in pGEM-T and sequenced using an ABI 3700 sequencer. The BLASTX and BLASTN against NR databases showed sequence similarity to the Movement protein of Sorghum Chlorotic Spot Virus (BAA94804).