Patent Application
20060024740
The present invention relates to the use of primers specific for races of pathogenic fungi which are resistant to certain fungicides in polymerase chain reaction assays for the detection of fungal pathogens. The use of these primers enables the detection of specific isolates of fungal pathogens and the monitoring of disease development in plant populations.
Diseases in plants cause considerable crop loss from year to year resulting both in economic deprivation to farmers and additionally in many parts of the world to shortfalls in the nutritional provision for local populations. The widespread use of fungicides has provided considerable security against plant pathogen attack. However, despite $1 billion worth of expenditure on fungicides, worldwide crop losses amounted to approximately 10% of crop value in 1981 (James, 1981; Seed Sci. &Technol. 9: 679-685).
The severity of the destructive process of disease depends on the aggressiveness of the pathogen and the response of the host. One aim of most plant breeding programs is to increase the resistance of host plants to disease. Typically, different races of pathogens interact with different varieties of the same crop species differentially, and many sources of host resistance only protect against specific pathogen races. Furthermore, some pathogen races show early signs of disease symptoms, but cause little damage to the crop. Jones and Clifford (1983; Cereal Diseases, John Wiley) report that virulent forms of the pathogen are expected to emerge in the pathogen population in response to the introduction of resistance into host cultivars and that it is therefore necessary to monitor pathogen populations. In addition, there are several documented cases of the evolution of fungal strains which are resistant to particular fungicides. As early as 1981, Fletcher and Wolfe (1981; Proc. 1981 Brit. Crop Prot. Conf) contended that 24% of the powdery mildew populations from spring barley, and 53% from winter barley showed considerable variation in response to the fungicide triadimenol and that the distribution of these populations varied between varieties with the most susceptible variety also giving the highest incidence of less susceptible types. Similar variation in the sensitivity of fungi to fungicides has been documented for wheat mildew (also to triadimenol), Botrytis (to benomyl), Pyrenophora (to organomercury), Tapesia (to MBC-type fungicides) and Mycosphaerella fijiensis to triazoles to mention just a few (Jones and Clifford; Cereal Diseases, John Wiley, 1983).
Cereal species are grown world-wide and represent a major fraction of world food production. Although yield loss is caused by many pathogens, the necrotizing pathogens Septoria and Tapesia are particularly important in the major cereal growing areas of Europe and North America (Jones and Clifford; Cereal Diseases, John Wiley, 1983). In particular, the differential symptomology caused by different isolates and species of these fungi make the accurate predictive determination of potential disease loss difficult. Consequently, the availability of improved diagnostic techniques for the rapid and accurate identification of specific pathogens will be of considerable use to field pathologists.
The eyespot disease of cereals is caused by the fungi Tapesia yallundae and Tapesia acuformis is restricted to the basal culm of the plant. The two causal pathogens were previously classified as two subspecies of Pseudocercosporella herpotrichoides (Fron) Deighton (amamorph). T. yallundae refers to the variety herpotrichoides and the SF-, L-, I- or W-types. T. acuformis corresponds to the variety acuformis and the FE-, N-, II- or R-types (Leroux and Gredt, 1997; 51:321-327). Wheat, rye, oats and other grasses are susceptible to the eyespot disease which occurs in cool, moist climates and is prevalent in Europe, North and South America, Africa and Australia. Wheat is the most susceptible cereal species, but isolates have been identified which are also virulent on other cereals. The R-strain (T. acuformis) of the fungus, for example, has also been isolated from rye and grows more slowly on wheat than the W-strain (T. yallundae) which has been isolated from wheat. Although eyespot may kill tillers or plants outright, it more usually causes lodging and/or results in a reduction in kernel size and number. Yield losses associated with eyespot are of even greater magnitude than those associated with Septoria tritici and Septoria nodorum. Typical control measures for eyespot include treatment with growth regulators to strengthen internodes, and fungicide treatment. However, the differing susceptibility of cultivars to different strains of the fungus render the predictive efficacy of fungicide treatments difficult. In addition, both Leroux et al (1997; Pesticide Science, 51:321-327) and Dyer et al (2000; Appl. and Environ. Microbiol. 66:4599-4604) have reported on isolates of T. yallundae with reduced sensitivity to the imidazole DMI fungicide prochloraz (1-[N-propyl-N-[2-92,4,6-trichlorophenoxy)ethyl]carbamoyl]-imidazole). Following heavy treatments of benzimidazole fungicides such as benomyl, carbendazim and thiabendazole, acquired resistance to this class of fungicides was determined in both T. acuformis and T. yallundae (Leroux and Cavelier, 1983; Phytoma 351:40) and (Cavelier et al, 1985; Bull. OEPP 85:495).
Thus, there is a real need for the development of technology which will allow the identification of specific races of pathogen fungi which are resistance to certain fungicides early in the infection process. By identifying the specific race of a pathogen before disease symptoms become evident in the crop stand, the agriculturist can assess the likely effects of further development of the pathogen in the crop variety in which it has been identified and can choose an appropriate fungicide if such application is deemed necessary.
The present invention relates to the use of primers specific for races of pathogenic fungi which are resistant to certain fungicides in polymerase chain reaction assays for the detection of fungal pathogens. The invention provides DNA sequences which show variability between different fungal pathotypes. Such DNA sequences are useful in the method of the invention as they can be used to derive primers for use in polymerase chain reaction (PCR)-based diagnostic assays. These primers generate unique fragments in PCR reactions in which the DNA template is provided by specific fungal pathotypes and can thus be used to identify the presence or absence of specific pathotypes in host plant material before the onset of disease symptoms.
This invention provides the possibility of assessing potential damage in a specific crop variety-pathogen strain relationship and of utilizing judiciously the diverse armory of fungicides which is available. Furthermore, it can be used to provide detailed information on the development and spread of specific pathogen races over extended geographical areas.
Kits useful in the practice of the invention are also provided. The kits find particular use in the identification of Tapesia pathogens.
The present invention provides a nucleic acid molecule comprising the nucleic acid sequence of SEQ ID NOS: 3-13 or 14. In a more preferred embodiment, the nucleic acid molecule has sequence identity with at least 10 contiguous nucleotides of SEQ ID NOS: 2-13 or 14. In another preferred embodiment, the nucleic acid molecule comprises a nucleotide sequence of SEQ ID NOs: 3-76 or 77.
The invention also provides a pair of oligonucleotide primers wherein at least one primer consists of the nucleotide sequence of SEQ ID NOS: 3-76 or 77. In a preferred embodiment, the pair of oligonucleotide primers comprises:
The invention also provides a method for the detection of a fungal pathogen, comprising the steps of:
The invention further provides a method for the detection of a fungal pathogen, comprising the steps of:
The invenion also provides a diagnostic kit used in detecting a fungal pathogen comprising at least one primer having at least 10 contiguous nucleotides of a nucleic acid molecule of the nucleic acid molecules described above. In a preferred embodiment, at least one primer comprises SEQ ID NO: 3-76 or 77. In more preferred embodiments, the pair of primers are:
The present invention provides unique DNA sequences which are useful in identifying different pathotypes of plant pathogenic fungi. Particularly the DNA sequences can be used as primers in PCR based analysis for the identification of fungal pathotypes. The DNA sequences of the invention include product, cloned from RAPD primer analysis of particular fungal pathogens as well as primers which are derived from these regions which are capable of identifying the particular pathogen. These DNA sequences from different pathotypes within a pathogen species or genus which vary between the different members of the species or genus based on different fungicides' susceptibility can be used to identify those specific members.
Biomedical researchers have used PCR-based techniques for some time and with moderate success to detect pathogens in infected animal tissues. Only recently, however, has this technique been applied to detect plant pathogens. The presence of Gaumannomyces graminis in infected wheat has be en detected using PCR of sequences specific to the pathogen mitochondrial genome (Schlesser et al., 1991; Applied and Environ. Microbiol 57: 553-556) and random amplified polymorphic DNA (i.e. RAPD) markers were able to distinguish numerous races of Gremmeniella abietina, the causal agent of scleroderris canker in conifers.
The DNA sequences of the invention are from randomly amplified polymorphic DNA (RAPD) of different plant pathogens. The RAPD sequences from different pathotypes within a pathogen species or genus vary between the different members of the species or genus. Once having determined the unique RAPD sequences of a pathogen, primers can be derived from the sequences. That is, primers can be designed based on regions within the uniquely identified RAPD fragment sequence among the fungal pathotypes. These sequences and primers based on these sequences can be used to identify specific pathogen members.
Particular DNA sequences of interest include uniquely identified RAPD sequences from Tapesia, particularly, Tapesia acuformis and Tapesia yallundae, more particularly for the identification of T. acuformis subtypes IIs and IIp and T. yallundae subtypes Ia, Ib and Ic. Such DNA sequences as well as primers of interest are given in SEQ ID NO: 3-77. The sequences find use in the PCR-based identification of the pathotypes of interest.
Sequences from RAPD analysis of uniquely identified fragments include SEQ-ID NOs: 3-14. The sequences find use in the PCR-based identification of pathogens of interest. In a preferred embodiment the sequence disclosed as SEQ-ID-NO: 10 is useful in the development of primers for differentiating T. acuformis subtypes IIs and IIp. In another preferred embodiment the sequence disclosed as SEQ-ID-NO: 8 is useful in the development of primers for the detection of T. yallundae subtype Ic.
Sequences from oligonucleotide primers derived from the uniquely identified RAPD analysis fragments are disclosed as SEQ-ID-Nos: 15-77. In a preferrred embodiment, the pair of oligonucleotide primers consists of SEQ-ID-NO: 59 and SEQ-ID-NO: 58 is used for the detection of T. yallundae Ic. In another preferrred embodiment, the pair of oligonucleotide primers consists of SEQ-ID-NO: 59 and SEQ-ID-NO: 60 is used for the detection of T. yallundae Ic. In yet other embodiments, T. acuformis subtype IIs can be differentiated from T. acuformis subtype IIp using the primer combination consisting of oligonucleotide primers with SEQ-ID-NO: 49 and SEQ-ID-NO: 50 and the primer combination consisting of oligonucleotide primers with SEQ-ID-NO: 52 and SEQ-ID-NO: 50.
The present invention lends itself readily to the preparation of “kits” containing the elements necessary to carry out the process. Such a kit may comprise a carrier being compartmentalized to receive in close confinement therein one or more container means, such as tubes or vials. One of said container means may contain unlabeled or detectably labeled DNA primers. The labeled DNA primers may be present in lyophilized form, or in an appropriate buffer as necessary. One or more container means may contain one or more enzymes or reagents to be utilized in PCR reactions. These enzymes may be present by themselves or in admixtures, in lyophilized form or in appropriate buffers.
Finally, the kit may contain all of the additional elements necessary to carry out the technique of the invention, such as buffers, extraction reagents, enzymes, pipettes, plates, nucleic acids, nucleoside triphosphates, filter paper, gel materials, transfer materials, autoradiography supplies, and the like.
The examples below show, without limitation, typical experimental protocols which can be used in the isolation of unique RAPD sequences, the selection of suitable primer sequences, the testing of primers for selective and diagnostic efficacy, and the use of such primers for disease and fungal isolate detection. Such examples are provided by way of illustration and not by way of limitation.
Standard recombinant DNA and molecular cloning techniques used here are well known in the art and are described by J. Sambrook, E. F. Fritsch and T. Maniatis, Molecular Cloning: A Laboratory manual, Cold Spring Harbor laboratory, Cold Spring Harbor, N.Y. (1989) and by T. J. Silhavy, M. L. Berman, and L. W. Enquist, Experiments with Gene Fusions, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1984) and by Ausubel, F. M. et al., Current Protocols in Molecular Biology, pub. by Greene Publishing Assoc. and Wiley-Interscience (1987).
See Table 1 for a listing of the fungal isolates used. Isolates for which fungicide sensitivity was characterized were obtained from Institut National de la Recherche Agronomique (INRA, Le Rheu, France). Fungi are grown on PDA (Potato Dextrose Agar) plates. Cultures are incubated for up to 10 days at 28° C. Mycelia are ground in liquid nitrogen, and total genomic DNA is extracted using the following modified CTAB protocol.
16. Resuspend in 50 μl TE
1S = Sensitive, R = Resistant
Polymerase chain reactions are performed to obtain Randomly Amplified Polymorphic DNA (RAPD) profiles for each of the Tapesia spp. subtypes. Forty different RAPD 10-mer primers from Qiagen Operon (Operon Technologies Inc., Alameda, Calif., USA) kits AA and J identified individually as OPAA-01-OPAA-20 and OPJ01-OPJ-20 are used in amplifications to find RAPD products specific to subtypes Ic and IIp. A single 10-mer RAPD primer is used in RAPD-PCR reactions. Reactions are prepared using the GeneAmp Kit from Perkin-Elmer (Foster City, Calif.; part no. N8O8-0009) using 50 mM KCl, 2.0 mM MgCl2, 10 mM Tris-HCl, pH8.3 and containing 100 μM of each dTTP, dATP, dCTP, and dGTP in 25 μL reactions. In each reaction, 25 pmol of RAPD primer is used with 0.5 Units of AmpliTaq Polymerase. Approximately 25 ng of genomic DNA from the subtypes listed in Example 1 are used as template. Reactions are run in a GeneAmp Model 9700 thermal cycler (Applied Biosystems, Foster City, Calif.). Thermal cycling is run for 45 cycles of 30 s at 94° C., 30 s at 34° C., and 60 s at 72° C. and is proceeded by a hold at 94° C. for one minute and followed by a final hold at 72° C. for ten minutes before being stored at 4° C. The products are analyzed by loading 10 μl of each PCR sample with loading buffer on a 1.0% agarose gel and electrophoresing.
The gel is stained with ethidium bromide and separated RAPD-PCR bands are observed under ultraviolet light.
RAPD-PCR products for each Tapesia spp. subtype are compared. Bands that appear to be specific to a certain subtype are selected for further analysis by DNA sequencing. These bands are cut from the agarose gel by a sterile scalpel. The RAPD-PCR product is purified from the agarose using GenElute Minus EtBr Spin Columns (Product Code 5-6501, Sigma-Aldrich, St. Louis, Mo., USA). The purified product is cloned into the pCR4-TOPO vector and transformed into One Shot chemically compentent bacterial cells using the TOPO TA Cloning Kit for Sequencing (Invitrogen Corporation, Carlsbad, Calif., USA) under manufacturer's protocol. Transformed cells containing the vector plus RAPD-PCR product insert are identified by endonuclease digestion of minipreped DNA of isolated colonies. Minipreps of vector DNA containing the RAPD-PCR product are sequenced. Sequencing is performed on an ABI PRISM 377™ DNA sequencer (Applied Biosystems, Foster City, Calif.) using primers in the pCR4TOPO cloning vector: FORWARD (5′-gtaaaacgacggccagt-3′; SEQ ID NO: 1) and REVERSE (5′-caggaaacagctatgac-3′; SEQ ID NO:2). Sequences obtained for each Tapesia spp. subtype are identified in Table 2.
PCR Primers are designed to amplify within the sequences of RAPD-PCR products obtained according to Example 3. Primers are designed to be used either in conventional PCR reactions or with an oligonucleotide probe in TaqMan PCR reactions. Multiple primers are developed to target each RAPD-PCR Tapesia spp. subtype-specific sequence. These primers are listed in Table 3.
Oligonucleotides (primers) are synthesized by, for example, either Integrated DNA Technologies (Coralville, Iowa) or Midland Certified Reagent Company (Midland, Tex.). Primer sequences labeled as “probe” are synthesized with a fluorescent-reporter group attached at the 5′ end for example 6-carboxy-fluorescein or “FAM” and a fluorescence quenching group attached at the 3′ end for example 6-carboxy-tetramethul-rhodamine or “TAMRA” or for example a dark quencher such as the proprietary Black Hole Quencher or “BHQ™” from Biosearch Technologies (Novato, Calif.).
Polymerase chain reactions are performed with the GeneAmp Kit from Perkin-Elmer (Foster City, Calif.; part no. N808-0009) using 50 mM KCl, 2.5 mM MgCl2, 10 mM Tris-HCl, pH8.3, containing 200 μM of each dTTP, dATP, dCTP, and dGTP in 25 μL reactions containing 50 μM each primer, 0.25 U/μL of Taq polymerase and approximately 25 ng of genomic DNA per reaction. Reactions are run for 30-35 cycles of 15 s at 94° C., 15 s at 50° C.-70° C., and 45 s at 72° C. in a Perkin-Elmer Model 9600 or 9700 thermal cycler. The products are analyzed by loading 10 μl of each PCR sample on a 1.0% agarose gel and electrophoresing. The gel is stained with ethidium bromide and products are visualized under ultraviolet light.
PCRs are performed according to Example 6 using different primer combinations (Table 4) in an attempt to amplify single specific fragments. Specific PCR amplification products are produced from primers designed from RAPD-PCR product sequences of each Tapesia spp. subtype.
In an initial screen for specificity, PCR reaction mixtures are made according to Example 6 for each of the primer combinations in Table 4. These are run against a negative control (no DNA added) and approximately 25 ng of fungal DNA for each of the Tapesia spp. subtypes listed in Table 1 prepared as described in example 1.
When visualized on an ethidium bromide stained gel several primer pairs give satisfactory results: good amplification of target DNA from multiple isolates of the target species subtype with all other reactions (negative control and other fungal DNAs) free of both specific and nonspecific reaction products. Some give unsatisfactory results including nonspecific amplification, no amplification of target DNA, and amplification of DNAs from fungal species other that the target. The primer pairs that give good specific amplification for T. yallundae subtype Ic target DNA with no cross-amplification are summarized in Table 5.
When primers JB944 and JB943 or primers JB944 and JB945 are run against DNA preparations of the eyespot subtypes listed in Table 1 the following results are recorded (Table 6).
Thus, primer pairs JB944 and JB943 and primers JB944 and JB945 are useful in the differentiation of subtypes within the Tapesia yallundae species.
For specificity to T. acuformis subtypes IIs and Up primer pairs were selected that amplify from both T. acuformis subtypes but with different sized PCR products that allow differentiation of the subtype by product size. Again, primers were selected based on good amplification of target DNA from multiple isolates of the target species subtype with all other reactions (negative control and other fungal DNAs) free of both specific and nonspecific reaction products. Some give unsatisfactory results including nonspecific amplification, no amplification of target DNA, and amplification of DNAs from fungal species other that the target. The primer pairs that give good specific amplification for T. acuformis subtypes IIs and IIp with different sized products for each subtype with no cross-amplification are summarized in Table 7.
Primers JB934 and JB935 are run against DNA preparations of the eyespot subtypes listed in Table 1. The results are presented in Table 8).
Thus, primer pairs JB934 and JB935 are specific to Tapesia acuformis at the species level and provide differently sized PCR products that are useful in the differentiation of IIs and IIp subtypes within the Tapesia acuformis species.
Some of the primers detailed in Table in 3 were designed for the additional possible use in TaqMan reactions for detection of specific Tapesia spp. subtypes. Possible primer combinations for these reactions are listed in Table 9.
The combinations listed in Table 7 are tested in initial TaqMan™ screens for subtype level specificity. Primer and probe combinations are tested for their ability to amplify from the target subtypes's DNA. Reaction conditions are held constant (1X TaqMan™ Universal Master Mix (Perkin Elmer, Norwalk, Conn.; part no. N430-4447), 300 nM each primer, 200 nM probe, approximately 25 ng pre reaction of fungal target genomic DNA, thermal cycling: 50° C. for 2 min., 95° C. for 10 min., 40 cycles of 95° C. for 15 s, 60° C. for 60 s). In initial screens for specificity under these conditions no primer/probe combination provides absolute specificity. It is prophetic that further experimentation with reaction conditions will provide subtype specific tests using these primers that are designed for specificity.
This invention also provides the possibility of assessing potential damage in a specific crop variety-pathogen strain relationship and of utilizing judiciously the diverse armory of fungicides which is available. Furthermore, it can be used to provide detailed information on the development and spread of specific pathogen races over extended geographical areas.
Kits useful in the practice of the invention are also provided. The kits find particular use in the identification of Tapesia pathogens.
While the present invention has been described with reference to specific embodiments thereof, it will be appreciated that numerous variations, modifications, and further embodiments are possible, and accordingly, all such variations, modifications and embodiments are to be regarded as being within the scope of the present invention.
Numerous patents, applications and references are discussed or cited within this specification, and all are incorporated by reference in their entireties.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/369,796 filed Apr. 3, 2002, which is incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60369796 | Apr 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10401343 | Mar 2003 | US |
| Child | 11237807 | Sep 2005 | US |
