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Published online 8 September 2006
Published in Crop Sci 46:2155-2160 (2006)
© 2006 Crop Science Society of America
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CROP BREEDING & GENETICS

Introgression of a Strawbreaker Foot Rot Resistance Gene from Winter Wheat into Jointed Goatgrass

A. Perez-Jonesa,*, C. A. Mallory-Smitha, O. Riera-Lizarazua, C. J. W. Watsona, Z. Wangb, M. Rehmanb and R. S. Zemetrab

a Dep. of Crop and Soil Science, Oregon State Univ., Corvallis, OR 97331-3002
b Dep. of Plant, Soil, and Entomological Sciences, Univ. of Idaho, Moscow, ID 83844-2339

* Corresponding author (perezjoa{at}oregonstate.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Strawbreaker foot rot (SFR) [Pseudocercosporella herpotrichoides (Fron) Deighton] is a disease of winter wheat (Triticum aestivum L.) in many wheat growing regions in the world. Resistance to SFR can be conferred by a single dominant gene (Pch1) from Aegilops ventricosa Tausch that was transferred onto chromosome 7D of wheat. ‘Madsen’, a hexaploid winter bread wheat, carries Pch1 and is highly resistant to SFR. Jointed goatgrass (Ae. cylindrica Host.) is a winter annual grass weed. Wheat and jointed goatgrass have the D genome in common and have been found to hybridize and backcross under field conditions. Since SFR resistance in winter wheat is controlled by Pch1 on the D genome, it is theoretically possible for resistance to be transferred to jointed goatgrass via backcrossing. A SFR resistant jointed goatgrass population would potentially have an ecological advantage in the presence of the disease. To evaluate the likelihood of gene introgression, Madsen, ‘Stephens’ (a SFR susceptible winter wheat), three jointed goatgrass accessions, and 15 artificially produced backcross progenies (BC2S2) were inoculated with SFR. The percentage of infection in Stephens, the joined goatgrass accessions, and the backcross progenies was 80% or greater except for one BC2S2 progeny that had only 20% infection. Madsen had 0% infection. The presence of Pch1 in the BC2S2 progeny was confirmed using a biochemical marker linked to the resistance gene. These results show that a SFR resistance gene from winter wheat can be transferred to jointed goatgrass.

Abbreviations: PNW, U.S. Pacific Northwest • SFR, strawbreaker foot rot


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
STRAWBREAKER foot rot (eyespot), caused by Tapesia yallundae Wallwork and Spooner and T. acuformis (Boerema, Pieters, and Hamers) Crous [anamorphic species Pseudocercosporella herpotrichoides (Fron) Deighton] is a chronic, yield-limiting disease of winter wheat in the U.S. Pacific Northwest (PNW) (northeastern Oregon, eastern Washington, and northern Idaho) (de la Peña and Murray, 1994; Douhan et al., 2003). Pseudocercosporella herpotrichoides is dormant or least active during the summer and survives by colonizing host debris. During cool and moist conditions in the fall and spring, the pathogen sporulates, infecting and colonizing the plant base (foot) near the soil level. The lesions appear superficially on leaf sheaths and with time grow wider and longer and penetrate the culm, resulting in tiller weakening and breakage (Wiese, 1987). Thus, strawbreaker foot rot causes significant yield loses by reducing tiller number, lodging, and decreasing seed set (Cook and Veseth, 1991). Wheat is more susceptible to P. herpotrichoides, but other cereals can also be infected, including oat (Avena sativa L.), barley (Hordeum vulgare L.), and rye (Secale cereale L.), as well as many other grasses (Murray, 1986). Strawbreaker foot rot is most severe where wheat is grown continuously and the climate is cool and moist (Nyvall, 1999).

Historically, strawbreaker foot rot has been effectively controlled with chemicals. However, isolates of P. herpotrichoides resistant to benzimidazole fungicides (e.g., benomyl, thiobendazole, and propiconazole) were detected in commercial winter wheat fields in the PNW region for the first time in the spring of 1989 (Murray et al., 1990; Murray, 1996). This prompted plant breeders to focus on genetic control, making disease resistant cultivars the most economical and desirable method of controlling strawbreaker foot rot (Yildirim et al., 1995; Cadle et al., 1997). A few strawbreaker foot rot resistance genes are known to occur in wheat germplasm. The most effective is Pch1. This major dominant gene that confers strawbreaker foot rot resistance was transferred from tetraploid Ae. ventricosa (2n = 4x = 28; genomes DvDvMvMv) onto chromosome 7D of hexaploid wheat (2n = 6x = 42; genomes AABBDD), using tetraploid wheat T. turgidum (2n = 4x = 28; genomes AABB) as a bridge species. Thus, the strawbreaker foot rot resistant line VPM1 (Ventricosa x Persicum x Marne) was obtained by crossing a synthetic Ae. ventricosa by T. persicum amphiploid (2n = 8x = 56; genomes AABBDvDvMvMv) with the hexaploid wheat cultivar Marne (Maia, 1967; Doussinault et al., 1983). Madsen, a hexaploid soft white winter wheat (T. aestivum) cultivar adapted to the PNW, carries Pch1 (derived from VPM1) and is highly resistant to strawbreaker foot rot (Allan et al., 1989).

Jointed goatgrass is a winter annual grass weed that infests over 3 million ha of winter wheat in the PNW and Great Plains regions of the United States (Dewey, 1996). Jointed goatgrass reduces winter wheat yields by interference and lowers harvested grain quality. Average yield loss with moderate to dense infestations has been estimated to be 25% (Donald and Ogg, 1991). It was also estimated that the economic cost of jointed goatgrass to winter wheat producers in the western United States is $145 million annually (Ogg, 1993).

Jointed goatgrass is an allotetraploid (2n = 4x = 28; genomes CCDD) of the Triticeae tribe (Poaceae family) with two genomes and 28 chromosomes that originated from two species. The C genome was contributed by Ae. markgrafii (Greuter) Hammer (2n = 2x = 14; genome CC) and the D genome was contributed by Ae. tauschii Coss. (2n = 2x = 14; genome DD) (Johnson, 1967; Linc et al., 1999). Wheat is an allohexaploid with three genomes and 42 chromosomes. As in jointed goatgrass, each genome contains seven chromosomes and originated from a different ancestor. Thus, T. aestivum arose from a combination of AB genomes from T. turgidum and the D genome from Ae. tauschii (Feldman and Sears, 1981; Kimber and Sears, 1987).

Jointed goatgrass and wheat are related genetically, and they can hybridize under natural field conditions (Mallory-Smith et al., 1996; Zemetra et al., 1998; Guadagnuolo et al., 2001). Since jointed goatgrass and wheat have a common ancestor (Ae. tauschii), D genome chromosomes in F1 hybrids between these two species form bivalents during meiosis (Kimber and Zhao, 1983) and genetic studies have shown normal segregation of loci on these chromosomes (Kroiss et al., 2004). Hybrids between winter wheat and jointed goatgrass are partially female fertile and have been shown to backcross under natural field conditions and set seed (Seefeldt et al., 1998; Snyder et al., 2000; Morrison et al., 2002a, 2002b).

The presence of homologous D genome chromosomes in wheat by jointed goatgrass F1 hybrids results in normal bivalent formation (Kimber and Zhao, 1983) and chromosome segregation during meiosis (Kroiss et al., 2004). Since strawbreaker foot rot resistance in winter wheat is controlled by a single dominant gene (Pch1) on the D genome, it is theoretically possible for resistance to be transferred to jointed goatgrass via backcrossing. A strawbreaker foot rot resistant jointed goatgrass population would potentially have an ecological advantage in the PNW in the presence of the disease.

The objectives of this research were to determine if unselected BC2S2 progeny, derived from artificial crosses between Madsen and a jointed goatgrass accession, were resistant to strawbreaker foot rot, and if so to determine if the strawbreaker foot rot resistant BC2S2 progeny carry the resistance gene (Pch1) derived from Madsen.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Material
Winter wheat by jointed goatgrass backcross progeny were tested for strawbreaker foot rot resistance. The winter wheat cultivars Madsen and Stephens, which are resistant and susceptible to strawbreaker foot rot, respectively, and three jointed goatgrass accessions (ID, OR, and WA) were also tested. The jointed goatgrass accessions were collected from winter wheat fields from Idaho, Oregon, and Washington, respectively. The backcross progeny were developed by Wang et al. (2001) as follows: Madsen was used as the female parent and crossed manually with the jointed goatgrass accession ID using controlled crosses in the greenhouse; the F1 hybrids were backcrossed twice using the same jointed goatgrass accession as the male recurrent parent to restore self-fertility; spikes from the second backcross (BC2) generations were bagged and allowed to self twice, to produce the second backcross-second self (BC2S2) progeny.

Test for Strawbreaker Foot Rot Resistance
The test for strawbreaker foot rot resistance was conducted according to Macer (1966) with some modifications. The winter wheat cultivars Madsen and Stephens, three jointed goatgrass accessions (ID, OR, and WA), and 15 BC2S2 progenies (731, 732, 806, 808, 814, 823, 825, 831, 832, 837, 842, 844, 847, 851, and 854) were grown in flats (52 by 26 by 7 cm) containing potting mix (Sunshine Mix #1, Sun Gro Horticulture, Inc., Bellevue, WA) in the greenhouse at 24°C. There were a total of six flats, and each flat contained a total of 10 rows with 10 plants per row. For each line, there were three replications that were randomly distributed among the flats. Thus, for each line, a total of 30 plants were infected. One week after planting, a 3-cm plastic straw cylinder was placed over the coleoptiles to hold the inoculum and to improve the uniformity of contact with the entire stem base. Two weeks later, the plants were individually inoculated by adding 500 µL of P. herpotrichoides inoculum (approximately 1 x 105 conidia mL–1) per cylinder. The inoculum slurry was produced by blending one plate of actively growing P. herpotrichoides on potato dextrose broth/bactoagar medium (DIFCO Laboratories, Detroit, MI) in 250 mL sterile water. One week after inoculation, the plants were transferred to a growth chamber set at 6°C and 8 h of light, as low temperatures provide suitable conditions for the disease infection (Mena et al., 1992; de la Peña and Murray, 1994). Ten weeks later, the plants were evaluated for disease infection. A plant was scored as susceptible when the mycelium penetrated all of the leaf sheaths and infected the main stem causing the plant to die. A plant was scored as resistant when the mycelium only grew on the external leaf sheaths and did not affect the main stem.

Endopeptidase Assay
The starch gel electrophoresis was conducted according to McMillin et al. (1986). Fifty seeds from Madsen, Stephens, the jointed goatgrass accession ID, and the putative strawbreaker foot rot resistant BC2S2 progeny 823 were placed in germination boxes containing blotter paper for 6 d at 22/18°C and 12 h of light. Roots were harvested and homogenized in 0.025 M glycineglycine buffer (pH 7.4) using a mortar and pestle chilled on ice. Twelve microliters of each extract was added to the wells of a 12% starch gel (Starch Art Corporation, Smithville, TX). A Tris-citrate buffer (1.0 M, pH 7.0) system was used for electrophoresis, which was conducted at 38 mA for 7 h at 4°C. Following electrophoresis, the gel was sliced 3 mm thick and placed in a gel box containing an endopeptidase activity staining solution (0.1 M Tris–maleate–NaOH, pH 6.4, Fast Black K, and sodium-benzoyl-DL-arginine b-naphthylamide hydrochloride dissolved in dimethyl sulfoxide) for 1 h in the dark.

PCR-based Markers
The DNA-based marker XustSSR2001–7DL was tested for co-segregation with the Ep-D1b allele in the winter wheat cultivars Madsen and Stephens, the jointed goatgrass accession ID, and the putative strawbreaker foot rot resistant BC2S2 progeny 823, as described by Groenewald et al. (2003). A total of eight plants per line were analyzed. DNA was extracted from 80 to 100 mg of leaf tissue using a DNA isolation kit (DNeasy Plant Mini Kit, Qiagen, Inc., Valencia, CA). Polymerase chain reactions (PCR) were performed in a volume of 10 µL in a Primus 96 Plus thermocycler (MWG Biotech, Inc., High Point, NC). The reaction mixture contained 1x PCR buffer with 1.5 mM of MgCl2, 0.5 µM of each primer, 0.2 mM of each deoxynucleotide, 2% sucrose in 0.04% cresol red, 0.3 units of Taq DNA polymerase (Qiagen, Inc.), and 80 to 100 ng of template DNA. The cycling program consisted of one denaturation step of 3 min at 94°C, 40 cycles of 30 sec at 94°C, 30 sec at 58°C, and 30 sec at 72°C, followed by a final extension step of 5 min at 72°C. Reverse primers were labeled with a fluorescent dye (6-carboxyfluorescein [FAM]) and fragment analysis was conducted using an automated fragment analyzer (ABI Prism 3100 genetic analyzer). ABI GeneScan 2.1 and Genotyper 2.0 software (Applied Biosystems, Foster City, CA) were used to size PCR-amplified fragments based on internal lane standards (N,N,N',N'-tetramethyl-6-carboxyrhodamine [TAMARA] or 6-carboxy-x-rhodamine [ROX]).

Chromosome Analysis
Somatic chromosome counts were conducted using root-tip cells on two BC2S2 progenies, including one putative strawbreaker foot rot resistant progeny. Seeds were germinated in Petri dishes, and root-tips were collected from primary roots and placed in 1°C water for 24 h, before fixing in Farmer's solution (3:1 ethanol/acetic acid). After a minimum of 3 d, the roots were transferred to 2% acetocarmine for staining. Chromosome counts for six plants from each progeny were performed using a light microscope (Zeiss Axioskop 2, Carl Zeiss Inc., Thornwood, NY).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Test for Strawbreaker Foot Rot Resistance
The percentage of plants infected by strawbreaker foot rot in the winter wheat cultivar Stephens, the three jointed goatgrass accessions, WA, OR, and ID, and the backcross progenies was 80% or greater, except for one BC2S2 progeny that had only 20% infection. None of the plants of the winter wheat cultivar Madsen were infected (Fig. 1 ). In the susceptible plants (e.g., the winter wheat cultivar Stephens and the jointed goatgrass accessions ID, OR, and WA), the mycelium penetrated all the leaf sheaths infecting the main stem and killing the plant. However, in the resistant plants (e.g., the winter wheat cultivar Madsen and the BC2S2 progeny 823), the mycelium only grew on the external leaf sheaths and did not reach the main stem (Fig. 2 ).


Figure 1
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  		Fig. 1. Percentage of strawbreaker foot rot (SFR) infection in two winter wheat cultivars, Madsen (MD) and Stephens (ST), three jointed goatgrass accessions, Washington (WA), Oregon (OR), and Idaho (ID), and 15 BC2S2 progenies. Vertical bars represent standard errors of the mean.

 

Figure 2
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  		Fig. 2. Strawbreaker foot rot symptoms in (a) the resistant winter wheat cultivar Madsen and (b) the BC2S2 progeny 823, and (c) the susceptible winter wheat cultivar Stephens and (d) the jointed goatgrass accession ID.

 
Endopeptidase Assay
Starch gel electrophoresis for the endopeptidase marker indicated that the winter wheat cultivar Madsen and the BC2S2 progeny 823 carried the endopeptidase enzyme Ep-D1b allele from VPM1, while the winter wheat cultivar Stephens and the jointed goatgrass accession ID did not (Fig. 3 ). Since the Ep-D1b allele is tightly linked to the strawbreaker foot rot resistance Pch1 gene, these results indicate that the BC2S2 progeny 823 carries the Pch1 gene from Madsen.


Figure 3
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  		Fig. 3. Starch gel showing the presence of the endopeptidase enzyme Ep-D1b allele from VPM1 in the resistant winter wheat cultivar Madsen and the BC2S2 progeny 823, and its absence in the susceptible jointed goatgrass accession ID and the winter wheat cultivar Stephens. The Ep-D1b allele is tightly linked with the strawbreaker foot rot resistance Pch1 gene.

 
PCR-based Markers
Amplification of the simple sequence repeat locus XustSSR2001–7DL yielded a fragment of 238 bp in the winter wheat cultivar Madsen and 204 bp in the BC2S2 progeny 823 and the jointed goatgrass accession ID (Table 1). Thus, the BC2S2 progeny 823 received the Ep-D1b allele and the Pch1 gene from the winter wheat cultivar Madsen, as indicated by the test for resistance and the endopeptidase assay, and the XustSSR2001–7DL allele from the jointed goatgrass accession ID.


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  		Table 1. Fragment size analysis of the PCR-based marker XustSSR2001–7DL in two winter wheat cultivars (Madsen and Stephens), one BC2S2 progeny (823), and one jointed goatgrass accession (ID).

 
Chromosome Analysis
The chromosome number for two BC2S2 progenies, including the strawbreaker foot rot resistant progeny 823, was determined. In the BC2S2 progeny 823, three out of six plants had 28 chromosomes, while the remaining three plants had 29 chromosomes. In the BC2S2 progeny 831, a strawbreaker foot rot susceptible progeny, all six plants had 28 chromosomes (Fig. 4 ).


Figure 4
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  		Fig. 4. Somatic mitotic metaphase of a BC2S2 progeny after acetocarmine staining showing 2n = 4x = 28 chromosomes.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The greenhouse screening method successfully distinguished between the strawbreaker foot rot susceptible and resistant progenies. Out of the 15 BC2S2 progenies tested, only one progeny (BC2S2–823) was found to be resistant to strawbreaker foot rot. During the production of the backcross progenies, BC1 and BC2 generations did not undergo selection for the strawbreaker foot rot resistance gene. Therefore, the observed low percentage (6.6%) of resistant lines carrying the unselected resistance gene was expected.

Chromosome counts of two BC2S2 progenies showed a continued restoration of the chromosome number of jointed goatgrass, with 9 out of 12 plants examined having 28 chromosomes. This was expected since the F1 hybrids were backcrossed twice using jointed goatgrass as the male recurrent parent to restore self-fertility. Nevertheless, individuals with one additional chromosome (29 chromosomes in total) are expected to have high percentages of self-fertility (over 80%) as indicated in a previous study (Wang et al., 2001). Only individuals from the BC2S2 progeny 823 had 29 chromosomes. However, there is no relationship between resistance to strawbreaker foot rot and the presence of one extra chromosome from A or B genomes since Pch1 is located on chromosome 7DL. Moreover, backcross progenies, BC1 and BC2, did not undergo selection for the strawbreaker resistance gene.

Pch1 is located on chromosome 7DL in the Ae. ventricosa–derived strawbreaker foot rot resistant VPM1 line, and has been shown to be tightly linked with the endopeptidase enzyme Ep-D1b allele (McMillin et al., 1986; Mena et al., 1992). The winter wheat cultivar Madsen carries the Pch1 gene and the Ep-D1b allele and is highly resistant to strawbreaker foot rot (Allan et al., 1989). Starch gel electrophoresis indicated that both Madsen and the BC2S2 progeny 823 carry the Ep-D1b allele. However, Madsen carried the 238-bp XustSSR2001–7DL allele whereas the BC2S2 progeny 823 carried the 204-bp XustSSR2001–7DL allele. Thus, the BC2S2 progeny 823 had a recombinant chromosome with the Ep-D1b allele and the Pch1 gene from the winter wheat cultivar Madsen, as indicated by the endopeptidase assay and the greenhouse test for strawbreaker foot rot resistance, and the XustSSR2001–7DL allele from the jointed goatgrass accession ID. The recombination frequency between the Ep-D1b allele and the XustSSR2001–7DL allele was previously estimated to be 2% in wheat (Groenewald et al., 2003). However, based on the limited number of progeny studied, recombination in the Ep-D1b– XustSSR2001–7DL interval may be higher in winter wheat by jointed goatgrass derivates. This finding is consistent with a previous report (Kroiss et al., 2004) that showed recombination between microsatellite marker loci on homologous D genome chromosomes of wheat and jointed goatgrass during two successive backcrosses.

It has been shown previously that recombination between homologous D genome chromosomes of wheat and jointed goatgrass in backcross progenies can occur, which indicates that a gene in wheat on the D genome can be retained and integrated into the D genome of jointed goatgrass, as demonstrated with the integration of wheat D genome chromatin into jointed goatgrass through normal genetic recombination (Kroiss et al., 2004). Moreover, a recent report described spontaneous DNA introgression from hexaploid bread wheat into distantly related wild tetraploid Ae. peregrina and the stabilization of this introgression in wild populations despite not having homologous chromosomes (Weissmann et al., 2005). In this study, we have shown that the strawbreaker foot rot resistance gene Pch1 can be moved from winter wheat into jointed goatgrass. Overall, chromosome pairing evidence, genetic analysis, and the present study irrefutably show that a gene on the D genome of wheat can move into jointed goatgrass through hybridization and backcrossing even in the absence of selection.

Gene flow from cultivated crops to wild relatives has become an issue since the introduction of genetically modified crops (Raybould and Gray, 1993). Gene introgression from genetically modified crops to their wild relatives (i.e., transgene introgression) requires the hybridization between the crop and the wild relative, followed by repeated backcrosses to the wild relative and the stabilization of the transgene in the new host genome (Stewart et al., 2003). In fact, a transgene introgression event from a commercially released genetically modified crop to a wild relative has been already reported. There is good molecular evidence that canola (Brassica napus L.) can hybridize with field mustard (B. rapa L.) and gene introgression can occur in the field under natural conditions (Warwick et al., 2003). However, non-transgene introgression has been of little concern. The movement and establishment of genes from crops to their wild weedy relatives under natural field conditions have been reported in several crop species, including sunflower (Helianthus annuus L.) (Whitton et al., 1997), rice (Oryza sativa L.) (Langevin et al., 1990), and sorghum [Sorghum bicolor (L.) Moench] (Arriola and Ellstrand, 1996). Introgression of genes from crops into their wild relatives after hybridization can increase the ability of the wild relatives to adapt to certain agricultural environments, and also their ability to compete with domesticated crops or other wild species (Ellstrand et al., 1999). In fact, introgression of genes by natural hybridization has played a significant role in the evolutionary process of adaptation and diversification in many plant species (Arnold, 1997), especially in polyploid species such as wheat (Zohary and Feldman, 1962). The results presented here provide evidence for the introgression of a strawbreaker foot rot resistance gene from winter wheat to jointed goatgrass after artificial hybridization and backcrossing. The introgression of Pch1 from winter wheat to jointed goatgrass could occur in the field under natural conditions and this could improve the competitive advantage of the weed in the PNW where there is high pressure from the disease. In a future study, we plan to collect different populations of jointed goatgrass from fields in the PNW where the winter wheat cultivar Madsen has been grown intensively for several years and determine if the introgression of the strawbreaker foot rot resistance gene has already occurred under natural conditions. Also, we plan to evaluate the ecological advantage conferred by the strawbreaker foot rot resistance gene through a fitness study, in which winter wheat will be grown in the field in competition with both susceptible and strawbreaker foot rot resistant BC2S2 progenies under conditions of high pressure of the disease.

Received for publication March 24, 2006.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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