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Crop Science 42:105-110 (2002)
© 2002 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Field Evaluation of Transgenic and Classical Sources of Wheat streak mosaic virus Resistance

G. L. Sharp, J. M. Martin, S. P. Lanning, N. K. Blake, C. W. Brey, E. Sivamani, R. Qu and L. E. Talbert*

Dep. of Plant Sciences and Plant Pathology, Montana State University, Bozeman, MT 59717

* Corresponding author (usslt{at}montana.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The development of wheat (Triticum aestivum L.) cultivars that are resistant to Wheat streak mosaic virus (WSMV), yet competitive in yield under nondiseased conditions, is an objective for breeding programs in the Great Plains. This field study was conducted to compare classical and transgenic sources of resistance to WSMV. Three sets of germplasm were evaluated. These included adapted cultivars with various levels of tolerance, transgenic wheat lines containing viral coat protein or replicase sequences from WSMV that showed resistance in greenhouse trials, and germplasm with resistance to WSMV due to a translocated segment of chromosome 4Ai-2 from Thinopyrum intermedium (Host) Barkworth and Dewey containing Wsm1. A replicated field trial was conducted at Bozeman, MT, over a two-year period to evaluate the effectiveness of these different sources of resistance to mechanical inoculation of WSMV. Adapted cultivars differed in their ability to tolerate WSMV with mean reductions in yield over the two years ranging from 41 to 74%. Incorporation of the replicase or coat protein gene from WSMV did not provide field resistance to viral infection and in general, transgenic lines yielded less than their parent cultivar, ‘Hi-Line’. Wheat-Thinopyrum lines positive for a DNA marker linked to the Wsm1 gene had significantly reduced yield losses ranging from 5 to 39% compared with yield losses of 57 to 88% in near isogenic lines not having the Wsm1 gene. Yield of lines with Wsm1 in the absence of disease ranged from 11 to 28% less than yield of lines without Wsm1. Our results suggest Wsm1 provides the best source of WSMV resistance but a yield penalty may exist because of the presence of the translocation.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
WHEAT STREAK MOSAIC VIRUS (WSMV) is an important disease in wheat (Triticum aestivum L.). The disease occurs in many areas of the world including Canada, Europe, and Russia, and is becoming increasingly more common in the Great Plains of North America (Wiese, 1987). WSMV also infects barley (Hordeum vulgare L.), oats (Avena sativa L.), rye (Secale cereale L.), corn (Zea mays L.), millets (Panicum, Setaria, and Echinochloa spp.), and many wild grasses (Wiese, 1987). WSMV had previously been assigned to the family Potyviridae of the genus Rymovirus but recent phylogenetic studies place the virus in the newly formed genus, Tritimovirus (Stenger et al., 1998; Fauquet and Mayo, 1999). The complete 9, 384-nucleotide sequence of WSMV has been determined and reassignment to the new genus is based on sequence similarities to Brome streak mosaic virus (BrSMV). WSMV is vectored by the wheat curl mite (Aceria tosichella Kiefer) as it feeds on young plants. The virus is a flexuous rod of single stranded RNA 700 nm long and 15 nm wide. Symptoms of the disease include light green and yellow streaking of the leaves as well as curling of the leaves due to mite activity. Infected plants typically exhibit stunted growth, reduced tillering, and sterile or partially filled heads.

To date, no resistance has been discovered in cultivated wheat. Resistance does, however, exist in wild relatives of wheat (Friebe et al., 1991), and the process of introgressing the resistance gene into agronomically acceptable backgrounds has been accomplished (Friebe et al., 1996). CI 17884 contained a translocation whereby the short arm of chromosome 4D from wheat was replaced by the short arm of chromosome 4 from Thinopyrum intermedium (Host) Barkworth and Dewey. The Thinopyrum segment contains the resistance gene Wsm1 (Friebe et al., 1996). Improvements to CI 17884 have been accomplished by backcrossing the germplasm line with ‘Karl’ hard red winter wheat (Gill et al., 1995). KS93WGRC27 was derived from the selfed progeny of a Karl*4/CI 17884 plant that was homozygous for the Thinopyrum translocation. This germplasm has been used by breeders to incorporate the Wsm1 gene into adapted cultivars.

Due to the lack of resistance in wheat, transgenic approaches to developing resistance have been pursued. In the early 1980s expression of viral gene sequences in plants was suggested as a means of providing resistance by disrupting the life cycle of the virus. Viral mediated resistance, or pathogen derived resistance, has since become a popular method for the development of resistant crop species. Sivamani et al. (2000)(2001) developed potential resistance to WSMV in experimental lines by incorporating viral transgenes into wheat. Wheat plants of ‘Hi-Line’ (Lanning et al., 1992) and ‘Bobwhite’ were stably transformed with a viral replicase gene (NIb) or a viral coat protein (CP) gene via biolistic particle bombardments of immature wheat embryos. The nucleotide sequences of the WSMV NIb and CP genes were synthesized by PCR (polymerase chain reaction) from cDNA clones of the ‘Conrad’ isolate of WSMV (Carroll et al., 1982). The sequences were ligated into vector plasmids containing the maize ubiquitin-1 promoter, the nopaline synthase terminator, and an ampicillin resistance gene. These plasmids were cotransformed into wheat with the herbicide resistance gene, bar (Sivamani et al., 2000, 2001). Greenhouse tests of the progeny indicated that some of the lines exhibited a delayed resistance phenotype characteristic of protein-mediated resistance (Sivamani et al., 2000, 2001).

In addition to Wsm1 and viral transgene resistance, varying degrees of tolerance to WSMV have been observed in cultivated wheat (Carroll et al., 1982; Shahwan and Hill, 1984; Seifers and Martin, 1988; and M. Young, personal communication, 2000). Cultivars classified as tolerant become infected with WSMV but symptoms are less severe and yield reductions are less than in susceptible cultivars (Seifers and Martin, 1988).

The objective of this study was to compare three sources of resistance or tolerance to WSMV. We conducted field studies in 1999 and 2000, utilizing wheat-Thinopyrum germplasm, transgenic lines, and adapted cultivars. Our results provide direction for breeding programs seeking to incorporate resistance to WSMV into locally adapted germplasm.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
KS93WGRC27 (Gill et al., 1995), containing the WSMV resistance gene Wsm1 from T. intermedium, was crossed with Montana-adapted cultivars ‘Amidon’ and ‘McNeal’ (Lanning et al., 1994). Resultant progenies were crossed with ‘McNeal’ or experimental line MT9328. F4-derived F6 lines were tested for the presence of the Thinopyrum chromosome segment by PCR (Talbert et al., 1996) and for resistance by mechanical inoculation of seedlings with WSMV. Six wheat lines containing the Wsm1 resistance gene and six sister lines without the gene were selected from F7 populations to be grown in the first year of this study (Table 1).


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  		Table 1. Wheat lines evaluated for resistance to Wheat steak mosaic virus at Bozeman, MT, in 1999 and 2000.

 
Seven transgenic lines testing PCR positive for the viral NIb or CP genes were obtained for the 1999 field trial. Six of the transgenic lines were derived from the cultivar ‘Hi-Line’, while the remaining transgenic line was derived from the cultivar, ‘Bobwhite’. Two additional transgenic lines, derived from ‘Hi-Line’, were field tested in 2000.

From transformation experiments, Sivamani et al. (2000)(2001) evaluated CP and NIb lines for presence of the WSMV transgenes and for expression of the glufosinate-resistant transgene, bar. PCR positive plants were resistant to herbicide application suggesting that the viral transgenes were co-integrated with the bar gene at a single locus and were segregating together in progeny. Northern blot analysis revealed the coat protein transcript was detectable in only one of the CP lines, 25.14. This line also showed the insertion of a high number of transgene copies. NIb 4.8 had detectable transgene expression of mRNA in Northern blots, but was not resistant to WSMV. NIb 4.4 and NIb 7.21 were highly resistant to WSMV, but showed no transgene expression in Northern blots. Southern blot analysis showed a low copy number for the NIb 4.4 line. In greenhouse inoculations of transgenic lines, CP 25.14, NIb 4.4, and NIb 7.21 exhibited a recovery type resistance to WSMV. These lines had milder symptoms and lower virus titers than nontransgenic controls. New growth of the plants had a significant delay in symptom development. Other transgenic lines showed nondetectable resistance in the greenhouse.

For our experiment, verification of genotype for transgenic and Wsm1 entries was conducted. Total genomic DNA was extracted from young leaves following the method of Riede and Anderson (1996). PCR amplifications were performed as previously described (Talbert et al., 1994). For detection of the Wsm1 gene, the sequence tagged site primers, STSJ15, developed by Talbert et al. (1996) were used. This primer set amplifies a 500 base pair segment associated with the translocation from T. intermedium. For entries containing the coat protein or replicase gene, primers designed and reported by Sivamani et al. (2000)(and 2001) were used. The nucleotide sequences of the primers flank the 5' and 3' ends of the corresponding gene sequences of WSMV and PCR produces diagnostic bands approximately 1.3 kilobase pairs for each transgene.

Nine adapted cultivars (including parents of the transgenic and Wsm1 lines), exhibiting various degrees of tolerance to WSMV (L. Talbert and M. Young, unpublished data, 2000) were included in the study (Table 1).

Field trials were conducted at the Arthur H. Post Field Research Farm eight miles west of Bozeman in 1999 and 2000. The elevation is 1,439 m and the soil is an Amsterdam silt loam. Available soil nitrogen for the crop was approximately 250 kg ha-1 each year.

In 1999, twenty-eight entries were planted in a randomized complete block design with three replications. Entries were planted in paired rows 3 m long with row spacing of 30 cm. One row per entry was mechanically inoculated with WSMV and the second row was the non-inoculated control. In 2000, thirty entries were planted using the same experimental design as the previous year. Additional entries were CP 25.13 and CP 25.14.

Seed was space planted in 1999 with ninety seeds of each line planted per row. Transgenic entries were sprayed with glufosinate and thinned to equal numbers of plants in paired rows. Eight grams of seed were planted per row in 2000 for 28 of the entries. Coat protein lines, 25.13 and 25.14, grown in the greenhouse for seed increase in January 2000 were inadvertently infested with viruliferous wheat curl mites and subsequent infection with WSMV resulted in poor seed set. These two CP lines were planted at seven grams per row for the 2000 field trial. A greater abundance of seed in 2000 enabled us to plant four replicates of paired rows in 2000.

Planting occurred on 12 May 1999 and 3 May 2000. Harvest occurred in August of both years. Individual rows were cut with a hand sickle and threshed with a stationary Vogel thresher.

At the two- to three-leaf stage, plants in one row from each plot were inoculated with WSMV by wiping the leaves with inoculum prepared from infected greenhouse seedlings inoculated with the ‘Conrad’ isolate of WSMV. Field inoculum was prepared by grinding diseased leaf tissue in a Waring blender with phosphate buffer pH 6 (1 g infected leaf tissue/10 mL buffer). A small amount of carborundum was added to facilitate leaf injury and viral introduction during application. In 2000, non-inoculated control rows were mock inoculated with buffer and carborundum. Disease symptoms were rated on a scale of 0–5, with 0 denoting no visible signs of disease and 5 denoting severe symptoms, based on the method of Shahwan and Hill (1984) as noted in Table 2. Presence of virus in infected wheat tissue was determined by protein A sandwich enzyme-linked immunosorbent assay (PAS-ELISA) as described by Edwards and Cooper (1985). Polyclonal antibodies to the WSMV virus had been previously prepared from the ‘Conrad’ isolate of WSMV (Baley, 1999). A 6 cm leaf section, ground and resuspended in 800 ul of buffer was used as substrate.


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  		Table 2. Rating system used to evaluate symptoms of Wheat streak mosaic virus in spring wheat lines.

 
At 20 and 40 d post inoculation, the newest leaf from ten plants per entry was analyzed for virus by PAS-ELISA. Optical density of ELISA samples was measured on a Kinetic Microplate Reader (Molecular Devices Corp., Sunnyvale, CA) at 405 nm after a 20-min incubation with p-nitrophenyl phosphate. In this study, positive assessment of infection with optical density readings was accomplished by establishing the positive-negative threshold as three times the absorbancy reading of the negative controls (Khetarpal and Kumar, 1995).

Wheat plants from each row, both inoculated and non-inoculated, were evaluated for morphological characteristics and quality. Heading date was recorded as the number of days from 1 Jan. when 50% of the heads had emerged from the boot. Height of each row was taken as the average of two measurements made from the soil surface to the top of the spikes, excluding awns, of 3–5 main tillers gathered together. Grain yield from each row was expressed as g m-2. A Contador E electronic seed counter (Hoffman Mfg., Inc.) was used to count a random sample of 30 mL of seed and the weight of 1000 kernels was calculated.

Quality evaluations were conducted by approved methods (AACC, 1995). Whole flour meal was prepared in a UDY Cyclone Mill fitted with a 0.5 mm screen. Protein quality was assessed thru sodium dodecyl sulfate sedimentation (SDSS) tests on flour meal and reported in ml. Grain protein was measured on flour samples with a LECO-328 or a LECO FP-528 protein/nitrogen analyzer (AACC Method 46-30). Protein was adjusted for moisture content and reported at 14% moisture.

Analysis of variance was computed for each measured trait combined over years with Statistical Analysis Systems (SAS) version 7.0 (SAS Institute, 1998). Entry means were computed for each year and combined for the two years. Entry means were compared by Fisher's least significant difference (LSD). Contrasts were used to compare the average of grouped entries. A significant interaction existed between year and entry for most traits measured. However, conclusions drawn from the analyses were the same for both years, and thus data is presented averaged over the two years.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Adapted cultivars were all susceptible to WSMV. ELISA values in new leaf tissue collected from cultivars at both 20 and 40 d post inoculation indicated a high level of virus accumulation regardless of symptom expression (Table 3). Tolerance is described as a low level resistance (Seifers and Martin, 1988) whereby plants are infected and develop symptoms but yield is reduced less than in susceptible cultivars. Results of our field trial suggest that Montana-adapted cultivars differ in their ability to tolerate WSMV. However, all varieties were severely affected by WSMV infection and differences between inoculated and non-inoculated were significant for all varieties.. Average yield reduction was 58% with the least percent reduction shown by ‘McNeal’ (44%), ‘Hi-Line’ (51%), and ‘Amidon’ (53%). Ernest showed a severe yield loss of 77%, with ‘MTHW9420’ and ‘Rambo’ also being severely effected. Symptom expression was predictive of yield loss with a correlation between these traits of 0.94 (P < 0.01) at 4 wk post inoculation. These data are similar to previous field observations made on subset of these varieties, where the most to least tolerant varieties were Scholar, McNeal, Fortuna, Rambo, Ernest, and MTHW9420, respectively (L. Talbert and M. Young, unpublished data, 2000).


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  		Table 3. Means for agronomic data of spring wheat lines inoculated (Inoc) and non-inoculated (Non) with Wheat streak mosaic virus grown at Bozeman, MT in 1999 and 2000. Data are averaged over 2 yr. Lines tested in year 2000 only (CP 25.13 and CP 25.14) are not included.

 
All transgenic lines developed symptoms and had high virus titers suggesting that the transgenes did not confer resistance to WSMV (Tables 3 and 4). Yield loss in the transgenic lines ranged from 46% (NIb 7.17) to 67% (CP 25.8). The recovery type resistance demonstrated by NIb 4.4 and CP 25.14 in earlier greenhouse studies (Sivamani et al., 2000, 2001) was not observed under field conditions. NIb 4.4 and CP 25.14 did not differ significantly from ‘Hi-Line’ in most traits measured (Table 5). High ELISA values from extracts of the newest leaves at 20 and 40 d post inoculation showed that spread of WSMV was systemic throughout the plant. There are several possibilities for the differences seen in our field trial and the previous greenhouse results. First, it is possible that expression of the transgenes was silenced in our transgenic seedlings as seed used in these experiments was from later generations than that used in previous greenhouse trials. Gene silencing is commonly recognized in many species and under varying conditions (Demeke et al., 1999; Cogoni and Macino, 2000; Flavell, 1994). This is unlikely to be true for NIb 4.4 which exhibited no detectable transgene expression in previous experiments and yet showed resistance (Sivamani et al., 2000). Sivamani et al. (2001) found that CP 25.14 highly expressed the coat protein gene in early generations of line development. However, analysis of later generations of 25.14 by reverse transcriptase PCR showed no detectable levels of transcript (data not shown). A second possibility for the nonresistance observed in our field study regards the different inoculation techniques employed in the greenhouse and field experiments. Inoculum used in greenhouse trials was purified virus (100 µg/ml) while field inoculation was with fresh infected wheat seedlings. Finally, growth conditions in the field versus the greenhouse may have influenced results. This explanation is not likely for CP 25.14. During the course of seed increase in the greenhouse in 2000, viruliferous wheat curl mites inadvertently infected wheat seedlings of the CP 25.13 and CP 25.14 lines. Both lines became infected with WSMV developing symptoms and positive ELISA values comparable to that observed in the field trial.


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  		Table 4. Agronomic data for spring wheat lines inoculated (Inoc) and non-inoculated (Non) with Wheat streak mosaic virus at Bozeman, MT, in 2000.

 

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  		Table 5. Level of significance for group contrasts made in spring wheat lines inoculated (Inoc) and non-inoculated (Non) with Wheat streak mosaic virus grown at Bozeman, MT.

 
The yield of transgenic lines was lower than that observed for the parent cultivar, ‘Hi-Line’, both in the presence or absence of disease (Tables 3, 4, and 5). When evaluated as a group, transgenic lines yielded significantly less than nontransformed ‘Hi-Line’ and had significantly higher protein content (Table 5). Somaclonal variation due to the transformation and regeneration process also produced plants that headed later and were shorter than ‘Hi-Line’. Bregitzer and Poulson (1995) examined somaclonal variation within barley lines derived from tissue culture. Their results indicated that the agronomic performance for most lines was altered by passage through tissue culture and that alterations were generally undesirable. In another experiment, Bregitzer et al. (1998) compared agronomic traits in 44 segregating transgenic lines to their nontransformed parent. A third experiment compared the agronomic traits of seven transgenic-derived, null (nontransgenic) segregant lines. In both cases, compared with their uncultured parent, most lines were shorter, lower yielding, and had smaller seed. The transformation procedure appeared to cause greater somaclonal variation than tissue culture alone. Differences in performance between cultured and uncultured plants seemed to be cultivar-dependent with some cultivars showing less variation. They concluded the decreased performance of transgenic lines due to somaclonal variation may slow the introduction of beneficial transgenes into adapted cultivars and that recovery of economically competitive cultivars might be cultivar dependent and require the screening of large populations of regenerated plants. Our findings in this study on the altered performance of transgenic wheat lines supports the conclusions drawn by Bregitzer et al. (1998).

Wheat-Thinopyrum lines PCR positive for the Wsm1 gene were resistant to WSMV as indicated by lack of symptoms, reduced ELISA values, and yield effect (Table 3). The lines with Wsm1 had ELISA values significantly lower than those for the sister lines both at 20 dpi and 40 dpi. This is in agreement with Seifers and Martin (1988), who demonstrated high levels of WSMV resistance under field conditions in wheat containing Wsm1. However, in the absence of disease in our experiments, the susceptible wheat-Thinopyrum lines headed earlier, were taller, and yielded significantly better than the Wsm1 entries (Table 3). Yields of Wsm1 lines in the absence of disease were 11 to 28% less than yields of sister lines not having the Wsm1 gene (Table 3). This is in agreement with a larger study of Wsm1 lines conducted by Baley (1999), who showed a 5% yield difference between Wsm1 and near isogenic lines. The Wsm1 lines had higher 1000 kwt than susceptible sister lines when inoculated, but lower 1000 kwt when non-inoculated (Table 5). Sodium dodecyl sulfate sedimentation were higher in the Wsm1 lines and their susceptible sister lines than in adapted varieties (Tables 3 and 5).

All Wsm1 lines were symptom free in 2000 and yield losses ranged from 6 to 28% (data not shown). In 1999, it was noted that four of the Wsm1 lines exhibited symptoms consistent with WSMV and yield reductions of 5 to 56% (data not shown). However, these lines had negative virus titers at 20 d post inoculation. At 40 d post inoculation, virus titers of Wsm1 entries were higher than titers obtained at 20 d post inoculation. A review of available literature regarding the temperature sensitivity of Wsm1 derived resistance revealed that resistance is effective at 20°C but not at 25°C (Seifers et al., 1995). In their study, CI 17884 was used as a parent in a randomly mated, recurrently selected population. Progeny were symptom free and negative in ELISA at 20°C, but were positive in ELISA at 27°C. All lines that developed WSMV symptoms and titers carried only the short arm of the T. intermedium chromosome 4Ai-2. Progeny lines that remained resistant at 27°C had been derived from crosses involving CI 15092, which has the short arm of 4Ai-2 in addition to the long arm of 4Ai-2. Apparently, genes on the long arm allow effective resistance to WSMV at the higher temperature. The source of resistance to WSMV used in this study is from CI 17884, thus temperature sensitivity could be a contributing factor to virus accumulation and yield loss in inoculated Wsm1 entries. The average temperature during grain fill in July of 1999 and 2000 was near 20°C and reached 36°C on 29 July 1999 and 31 July 2000. Seifers et al. (1995) concluded that the resistance to WSMV conferred by the translocated segment of the short arm of chromosome 4Ai-2 is temperature sensitive, but stable enough to provide protection from WSMV under field conditions. Our data indicates that Wsm1 lines incur some yield loss under field conditions; however, this loss is significantly less than the reduction in yield of susceptible lines.

In conclusion, our results suggest that transgenic wheat lines that previously showed WSMV resistance in greenhouse trials do not express resistance under field conditions. The Wsm1 gene from Thinopyrum does offer a degree of resistance greater than that observed in presently grown cultivars, however, a yield penalty may exist for the presence of the translocation.

Received for publication March 8, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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