Resistance evaluation of wheat germplasm containing Dn4 or Dny against Russian wheat aphid biotype RWASA3

Host plant resistance can effectively manage Russian wheat aphid (Diuraphis noxia) Kurdjumov (Homoptera: Aphididae) in areas where it is an economically important pest of wheat. However, biotypes of D. noxia virulent on wheat containing resistance gene Dn4 have been reported in both the United States and South Africa. Thirty wheat genotypes, including susceptible Yuma, resistant CItr2401, as well as 25 genotypes containing Dn4 and three genotypes containing Dny were planted under greenhouse conditions in Bethlehem, South Africa, and screened with D. noxia biotype RWASA3. RWASA3 caused susceptible damage symptoms in MTRWA92‐145, Ankor, Halt, Bond CL, 18FAWWON‐SA 262, Prowers99, 18FAWWON‐SA 264, Hatcher, Yumar, Corwa and Thunder CL all reported to contain the Dn4 resistance gene. Genotypes PI586956, Stanton and 18FAWWON‐SA 257, containing the Dny‐resistance gene were susceptible to RWASA3. Similarly, coinciding development of virulence to resistance genes Dn4 and Dny was reported in the United States. However, in this study, 13 Dn4‐containing genotypes showed moderate resistance when screened with RWASA3 alluding to a more complex biotype‐gene‐interaction. These findings could indicate that Dn4 and Dny may be related and possibly share a similar or common resistance factor. Further studies will be aimed at explaining these results investigating the possibility of an allelic cluster or series for Dn4, possibly including Dny.     

Possible roles of esterase, glutathione S-transferase, and superoxide dismutase activities in understanding aphid–cereal interactions

Activities of the detoxification enzymes esterase, glutathione S‐transferase, and of superoxide dismutase in aphids and aphid‐infested cereal leaves were assayed using polyacrylamide gel electrophoresis and a spectrophotometer to elucidate the enzymatic mechanisms of aphid resistance in cereal plants. A chlorosis‐eliciting Russian wheat aphid, Diuraphis noxia (Mordvilko), and non‐chlorosis‐eliciting bird cherry‐oat aphid, Rhopalosiphum padi (L.), and four cereals were used in this study. The four cereal genotypes were ‘Arapahoe’ (susceptible) and ‘Halt’ (resistant) wheat (Triticum aestivum L.), ‘Morex’ (susceptible) barley (Hordeum vulgare L.), and ‘Border’ (resistant) oat (Avena sativa L.). Esterase isozymes differed between the two aphid species, although glutathione S‐transferase and superoxide dismutase did not. Esterase, glutathione S‐transferase, and superoxide dismutase activities in either aphid species were not affected by the level of resistance of a cereal to D. noxia. The assays of cereal leaf samples showed that D. noxia feeding elicited an increase in esterase activity in all four cereal genotypes, although R. padi feeding did not. The increase of esterase activity in cereals, however, was not correlated to aphid resistance in the cereals. The time‐series assays of aphid‐infested cereal leaves showed that D. noxia‐infested Morex barley had a significant increase in esterase activity on all sampling dates (3, 6, and 9 days) in comparison with either uninfested or R. padi‐infested barley. No difference in glutathione S‐transferase activity was detected among either aphid infestations or sampling dates. The electrophoretic assays, however, revealed that aphid feeding elicited a significant increase in superoxide dismutase activity, which served as the control of glutathione S‐transferase activity assays. The increase in esterase and superoxide dismutase activities suggested that D. noxia feeding imposes not only toxic, but also oxidative stresses on the cereals. The ramification of using these enzyme activity data to understand the etiology of D. noxia‐elicited chlorosis is discussed.     

Population growth rate and relative virulence of the two South African biotypes of Russian wheat aphid,Diuraphis noxia,and bird cherry‐oat aphid,Rhopalosiphum padi,on resistant and non‐resistant barley

In South Africa a new biotype of the Russian wheat aphid (RWA), Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), RWASA2, has appeared which exhibits an improved performance compared to the original biotype (RWASA1) on wheat containing the Dn1 resistance gene. We examined population growth rates as well as damage caused by RWASA1 and RWASA2, in addition to a different aphid species, the bird cherry‐oat aphid (BCA), Rhopalosiphum padi L. (Hemiptera: Aphididae), on three RWA‐resistant barley [Hordeum vulgare L. (Poaceae)] lines (STARS‐9577B, STARS‐0502B, and STARS‐9301B) and one susceptible control (PUMA). RWASA2 had a higher reproductive rate than RWASA1 on all barley lines tested, which is consistent with previous results on wheat. Two of the RWA‐resistant lines (STARS‐0502B and STARS‐9301B) also exhibited a similar resistance phenotype against BCA. In our experiments, severe chlorosis and leaf roll appeared earlier on the control PUMA barley variety as a result of RWASA2 feeding than was the case with RWASA1, probably due to the differences in reproductive rate. Although chlorosis appeared earlier on resistant plants after RWASA2 feeding, this symptom developed much faster during RWASA1 feeding on all three resistant lines tested. As chlorosis did not correlate well with aphid population numbers, we surmise that the differential chlorosis effects may be related to differences in the amount of saliva introduced by the two aphid clones during feeding. Our results indicate that the difference between RWASA2 and RWASA1 are broader than a ‘gene for gene’ interaction with the Dn1 resistance (R) gene in wheat, and that these biotypes also differ in important aspects of their biology.     

Microsatellite markers linked to six Russian wheat aphid resistance genes in wheat

The Russian wheat aphid (RWA), Diuraphis noxia Mordvilko, is a serious economic pest of wheat and barley in North America, South America, and South Africa. Using aphid-resistant cultivars has proven to be a viable tactic for RWA management. Several dominant resistance genes have been identified in wheat, Triticum aestivum, including Dn1 in PI 137739, Dn2 in PI 262660, and at least three resistance genes (Dn5+) in PI 294994. The identification of RWA-resistant genes and the development of resistant cultivars may be accelerated through the use of molecular markers. DNA of wheat from near-isogenic lines and segregating F₂ populations was amplified with microsatellite primers via PCR. Results revealed that the locus for wheat microsatellite GWM111 (Xgwm111), located on wheat chromosome 7DS (short arm), is tightly linked to Dn1, Dn2 and Dn5, as well as Dnx in PI 220127. Segregation data indicate RWA resistance in wheat PI 220127 is also conferred by a single dominant resistance gene (Dnx). These results confirm that Dn1, Dn2 and Dn5 are tightly linked to each other, and provide new information about their location, being 7DS, near the centromere, instead of as previously reported on 7DL. Xgwm635 (near the distal end of 7DS) clearly marked the location of the previously suggested resistance gene in PI 294994, here designated as Dn8. Xgwm642 (located on 1DL) marked and identified another new gene Dn9, which is located in a defense gene-rich region of wheat chromosome 1DL. The locations of markers and the linked genes were confirmed by di-telosomic and nulli-tetrasomic analyses. Genetic linkage maps of the above RWA resistance genes and markers have been constructed for wheat chromosomes 1D and 7D. These markers will be useful in marker-assisted breeding for RWA-resistant wheat. Received: 17 May 2000 / Accepted: 13 June 2000     

Comparison of categories of resistance in wheat and barley genotypes against biotype 2 of the Russian wheat aphid, Diuraphis noxia (Kurdjumov)

The Russian wheat aphid Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae) is a global pest of wheat and barley. This arthropod is difficult to manage with pesticides or biological control agents due to the aphid’s ability to seek shelter in rolled leaves and also to develop virulent biotypes. During the past 20 years, the use of aphid-resistant cereal cultivars has proven to be an economically and ecologically beneficial method of protecting crops from D. noxia damage. Our research reports the results of experiments to determine the categories of D. noxia biotype 2 resistance present in Cereal Introduction Triticeae (CItr) 2401, and a barley genotype (IBRWAGP4-7), compared to control resistant and susceptible wheat and barley genotypes. CItr2401 and IBRWAGP4-7 exhibit no antixenosis, but both genotypes demonstrated antibiosis to D. noxia in the form of reduced aphid populations. Reduced leaf dry weight change, a measure of plant tolerance of D. noxia feeding, was significantly less in CItr2401 and IBRWAGP4-7 plants than in plants of susceptible control varieties. However, tolerance was negated when a tolerance index was calculated to correct for differences in aphid populations. Barley IBRWAGP4-7 is a new source of D. noxia biotype 2 resistance. D. noxia foliar leaf damage and population growth were significantly less on IBRWAGP4-7 plants than on plants of the susceptible barley variety Morex. IBRWAGP4-7 plants were equal in resistance to plants of the resistant barley STARS 9301 and wheat genotype CItr2401. Handling editor: Heikki Hokkanen     

Physiological responses of resistant and susceptible barley, Hordeum vulgare to the Russian wheat aphid, Diurpahis noxia (Mordvilko)

Knowledge of the physiological responses of barley, Hordeum vulgare L., to the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae) is critical to understanding the defense response of barley to aphid injury and identifying resistance mechanisms. This study documented the impact of D. noxia feeding on resistant (‘Sidney’) and susceptible (‘Otis’) barley through chlorophyll fluorescence measurements, chlorophyll content, and carbon assimilation (A–C_i) curves recorded at 1, 3, 6, 10, and 13 days after aphid introduction. All chlorophyll fluorescence parameters evaluated were similar between aphid-infested and control plants for both cultivars. A–C_i curves showed that D. noxia feeding negatively impacts the photosynthetic capacity in both cultivars, but this effect was greater in the susceptible plants. From the A–C_i curves, it is apparent that compensation occurs in resistant barley by day 10, but by the conclusion of the experiment, aphid populations reached levels that overwhelmed the resistant barley seedlings. Differences observed in carbon assimilation curves between control and infested plants show that D. noxia feeding impacts the dark reaction, specifically rubisco activity and RuBP regeneration. It is likely that declines in the photochemical efficiency and chlorophyll content of the plants may be a secondary effect and not the primary trigger of declines in host plant function.     

Identification of Russian wheat aphid (Homoptera: Aphididae) populations virulent to the Dn4 resistance gene

The Russian wheat aphid, Diuraphis noxia (Mordvilko), is a serious worldwide pest of wheat and barley. Russian wheat aphid populations from Hungary, Russia, and Syria have previously been identified as virulent to D. noxia (Dn) 4, the gene in all Russian wheat aphid-resistant cultivars produced in Colorado. However, the virulence of Russian wheat aphid populations from central Europe, North Africa, and South America to existing Dn genes has not been assessed. Experiments with plants containing several different Dn genes demonstrated that populations from Chile, the Czech Republic, and Ethiopia are also virulent to Dn4. The Czech population was also virulent to plants containing the Dnx gene in wheat plant introduction PI220127. The Ethiopian population was also virulent to plants containing the Dny gene in the Russian wheat aphid-resistant 'Stanton' produced in Kansas. The Chilean and Ethiopian populations were unaffected by the antibiosis resistance in Dn4 plants. There were significantly more nymphs of the Chilean population on plants of Dn4 than on Dn6 plants at both 18 and 23 d postinfestation, and the Ethiopian population attained a significantly greater weight on Dn4 plants than on plants containing Dn5 or Dn6. These newly characterized virulent Russian wheat aphid populations pose a distinct threat to existing or proposed wheat cultivars possessing Dn4.     

Aphid (Hemiptera: Aphididae) resistance in wheat near-isogenic lines

Plant and aphid biomass, photosynthetic pigment (chlorophylls a and b and carotenoids) concentrations, and chlorophyll a/b and chlorophyll/carotenoid ratios were quantified in aphid-infested 'Tugela' near-isogenic lines (Tugela, Tugela-Dn1, Tugela-Dn2, and Tugela-Dn5). The objectives were to quantify changes of photosynthetic pigments (chlorophylls a and b, and carotenoids) caused by aphid feeding and assess resistance of wheat isolines through aphid and plant biomass analysis. Biomass of bird cherry-oat aphid, Rhopalosiphum padi (L.) (Hemiptera: Aphididae)-infested plants was lower than Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae),- infested plants. When infested by D. noxia, all lines showed increased biomass over time, except Tugela where biomass decreased on day 12. No difference in plant biomass was detected among R. padi-infested and uninfested wheat lines. Biomass of D. noxia from Tugela (D. noxia-susceptible) was significantly higher than from plants with Diuraphis noxia-resistant Dn genes. Diuraphis noxia biomass from Tugela-Dn1 and Dn2 lines was not different from each other, but they were lower than from Tugela-Dn5. In contrast, there was no difference in R. padi biomass among wheat lines. Concentrations of chlorophylls a and b and carotenoids were significantly lower in D. noxia-infested plants compared with R. padi-infested and uninfested plants. When infested by D. noxia, chlorophyll a and b concentrations were not different among wheat lines on day 3, but they were lower in Tugela and Tugela-Dn1 than in Tugela-Dn2 and -Dn5 plants on days 6 and 12. However, no difference was detected in chlorophyll a/b or chlorophyll/carotenoid ratio among Tugela lines. The study demonstrated that Dn genes in the Tugela isolines conferred resistance to D. noxia but not to R. padi. Tugela-Dn1 was antibiotic, Tugela-Dn2 was tolerant and antibiotic, and Tugela-Dn5 was moderately antibiotic.     

Generation of marker‐free transgenic hexaploid wheat via an Agrobacterium‐mediated co‐transformation strategy in commercial Chinese wheat varieties

Genotype specificity is a big problem lagging the development of efficient hexaploid wheat transformation system. Increasingly, the biosecurity of genetically modified organisms is garnering public attention, so the generation of marker‐free transgenic plants is very important to the eventual potential commercial release of transgenic wheat. In this study, 15 commercial Chinese hexaploid wheat varieties were successfully transformed via an Agrobacterium‐mediated method, with efficiency of up to 37.7%, as confirmed by the use of Quickstix strips, histochemical staining, PCR analysis and Southern blotting. Of particular interest, marker‐free transgenic wheat plants from various commercial Chinese varieties and their F₁ hybrids were successfully obtained for the first time, with a frequency of 4.3%, using a plasmid harbouring two independent T‐DNA regions. The average co‐integration frequency of the gus and the bar genes located on the two independent T‐DNA regions was 49.0% in T₀ plants. We further found that the efficiency of generating marker‐free plants was related to the number of bar gene copies integrated in the genome. Marker‐free transgenic wheat plants were identified in the progeny of three transgenic lines that had only one or two bar gene copies. Moreover, silencing of the bar gene was detected in 30.7% of T₁ positive plants, but the gus gene was never found to be silenced in T₁ plants. Bisulphite genomic sequencing suggested that DNA methylation in the 35S promoter of the bar gene regulatory region might be the main reason for bar gene silencing in the transgenic plants.     

The molecular bases of plant resistance and defense responses to aphid feeding: current status

Plant genes participating in the recognition of aphid herbivory in concert with plant genes involved in defense against herbivores mediate plant resistance to aphids. Several such genes involved in plant disease and nematode resistance have been characterized in detail, but their existence has only recently begun to be determined for arthropod resistance. Hundreds of different genes are typically involved and the disruption of plant cell wall tissues during aphid feeding has been shown to induce defense responses in Arabidopsis, Triticum, Sorghum, and Nicotiana species. Mi‐1.2, a tomato gene for resistance to the potato aphid, Macrosiphum euphorbiae (Thomas), is a member of the nucleotide‐binding site and leucine‐rich region Class II family of disease, nematode, and arthropod resistance genes. Recent studies into the differential expression of Pto‐ and Pti1‐like kinase genes in wheat plants resistant to the Russian wheat aphid, Diuraphis noxia (Mordvilko), provide evidence of the involvement of the Pto class of resistance genes in arthropod resistance. An analysis of available data suggests that aphid feeding may trigger multiple signaling pathways in plants. Early signaling includes gene‐for‐gene recognition and defense signaling in aphid‐resistant plants, and recognition of aphid‐inflicted cell damage in both resistant and susceptible plants. Furthermore, signaling is mediated by several compounds, including jasmonic acid, salicylic acid, ethylene, abscisic acid, giberellic acid, nitric oxide, and auxin. These signals lead to the development of direct chemical defenses against aphids and general stress‐related responses that are well characterized for a number of abiotic and biotic stresses. In spite of major plant taxonomic differences, similarities exist in the types of plant genes expressed in response to feeding by different species of aphids. However, numerous differences in plant signaling and defense responses unique to specific aphid–plant interactions have been identified and warrant further investigation.     

Implications of Russian wheat aphid, <Emphasis Type="Italic">Diuraphis noxia</Emphasis>, falling rates for biological control in resistant and susceptible winter wheat

The restriction of aphid reestablishment onto plants by epigeal predators represents a critical component of integrated pest management. To further realize the potential that these predators might have in control programs, it is necessary to quantify such behavior as aphid falling rate to reveal the number of aphids that are available as potential prey. This study calculated the falling rate of the Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Sternorrhyncha: Aphididae), and tested whether this aphid more likely fell from wheat plants that differed between flat leaf architecture versus those with furled leaves. Specifically, the hypothesis was tested that a resistant wheat line (flat leaves) will have a higher aphid falling rate than a susceptible closely related line (furled leaves). The experiment was performed at Fort Collins and Akron, Colorado, USA, from May through July, 2008. Aphids were sampled from infested wheat rows to estimate aphid density, and sticky traps were used to capture falling aphids and to measure falling rate. Falling rates ranged from 0.7 to 69.5% in Fort Collins and from 1.4 to 59.5% in Akron. The falling rate of D. noxia was more influenced by plant growth stage than aphid densities, with the highest falling rate occurring after wheat senescence. Wheat plants with flat leaf architecture did not significantly increase aphid falling rate. Diuraphis noxia falls at a higher rate at lower aphid densities, which is when epigeal predators could have their greatest biological control impact.     

Evidence for regional diversity and host adaptation in Iranian populations of the Russian wheat aphid

Effective pest management is greatly facilitated by knowledge of the genetic structure and host adaptation of the pest species in question. The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko) (Homoptera: Aphididae: Macrosiphini), is an important economic pest in many cereal‐growing areas of the world, and in this study we investigated these aspects of its populations, using microsatellite markers and host plant response assays. Diuraphis noxia was sampled from 38 locations in Iran and genotyped at four polymorphic microsatellite loci that had been isolated from various Sitobion species. We identified 50 multilocus genotypes in 376 individuals. The overall observed heterozygosity was 0.134. F‐statistics showed a regional partitioning in D. noxia populations with overall F_ST = 0.231. In addition, there was a significant correlation between genetic and geographic distances. In order to test for the ecological consequences of genetic variability in D. noxia, biotypic variation amongst the isolates collected from wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) was evaluated on a number of resistant and susceptible wheat varieties. The plant variables we measured were damage rating (based on leaf chlorosis, leaf rolling, wilting, and death of the host plant), host plant dry weight, and root length. Damage rating was the best criterion for detecting biotypic variation in D. noxia. Discriminant analysis correctly classified the isolates in respective groups in 80–91.8% of the cases. The barley isolate showed no differences in performance on resistant and susceptible wheat, indicating a lack of gene‐by‐gene relationship with wheat plants. In contrast, wheat isolates differentially damaged the resistant and susceptible plants and showed moderate to severe virulence.     

In vitro enzymatic chlorophyll catabolism in wheat elicited by cereal aphid feeding

Chlorophyll degradation is a complex phenomenon that often accompanies insect feeding damage to plants. Loss of chlorophyll can be initiated by several reactions, including oxidative bleaching, chlorophyllase activity, and Mg-dechelatase activity. Extracts from the Russian wheat aphid [Diuraphis noxia (Mordvilko)], the bird cherry-oat aphid [Rhopalosiphum padi (L.)], and aphid-infested and uninfested wheat plants were assayed in vitro for activities involved in chlorophyll degradation. Although the initial infestation was the same (10 apterous adults) for both aphid species, D. noxia weight was significantly higher than R. padi after feeding for 12 days. Consequently, D. noxia feeding caused greater fresh leaf weight reduction than R. padi feeding. Chlorophyll degradation assays showed no activity from either D. noxia or R. padi extracts. Plant extract assays showed a significant difference in Mg-dechelatase activity, while no difference was detected in either the chlorophyllase or oxidative bleaching pathways among the aphid-infested or uninfested plant extracts. Diuraphis noxia-infested leaf extracts showed a greater increase of Mg-dechelatase activity than either R. padi-infested or the uninfested plants. The findings suggest that leaf chlorosis elicited by D. noxia feeding is different from the chlorophyll degradation that occurs in natural plant senescence. Aphid-elicited chlorosis might be the result of a Mg-dechelatase-driven catabolism of chlorophyll in challenged wheat seedlings, however, the factor(s) from D. noxia that elicited the increase of Mg-dechelatase activity still remain to be determined.     

Genetic mapping of Dn7, a rye gene conferring resistance to the Russian wheat aphid in wheat

The Russian wheat aphid is a significant pest problem in wheat and barley in North America. Genetic resistance in wheat is the most effective and economical means to control the damage caused by the aphid. Dn7 is a rye gene located on chromosome 1RS that confers resistance to the Russian wheat aphid. The gene was previously transferred from rye into a wheat background via a 1RS/1BL translocation. This study was conducted to genetically map Dn7 and to characterize the type of resistance the gene confers. The resistant line '94M370' was crossed with a susceptible wheat cultivar that also contains a pair of 1RS/1BL translocation chromosomes. The F₂ progeny from this cross segregated for resistance in a ratio of 3 resistant: 1 susceptible, indicating a single dominant gene. One-hundred and eleven RFLP markers previously mapped on wheat chromosomes 1A, 1B and 1D, barley chromosome 1H and rye chromosome 1R, were used to screen the parents for polymorphism. A genetic map containing six markers linked to Dn7, encompassing 28.2 cM, was constructed. The markers flanking Dn7 were Xbcd1434 and XksuD14, which mapped 1.4 cM and 7.4 cM from Dn7, respectively. Dn7 confers antixenosis, and provides a higher level of resistance than that provided by Dn4. The applications of Dn7 and the linked markers in wheat breeding are discussed.Communicated by J. Dvorak     

Development of RAPD and SCAR markers linked to the Russian wheat aphid resistance gene Dn2 in wheat

 RAPD (random amplified polymorphic DNA) analysis was used to identify molecular markers linked to the Dn2 gene conferring resistance to the Russian wheat aphid (Diuraphis noxia Mordvilko). A set of near-isogenic lines (NILs) was screened with 300 RAPD primers for polymorphisms linked to the Dn2 gene. A total of 2700 RAPD loci were screened for linkage to the resistance locus. Four polymorphic RAPD fragments, two in coupling phase and two in repulsion phase, were identified as putative RAPD markers for the Dn2 gene. Segregation analysis of these markers in an F₂ population segregating for the resistance gene revealed that all four markers were closely linked to the Dn2 locus. Linkage distances ranged from 3.3 cM to 4.4 cM. Southern analysis of the RAPD products using the cloned RAPD markers as probes confirmed the homology of the RAPD amplification products. The coupling-phase marker OPB10_880c and the repulsion-phase marker OPN1_400r were converted to sequence characterized amplified region (SCAR) markers. SCAR analysis of the F₂ population and other resistant and susceptible South African wheat cultivars corroborated the observed linkage of the RAPD markers to the Dn2 resistance locus. These markers will be useful for marker-assisted selection of the Dn2 gene for resistance breeding and gene pyramiding. Received: 1 July 1997 / Accepted: 20 October 1997