D. R. Porter

Efficacy of Pyramiding Greenbug (Homoptera: Aphididae) Resistance Genes in Wheat

Abstract Durable resistance to greenbug, Schizaphis graminum (Rondani), in wheat is a goal of wheat improvement teams, and one that has been complicated by the regular occurrence of damaging biotypes. Simulation modeling studies suggest that pyramiding resistance genes, i.e., combining more than one resistance gene in a single cultivar or hybrid, may provide more durable resistance than sequential releases of single genes. We examined this theory by pyramiding resistance genes in wheat and testing a series of greenbug biotypes. Resistance genes Gb2, Gb3, and Gb6, and pyramided genes Gb2/Gb3, Gb2/Gb6, and Gb3/Gb6 were tested for effectiveness against biotypes E, F, G, H, and I. By comparing reactions of plants with pyramided genes to those with single resistance genes, we found that pyramiding provided no additional protection over that conferred by the single resistance genes. Based on the results of this test, we concluded that the sequential release of single resistance genes, combined with careful monitoring of greenbug population biotypes, is the most effective gene deployment strategy for greenbug resistance in wheat.

Distribution and diversity of russian wheat aphid (Hemiptera: Aphididae) biotypes in North America.

Abstract Wheat, Triticum aestivum L., with Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae) resistance based on the Dn4 gene has been important in managing Russian wheat aphid since 1994. Recently, five biotypes (RWA1–RWA5) of this aphid have been described based on their ability to differentially damage RWA resistance genes in wheat. RWA2, RWA4, and RWA5 are of great concern because they can kill wheat with Dn4 resistance. In 2005, 365 Russian wheat aphid clone colonies were made from collections taken from 98 fields of wheat or barley, Hordeum vulgare L., in Oklahoma, Texas, New Mexico, Colorado, Kansas, Nebraska, and Wyoming to determine their biotypic status. The biotype of each clone was determined through its ability to differentially damage two resistant and two susceptible wheat entries in two phases of screening. The first phase determined the damage responses of Russian wheat aphid wheat entries with resistance genes Dn4, Dn7, and susceptible ‘Custer’ to infestations by each clone to identify RWA1 to RWA4. The second phase used the responses of Custer and ‘Yuma’ wheat to identify RWA1 and RWA5. Only two biotypes, RWA1 and RWA2, were identified in this study. The biotype composition across all collection sites was 27.2% RWA1 and 72.8% RWA2. RWA biotype frequency by state indicated that RWA2 was the predominant biotype and composed 73–95% of the biotype complex in Texas, Oklahoma, Colorado, and Wyoming. Our study indicated that RWA2 is widely distributed and that it has rapidly dominated the biotype complex in wheat and barley within its primary range from Texas to Wyoming. Wheat with the Dn4 resistance gene will have little value in managing RWA in the United States, based on the predominance of RWA2.

Genetic control of acquired high temperature tolerance in winter wheat

SummaryThe development of high temperature-tolerant wheat (Triticum aestivum L.) germplasm is necessary to improve plant productivity under high-temperature stress environments. The quantification of high temperature tolerance and the characterization of its genetic control are necessary for germplasm enhancement efforts. This study was conducted to determine the genetic control of acquired high temperature tolerance in common bread wheat cultivars. Reduction of 2,3,5-triphenyltetrazolium chloride (TTC) by heat-stressed seedling leaves was used as a quantitative measure to characterize acquired high temperature tolerance. Eleven-day-old seedlings of 20 F1 progeny produced through a complete 5×5 (‘Payne’, ‘Siouxland’, ‘Sturdy’, ‘TAM W-101’, and ‘TAM 108’) diallel mating design were acclimated at 37° C for 24 hours, followed by a 2-hour incubation at 50° C. Under these test conditions, acquired high temperature tolerance ranged from a high of 75.7% for the genotype TAM W-101 × TAM 108, to a low of 37.3% for the genotype Payne × Siouxland. Partitioning of genotypic variance revealed that only the general combining ability component effect was statistically highly significant, accounting for 67% of the total genotypic variation. These results suggest that enhancing the level of high temperature tolerance in wheat germplasm is feasible utilizing existing levels of genetic variability and exploiting additive genetic effects associated with high temperature tolerance.

Plant Resistance Components of Two Greenbug (Homoptera: Aphididae) Resistant Wheats

Abstract Several biotypes of the greenbug, Schizaphis graminum (Rondani), attack winter wheat, Triticum aestivum L., on the Southern Plains every year. Two wheat germplasm sources of resistance (‘Largo’ and ‘GRS 1201’) have been developed that provide protection against the three predominant greenbug biotypes (E, I, and K). Each source has agronomic and end-use quality advantages and disadvantages for the breeder to consider in choosing a greenbug-resistant breeding line. We compared these two germplasms to determine their levels of resistance against biotype E. Components of resistance (i.e., antibiosis, antixenosis, and tolerance) were measured on seedlings of GRS 1201, Largo, and ‘TAM W-101’ (a susceptible control). Several aphid and plant measurements (e.g., total number of aphids produced per plant, aphid selection preferences, and plant damage ratings) were recorded for each plant entry. Select data recorded for each resistance component were normalized and combined to derive a plant resistance index for each wheat entry. Results indicated that GRS 1201 had a higher level of combined resistance components than did Largo, followed by TAM W-101, the susceptible control. These data provide additional information for the breeder to consider in selecting a greenbug-resistant breeding line.

Differentiating greenbug resistance genes in barley

The greenbug [Schizaphis graminum (Rondani)] is an extremely damaging pest of barley (Hordeum vulgare L), particularly in the southern Great Plains of the USA. Two greenbug resistance genes, Rsg1a (in ‘Post 90’) and Rsg2b (in PI 426756), available for developing resistant barley cultivars, have similar phenotypes when challenged by various greenbug biotypes. This study was conducted to separate these two resistance genes via differential plant reactions to a recently collected field isolate of greenbug. Four barley entries and one wheat germplasm were challenged with two greenbug isolates and damage ratings were recorded for each combination. One greenbug isolate used in this study (TX1) was able to differentiate Rsg1a from Rsg2b through dramatically different plant responses (Rsg2b conferred resistance, Rsg1a did not). The results indicate the potential vulnerability of greenbug resistance genes in barley. Based on these and other reported results, we propose that gene symbol designations for greenbug resistance in barley be changed from Rsg1a to Rsg1 and Rsg2b to Rsg2.

Marker proteins associated with somatic embryogenesis of wheat callus cultures

Summary A marker protein for embryogenic potential could be useful in determining if target tissue for microprojectile bombardment has the ability to regenerate plants. The identification of such a protein in wheat callus cultures was approached by using isoelectric focusing and SDS-PAGE of proteins labeled in vivo with 35[S]-methionine and cysteine. Protein profile differences were examined in embryogenic (E-callus) and non-embryogenic (NE-callus) wheat callus, 105 or 271 d after callus initiation. Callus was maintained on medium containing either 5.6 or 9 μmol/L 2,4-dichlorophenoxyacetic acid (2,4-D). Proteins unique to E-callus (E-proteins) were identified by computer assisted analysis of scanned images of fluorographs of in vivo-labeled proteins of E- and NE-callus. Thirty-three E-proteins were identified in 105-day-old E-callus growing on 5.6 μmol/L 2,4-D, 71 E-proteins in 105-day-old callus on 9 μmol/L 2,4-D, 43 E-proteins in 271-day-old callus on 5.6 μmol/L 2,4-D, and 39 E-proteins in 271-day-old callus growing on 9 μmol/L 2,4-D. Of these E-proteins, 10 were in 105-day-old callus regardless of 2,4-D concentration. One E-protein was present in 271-day-old callus from both 2,4-D concentrations. Two E-proteins with relative molecular masses/pis of 43.0/7.6 and 27.0/8.2 were present in E-callus from three of the four treatments. These proteins could be used as markers for determining if tissue has embryogenic potential.

Identification of a Major Quantitative Trait Locus Conditioning Resistance to Greenbug Biotype E in Sorghum PI 550610 Using Simple Sequence Repeat Markers

Abstract Greenbug, Schizaphis graminum (Rondani), represents the most important pest insect of sorghum, Sorghum bicolor (L.) Moench, in the Great Plains of the United States. Biotype E is the most widespread and dominant type not only in sorghum and wheat, Triticum aestivum L., fields, but also on many noncultivated grass species. This study was designed to determine sorghum accession PI 550610 resistance to greenbug biotype E, to map the resistance quantitative trait loci (QTLs) by using an established simple sequence repeat (SSR) linkage map and to identify SSR markers closely linked to the major resistance QTLs. In greenhouse screening tests, seedlings of PI 550610 showed strong resistance to the greenbug at a level similar to resistant accession PI 550607. For QTL mapping, one F2 population containing 277 progeny and one population containing 233 F2:3 families derived from Westland A line × PI 550610 were used to genotype 132 polymorphic SSR markers and to phenotype seedling resistance to greenbug feeding. Phenotypic evaluation of sorghum seedling damage at 7, 12, 17, and 21 d postinfestation in the F2:3 families revealed that resistance variation was normally distributed. Single marker analysis indicated 16 SSRs spread over five chromosomes were significant for greenbug resistance. Composite interval and multiple interval mapping procedures indicated that a major QTL resided in the interval of 6.8 cM between SSR markers Xtxp358 and Xtxp289 on SBI-09. The results will be valuable in the development of new greenbug biotype E resistant sorghum cultivars and for the further characterization of major genes by map-based cloning.

Non-homoeologous wheat-rye chromosomal translocations conferring resistance to greenbug

SummaryC-banding andin situ hybridization were used to determine the chromosomal constitution of the greenbug-resistant germplasm GRS 1204. The results showed that this line had the radiation-induced non-homoeologous wheat-rye translocation chromosomes T2AS-1RS·1RL and T2AL·2AS-1RS. C-banding analysis further revealed the presence of a wheat-Agropyron elongatum translocation chromosome T1BL·1BS-3Ae#1L in line GRS 1204, that was derived from ‘Teewon’. The greenbug resistance of line GRS 1204 is similar to that of line GRS 1201 that was earlier shown to have the greenbug resistance geneGb6 located on the 1RS arm of the wheat-rye translocation chromosome T1AL·1RS. BecauseGb6 in line GRS 1204 is present on one of the non-homoeologous translocation chromosomes, agronomically line GRS 1201 should be the better adapted source ofGb6 resistance and be used in cultivar improvement.

Russian wheat aphid-induced protein alterations in spring wheat

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), has become a perennial, serious pest of wheat (Triticum aestivum L.) in the western United States. Current methodologies used to enhance RWA resistance in wheat germplasm could benefit from an understanding of the biochemical mechanisms underlying resistance to RWA. This study was initiated to identify specific polypeptides induced by RWA feeding that may be associated with RWA resistance. The effects of RWA feeding on PI 140207 (a RWA-resistant spring wheat) and Pavon (a RWA-susceptible spring wheat) were examined by visualizing, silver-stained denatured leaf proteins separated by two-dimensional polyacrylamide gel electrophoresis. Comparisons of protein profiles of noninfested and RWA-infested Pavon and PI 140207 revealed a 24-kilodalton-protein complex selectively inhibited in Pavon that persisted in PI 140207during RWA attack. No other significant qualitative or quantitative differences were detected in RWA-induced alterations of protein profiles. These results suggest that RWA feeding selectively inhibit synthesis and accumulation of proteins necessary for normal metabolic functions in susceptible plants.

Variation to Cause Host Injury Between Russian Wheat Aphid (Homoptera: Aphididae) Clones Virulent to Dn4 Wheat

Abstract Biotypes are infraspecific classifications based on biological rather than morphological characteristics. Cereal aphids are managed primarily by host plant resistance, and they often develop biotypes that injure or kill previously resistant plants. Although molecular genetic variation within aphid biotypes has been well documented, little is known about phenotypic variation, especially virulence or the biotype’s ability to cause injury to cultivars with specific resistance genes. Five clones (single maternal lineages) of Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), determined to be injurious to wheat, Triticum aestivum L., with the Dn4 gene, were evaluated on resistant and susceptible wheat and barley, Hordeum vulgare L., for their ability to cause chlorosis, reduction in plant height, and reduction in shoot dry weight. Variation to cause injury on resistant ‘Halt’ wheat, susceptible ‘Jagger’ wheat, and resistant ‘STARS-9301B’ barley was found among the Dn4 virulent clones. One clone caused up to 30.0 and 59.5% more reduction in plant height and shoot dry weight, respectively, on resistant Halt than other clones. It also caused up to 29.9 and 55.5% more reduction in plant height and shoot dry weight, respectively, on susceptible Jagger wheat. Although STARS-9301B barley exhibited an equal resistant response to feeding by all five clones based on chlorosis, two clones caused ≈20% more reduction in plant height and shoot dry weight than three other clones. The most injurious clones on wheat were not the most injurious clones on barley. This is the first report of variation to cause varying degrees of plant damage within an aphid biotype virulent to a single host resistance gene. A single aphid clone may not accurately represent the true virulent nature of a biotype population in the field.