Noxia Resistance Research Articles

Silencing of a Unique Integrated Domain Nucleotide-Binding Leucine-Rich Repeat Gene in Wheat Abolishes Diuraphis noxia Resistance.

Plants respond in a similar manner to aphid feeding as to pathogen attack. Diuraphis noxia is a specialist aphid, feeding only on selected grasses that include wheat, barley, and oats. The wheat-Diuraphis noxia interaction is characterized by responses very similar to those seen in wheat-pathogen interactions with none of the underlying resistance pathways and genes characterized yet. From wheat harboring the Dn1 resistance gene, we have identified a nucleotide-binding leucine-rich repeat (NLR) gene containing two integrated domains (IDs). These are three C-terminus ankyrin repeat domains and an N-terminus WRKY domain. The NLR core of the gene can be traced through speciation events within the grass family, with a recent WRKY domain integration that is Triticum-specific. Virus-induced gene silencing of the gene in a resistant wheat line resulted in the abolishment of the resistance response and induced a highly susceptible phenotype. Silenced plants supported a higher number of aphids, similar to the susceptible near-isogenic line (NIL), and the intrinsic rate of increase of the aphids matched that of aphids feeding on the susceptible NIL. The presence of the gene is necessary for Dn1 resistance and we have named the gene Associated with Dn resistance 1 (Adnr1) to reflect this function.

Virulent Diuraphis noxia Aphids Over-Express Calcium Signaling Proteins to Overcome Defenses of Aphid-Resistant Wheat Plants.

The Russian wheat aphid, Diuraphis noxia, an invasive phytotoxic pest of wheat, Triticum aestivum, and barley, Hordeum vulgare, causes huge economic losses in Africa, South America, and North America. Most acceptable and ecologically beneficial aphid management strategies include selection and breeding of D. noxia-resistant varieties, and numerous D. noxia resistance genes have been identified in T. aestivum and H. vulgare. North American D. noxia biotype 1 is avirulent to T. aestivum varieties possessing Dn4 or Dn7 genes, while biotype 2 is virulent to Dn4 and avirulent to Dn7. The current investigation utilized next-generation RNAseq technology to reveal that biotype 2 over expresses proteins involved in calcium signaling, which activates phosphoinositide (PI) metabolism. Calcium signaling proteins comprised 36% of all transcripts identified in the two D. noxia biotypes. Depending on plant resistance gene-aphid biotype interaction, additional transcript groups included those involved in tissue growth; defense and stress response; zinc ion and related cofactor binding; and apoptosis. Activation of enzymes involved in PI metabolism by D. noxia biotype 2 aphids allows depletion of plant calcium that normally blocks aphid feeding sites in phloem sieve elements and enables successful, continuous feeding on plants resistant to avirulent biotype 1. Inhibition of the key enzyme phospholipase C significantly reduced biotype 2 salivation into phloem and phloem sap ingestion.

Near-isogenic lines of Triticum aestivum with distinct modes of resistance exhibit dissimilar transcriptional regulation during Diuraphis noxia feeding.

ABSTRACTRussian wheat aphid (Diuraphis noxia, Kurdjumov) feeding on susceptible Triticum aestivum L. leads to leaf rolling, chlorosis and plant death – symptoms not present in resistant lines. Although the effects of several D. noxia (Dn) resistance genes are known, none have been isolated or characterized. Wheat varieties expressing different Dn genes exhibit distinct modes of D. noxia resistance, such as antibiosis (Dn1), tolerance (Dn2), and antixenosis (Dn5). However, the mechanism whereby feeding aphids are perceived, and how subsequent transcriptional responses are partitioned into resistance categories, remains unclear. Here we report on downstream events in near-isogenic wheat lines containing different Dn genes after D. noxia biotype SA1 feeding. Transcripts involved in stress, signal transduction, photosynthesis, metabolism and gene regulation were differentially regulated during D. noxia feeding. Expression analyses using RT-qPCR and RNA hybridization, as well as enzyme activity profiling, provide evidence that the timing and intensity of pathways induced are critical in the development of particular modes of resistance. Pathways involved include the generation of kinase signalling cascades that lead to a sustained oxidative burst, and a hypersensitive response that is active during antibiosis. Tolerance is a passive resistance mechanism that acts through repair or de novo synthesis of photosystem proteins. Results further suggest that ethylene-mediated pathways are possibly involved in generating volatile compounds and cell wall fortification during the antixenosic response.

Phi-class glutathione-S-transferase is involved in Dn1-mediated resistance.

Wheat Dn genes afford resistance to the economically important pest, Diuraphis noxia (Kurdjumov, Russian wheat aphid, RWA) and have been the topic of transcriptomic and proteomic studies aimed at unraveling the pathways involved in resistance. The antibiotic resistance conveyed by Dn1 is characterized by a hypersensitive response (HR) followed by systemic acquired resistance (SAR). Although many candidate genes differentially expressed during the Dn1-mediated resistance response have been identified, few have been functionally verified. The aim of this study was to silence three HR-associated candidate genes in Dn1 containing wheat using virus-induced gene silencing (VIGS): thylakoid-associated ascorbate peroxidase (tAPX), phi-class glutathione-S-transferase (TaGSTF6) and superoxide dismutase Cu/Zn (SOD). D. noxia fertility was used as a measure of antibiotic resistance. Silencing of SOD Cu/Zn had little effect on D. noxia fertility, while increased aphid reproduction was recorded on tAPX- and TaGSTF6-silenced plants. However, tAPX-silencing only affected early measurements and did not have a prolonged effect on resistance. TaGSTF6-silenced plants expressed lowered H2 O2 production in resistant wheat under infestation conditions, suggesting that TaGSTF6 and H2 O2 play an integral role in Dn1-mediated D. noxia resistance in wheat plants.

Comparative gut transcriptome analysis reveals differences between virulent and avirulent Russian wheat aphids, Diuraphis noxia

The Russian wheat aphid, Diuraphis noxia, is a destructive pest of cereal crops that exhibits virulence to D. noxia resistance genes in wheat. Therefore, it is important to identify D. noxia virulence factors. The insect gut, the primary site of defense to ingested toxins, is also a likely site of differential gene expression in virulent insects. Comparative analyses of gut transcriptomes from virulent and avirulent D. noxia can improve an understanding of aphid gut physiology and may reveal factors critical to compatible D. noxia–wheat interactions. A total of 4,600 clones were sequenced from gut cDNA libraries prepared from avirulent (biotype 1) and virulent (biotype 2) D. noxia feeding on biotype 1-resistant wheat. A majority of the sequences (66 % in biotype 1, 64 % in biotype 2) matched those from the NR database. BLASTx analysis of sequences with the highest E-values revealed that 59 % of the biotype 1 sequences matched those of the pea aphid, Acyrthosiphon pisum. However, only 17 % of the biotype 2 sequences were similar to those of A. pisum. RT-qPCR expression analyses confirmed that the biotype 2 gut transcriptome differs significantly from that of biotype 1. A transcript coding the tRNA-Leu gene was significantly up-regulated in the biotype 2 transcriptome, strongly suggesting that leucine metabolism is a critical factor in biotype 2 survival. Many more transcripts encoding protease inhibitors occurred in the avirulent biotype 1 gut than in the gut of virulent biotype 2. However, more protease transcripts occurred in the biotype 2 gut than in the biotype 1 gut, suggesting that the avirulent biotype produces protease inhibitors in response to plant proteases. The virulent biotype 2 produces trypsin-like and chymotrypsin-like serine protease counter-defenses to overcome biotype 1-resistant plants.

Wheat Gene Expression is Differentially Affected by a Virulent Russian Wheat Aphid Biotype

An improved understanding of the complex interactions between plants and aphids is emerging. Recognition of aphid feeding in plant tissues involves production of several defense response signaling pathways and downstream production of defense and detoxification compounds. Feeding by Russian wheat aphid, Diuraphis noxia (Kurdjumov), a serious pest of cereal crops worldwide, induces foliar deformity and chlorophyll loss during compatible wheat-D. noxia interactions. Experiments described here revealed significant differences in level and pattern of gene expression in defense response signaling and metabolic pathways between compatible and incompatible D. noxia-wheat interactions. The jasmonate (JA)-signaling genes LOX, AOS, and AOC were significantly more upregulated (~3- to 7 fold) in incompatible interactions than in compatible interactions (~2.5 to 3.5 fold) as early as 1 h post D. noxia infestation (hpi). Cellulose synthase, responsible for strengthening plant cell walls via cellulose production, was also more upregulated in incompatible interactions (4 to 7 fold) than in compatible interactions (1 to 3.5 fold). In contrast, glycolysis and citric acid cycle genes were significantly downregulated (~1.5 to 2 fold) in incompatible interactions and upregulated or less downregulated in compatible interactions from 6 to 72 hpi. Differences in expression of JA-signaling genes between feeding site tissues and non-feeding site tissues suggest that D. noxia defense response signals in wheat are restricted primarily to aphid feeding sites in the initial 6 hpi. This is the first report of differential upregulation of plant genes at 1 hpi in incompatible interactions involving aphid herbivory. Early wheat plant defense responses in incompatible D. noxia interactions at 1, 3, and 6 hpi appear to be important aspects of D. noxia resistance in wheat.

Variation of Resistance in Barley Against Biotypes 1 and 2 of the Russian Wheat Aphid (Hemiptera: Aphididae)

The Russian wheat aphid, Diruaphis noxia (Kurdjumov) (Hemiptera: Aphididae), is globally one of the most devastating pests of bread wheat, Tritium aestivum L.; durum wheat, Triticum turgidum L.; and barley, Hordeum vulgare L. Host plant resistance is the foundation for cereal insect pest management programs, and several sources of D. noxia resistance have been incorporated in cultivars to manage D. noxia damage. The emergence of D. noxia North American biotype 2 (RWA2) in Colorado has made all known Dn genes vulnerable except the Dn7 gene from rye, Secale cereale, and has warranted exploration for sources of resistance to both RWA1 and RWA2. The category of resistance in resistant donor plants may exert selection pressure over the aphid population to form a new virulent population. In the current study, we report tolerance and antibiosis resistance to RWA1 and RWA2 in the barley genotype 'Stoneham'. The rate and degree of expression of resistance in Stoneham against RWA1 and RWA2, although not similar, are greater than the partial resistance in 'Sidney'. Antixenosis resistance to RWA1 or RWA2 was not observed in Sidney or Stoneham. The tolerance identified in Stoneham is encouraging because it may delay D. noxia biotype selection and fits well in a dryland barley cropping system.

Global Phylogenetics of <I>Diuraphis noxia</I> (Hemiptera: Aphididae), an Invasive Aphid Species: Evidence for Multiple Invasions Into North America

The Russian wheat aphid, Diruaphis noxia (Kudjumov) (Hemiptera: Aphididae), is globally one of the most devastating pests of bread wheat, Tritium aestivum L., durum wheat, Triticum turgidum L., and barley, Hordeum vulgare L. Several sources of D. noxia resistance have been incorporated in commercial wheat and barley genotypes, but up to eight virulent biotypes, defined based on their ability to damage different wheat and barley genotypes, now occur across the western United States since the first appearance of D. noxia in North America in 1986. Critical to the study of D. noxia and other invasive species is an understanding of the number and origin of invasions that have occurred, as well as the rate or potential of postinvasion adaptation and geographic range expansion. The goal of this study was to determine whether D. noxia biotypes are by-products of a single invasion or multiple invasions into North America. We used the genome-wide technique of amplified fragment length polymorphisms, in combination with 22 collections of D. noxia from around the world, to assess this question, as well as patterns of genetic divergence. We found multiple lines of evidence that there have been at least two D. noxia invasions of different origin into North America, each resulting in subsequent postinvasion diversification that has since yielded multiple biotypes.

Inheritance and Categories of Resistance in Wheat to Russian Wheat Aphid (Hemiptera: Aphididae) Biotype 1 and Biotype 2

The Russian wheat aphid, Diruaphis noxia (Kudjumov) (Hemiptera: Aphididae), is globally one of the most devastating pests of wheat, Tritium aestivum L., and barley, Hordeum vulgare L. Host plant resistance is the foundation of cereal insect pest management programs, and several D. noxia resistance (Dn) genes from wheat have been introduced in commercial cultivars of wheat to manage Russian wheat aphid (RWA). Emergence of D. noxia biotype 2 (RWA2) in Colorado has made all known Dn genes, except the Dn7 gene from rye, Secale cereale L., vulnerable and has warranted exploration for sources of resistance to RWA1 and RWA2. Here, we report antibiosis resistance to RWA1 and RWA2 identified in the wheat breeding line KS94H871. Additional experiments indicated that tolerance and antixenosis are not operating in KS94H871. Segregation studies involving F2-derived F3 families indicated that KS94H871 resistance to RWA1 is controlled by one dominant gene and one recessive gene, whereas resistance to RWA2 is controlled by only one dominant gene. This new genetic resource may serve as a good source of resistance in future breeding programs with proper understanding of the genetics of resistance.

Categories and Inheritance of Resistance to Russian Wheat Aphid (Homoptera: Aphididae) Biotype 2 in a Selection from Wheat Cereal Introduction 2401

The Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), is one of the most devastating insect pests of wheat (Triticum spp.) and barley (Hordeum spp.) in the world. Yield losses and control costs are valued at several hundred million dollars each year. The use of D. noxia-resistant cultivars is an ecologically, economically, and biologically sound method of managing this pest. Several D. noxia resistance (Dn) genes from wheat have been used to develop cultivars resistant to D. noxia. However, a new U.S. D. noxia biotype (biotype 2) in Colorado is virulent to all known Dn genes except the Dn7 gene from rye (Secale spp.). Hence, there is an immediate need to identify and characterize unique sources of D. noxia resistance to biotypes. In this article, we report resistance to D. noxia biotype 2, identified in a selection from wheat cereal introduction (CItr) 2401, that is controlled by two dominant genes. CItr2401 has a strong antibiosis effect that is exhibited as a reduced intrinsic rate of increase of D. noxia biotype 2. CItr2401 plants also exhibit tolerance to leaf rolling and chlorosis. No antixenosis was detected in CItr2401.

Resistance to Biotype 2 Russian Wheat Aphid (Homoptera: Aphididae) in Two Wheat Lines

Biotype 2 of the Russian wheat aphid, Diuraphis noxia (Mordvilko), was identified in the United States in 2003 and is virulent to all commercially available cultivars of winter wheat, Triticum aestivum L., that are resistant to biotype 1. We compared the development and reproduction of biotype 2 D. noxia at 21.7 ± 0.12°C on ‘Trego’ (PI 612576), a susceptible commercial cultivar, and on lines CI 2401 and 03GD1378027 that represent putative resistance sources. CI 2401 is a pure wheat line originating in the former USSR (Tajikistan), whereas 03GD1378027 is a USDA–ARS breeding line originally developed from crosses with a South African line that carried a large rye translocation conferring D. noxia resistance. Both lines previously showed resistance to biotype 1 and are currently being used in the development of D. noxia-resistant wheat cultivars. Both solitary apterous virginoparae of biotype 2 and their progeny experienced a reduction in survival and prolonged developmental times on CI 2401 and 03GD1378027 compared with Trego, but the former lines did not differ significantly from each other with respect to either measure of aphid performance. Progeny developed faster than did their foundress mothers on CI 2401 and Trego, but not on 03GD1378027. Mean foundress fecundity did not differ between CI 2401 and 03GD1378027 but was reduced on these lines relative to Trego. Foundresses also were more often found off plants of CI 2401 and 03GD1378027 than Trego. Estimates of intrinsic rate of increase were higher on Trego than on either CI 2401 or 03GD1378027, the latter two lines yielding similar values. The negative impacts of CI 2401 and 03GD1378027 on development and reproduction of biotype 2 indicate that these lines represent sources of resistance effective against this biotype.

Resistance to Biotype 2 Russian Wheat Aphid (Homoptera: Aphididae) in Two Wheat Lines

Biotype 2 of the Russian wheat aphid, Diuraphis noxia (Mordvilko), was identified in the United States in 2003 and is virulent to all commercially available cultivars of winter wheat, Triticum aestivum L., that are resistant to biotype 1. We compared the development and reproduction of biotype 2 D. noxia at 21.7 +/- 0.12 degrees C on 'Trego' (PI 612576), a susceptible commercial cultivar, and on lines CI 2401 and 03GD1378027 that represent putative resistance sources. CI 2401 is a pure wheat line originating in the former USSR (Tajikistan), whereas 03GD1378027 is a USDA-ARS breeding line originally developed from crosses with a South African line that carried a large rye translocation conferring D. noxia resistance. Both lines previously showed resistance to biotype 1 and are currently being used in the development of D. noxia-resistant wheat cultivars. Both solitary apterous virginoparae of biotype 2 and their progeny experienced a reduction in survival and prolonged developmental times on CI 2401 and 03GD1378027 compared with Trego, but the former lines did not differ significantly from each other with respect to either measure of aphid performance. Progeny developed faster than did their foundress mothers on CI 2401 and Trego, but not on 03GD1378027. Mean foundress fecundity did not differ between CI 2401 and 03GD1378027 but was reduced on these lines relative to Trego. Foundresses also were more often found off plants of CI 2401 and 03GD1378027 than Trego. Estimates of intrinsic rate of increase were higher on Trego than on either CI 2401 or 03GD1378027, the latter two lines yielding similar values. The negative impacts of CI 2401 and 03GD1378027 on development and reproduction of biotype 2 indicate that these lines represent sources of resistance effective against this biotype.

<I>Diuraphis noxia</I> and <I>Rhopalosiphum padi</I> (Hemiptera: Aphididae) Interactions and Their Injury on Resistant and Susceptible Cereal Seedlings

Interspecific interactions between the symptomatic (chlorosis-eliciting) Russian wheat aphid, Diuraphis noxia (Mordvilko), and the asymptomatic (nonchlorosis-eliciting) bird cherry-oat aphid, Rhopalosiphum padi (L.), on four cereal genotypes were examined by simultaneous infestations. Four cereals (i.e., Diuraphis noxia-susceptible 'Arapahoe' wheat and 'Morex' barley, and D. noxia-resistant 'Halt' wheat and 'Border' oat) and four infestations (i.e., control, D. noxia, R. padi, and D. noxia/R. padi) were used in the research. Whereas D. noxia biomass confirmed D. noxia resistance among the cereals, R. padi biomass indicated that the D. noxia-resistant cereals did not confer R. padi resistance. D. noxia biomass was significantly lower in D. noxia/R. padi infestation than that in D. noxia infestation on all cereals, except Border oat, which indicated an antagonistic effect of R. padi on D. noxia. All aphid infestations caused a significant plant biomass reduction in comparison with the control. In comparison with D. noxia infestation, D. noxia/R. padi caused a significant plant biomass reduction on all cereals, except Morex barley. Although D. noxia biomass in D. noxia/R. padi infestation was significantly less than that in D. noxia infestation, leaf chlorophyll reduction was the same between D. noxia/R. padi and D. noxia infestations, which suggested that the asymptomatic R. padi enhanced the D. noxia-elicited leaf chlorophyll loss. The regression between chlorophyll content and aphid biomass indicated that the asymptomatic R. padi in the D. noxia/R. padi infestation enhanced chlorophyll loss, but interspecific aphid interaction on plant biomass varied among the cereals.

Species diversity, abundance, and phenology of aphid natural enemies on spring wheats resistant and susceptible to Russian wheat aphid

The species composition, relativeabundance, and seasonal dynamics of selectednatural enemies of cereal aphids were monitoredin spring wheat fields in Moscow, Idaho in 1997and 1998. Trials also examined the potentialimpact of resistance to Russian wheat aphid(RWA), Diuraphis noxia (Mordvilko)(Homoptera: Aphididae) in wheat, on aphidbiological control agents. Natural enemypopulations were monitored on two springwheats: D. noxia susceptible variety`Centennial' and resistant genotype `IDO488'. Field plots were artificially infested withadult D. noxia, and sampled for cerealaphids and parasitoids weekly. Coccinellidpredators were monitored once in 1997 and twicein 1998. The coccinellids Hippodamiaconvergens Guerin, Coccinellaseptempunctata L., C. transversoguttataBrown and C. trifasciata Mulsant weredetected. No significant differences in adultor immature coccinellid densities were observedbetween the D. noxia resistant andsusceptible genotypes. During both years, themost abundant primary hymenopteran parasitoidswere Diaeretiella rapae (M'Intosh), Aphidius ervi Haliday, A. avenaphis(Fitch), and Lysiphlebus testaceipes(Cresson), Aphelinus varipes (Foerster),Aphidius colemani Viereck, Aphidiuspicipes (Nees), Aphidius sp., Monoctonus washingtonensis Pike & Stary, Praon gallicum Stary, Praon occidentaleBaker, and Praon sp. were also detected. Numbers of both D. noxia and D.rapae were significantly greater on Centennialthan on IDO488 in both years. When all speciesof cereal aphids and parasitoids areconsidered, the total percentage parasitism wasnot significantly different between thegenotypes. There was no interaction betweenD. noxia resistance and the populationdensity of the predators or parasitoidsmonitored. These results suggest that the D. noxia resistant line had no adverse impacton natural enemies under the conditions ofthese field experiments.

Comparison of DIMBOA concentrations among wheat isolines and corresponding plant introduction lines

AbstractThe concentration of a hydroxamic acid, also known as DIMBOA (2,4‐dihydroxy‐7‐methoxy‐1, 4‐benzoxazin‐3‐one), in 6‐d old wheat seedlings was examined using reverse‐phase high performance liquid chromatography (HPLC). Wheat plant introduction (PI) lines PI 137739 (Dn1 gene), PI 262660 (Dn2 gene), and PI 294994 (Dn5 gene), the corresponding near‐isogenic lines‘Betta’‐Dn1, Betta‐Dn2 and Betta‐Dn5, and susceptible Betta wheat were used in the study. The Dn2 gene conferring Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae), tolerance was not related to DIMBOA concentration in wheat. Of the lines with Dn1 and Dn5 genes that confer antibiosis to D. noxia, only lines with the Dn5 gene showed increased DIMBOA accumulation. However, the Dn5 and the DIMBOA biosynthesis genes are not located in the same chromosome group. Possible relationship between the Dn5 gene and DIMBOA accumulation was discussed. This study indicates that DIMBOA concentration does not completely explain D. noxia resistance in the wheat lines examined and a comprehensive examination of other allelochemicals (e.g., phenolics) is necessary.

Seasonal dynamics of cereal aphids on Russian wheat aphid (Homoptera: Aphididae) susceptible and resistant wheats.

Field experiments were conducted in 1997 and 1998 to evaluate the impact of resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko), on the cereal aphid complex in wheat. Two spring wheats were planted: the variety "Centennial" (Russian wheat aphid susceptible) and the advanced line IDO488 (Russian wheat aphid resistant). IDO488 incorporates the resistance found in PI 294994 into a Centennial background. Field plots were artificially infested with adult D. noxia and sampled weekly. The most abundant aphid species in 1997 were Metopolophium dirhodum (Walker), Sitobion avenae (F.), D. noxia, and Rhopalosiphum padi (L.). In 1998, the order of abundance was M. dirhodum, R. padi, S. avenae, and D. noxia. The resistant genotype had significantly fewer D. noxia than the susceptible one during both years. However, plant genotype had no significant effect on the other aphid species in either year. Both the initial density of D. noxia and plant growth stage, had a significant effect on D. noxia population development, but had no effect on the other aphid species. There was no interaction between D. noxia resistance and the population density of the other aphid species observed.

Comparison of Diuraphis noxia resistance in wheat isolines and plant introduction lines

AbstractThe Russian wheat aphid, Diuraphis noxia (Mordvilko), is one of the most important aphid pests of wheat, Triticum aestivum L., worldwide. Among the various pest management options, plant resistance is an economical management tactic to control D. noxia in cereal crops such as wheat. Researchers have identified D. noxia resistant germplasm and it has been incorporated into wheat. This study compared D. noxia resistance between the ’Betta‘ wheat isolines Betta‐Dn1, Betta‐Dn2, and Betta‐Dn5 and their corresponding donor gene plant introduction (PI) lines PI 137739 (Dn1), PI 262660 (Dn2), and PI 294994 (Dn5). Although the Betta isolines and PI lines showed D. noxia resistance when compared with Betta wheat, the degree of resistance in the isolines to D. noxia was different from their corresponding PI donors. Aphid number, aphid fecundity, and biomass per aphid were not different between Betta‐Dn1 and PI 137739 or Betta‐Dn2 and PI 262660; however, the same parameters were significantly lower on PI 294994 compared with Betta‐Dn5. This indicated that aphid resistance in PI 137739 and PI 262660 was probably governed by a single dominant gene, while the resistance in PI 294994 was controlled by more than one gene. Additionally, plant biomass reduction was aphid density dependent, which suggested that use of appropriate aphid infestation level is important when using plant biomass reduction as an indicator of resistance. Plant resistance categorization showed that there was no detectable difference in antixenosis among the seven lines evaluated. However, the higher aphid fecundity observed on PI 262660 compared with PI 137739 and PI 294994, in addition to no significant differences among the three PIs in plant biomass reduction, suggested PI 262660 was a tolerant line, while PI 137739 and PI 294994 were antibiotic lines. Plant tolerance could not be elucidated among the three Betta isolines using aphid fecundity and plant biomass reduction as indicators.

Inheritance of resistance to Russian wheat aphid, Diuraphis noxia (Homoptera: Aphididae) in two wheat lines

The Russian wheat aphid, Diuraphis noxia, is a serious pest of wheat grown in the summer rainfall regions of South Africa. The use of D. noxia resistant cultivars may reduce the impact of this pest on cereal production, at the same time reducing environmental risks and minimizing control costs. The objective of the study was to determine the inheritance of D. noxia resistance present in the lines OSU ID 2808 (Triticum aestivum) and Aus 22498 (Triticum aestivum var. vavilovi) in order to ensure their judicious use in a backcross breeding programme. Resistant lines OSU ID 2808 and Aus 22498 were crossed with the susceptible cultivar Tugela after being screened with D. noxia. Resistance reactions of the F1, BC1 and F2 plants, and individual F2 plant derived F3 families indicated that the resistance in both genotypes is controlled by single dominant genes. It is unknown whether these genes are identical to any known, designated resistance genes or each other. In Suid-Afrika is die Russiese koringluis, Diuraphis noxia, 'n ernstige plaag van aangeplante koring in die somer- reënvalgebied. Die gebruik van D. noxia weerstandbiedende kultivars kan die impak van hierdie plaag op graanproduksie verminder en terselfdertyd omgewingsrisikos en beheerkostes verlaag. Die doel van hierdie studie was om die oorerwing van D. noxia weerstand in die lyne OSU ID 2808 (Triticum aestivum) and Aus 22498 (Triticum aestivum var. vavilovi) te bepaal, sodat die lyne effektief in 'n terugkruisingsprogram benut kan word. Die weerstandslyne OSU ID 2808 en Aus 22498 is met die vatbare kultivar Tugela gekruis na toetsing met D. noxia. Weerstandsreaksies van die F1 TK1 en F2 plante asook die van die F3families vanaf enkel F2 plante toon dat die weerstand in albei genotipes deur enkel dominante gene beheer word. Dit is onbekend of die gene dieselfde is as ander bekende, benaamde gene, of mekaar.

Fungal endophytes of wild barley and their effects on Diuraphis noxia population development

AbstractLaboratory experiments were conducted to compare the expression of Diuraphis noxia (Mordvilko) (Homoptera: Aphididae) resistance in four plant introduction (PI) lines of wild barley (Hordeum) infected with different species or strains of endophytic fungi (tribe Balansieae, family Clavicipitaceae, Neotyphodium gen. nov. [formerly Acremonium]). Aphid densities were significantly lower on endophyte‐infected plants of PI 314696 (H. bogdanii Wilensky) and PI 440420 (H. brevisubulatum subsp. violaceum (Boissier & Hohenacker)), compared with densities on endophyte‐free plants of both PI lines in population growth experiments. This endophyte‐ associated resistance was the result of antibiosis effects or starvation. In other experiments, endophyte‐free plants of PI 269406 and PI 440413 (H. bogdanii) were not superior to endophyte‐infected conspecifics as host plants of D. noxia. Our results demonstrate the influence of host plant species/genotype and endophyte species/strain on expression of aphid resistance, provide an explanation of the high levels of D. noxia resistance in PI 314696 and PI 440420 previously reported in the literature, and underscore the potential importance of endophytic fungi in conferring insect resistance in wild barley.

Probing behaviour of Diuraphis noxia on five cereal species with different hydroxamic acid levels

AbstractThe feeding behaviour of Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae) was electronically monitored on five cereal species (Triticum aestivum L., T. turgidum L., Secale cereale L. and x Triticosecale Wittmack) containing different hydroxamic acids levels and on Hordeum vulgare L., lacking these compounds, by means of a DC‐System. With increasing DIMBOA (2,4‐dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one) levels D. noxia showed less probing particularly before reaching the phloem phase and a lower percentage of aphids achieved sustained phloem ingestion. However, no significant correlation was found between DIMBOA levels and the total time of phloem feeding. These findings indicate that factors from the mesophyll and vascular tissues appear to be involved in the effects of resistance. The underlying mechanism, however, remains unclear. Thus, DIMBOA seems to be only part of the chemical defenses of the cereal plants that might be implicated in D. noxia resistance.