Noxia Feeding Research Articles

Wheat Argonaute 5 Functions in Aphid-Plant Interaction.

Aphids feeding on plants experience similar responses to pathogens due to the prolonged and intimate contact with the plant. Diuraphis noxia is an economically important aphid pest on wheat that exhibits such an interaction. Studies on small RNA (sRNA) that regulate genes imparting resistance to wheat against D. noxia have predicted an Argonaute 5 (TaAGO5) gene as possible role player in the resistance response. Functional characterization revealed that TaAGO5 is crucial in regulating the response to infestation by D. noxia. Knockdown of TaAGO5 by 22% in D. noxia resistant wheat resulted in a completely susceptible phenotype. The fecundity and stress levels of D. noxia feeding on these silenced plants were similar to aphids feeding on the susceptible controls. Thus, TaAGO5 is crucial in the defense response by wheat plants during aphid feeding and this is similar to Nicotiana benthaminia plants experiencing arthropod herbivory. Additionally, TaAGO5 was differentially regulated by the Barley mosaic virus (BMV) used in the functional characterization. This provides evidence that TaAGO5 could play a role during virus infection of wheat. The role of AGO5 proteins in plant responses to arthropod herbivory and virus infection is known for dicotyledonous plants. Here, we present data that indicate that this role of TaAGO5 is conserved in wheat and possibly for monocotyledonous plants. These observations extend our knowledge on the roles of AGO proteins in plant resistance.

Bacteria Associated with Russian Wheat Aphid (Diuraphis noxia) Enhance Aphid Virulence to Wheat

Phenotypic responses to biotic stresses are often studied as the interactions between two species; however, in the phytobiome, these responses frequently result from complex interactions involving several organisms. Here, we show that variation in chlorosis caused by Russian wheat aphid (Diuraphis noxia) feeding is determined, in part, by aphid-associated bacteria. Proteomic analysis of fluids injected into a sterile medium by the aphid during feeding indicate that 99% of the proteins are of bacterial origin. Of these, the greatest proportion are produced by bacteria in the order Enterobacteriales. Bacteria from five genera in four families that have the capacity to produce these proteins were isolated directly from aphids as well as from wheat leaves only after D. noxia feeding. By themselves or in combination, these bacteria were not virulent to wheat, even at high inoculum levels. Metagenomic analysis showed that the same five D. noxia-associated genera dominated the non-Buchnera component of the aphid microbiome, and that representation of these genera was reduced in aphids from colonies established after isolation of newborn nymphs from their mothers prior to feeding (isolated aphids). Isolation or treatment with antibiotics reduced bacterial numbers, and these aphids caused less feeding damage on wheat than non-isolated or non-antibiotic-treated aphids. Our data show that bacterial proteins are a significant component of Russian wheat aphid saliva, that the bacteria producing these proteins are associated with aphids and plants fed upon by aphids, and that these aphid-associated bacteria facilitate aphid virulence to wheat.

Electrical Penetration Graph Recording of Russian Wheat Aphid (Hemiptera: Aphididae) Feeding on Aphid-Resistant Wheat and Barley.

Biotypes of Russian wheat aphid, Diuraphis noxia (Kurdjumov), have nullified D. noxia-resistant wheat. In this study, feeding of North American D. noxia was measured in aphids fed resistant and susceptible wheat and barley using electrical penetration graph (EPG) recordings. Interactions between barley genotypes and D. noxia biotypes were significant. EPG recordings of biotype 1 aphids fed on D. noxia-resistant IBRWAGP4-7 barley plants displayed significantly more non-phloem (pathway) phase movements and significantly less sieve element phase (SEP) feeding than on susceptible plants. EPG recordings of D. noxia biotype 2 feeding are the first ever recorded, but no differences between biotype 2-susceptible and -resistant barley plants were found for any EPG parameter in biotype 2 aphids fed barley. No wheat genotype-D. noxia biotype interactions were detected, but when responses were averaged across resistant and susceptible wheat genotypes, biotype 1 displayed a significantly longer pathway phase and significantly more SEP feeding than biotype 2, and biotype 2 engaged in significantly more xylem drinking than biotype 1. IBRWAGP4-7 barley resistance to biotype 1 appears to be controlled by both intercellular factors encountered during the pathway phase and intracellular factors ingested during SEP feeding. The lack of differences in EPG parameters displayed by biotype 2 feeding on barley suggests that biotype 2 resistance in IBRWAGP4-7 barley is based on tolerance to D. noxia feeding instead of altered feeding patterns. Resistance in 'KS94H871' wheat appears to be a function of phloem, non-phloem, and xylem factors that extend the duration of pathway feeding and limit SEP feeding.

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.

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.

Barley tolerance of Russian wheat aphid (Hemiptera: Aphididae) biotype 2 herbivory involves expression of defense response and developmental genes

The Russian wheat aphid, Diuraphis noxia (Kurdjumov), is an invasive insect pest that causes serious yield losses in bread wheat, Triticum aestivum L., durum wheat, T. turgidum L and barley, Hordeum vulgare L. Successful management of D. noxia has been achieved through resistant varieties via plant antixenosis (aphid non-preference), antibiosis (reduced aphid growth or fecundity), tolerance (plant compensatory growth after aphid feeding), or a combination of each. Previous phenotyping experiments revealed that plants of the variety Stoneham resist D. noxia damage via tolerance. In the present study, genes involved in upstream regulation of jasmonic acid (JA), salicylic acid (SA), ethylene (ET), auxin (AUX) and abscisic acid (ABA) biosynthetic pathways were monitored using qRT-PCR in Stoneham and susceptible Otis barley plants after D. noxia biotype 2 feeding. Results indicate that D. noxia tolerance in Stoneham plants is related to greater constitutive expression of JA-, ET- and AUX-biosynthetic pathway genes than in susceptible Otis plants, suggesting the possibility of immediate plant adjustments due to the stress of D. noxia feeding. There was limited induction of genes in the ET-(ACCS) and IAA (TDC) pathways in Stoneham tissues after D. noxia feeding. JA pathway genes upregulated in Otis tissues after D. noxia infestation failed to successfully defend Otis plants. AUX and ABA transcripts in Otis may be associated with developmental collapses resulting from source and sink adjustment failures.

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–Ci) 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–Ci 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–Ci 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.

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.

Entomological and genetic variation of cultivated barley (Hordeum vulgare) from Egypt

The Russian wheat aphid (Diuraphis noxia (Mordvilko) is one of the most notorious pests and greatly limits the productivity of wheat and barley worldwide. Assessment of the genetic diversity of new barley germplasm (landraces) at both the genotypic and phenotypic levels may increase the efficiency of plant breeding and marker-assisted selection of desirable plant traits. The objective of this work was to use amplified fragment length polymorphisms (AFLPs) to assess genetic diversity among 24 Egyptian barley landraces and to identify resistance to D. noxia in the landraces. Out of 273 polymorphic bands obtained using eight primer combinations, 35 bands were discriminative AFLP markers and could be used to distinguish the Egyptian barley landraces at the DNA level. An AFLP-based dendrogram, generated by UPGMA cluster analysis, indicated that there is a correlation between genetic similarities and location. The obtained dendrogram clustered the investigated landraces into three main groups. There was significantly less D. noxia feeding damage on plants of several landraces originating near Giza compared to those originating in Sinai or Marsa Matrouh.

Molecular Basis of Plant Gene Expression During Aphid Invasion: Wheat <I>Pto-</I> and <I>Pti-</I>Like Sequences Are Involved in Interactions Between Wheat and Russian Wheat Aphid (Homoptera: Aphididae)

The Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), is a major pest of bread wheat, Triticum aestivum L. (em Thell), in most wheat-growing areas worldwide. Aphid-resistant cultivars are used to combat this pest, but very little is known about the molecular basis of resistance. In this study, differential gene expression in D. noxia biotype 1-resistant wheat plants containing the Dnx gene and D. noxia biotype 1 feeding on Dnx plants was investigated using suppressive subtraction hybridization. The derived subtracted cDNA library includes sequences similar to Pto and Pti1, genes involved in gene-for-gene recognition of and resistance to bacterial speck disease in tomato, Lycopersicon esculentum (L.). Pto- and Pti1-like sequences contain an activation domain with conserved amino acid residues crucial for avr protein recognition and binding by Pto, and avr-Pto phosphorylation of Pti1. Wheat defense signaling is represented by sequences putatively involved in producing sterols, jasmonates, Ca2+, and abscisic and gibberellic acids. We suggest that reductions in populations of D. noxia fed Dnx plants are related to the expression of sequences involved in defensive chemical production, cellular transport, and exocytosis. Dnx plant tolerance of D. noxia feeding is proposed to be based on the expression of sequences putatively involved in self-defense against reactive oxygen species and toxins, and proteolysis; DNA, RNA, and protein synthesis; chloroplast and mitochondrial function; carbohydrate metabolism; and maintenance of cell homeostasis. D. noxia unsuccessfully counter Dnx by expressing sequences putatively involved in detoxification; proteolysis; DNA, RNA, protein, and lipid synthesis; carbohydrate metabolism; and mitochondrial function.

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

AbstractActivities 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.

Light activation of Russian wheat aphid-elicited physiological responses in susceptible wheat.

The impact of light and its role in Russian wheat aphid, Diuraphis noxia (Mordvilko), damage symptom formation, and photosynthetic capacity in 'Arapahoe' wheat (Triticum aestivum L.) were examined. After 72 h under continuous dark or continuous light regimes, the number of aphids (nymphs), leaf rolling and chlorosis ratings, fresh leaf weight, and chlorophyll contents were recorded. Photosynthetic rates, chlorophyll a, kinetics and chlorophyll extractions also were determined. Aphid infestation caused significant reductions in plant height, fresh weight, gas exchange, and chlorophyll fluorescence only under continuous light. Under the 72 h continuous dark regime, aphid infestation did not cause either damage symptom formation or reduction in plant growth or metabolism (photosynthesis). Furthermore, significantly more D. noxia nymphs were produced under continuous light condition than continuous dark. Our results demonstrate that the development of D. noxia feeding damage symptoms (i.e., leaf rolling and chlorotic streaks) on susceptible wheat seedlings is a light-activated process, even though the elicitor of the plant damage symptoms is aphid feeding.

Dynamic change in photosynthetic pigments and chlorophyll degradation elicited by cereal aphid feeding

AbstractThe concentration of photosynthetic pigments (i.e., chlorophylls a and b, and carotenoids) and chlorophyll degradation enzyme (i.e., chlorophyllase, oxidative bleaching, and Mg‐dechelatase) activities on aphid‐damaged and non‐damaged regions of the infested leaves were determined with two infestation periods (6 and 12 days). Russian wheat aphid [Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae)] feeding caused significant losses of chlorophylls a and b and carotenoids in the damaged regions. However, bird cherry‐oat aphid [Rhopalosiphum padi (L.) (Hemiptera: Aphididae)] feeding did not, except a significantly lower level of carotenoids was observed in the damaged regions from the short‐infestation (6‐day) samples. Interestingly, the non‐damaged regions of D. noxia‐infested leaves on both sampling dates had a significant increase of chlorophylls a and b and carotenoid concentrations when compared with the uninfested leaves. Although D. noxia feeding did not cause any changes in either chlorophyll a/b or chlorophyll (a+b)/carotenoid ratio between the damaged and non‐damaged leaf regions on short‐infestation (6‐day) samples, a significantly lower chlorophyll a/b ratio was detected in long‐infestation (12‐day) samples. The assays of chlorophyllase and oxidative bleaching activities showed no significant differences between the damaged and non‐damaged regions of the infested leaves on either sampling date. Mg‐dechelatase activity, however, was significantly higher in D. noxia‐damaged than non‐damaged leaf regions from the short‐infestation samples, while no differences were detected from the long‐infestation samples. Furthermore, the long‐infestation samples showed that Mg‐dechelatase activity from both D. noxia‐damaged and non‐damaged regions increased significantly in comparison with the respective regions of either uninfested or R. padi‐infested leaves. We infer that non‐damaged regions of D. noxia‐infested leaves compensate for the pigment losses in the damaged regions, and that Mg‐dechelatase activity changed dynamically from a localized response to a systemic response as infestation duration extends. The findings from this study on cereal aphid‐elicited chlorosis (or desistance) would help us to elucidate plant resistance mechanisms, in particular plant tolerance to non‐defoliating herbivory.

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

AbstractChlorophyll 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.

Oxidative responses of resistant and susceptible cereal leaves to symptomatic and nonsymptomatic cereal aphid (Hemiptera: Aphididae) feeding.

The impact of the leaf-chlorosis-eliciting Russian wheat aphid, Diuraphis noxia (Mordvilko), and the nonchlorosis-eliciting bird cherry-oat aphid, Rhopalosiphum padi (L.), feeding on D. noxia-susceptible and -resistant cereals was examined during the period (i.e., 3, 6, and 9 d after aphid infestation) that leaf chlorosis developed. After aphid number, leaf rolling and chlorosis ratings, and fresh leaf weight were recorded on each sampling date, total protein content, peroxidase, catalase, and polyphenol oxidase activities of each plant sample were determined spectrophotometrically. Although R. padi and D. noxia feeding caused significant increase of total protein content in comparison with the control cereal leaves, the difference in total protein content between R. padi and D. noxia-infested leaves was not significant. Although R. padi-feeding did not elicit any changes of peroxidase specific activity in any of the four cereals in comparison with the control leaves, D. noxia feeding elicited greater increases of peroxidase specific activity only on resistant 'Halt' wheat (Triticum aestivum L.) and susceptible 'Morex' barley (Hordeum vulgare L.), but not on susceptible 'Arapahoe' and resistant 'Border' oat (Avena sativa L.). D. noxia-feeding elicited a ninefold increase in peroxidase specific activity on Morex barley and a threefold on Halt wheat 9 d after the initial infestation in comparison with control leaves. Furthermore, D. noxia feeding did not elicit any differential changes of catalase and polyphenol oxidase activities in comparison with either R. padi feeding or control leaves. The findings suggest that D. noxia feeding probably results in oxidative stress in plants. Moderate increase of peroxidase activity (approximately threefold) in resistant Halt compared with susceptible Arapahoe wheat might have contributed to its resistance to D. noxia, whereas the ninefold peroxidase activity increase may have possibly contributed to barley's susceptibility. Different enzymatic responses in wheat, barley, and oat to D. noxia and R. padi feeding indicate the cereals have different mechanisms of aphid resistance.

Feeding damage by Diuraphis noxia results in a nutritionally enhanced phloem diet

AbstractThe feeding behavior of Diuraphis noxia Mordvilko (Homoptera: Aphididae) on susceptible hosts causes both ultrastructural and tissue level damage which may affect phloem composition. Genetic evidence suggests that endosymbiotic bacteria in most aphids overproduce limiting amino acids to benefit hosts but that D. noxia depends less on endosymbionts for these nutrients, possibly due to an enriched diet. To determine whether D. noxia feeding damage results in higher concentrations of essential amino acids, stylet exudates were analyzed from wheat (Triticum aestivum) damaged to different degrees. Comparison of samples from undamaged and damaged susceptible wheat revealed changes in amino acid composition and an increase in levels of essential amino acids, indicating a nutritionally enhanced ingesta. The changes in stylet exudates paralleled changes in leaf exudates, indicating that the effects are systemic. Feeding damage is not observed on a resistant wheat host, var. Halt, and leaf exudates from infested Halt did not show changes in amino acid composition. Mean relative growth of nymphs was significantly lower on Halt than on susceptible Arapahoe, indicating that Halt is a less suitable host. Both varieties show similar amino acid levels in non‐infested samples, suggesting that D. noxia infestation does not enhance the phloem environment in Halt. This study provides evidence that aphid feeding can generate a nutritionally enhanced phloem diet.

Changes in Chlorophyll a Fluorescence Induction Kinetics in Cereals Infested with Russian Wheat Aphid (Homopetra: Aphididea)

Plant entries that previously had tested resistant or susceptible to Russian wheat aphid, Diuraphis noxia (Mordvilko), were used to evaluate the effect of aphid feeding on leaf chlorophyll content and in vivo chlorophyll fluorescence induction kinetics. D. noxia feeding for 96 h caused significant reductions in chlorophyll a, chlorophyll b, and total chlorophyll content in susceptible wheat (‘Avon’, ‘TAM W-101’) and barley (‘Wintermalt’). Total chlorophyll content and constituent chlorophyll a and b levels were not significantly affected by D. noxia in resistant wheat (PI 366616, PI 372129) or barley (PI 366450) and in neither resistant (PI 386156) nor susceptible (‘Beagle 82’) critical. D. noxia infestation resulted in significant alterations of the primary fluorescence induction transients. When compared with non infested control plants, infested susceptible wheat and barley showed significantly increased non variable fluorescence (F0) and decreased maximal fluorescence (Fm) and variable fluorescence (Fv). In contrast, values for these parameters did not differ significantly among the infested and non infested controls of resistant plants. When compared with control plants, the photochemical efficiency of photo system II (F/Fm) and the half-rise time from Fo to Fm (t1/2) were reduced significantly in the infested susceptible entries but remained relatively unchanged for the infested resistant cultivates. No significant differences between infested and non infested plants were observed in the measured chlorophyll fluorescence parameters for either the resistant or the susceptible critical, which implied a much different resistance mechanism(s) to D. noxia The results of this investigation suggest that D. noxia damage goes beyond the simple removal of photosyntheses from the plant. The substantial decrease in F/Fm following aphid infestation for the susceptible wheat and barley indicated a significant decrease in the capacity and efficiency of the primary photochemistry of photo system II.

Interaction Between Russian Wheat Aphid (Homoptera: Aphididae) and Resistant and Susceptible Genotypes of Wheat

Interaction between the Russian wheat aphid, Diuraphis noxia (Mordvilko), and the resistant wheat, Triticum aestivum L., plant introduction lines PI 137739, PI 140207, and PI 262660 and the susceptible wheat cultivar ‘Stephens’ were studied. D. noxia reproductive rate and population development were higher when fed on Stephens compared with the resistant genotypes. Reproductive rate of D. noxia decreased as aphid density and length of aphid time on plant increased. Whereas, D. noxia populations increased with increasing aphid densities and length of aphid time on plant. Although the populations of D. noxia were lower when fed resistant genotypes. total chlorophyll content was reduced compared with the susceptible genotype. This could have been caused by increased aphid probing activity on resistant genotypes. There was no significant difference between the antibiotic line PI 140207 and the tolerant line PI 262660 in aphid reproductive rate, population density or reduction in leaf chlorophyll content. Expression of several polypeptides were altered by D. noxia feeding on PI 137739 and Stephens. Three polypeptides of sizes ≈32, 33, and 35 k were enhanced in PI 137739 but not in Stephens. Involvement of these proteins in plant defense is discussed.

Russian Wheat Aphid (Homoptera: Aphididae) Population Development and Plant Damage on Resistant and Susceptible Wheat

The resistant wheat plant introductions PI 137739 and PI 262660 and the susceptible wheat ( Triticum aestivum L.) cultivar ‘Stephens’ were used to determine the effects of varying Russian wheat aphid, Diuraphis noxia (Kuvdjumov), population densities (0, 10, and 40 aphids per plant) on plant growth and aphid population development. Aphid population development and plant growth were evaluated by destructively sampling plants and aphids at 1, 5, 10, and 15 d after infestation. Plant parameters measured included height, dry weight, and damage. D. noxia feeding damage was determined by counting the number of leaves that were rolled and were chlorotic. The numbers oflive and dead aphids were counted to measure D. noxia population development. D. noxia densities on susceptible ‘Stephens’ wheat were greater than on the resistant accessions at 5, 10, and 15 dafter infestation. The impact of aphid feeding was less on PI 137739 and PI 262660 than on ‘Stephens’ wheat. When the initial D. noxia density was 40 aphids per plant, the total number of aphids produced was higher at days 5 and 10 than the other aphid density, and the reproductive rates were lower on all dates. Plants intially infested with 10 aphids supported larger populations on day 15, because of lower mortality, higher aphid reproductive rates, and fewer dying plants.

Categories of Resistance in Plant Introductions of Wheat Resistant to the Russian Wheat Aphid (Homoptera: Aphididae)

Wheat cultivars previously identified as resistant to the Russian wheat aphid, Diumphis noxia (Mordvilko), in South Africa and in Idaho were evaluated in the greenhouse to determine the categories of resistance in operation. All test entries tested possessed a significant level of tolerance to D. noxia feeding, based on percentage reductions in plant height. Twenty-one days after infestation, D. noxia maintained on the three plant introductions from South Africa displayed reduced reproductive rates indicative of low-level antibiosis. However, none of the test entries reduced overall fecundity during the total (84-d) reproductive period of the experiment. None of the test entries displayed antixenosis.