C. Michael Smith

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.

Molecular Bases of Plant Resistance to Arthropods

Arthropod-resistant crops provide significant ecological and economic benefits to global agriculture. Incompatible interactions involving resistant plants and avirulent pest arthropods are mediated by constitutively produced and arthropod-induced plant proteins and defense allelochemicals synthesized by resistance gene products. Cloning and molecular mapping have identified the Mi-1.2 and Vat arthropod resistance genes as CC-NBS-LRR (coiled coil-nucleotide binding site-leucine rich repeat) subfamily NBS-LRR resistance proteins, as well as several resistance gene analogs. Genetic linkage mapping has identified more than 100 plant resistance gene loci and linked molecular markers used in cultivar development. Rice and sorghum arthropod-resistant cultivars and, to a lesser extent, raspberry and wheat cultivars are components of integrated pest management (IPM) programs in Asia, Australia, Europe, and North America. Nevertheless, arthropod resistance in most food and fiber crops has not been integrated due primarily to the application of synthetic insecticides. Plant and arthropod genomics provide many opportunities to more efficiently develop arthropod-resistant plants, but integration of resistant cultivars into IPM programs will succeed only through interdisciplinary collaboration.

Molecular Basis of Plant Gene Expression During Aphid Invasion: Wheat Pto- and Pti-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.

Identification of Russian Wheat Aphid (Homoptera: Aphididae) Populations Virulent to the Dn4 Resistance Gene

Abstract 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.

Plant resistance to aphid feeding: behavioral, physiological, genetic and molecular cues regulate aphid host selection and feeding.

Aphids damage major world food and fiber crops through direct feeding and transmission of plant viruses. Fortunately, the development of many aphid-resistant crop plants has provided both ecological and economic benefits to food production. Plant characters governing aphid host selection often dictate eventual plant resistance or susceptibility to aphid herbivory, and these phenotypic characters have been successfully used to map aphid resistance genes. Aphid resistance is often inherited as a dominant trait, but is also polygenic and inherited as recessive or incompletely dominant traits. Most aphid-resistant cultivars exhibit constitutively expressed defenses, but some cultivars exhibit dramatic aphid-induced responses, resulting in the overexpression of large ensembles of putative aphid resistance genes. Two aphid resistance genes have been cloned. Mi-1.2, an NBS-LRR gene from wild tomato, confers resistance to potato aphid and three Meloidogyne root-knot nematode species, and Vat, an NBS-LRR gene from melon, controls resistance to the cotton/melon aphid and to some viruses. Virulence to aphid resistance genes of plants occurs in 17 aphid species--more than half of all arthropod biotypes demonstrating virulence. The continual appearance of aphid virulence underscores the need to identify new sources of resistance of diverse sequence and function in order to delay or prevent biotype development.

Effects of diet on growth, fecundity and duration of tethered flight of Nezara viridula

The effects of diet during the fifth stadium on performance of adult N. viridula were determined through comparing growth, fecundity, and duration of tethered flight of insects reared on four axenic diets: fresh green bean, raw‐shelled peanut, a combination of the two and soaked soybean seed. Females reared on the combination diet were heavier than those reared on green beans and oviposited more than those reared on peanuts. Three‐day‐old adult females reared on peanuts flew longer than those reared on green beans. More females reared on diets containing mature seed showed a capacity for extended flight and had larger fat bodies at 34‐days postemergence. Thus, inadequate diet during the fifth stadium irreversibly affected growth, development, reproduction and flight capacity.

Categories of Resistance to Greenbug (Homoptera: Aphididae) Biotype I in Aegilops tauschii Germplasm

Abstract Categories of resistance to greenbug, Schizaphis graminum (Rondani), biotype I, were determined in goatgrass, Aegilops tauschii (Coss.) Schmal., accession 1675 (resistant donor parent), ‘Wichita’ wheat, Triticum aestivum L., (susceptible parent), and an Ae. tauschii-derived resistant line, ‘97-85-3’. Antibiosis was assessed using the intrinsic rate of increase (rm) of greenbugs confined to each of the three genotypes. Neither parent nor the resistant progeny expressed antibiosis. Mean rm values for greenbug I on Wichita (0.0956), and Ae. tauschii (0.10543) were not significantly different. Mean rm values for Wichita and 97-85-3 were also not significantly different. Antixenosis was determined by allowing aphids a choice to feed on plants of each of the three genotypes. Ae. tauschii 1675 exhibited antixenosis, but this resistance was not inherited and expressed in 97-85-3. In experiments comparing Wichita and Ae. tauschii 1675, greenbug I population distributions were not significantly different on Wichita at 24 h, but were shifted toward Wichita at 48 h. In the second antixenosis experiment, there were no significant differences in greenbug I population distributions on 97-85-3 or Wichita at 24 or 48 h. When all three lines were compared, there were no significant differences in greenbug biotype I populations at 24 or 48 h after infestation. Comparisons of proportional dry plant weight loss (DWT) and SPAD meter readings were used to determine tolerance to greenbug I feeding. Ae. tauschii 1675 and 97-85-3 were highly tolerant compared with Wichita. Infested and uninfested Ae. tauschii 1675 DWT was nonsignificant, and infested Wichita plants weighed significantly less than uninfested plants. When Wichita and 97-85-3 were contrasted, DWT of infested and uninfested Wichita plants were significantly different, but those of 97-85-3 were not. Mean percent leaf chlorophyll losses for the three genotypes, as measured by the SPAD chlorophyll meter, were as follows: Wichita = 65%; Ae. tauschii 1675 = 25%; and 97-85-3 = 39%. Percent leaf chlorophyll losses caused by greenbug feeding was significantly different in comparisons between Wichita and Ae. tauschii 1675, and comparisons between Wichita and 97-85-3, although feeding damage was not significantly different in comparisons between Ae. tauschii 1675 and 97-85-3. These data provided further evidence of the expression of tolerance to greenbug feeding in Ae. tauschii 1675 and 97-85-3.

Expression, mechanisms and chemistry of resistance in soybean, Glycine max L. (Merr.) to the soybean looper, Pseudoplusia includens (Walker)

The soybean plant introduction (PI)227687 is highly resistant to 16 different insect pests of soybean in Australia, Brazil, Taiwan and the United States. In most of the pest Lepidoptera, resistance is expressed as greatly increased mortality in the later larval instars. The soybean looper, Pseudoplusia includens (Walker) is most severely affected in the final larval instar. Antibiotic effects include reduced weight gain, growth rate and increased mortality. These effects are enhanced by mechanically wounding foliage 24 hr prior to larval consumption. Antibiosis appears to be due to the combined effects of a feeding deterrent and a growth inhibitor. Resistance is location-specific on (PI)227687 plants and occurs abruptly on the third fully-expanded leaf from the plant apex. Larval growth on leaves of the susceptible variety ‘Davis’ is similar to or greater than that of the upper, susceptible leaves of (PI)227687. Resistance appears to be chemically based, since larvae fed artificial diets containing leaf extracts exhibit antibiotic symptoms similar to those fed fresh (PI)227687 leaves.RésuméLa plante de soja (PI)227687 est résistante à 16 differents insectes qui lui sont nuisibles en Australie, au Brésil, au Taiwan et aux Etats Unis d’Amérique. Dans la plupart des insectes lépidoptères, la résistance est exprimée comme une mortalité accure au cours des phases larvaires secondaires. La résistance chez la plante de soja (PI)227687 à l’arpenteuse Pseudoplusia includens (Walker) est antixénotique à cause d’un préventif alimentaire et d’un antibiotique dû à un inhibiteur de croissance métabolique. Les effets de l’antibiotique comprennent une augmentation réduite du poids, une réduction du taux de croissance et une mortalité accrue. Ces effets sont accrus par l’ulcération mécanique des feuilles 24 hr avant la consommation larvaire. La résistance est spécifique à l’emplacement chez les plantes (PI)227687 et survient brusquement sur la troisième feuille complètement développée à partir du sommet de la plante. La croissance larvaire sur les feuilles de la variété susceptible ‘Davies’ est similaire ou plus importante que celle ayant lieu sur les feuilles susceptibles supérieures de (PI)227687. La résistance semble avoir une base chimique, car les larves nourris aux aliments artificiels contenant les extraits des feuilles présentent des symptômes antibiotiques similaires à ceux présentés par ceux qui sont nourris aux feuilles fraîches de (PI)227687.

Polymorphisms in salivary-gland transcripts of Russian wheat aphid biotypes 1 and 2

Abstract  The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), is a major pest of small grains. As with plant‐feeding aphids in general, the interaction between RWA and host plants is governed, on the insect side, by proteins and enzymes in saliva. In this work, we examined sequence variations in transcripts encoding proteins and enzymes of RWA salivary glands. We conducted reverse transcription – polymerase chain reaction in RWA biotypes 1 and 2 using primers derived from pea aphid orthologs, and cloned regions of 17 putative salivary gland transcripts. For four of the transcripts, we observed no difference in sequences between the two biotypes. For the other 13 transcripts, for example, the transcripts encoding sucrase, trehalase and protein C002, large amount of variations, both within each biotype and between the two biotypes, were observed. Usually the two biotypes shared only one variant, which was typically the most common variant in both biotypes. Most of the transcripts had more non‐synonymous than synonymous codon changes among their variants. Our results offer possible molecular markers for distinguishing the two biotypes and insights into their evolution.

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

ABSTRACT 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.