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 nucleotidebinding 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 aphidinflicted 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.

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.

No abstract

Molecular mapping of genes for crop resistance to the green bug, Schizaphis graminum Rondani, will facilitate selection of green bug resistance in breeding through marker-assisted selection and provide information for map-based gene cloning. In the present study, microsatellite marker and deletion line analyses were used to map green bug resistance genes in five newly identified wheat germ plasms derived from Aegilops tauschii. Our results indicate that the Gb genes in these germ plasms are inherited as single dominant traits. Microsatellite markers X wmc 157 and X gdm 150 flank G bx 1 at 2.7 and 3.3 cM, respectively. Xwmc 671 is proximately linked to G ba, G bb, G bc and G bd at 34.3, 5.4, 13.7, 7.9 cM, respectively. X barc 53 is linked distally to G ba and G bb at 20.7 and 20.2 cM, respectively. X gdm 150 is distal to G bc at 17.9 cM, and X wmc 157 is distal to G bd at 1.9 cM. G bx 1, G ba, G bb, G bc, G bd and the previously characterized G bz are located in the distal 18% region of wheat chromosome 7 DL. G bd appears to be a new green bug resistance gene different from G bx 1 or G bz. G bx 1, G bz G ba, G bb, G bc and G bd are either allelic or linked to Gb 3.

Aegilops tauschii (Coss.) Schmal. (2n = 2x = 14, DD) (syn. A. squarrosa L.; Triticum tauschii) is well known as the D-genome donor of bread wheat (T. aestivum, 2n = 6x = 42, AABBDD). Because of conserved synteny, a high-density map of the A. tauschii genome will be useful for breeding and genetics within the tribe Triticeae which besides bread wheat also includes barley and rye. We have placed 249 new loci onto a high-density integrated cytological and genetic map of A. tauschii for a total of 732 loci making it one of the most extensive maps produced to date for the Triticeae species. Of the mapped loci, 160 are defense-related genes. The retrotransposon marker system recently developed for cultivated barley (Hordeum vulgare L.) was successfully applied to A. tauschii with the placement of 80 retrotransposon loci onto the map. A total of 50 microsatellite and ISSR loci were also added. Most of the retrotransposon loci, resistance (R), and defense-response (DR) genes are organized into clusters: retrotransposon clusters in the pericentromeric regions, R and DR gene clusters in distal/telomeric regions. Markers are non-randomly distributed with low density in the pericentromeric regions and marker clusters in the distal regions. A significant correlation between the physical density of markers (number of markers mapped to the chromosome segment/physical length of the same segment in microm) and recombination rate (genetic length of a chromosome segment/physical length of the same segment in microm) was demonstrated. Discrete regions of negative or positive interference (an excess or deficiency of crossovers in adjacent intervals relative to the expected rates on the assumption of no interference) was observed in most of the chromosomes. Surprisingly, pericentromeric regions showed negative interference. Islands with negative, positive and/or no interference were present in interstitial and distal regions. Most of the positive interference was restricted to the long arms. The model of chromosome structure and function in cereals with large genomes that emerges from these studies is discussed.

The greenbug, Schizaphis graminum (Rondani), is one of the major pests of wheat worldwide. The efficient utilization of wheat genes expressing resistance to greenbug infestation is highly dependent on a clear understanding of their relationships. The use of such genes will be further facilitated through the use of molecular markers linked to resistance genes. The present study involved several F(2) wheat populations derived from crosses between susceptible cultivars and resistant germplasm carrying different greenbug resistance genes. These populations were used to characterize the inheritance of a wheat gene ( Gbz) conferring tolerance to greenbug biotype I, to identify molecular markers linked to Gbz, and to investigate the relationship between Gbz and Gb3, a previously identified greenbug resistance gene. Our results indicated that Gbz is inherited as a single dominant gene. Microsatellite marker Xwmc157 is completely linked to Gbz, and Xbarc53 and Xgdm46 flank Gbz at distances of 5.1 and 9.5 cM, respectively. Selection of Gbz using marker Xwmc157 alone gives breeders 100% selection accuracy. Gbz may be placed in the distal region of the long arm of the wheat chromosome 7D. The results of allelism tests indicated that Gbz is either allelic or tightly linked to Gb3.

No abstract

The greenbug, Schizaphis graminum (Rhodani),is one of the major insect pests of wheat worldwide and it is important to develop a basic understanding of the chromosomal locations of known and new greenbug resistance genes. Gby is a new greenbug resistance gene in the wheat line 'Sando's selection 4040'. A mapping population used in this study was derived from a cross of Sando's 4040 and PI220127, a greenbug susceptible wheat land race from Afghanistan. A progeny test indicated that Gby is inherited as a single semi-dominant gene. A genetic linkage map consisting of Gby, Xgwm322 (a wheat microsatellite marker), XksuD2 (an STS marker) and 18 restriction fragment length polymorphism (RFLP) loci was constructed. We used DNA from Chinese Spring 7A deletion lines to show that the gwm332 and ksuD2 amplified fragments mapped in this study are located on along arm of chromosome 7A. This suggests that Gby is located on wheat chromosome 7A. Gby was mapped to the area in the middle of the 'island' of putative defense response genes that are represented by RFLP markers(Xpsr l9, XZnfp, Xbcd98 and Prl b) previously mapped to the distal part of the short arm of wheat chromosome group 7. This region of chromosome 7A is characterized by a high recombination rate and a high physical density of markers which makes Gby a very good candidate for map-based cloning. The selection accuracy when theRFLP markers Xbcd98, Xpsrll9 or XZnfp and Prlb flanking Gby are used together to tag Gby is 99.78%,suggesting that they can be successfully used in marker assisted selection.