Greenbug Resistance Gene Research Articles

Characterization of a new greenbug resistance gene Gb9 in a synthetic hexaploid wheat.

Greenbug [Schizaphis graminum (Rondani)] is a serious insect pest that not only damages cereal crops, but also transmits several destructive viruses. The emergence of new greenbug biotypes in the field makes it urgent to identify novel greenbug resistance genes in wheat. CWI 76364 (PI 703397), a synthetic hexaploid wheat (SHW) line, exhibits greenbug resistance. Evaluation of an F2:3 population from cross OK 14319 × CWI 76364 indicated that a dominant gene, designated Gb9, conditions greenbug resistance in CWI 76364. Selective genotyping of a subset of F2 plants with contrasting phenotypes by genotyping-by-sequencing identified 25 SNPs closely linked to Gb9 on chromosome arm 7DL. Ten of these SNPs were converted to Kompetitive allele-specific polymerase chain reaction (KASP) markers for genotyping the entire F2 population. Genetic analysis delimited Gb9 to a 0.6-Mb interval flanked by KASP markers located at 599,835,668bp (Stars-KASP872) and 600,471,081bp (Stars-KASP881) on 7DL. Gb9 was 0.5cM distal to Stars-KASP872 and 0.5cM proximal to Stars-KASP881. Allelism tests indicated that Gb9 is a new greenbug resistance gene which confers resistance to greenbug biotypes C, E, H, I, and TX1. TX1 is one of the most widely virulent biotypes and has overcome most known wheat greenbug resistance genes. The introgression of Gb9 into locally adapted wheat cultivars is of economic importance, and the KASP markers developed in this study can be used to tag Gb9 in cultivar development.

Identification of a new Rsg1 allele conferring resistance to multiple greenbug biotypes from barley accessions PI 499276 and PI 566459.

Greenbug [Schizaphis graminum (Rondani)] is a major insect pest that significantly affects barley production worldwide. The identification of novel greenbug resistance genes is crucial for sustainable barley production and global food security. To identify greenbug resistance genes from a US breeding line PI 499276 and a Chinese cultivar PI 566459, two F6:7 recombinant inbred line (RIL) populations developed from crosses Weskan × PI 499276 and Weskan × PI 566459 were phenotyped for responses to greenbug biotype E and genotyped using genotyping-by-sequencing (GBS). Linkage analysis using single nucleotide polymorphism and kompetitive allele-specific polymorphism (KASP) markers delimited the greenbug resistance genes from PI 499276 and PI 566459 to a 1.2 Mb genomic region between 666.5 and 667.7 Mb on the long arm of chromosome 3H in the Morex Hordeum vulgare r1 reference sequence. Allelism tests based on responses of four F2 populations to greenbug biotype E indicated that the greenbug resistance gene in PI 499276 and PI 566459 is either allelic or very close to Rsg1. Given that PI 499276 and PI 566459 shared the same unique resistance pattern to a set of 14 greenbug biotypes, which is different from those of other Rsg1 alleles, they carry a new Rsg1 allele. The greenbug resistance genes in Post 90, PI 499276/PI 566459, and WBDC 336 were designated as Rsg1.a1, Rsg1.a2, and Rsg1.a3, respectively. KASP markers KASP-Rsg1a3-1, KASP-Rsg1a3-2, and KASP160 can be used to tag Rsg1.a2 in barley breeding.

Identification of bird cherry‐oat aphid and greenbug resistance sources from Aegilops tauschii for wheat improvement

AbstractBird cherry‐oat aphid (BCOA) and greenbug pose a persistent threat to wheat production in many wheat‐growing regions. Currently, greenbug resistance genes are limited for wheat improvement, and BCOA resistance sources are even more scarce. Thus, it is urgent to identify BCOA and greenbug resistance sources in wheat gene pools. An Aegilops tauschii (Ae. tauschii) panel of 260 accessions, which accounted for 48% of the total Ae. tauschii accessions available in global germplasm banks, was evaluated for BCOA resistance, and a set of BCOA‐resistant lines were identified. The same panel was evaluated for response to greenbug biotypes C, E, F, G, I, and TX1. A total of 102 lines exhibited 14 unique resistance profiles or phenotypes featuring resistance to at least one biotype. The frequency of resistant lines varied among lineages or subspecies, from 62% in Lineage 1 (subspecies tauschii) to 18% in Lineage 2 (subspecies stangulata), indicating conservation of genetic variance for greenbug resistance was nonrandom. Ae. tauschii accessions resistant to both BCOA and greenbug were identified for enhancing aphid resistance.

Detection of some Resistance Genes of Schizaphis graminum (Rondani) in some Wheat Cultivars using DNA Marker

Schizaphis graminum is one of the most important pests that infect wheat, causing great losses to the crop, This study included the detection of sensitive and resistant of some cultivars wheat to greenbug infestation as well as, the detection Gb 8 gene and Gb 3 gene which are responsible for the appearance of resistance in some wheat cultivars against wheat greenbug using two DNA primers. Four cultivars of wheat were used in the study, which is as follows Jehan, Oneha, Wafih, and Babil, The results showed that Oneha variety was the most sensitive cultivars, also the resistance gene Gb 8 gene was presented in Jehan and Babil cultivars while, DNA bands of Gb 3 gene appeared in Jehan, Wafih and Babil cultivars. The detection of greenbug resistance genes in wheat will help in accelerating the breeding program in the future.

Genomic location of Gb1, a unique gene conferring wheat resistance to greenbug biotype F

Greenbug (Schizaphis graminum, Rondani) is a serious insect pest in many wheat growing regions and has been infesting cereal crops in the USA for over a century. Continuous occurrence of new greenbug biotypes makes it essential to explore all greenbug resistant sources available to manage this pest. Gb1, a recessive greenbug resistance gene in DS28A, confers resistance to several economically important greenbug biotypes and is the only gene found to be resistant to greenbug biotype F. A set of 174 F2:3 lines from the cross DS28A × Custer was evaluated for resistance to greenbug biotype F in 2020 and 2022. Selective genotyping of the corresponding F2 population using single nucleotide polymorphism (SNP) markers generated by genotyping-by-sequencing (GBS) led to the identification of a candidate genomic region for Gb1. Thus, SSR markers previously mapped in this region were used to genotype the entire F2 population, and kompetitive allele specific PCR (KASP) markers were also developed from SNPs in the target region. Gb1 was placed in the terminal region of the short arm of chromosome 1A, and its location was confirmed in a second population derived from the cross DS28A × PI 697274. The combined data analysis from the two mapping populations delimited Gb1 to a < 1 Mb interval between 13,328,200 and 14,241,426 bp on 1AS.

Characterization of Rsg2.a3: A new greenbug resistance allele at the Rsg2 locus from wild barley (Hordeum vulgare ssp. spontaneum)

Greenbug (Schizaphis graminum Rondani) is a destructive insect pest that not only damages plants, but also serves as a vector for many viruses. Host plant resistance is the preferred strategy for managing greenbug. Two greenbug resistance genes, Rsg1 and Rsg2, have been reported in barley. To breed cultivars with effective resistance against various greenbug biotypes, additional resistance genes are urgently needed to sustain barley production. Wild barley accession WBDC053 (PI 681777) was previously found to be resistant to several greenbug biotypes. In this study, a recombinant inbred line (RIL) population derived from Weskan × WBDC053 was evaluated for response to two greenbug biotypes (E and TX1) and genotyped using genotyping by sequencing (GBS). A set of 3347 high quality GBS-derived single nucleotide polymorphisms (SNPs) were then used to map the greenbug resistance gene in this wild barley accession. Linkage analysis placed the greenbug resistance gene in a 2.35 Mb interval (0–2,354,645 bp) in the terminal region of the short arm of chromosome 2H. This interval harbors 15 genes with leucine-rich-repeat (LRR) protein domains. An allelism test indicated that the greenbug resistance gene in WBDC053, designated Rsg2.a3, is likely allelic or closely linked to Rsg2. GBS-SNPs 2H_1318811 and 2H_1839499 co-segregating with Rsg2.a3 in the RIL population were converted to Kompetitive allele specific PCR (KASP) markers KASP-Rsg2.a3-1 and KASP-Rsg2.a3-2, respectively. The two KASP markers can be used to select Rsg2.a3 and have the potential to tag Rsg2 in barley improvement programs.

Rsg1.a3: A new allele conferring unique resistance to greenbug Biotype H at the Rsg1 locus in Hordeum vulgare ssp. spontaneum

AbstractGreenbug (Schizaphisgraminum Rondani) is one of the aphids causing significant cereal yield losses worldwide, and greenbug Biotype H is virulent to all known greenbug resistance genes in barley (Hordeum L.). Wild barley (Hordeum vulgare ssp. spontaneum) accession WBDC336 (PI 682028) exhibits resistance to greenbug Biotypes C, E, H, I, WY81, WY12 MC, and WY86. To investigate the genetics of greenbug resistance in WBDC336, a recombinant inbred line (RIL) population was developed from the cross Weskan × WBDC336. The RIL population was evaluated for responses to greenbug Biotype E and genotyped using genotyping‐by‐sequencing. Among 67,238 single nucleotide polymorphisms (SNPs) identified, 3,907 high quality SNPs were selected for gene mapping after several filtering steps. Linkage analysis positioned the greenbug resistance gene to a 1.14‐Mb interval in the terminal region of the long arm of chromosome 3H in WBDC336, where the greenbug resistance gene Rsg1 resides. Greenbug assays and an allelism test indicated that the greenbug resistance gene in WBDC336 is likely an allele at the Rsg1 locus and was given the allele designation Rsg1.a3. Two Kompetitive allele‐specific polymerase chain reaction (KASP) markers were developed from SNPs closely linked to Rsg1.a3, and these markers can be used to tag both alleles at the Rsg1 locus in breeding program utilizing marker‐assisted selection.

Development of KASP markers for wheat greenbug resistance gene Gb5

AbstractGreenbug [Schizaphis graminum (Rondani)] is a destructive pest of wheat (Triticum aestivum L.) worldwide. Incorporation of wheat greenbug resistance genes in wheat‐breeding pipelines is critical to improve wheat production. The greenbug resistance gene Gb5, originally identified in Triticum speltoides chromosome 7S and transferred to wheat chromosome 7A, confers resistance to several economically important greenbug biotypes. The T. speltoides chromosome segment carrying Gb5 was previously shortened by means of homoeologous recombination induced by the ph1b gene, which minimized linkage drag and made Gb5 a useful gene for improvement of greenbug resistance in wheat. This study characterized the shortened T. speltoides chromosome segment in PI 603919 and developed kompetitive allele‐specific polymerase chain reaction (KASP) markers to facilitate its introgression in wheat breeding. A set of 21 simple sequence repeat (SSR) markers specific to wheat 7AL chromosome were developed to determine the length and location of the T. speltoides chromosome segment. The T. speltoides segment in PI 603919 was estimated to be about 79.5–87.8 Mb in the Chinese Spring reference sequence IWGSC RefSeq v.1.0. A recombinant inbred line (RIL) population derived from a cross between CI 17884, which carries the original wheat‐T. speltoides 7A‐7S translocation chromosome segment, and Bainong 418 was genotyped using the genotyping‐by‐sequencing (GBS) approach and evaluated for responses to greenbug biotype E. Three GBS‐derived single nucleotide polymorphisms (SNPs) on the shortened translocation segment were converted to KASP markers. These KASP markers were evaluated for efficacy to select Gb5 in wheat breeding.

GREENBUG RESISTANCE OF OAT LANDRACES FROM CENTRAL ASIA&#x0D; К ОБЫКНОВЕННОЙ ЗЛАКОВОЙ ТЛЕ

The greenbug (Sсhizaphis graminum) is a dangerous pest of cereals in Southern Russia. Breeding of resistant varieties is an effective and eco-friendly way to control this insect. Its differential interaction with host plants substantiates the search for new resistance donors. We evaluated 276 accessions of oat landraces from Central Asian countries (Kazakhstan, Uzbekistan, Kyrgyzstan, and Turkmenistan) to the Krasnodar population and respective isolated clones of the aphid. We identified two pest resistant accessions from Kazakhstan (k-6945 and k-8691) and found 133 accessions from Kazakhstan being heterogeneous including 77 forms with high and moderate resistance and 56 – with only moderate resistance. All accessions from Uzbekistan and Turkmenistan were susceptible to Sсh. graminum. Accession k-9993 from Kyrgyzstan was heterogeneous in terms of resistance. A wide variation in the damage degree of the most oat forms was mostly due to the virulence heterogeneity of the aphid population. Damage evaluation of 15 accessions from Kazakhstan by Sсh. graminum clones showed that the alleles of greenbug resistance genes of these forms differ from the previously identified Grb1 and Grb3 genes.

Gb8, a new gene conferring resistance to economically important greenbug biotypes in wheat.

A new greenbug resistance gene Gb8 conferring broad resistance to US greenbug biotypes was identified in hard red winter wheat line PI 595379-1 and was mapped to the terminal region of chromosome 7DL. Greenbug [Schizaphis graminum (Rondani)] is a worldwide insect pest that poses a serious threat to wheat production. New greenbug resistance genes that can be readily used in wheat breeding are urgently needed. The objective of this study was to characterize a greenbug resistance gene in PI 595379-1, a single plant selection from PI 595379. Genetic analysis of response to greenbug biotype E in an F2:3 population derived from a cross between PI 595379-1 and PI 243735 indicated that a single gene, designated Gb8, conditioned resistance. Linkage analysis placed Gb8 in a 2.7-Mb interval in the terminal bin of chromosome 7DL (7DL3-082-1.0), spanning 595.6 to 598.3Mb in the Chinese Spring IWGSC RefSeq version 1.0 reference sequence. Gb8 co-segregated with a newly developed SSR marker Xstars508, positioned at 596.4Mb in the reference sequence. Allelism tests showed that Gb8 was different from three permanently named genes on the same chromosome arm and the estimated genetic distance between Gb8 and Gb3 was 15.35 ± 1.35cM. Gb8 can be directly used in wheat breeding to enhance greenbug resistance.

Development and validation of KASP markers for the greenbug resistance gene Gb7 and the Hessian fly resistance gene H32 in wheat

Greenbug and Hessian fly are important pests that decrease wheat production worldwide. We developed and validated breeder-friendly KASP markers for marker-assisted breeding to increase selection efficiency. Greenbug (Schizaphis graminum Rondani) and Hessian fly [Mayetiola destructor (Say)] are two major destructive insect pests of wheat (Triticum aestivum L.) throughout wheat production regions in the USA and worldwide. Greenbug and Hessian fly infestation can significantly reduce grain yield and quality. Breeding for resistance to these two pests using marker-assisted selection (MAS) is the most economical strategy to minimize losses. In this study, doubled haploid lines from the Synthetic W7984×Opata M85 wheat reference population were used to construct linkage maps for the greenbug resistance gene Gb7 and the Hessian fly resistance gene H32 with genotyping-by-sequencing (GBS) and 90K array-based single nucleotide polymorphism (SNP) marker data. Flanking markers were closely linked to Gb7 and H32 and were located on chromosome 7DL and 3DL, respectively. Gb7-linked markers (synopGBS773 and synopGBS1141) and H32-linked markers (synopGBS901 and IWB65911) were converted into Kompetitive Allele Specific PCR (KASP) assays for MAS in wheat breeding. In addition, comparative mapping identified syntenic regions in Brachypodium distachyon, rice (Oryza sativa), and sorghum (Sorghum bicolor) for Gb7 and H32 that can be used for fine mapping and map-based cloning of the genes. The KASP markers developed in this study are the first set of SNPs tightly linked to Gb7 and H32 and will be very useful for MAS in wheat breeding programs and future genetic studies of greenbug and Hessian fly resistance.

Variation in the salivary proteomes of differentially virulent greenbug (Schizaphis graminum Rondani) biotypes

Greenbug (Schizaphis graminum Rondani) biotypes are classified by their differential virulence to wheat, barley, and sorghum varieties possessing greenbug resistance genes. Virulent greenbug biotypes exert phytotoxic effects upon their hosts during feeding, directly inducing physiological and metabolic alterations and accompanying foliar damage. Comparative analyses of the salivary proteomes of four differentially virulent greenbug biotypes C, E, G, and H showed significant proteomic divergence between biotypes. Thirty-two proteins were identified by LC-MS/MS; the most prevalent of which were three glucose dehydrogenase paralogs (GDH), lipophorin, complementary sex determiner, three proteins of unknown function, carbonic anhydrase, fibroblast growth factor receptor, and abnormal oocyte (ABO). Seven nucleotide-binding proteins were identified, including ABO which is involved in mRNA splicing. Quantitative variation among greenbug biotypes was detected in six proteins; two GDH paralogs, carbonic anhydrase, ABO, and two proteins of unknown function. Our findings reveal that the greenbug salivary proteome differs according to biotype and diverges substantially from those reported for other aphids. The proteomic profiles of greenbug biotypes suggest that interactions between aphid salivary proteins and the plant host result in suppression of plant defenses and cellular transport, and may manipulate transcriptional regulation in the plant host, ultimately allowing the aphid to maintain phloem ingestion. Greenbug (Schizaphis graminum Rondani, GB) is a major phytotoxic aphid pest of wheat, sorghum, and barley. Unlike non-phytotoxic aphids, GB directly damages its host, causing uniformly characteristic symptoms leading to host death. As saliva is the primary interface between the aphid and its plant host, saliva is also the primary aphid biotypic determinant, and differences in biotypic virulence are the result of biotypic variations in salivary content. This study analyzed the exuded saliva of four distinct Greenbug biotypes with a range of virulence to crop lines containing greenbug resistance traits in order to identify differences between salivary proteins of the examined biotypes. Our analyses confirmed that the salivary proteomes of the examined greenbug biotypes differ widely, identified 32 proteins of the greenbug salivary proteome, and found significant proteomic variation between six identified salivary proteins. The proteomic variation identified herein is likely the basis of biotypic virulence, and the proteins identified can serve as the basis for functional studies into both greenbug-induced phytotoxic damage and into the molecular basis of virulence in specific GB biotypes. This article is part of a Special Issue entitled: SI: Proteomics of non-model organisms.

Mapping and candidate gene identification of loci determining tolerance to greenbug (Schizaphis graminum, Rondani) in barley

Greenbug is one of the most aggressive pests of barley and wheat. In Argentina, yield losses of wheat, barley, oat and sorghum crops caused by greenbug are chronic and at times severe. Since Marker Assisted selection for greenbug resistance genes in barley is very limited, the purpose of the current study was to map greenbug resistance genes in doubled haploid (DH) lines and to identify candidate genes. A set of DH lines of the Oregon-Wolfe Barley (OWB) mapping population derived from the cross between OWBDOM and OWBREC and both parental lines were screened for tolerance to greenbug. There was significant variation among the DH lines in foliar area (FA), dry weight (DW) and chlorophyll contents (Ch) between infested and control DH lines. Three main QTLs were identified. These QTLs explained 82 % of the FA, 80 % of DW and 58 % of Ch variability of infested plants. The initial and final FA and DW of controls and final DW of infested plants were associated with the same molecular markers on chromosome 2H (Vrs1, BmAc0144f, GBR259, GBS705). The final FA of infested plants was significantly linked to molecular markers on chromosome 5H (GBRO986, GBR518, GBM1483, GBR1082). The positive alleles were provided by OWBDOM. The content of chlorophyll of infested plants was associated with the marker loci Ris44, GBR1608, GBR1637N and GBS0785 on chromosome 7H, with the positive alleles provided by OWBREC. Both parents contributed to different tolerance traits. The QTLs found in this population are new greenbug resistance loci. A sequence homology search was performed to derive the putative function of the genes linked to the QTLs.

Fine genetic mapping of greenbug aphid-resistance gene Gb3 in Aegilops tauschii

The greenbug, Schizaphis graminum (Rondani), is an important aphid pest of small grain crops especially wheat (Triticum aestivum L., 2n = 6x = 42, genomes AABBDD) in many parts of the world. The greenbug-resistance gene Gb3 originated from Aegilops tauschii Coss. (2n = 2x = 14, genome D(t)D(t)) has shown consistent and durable resistance against prevailing greenbug biotypes in wheat fields. We previously mapped Gb3 in a recombination-rich, telomeric bin of wheat chromosome arm 7DL. In this study, high-resolution genetic mapping was carried out using an F(2:3) segregating population derived from two Ae. tauschii accessions, the resistant PI 268210 (original donor of Gb3 in the hexaploid wheat germplasm line 'Largo') and susceptible AL8/78. Molecular markers were developed by exploring bin-mapped wheat RFLPs, SSRs, ESTs and the Ae. tauschii physical map (BAC contigs). Wheat EST and Ae. tauschii BAC end sequences located in the deletion bin 7DL3-0.82-1.00 were used to design STS (sequence tagged site) or CAPS (Cleaved Amplified Polymorphic Sequence) markers. Forty-five PCR-based markers were developed and mapped to the chromosomal region spanning the Gb3 locus. The greenbug-resistance gene Gb3 now was delimited in an interval of 1.1 cM by two molecular markers (HI067J6-R and HI009B3-R). This localized high-resolution genetic map with markers closely linked to Gb3 lays a solid foundation for map based cloning of Gb3 and marker-assisted selection of this gene in wheat breeding.

Improvement of crop protection against greenbug using the worldwide sorghum germplasm collection and genomics-based approaches

Successful development of new sorghum cultivars and hybrids to ensure sustainable production depends largely on the availability of genetic resources with desirable traits such as pest resistance. Our recent research has focused on improvement of crop protection against greenbugs using the worldwide germplasm collection and genomics-based approaches. First, we conducted the systematic evaluation of a worldwide germplasm collection in order to identify new sources of greenbug resistance. Twenty-one resistant lines were identified, which offered new sources of resistance to sorghum breeding. Molecular markers used to assess the genetic diversity among those resistant lines suggested relatively diverse resistant sources in the sorghum germplasm collection. More recently, a mapping project was executed to associate the resistance genes with sorghum chromosomes. The mapping data indicated one major and a minor quantitative trait loci reside on chromosome 9 and are responsible for resistance to greenbug. In addition, cDNA microarrays were used to monitor greenbug-induced gene expression in sorghum plants. This study has developed a transcriptional profile for sorghum in response to greenbug attack, which provides us with useful molecular information for discovery of greenbug resistance genes and a better understanding of the genetic mechanisms controlling host defences in sorghum.

Molecular mapping of greenbug resistance genesGb2andGb6in T1AL.1RS wheat-rye translocations

With 2 figures and 2 tables Abstract Two greenbug resistance genes Gb2 and Gb6 derived from the same donor rye line ‘Insave’, are present in wheat germplasm lines ‘Amigo’ and ‘GRS1201’, respectively as 1AL.1RS wheat-rye translocations. The allelic relationship between the two genes has not been determined, and no molecular markers for Gb6 are available. In this study, molecular mapping of Gb2 and Gb6 was performed in a mapping population derived from a cross between ‘N96L9970’ (Gb6Gb6) and ‘TAM 107’ (Gb2Gb2). Segregation among F2:3 families of host responses to infestation of greenbug biotypes E and KS-1 revealed that Gb2 and Gb6 were different linked loci in 1RS. Gb2 and Gb6 were 15.8 cM apart with Gb6 being distal to Gb2. Despite the low number of marker polymorphisms between the parental lines, eight markers linked with Gb2 and Gb6 were identified. The closest marker, XIA294, was 11.4 cM proximal to Gb2. Deletion mapping indicated that both Gb2 and Gb6 were physically located in the satellite region of 1RS.

Genetic diversity of sorghum in greenbug resistance

Resistance of the sorghum gene pool to the Krasnodar greenbug population is investigated. A high level of resistance is established in accessions of cultivated sorghum species. Genetic uniformity of resistance donors widely used in Russian and US breeding programs is shown. Fifteen greenbug resistance genes are identified.

Differentiating greenbug resistance genes in barley

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

Inheritance and molecular mapping of new greenbug resistance genes in wheat germplasms derived from Aegilops tauschii

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.

Spatial and Temporal Distribution of Induced Resistance to Greenbug (Homoptera: Aphididae) Herbivory in Preconditioned Resistant and Susceptible near Isogenic Plants of Wheat

Interactions between biotype E greenbugs, Schizaphis graminum (Rodani), and two near isogenic lines of the greenbug resistance gene Gb3 of wheat, Triticum aestivum L., were examined for 62 d after infestation. By comparing aphid performance and host responses on control and greenbug-preconditioned plants, we demonstrated that systemic resistance to greenbug herbivory was inducible in the resistant genotype with varying intensities and effectiveness in different parts of the plants. Preconditioning of susceptible plants resulted in modification of within-plant aphid distribution and reduction of cumulative greenbug densities, but it showed no effect on reducing greenbug feeding damage to host plant. Preconditioning of resistant plants altered greenbug population dynamics by reducing the size and buffering the fluctuation of the aphid population. Preconditioning in the first (oldest) leaf of the resistant plant had no phenotypically detectable effect in the stem and induced susceptibility locally in the first leaf within the first 2 d after infestation. The preconditioning-induced resistance reduced greenbug density, delayed aphid density peaks and extended the life of younger leaves in resistant plants. Expression of induced resistance was spatially and temporally dynamic within the plant, which occurred more rapidly, was longer in duration, and stronger in intensity in younger leaves. Host resistance gene-mediated induced resistance was effective in lowering greenbug performance and reducing damage from greenbug herbivory in host plants. Results from this study supported the optimal defense theory regarding within-plant defense allocation.