| Russian wheat aphids (RWA), Diuraphis noxia (Kurdjumov), are phloem-feeder insect pests that impart significant damage and yield losses upon infestation. Continual emergence of new biotypes challenges the production of RWA resistant cultivars, since the race-specific, gene-for-gene resistance conferred by R-genes may be rendered ineffective with biotypic variation. Therefore, identification of durable resistance effective against multiple biotypes would be a significant advantage.;The first approach examined in this research was a reverse genetics investigation into susceptible RWA-wheat interactions. The goal was to test whether silencing a candidate gene predicted to be involved in compatible interactions between RWA and wheat would confer resistance to a susceptible wheat genotype. Several genes were identified as differentially expressed between the susceptible line, 'Gamtoos-S' (GS), and the near-isogenic resistant line, 'Gamtoos-R' (GR; carrying Dn7), in a previous transcript profiling study (Botha et al., 2010). The goals of the current research were to identify a candidate gene up-regulated in compatible RWA-wheat interactions (GS compared to GR) and determine whether the gene is involved in these compatible interactions by assessing the effects of gene silencing on host symptom development and aphid reproduction. Barley stripe mosaic virus (BSMV)-mediated virus-induced gene silencing (VIGS) was employed to test whether (1,3;1,4)-beta-glucanase is involved in the susceptible reaction of GS during RWA biotype 2 infestation. (1,3;1,4)-beta-glucanase transcript abundance in the silenced treatment was reduced to levels similar to GR (P=0.600). Aphids on the silenced treatment reproduced less per day (P<0.0001) and had longer pre-nymphipositional periods than those on GS (P=0.003). Compared to GS, the silenced treatment exhibited less chlorosis ( P<0.0001), greater dry weight (P=0.044) and had lower aphid to dry weight ratios (P=0.039). However, aphid-induced leaf rolling appeared to be unaffected by reduction of (1,3;1,4)-beta-glucanase expression (P=1.000), suggesting separate mechanisms for leaf rolling and chlorosis. Aphid reproduction and host symptom development exhibitted linear relationships with (1,3;1,4)-beta-glucanase transcript levels. VIGS construct sequence analyses indicated the possibility of non-target silencing of (1,3)-beta-glucanase, but how transcript abundance of (1,3)-beta-glucanase was altered by the VIGS construct designed for this study is yet to be determined. (1,3;1,4)-beta-glucanase may be a susceptibility factor that could be exploited as a potential avenue of aphid resistance since suppression in the susceptible GS background to GR levels was related to a more RWA-resistant phenotype. Subsequent investigation into the possible co-silencing of (1,3)-beta-glucanase and a more detailed examination of the role of (1,3;1,4)-beta-glucanase in RWA susceptibility will provide insight into how (1,3;1,4)-beta-glucanase suppression might be used in the implementation of novel RWA resistance in wheat breeding programs.;A forward genetic approach involved examination of potentially novel resistance found in an Iranian wheat landrace accession that is resistant to RWA biotype 1 and the most virulent RWA biotype to date, RWA biotype 2. The objectives of this study were to determine the inheritance of resistance, identify closely linked markers, and map the chromosomal location of resistance found in Iranian landrace accession PI 626580. Bulked segregant analysis with a mapping population of 154-F2 individuals, developed from a single plant selection made from PI 626580 and 'Yuma' (a susceptible wheat cultivar), was employed. Based on chi-square goodness-of-fit tests, RWA resistance in PI 626580 appears to be conferred by a single dominant gene, provisionally designated as Dn10. Linkage mapping analysis identified three SSR markers, Xbarc214, Xgwm473 and Xgwm437, proximally linked to Dn10 near the centromere on the short arm of chromosome 7D, at distances of 12.9 cM, 16.0 cM, and 19.2 cM, respectively. The marker order, with respect to resistance in PI 626580, was verified by a quantitative single factor analysis of variance which revealed significant marker associations with chlorosis and leaf rolling (P<0.0001 for all three markers). Dn10, a new resistance gene found in PI 626580 could be used alone or by pyramiding with other Dn-genes to develop cultivars with improved RWA resistance. (Abstract shortened by UMI.)... |
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