A host-pathogen study of stripe rust resistance in Triticum aestivum

English: The successful introgression of the stripe rust seedling resistance gene YrSp into Kariega/ Avs/YrSp is towards the continued pursuit of durable resistance in this cultivar. Following the development of Kariega/ Avs/YrSp hybrid, an F2 and F3 Kariega/YrSp population was obtained and screened for virulence to pathotype 6E22A- of P. striiformis f. sp. tritici. The disease screening of the F2 population deviated from the expected single dominant gene ratio and did not fit a 3:1 gene segregation ratio determined by Chi-square analysis. Disease screening of the resistant F3 families exhibited segregation while susceptible F3 families was not considered to segregate with the observation of singular resistant plants in two susceptible F3 families. Histological studies firstly confirmed the infection pathway of P. striiformis f. sp. tritici as is described for other cereal rusts with the absence of the formation of appressoria and the observation of pseudo-SSVIs the most marked differences. The pseudo-SSVIs are thought to be due to the unsuccessful stomatal penetration by germ tubes as successful penetration results in a single SSVI that is generally larger in size than the vesicles observed but this requires further study. Secondly, quantitative analysis indicated that urediospores of P. striiformis f. sp. tritici have good germination efficiency but germ tubes were rarely observed to penetrate stomata. After penetration, the infection efficiency of the pathogen is high in susceptible cultivars due to systemic colonisation. The resistance conferred by the YrSp gene is considered posthaustorial as no significant difference in infection efficiency could be determined before haustorium formation. An increase in the number of internal infection structures after 7 dpi in the susceptible cultivar was observed while the resistant line reflected a decrease. Thirdly, fluorochrome assessment indicated uvitex 2B and the orange G probe hold the best potential of the fluorochromes tested for use with the confocal laser scanning microscopy (CLSM). Furthermore, the CLSM holds great potential to elucidate host-pathogen interactions if more research into fluorochromes selection and development occurs. Using molecular markers obtained from AFLPs and SSRs the YrSp gene was mapped to the short arm of chromosome 2B. The YrSp gene was located on the short arm of chromosome 2B in the Kariega/ Avs/YrSp hybrid. Two AFLP markers were found to flank the gene namely L15 and L68 at 19.5 and 21.4 cM respectively. In addition to this, 2 QTL markers, QYrSgi-2B.1and QYrSgi-7D for adult plant resistance was present in the Kariega /YrSp population. The previous report of an introgression on chromosome 6A into Avocet/YrSp was not found to contribute to the introgressed trait. The integration of various disciplines provided insight into the hostpathogen interaction of P. striiformis f. sp. tritici on wheat at different levels towards a holistic understanding of such interactions and the mechanisms involved in conditioning resistance.