John Innes Centre, Norwich Research Park, Norwich NR4 7UH, UK

Background and objectives
The need to combat pathogen attacks on crop plants and reduce the large yield losses means plant breeders are constantly looking for new and durable sources of resistance. An area of study that may provide novel forms of resistance is that of non-host resistance. The genetics and mechanisms of this form of resistance are poorly understood. The wheat cultivar Chinese 166 has been found to be susceptible, at the seedling stage, to some isolates of the barley-attacking form of yellow rust (Puccinia striiformis f.sp. hordei, Psh), whereas the wheat cultivar Lemhi is resistant to this isolate. Johnson and Lovell [1] identified two dominant genes in Lemhi, by crossing Lemhi with Chinese 166, which conferred non-host resistance to Psh.

The current project involves the study of the genetics and mechanisms of resistance in Lemhi to Psh. Amplified fragment length polymorphism (AFLPs) are being used to map the two resistance genes to Psh, BYr1 and BYr2, and a known wheat yellow rust host resistance gene in Lemhi, Yr21, to establish whether BYr1 and BYr2 are located at distinct loci from Yr21. A histological examination of yellow rust development for virulent and avirulent interactions will allow fungal development to be related to defence responses. This will be carried out for both host and non-host resistance. Expression of defence-related genes in wheat leaves inoculated with the wheat and barley-attacking forms of yellow rust are being studied and will allow a better understanding of the molecular and biochemical changes that occur in cereal host and non-host plant/pathogen interactions.

Materials and methods
AFLPs were used to examine F3 families from a cross between the cultivars Lemhi and Chinese 166. RNA extractions and Northern analysis were according to [2]. Initial probes used for gene expression were a chitinase and a PR-R protein from barley and WIR3 (peroxidase) and WIR5 (glutathione-s-transferase) from wheat. The histological examinations of Puccinia striiformis species involved clearing inoculated leaf segments with a solution containing 800 g chloral hydrate, 30O mI 95% ethanol, 125 ml 90% lactic acid and chloroform to 1 litre and staining with 0.2% trypan blue.

Results and conclusions
Results so far confirm the two dominant genes identified in Lemhi, BYr1 and BYr2. A range of clearing and staining techniques, for histological observations, were experimented with. The best results were obtained using the above-mentioned solution for clearing and trypan blue for staining. This allowed structures such as the haustorial mother cell and even the infection peg to be observed. Calcofluor also proved useful for rapid staining when only spores and germ tubes needed to be clearly identified. AFLPs between the parents Lemhi and Chinese 166: approximately 16 polymorphic bands were found for each primer pair (7% polymorphism) and presently 3875 polymorphic bands have been identified between Lemhi and Chinese 166 with the initial 255 primer pairs tested. F3 families, segregating for resistance to the barley-attacking Yr isolate, BYr80-1, have been identified and are currently being screened with selected AFLP primer pairs showing polymorphism between the parents.

1. Johnson R, Lovell NK, 1994. Mildew Bulletin 22, 3240.
2. Boyd LA, Smith PH, Green RM, Brown JKM, 1994. Molecular Plant-Microbe Interactions 7, 401-410.