THE EFFECTS OF ALLELIC VARIATION AT THE MLA RESISTANCE LOCUS IN BARLEY ON THE EARLY DEVELOPMENT OF ERYSIPHE-GRAMINIS F SP HORDEI AND HOST RESPONSES

The development of Erysiphe graminis f.sp. hordei isolate CC1 was foll owed on the susceptible barley variety Pallas, and the near-isogenic r esistant varieties, P-01 (Mla1), P-02 (Mla3), P-03 (Mla6) and P-04B (M la7). The development of CC1 was examined in relation to the host's ce llular resistance responses, papilla formation and hypersensitive cell death, and to the transcription of five barley defence-related genes. The infection type exhibited by each variety was positively correlate d with the time of CC1 developmental arrest and with the timing of the hypersensitive response, indicating a positive role for hypersensitiv e cell death in Mla resistance. On P-01 and P-03, a rapid hypersensiti ve response in the epidermis was associated with a higher percentage o f germlings arrested in their development at the haustorial stage. On P-02 and P-04B, a slower hypersensitive response was associated with h austoria, and more elongating secondary hyphae were formed. There was a later, and more extensive combined epidermal and mesophyll hypersens itive response associated with elongating secondary hyphae on these tw o varieties. A significant papilla response by P-03 corresponded to a larger number of germlings being arrested at the secondary lobed stage on this variety. In general, defence gene transcript levels were lowe r in Pallas than in the four resistant varieties. Levels of defence ge ne transcripts were often greater in tissues where a larger number of cells were undergoing hypersensitive cell death, suggesting an associa tion between defence gene expression and the hypersensitive response.