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 followed on the susceptible barley variety Pallas, and the near-isogenic resistant varieties, P-01 (Mla1), P-02 (Mla3), P-03 (Mla6) and P-04B (Mla7). The development of CC1 was examined in relation to the host's cellular 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 correlated with the time of CC1 developmental arrest and with the timing of the hypersensitive response, indicating a positive role for hypersensitive cell death in Mla resistance. On P-01 and P-03, a rapid hypersensitive response in the epidermis was associated with a higher percentage of germlings arrested in their development at the haustorial stage. On P-02 and P-04B, a slower hypersensitive response was associated with haustoria, and more elongating secondary hyphae were formed. There was a later, and more extensive combined epidermal and mesophyll hypersensitive response associated with elongating secondary hyphae on these two 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 lower in Pallas than in the four resistant varieties. Levels of defence gene transcripts were often greater in tissues where a larger number of cells were undergoing hypersensitive cell death, suggesting an association between defence gene expression and the hypersensitive response.