INFLUENCE OF SPATIAL AND TEMPORAL VARIATIONS IN THE HYPERSENSITIVE RESPONSE ON THE ACCUMULATION OF DEFENSERELATED TRANSCRIPTS IN POWDERY MILDEWINFECTED BARLEY
C. Rayapuram, V. Hemzalová, M.F. Lyngkjær
Effective race-specific resistance genes (R-genes) against Blumeria graminis f. sp. hordei (Bgh) in barley determine spatial and temporal variation of the hypersensitive response (HR). This study investigated if different fast and slow acting HR, mediated by Mla R-genes in barley will influence the expression of nine commonly induced defense-response genes at time points that coincide with Bgh penetration and hyphal formation. Mla1 resistance was characterized by a fast and significant increase in the death of single Bgh-attacked epidermal cells in response to Bgh isolate C15, whereas Mla3 resistance was characterized by a slow increase in mesophyll multiple cell death in response to Bgh isolate A6. The reciprocal interactions result in compatible interactions. Using such experimental set-up, we partitioned the source of variation in gene expression and found that three genes were affected in a Bgh isolate-dependent manner, and five genes were affected in a host-dependent manner, regardless of the compatibility of the interaction. The results from this study show that the differential expression of many Bgh-responsive defense genes in barley does not necessarily correlate with the spatial and temporal variations in HR, but can be significantly influenced by: (i) the low genetic variability in the near isogenic lines or (ii) differences in the aggressiveness of pathogenic isolates. However, among the nine genes, expression of BAX inhibitor-1 (BI-1), correlated strongly with the timing of cellular events leading to HR in an interaction- specific manner, suggesting that BI-1 could be used as a HR marker in barley.