Comparative analysis of early H2O2 accumulation in compatible and incompatible wheat–powdery mildew interactions

Comparative analysis was carried out for hydrogen peroxide (H₂O₂) accumulation in response to powdery mildew (Blumeria graminisErysiphe graminis] f.sp. tritici) primary germ tube (PGT) and appressorial germ tube (AGT) contact, and in papilla (cell wall apposition) and hypersensitive responses (HR) in wheat (Triticum aestivum). Using primary leaves of three susceptible wheat lines (Bainong 3217; Beijing 837; Jingshuang 16) and five resistant lines Mardler (Pm2 + Pm6); Ulka/8 × Cc (Pm2); Mardler/7 × Bainong 3217 (Pm2); Pm16; Pm16/7 × Beijing 837 (Pm16)], early H₂O₂ accumulation between 10 and 48 h after inoculation was studied. Strong H₂O₂ accumulation was found in effective papillae and associated cytosolic vesicles in both susceptible and resistant wheat lines, suggesting the important role of H₂O₂ in effective papillae formed as a general plant defence against powdery mildew. High frequency of effective papilla formation was observed in all five resistant lines. Among resistant lines, hypersensitive cell death was detected earlier in Mardler and Ulka/8 × Cc than in Pm16 and Pm16/7 × Beijing 837. In all cases this was associated with H₂O₂ accumulation in attacked epidermal cells. Interestingly, penetration resistance but not HR appeared to be mediated by the Pm2 gene in the Mardler/7 × Bainong 3217 line, suggesting that Pm2 may govern an HR-independent defence pathway in this genetic background. As effective papillae and HR did not occur in the same cells, papilla deposition may be independent of the HR response, despite the fact that both defence mechanisms were associated with high H₂O₂ accumulation. In wheat powdery mildew-incompatible interactions, HR acts as a second line of defence to contain infection when the papilla defence fails.