Mechanisms underlying induced resistance in barley against powdery mildew

Chitosan (CHT), a deacetilated chitin derivative, and benzo iadiazole-y-carbothioic add S-methyl ester (BTH), a non toxic synthetic functional analogue of salicylic acid, were applied as foliar spray to barley plants (Hordeum vulgare L.), to compare their effectiveness in inducing resistance against Blumeria graminis f.sp. hordei and to investigate the mechanisms underlying the establishment of acquired resistance. In CHT-treated plants, a significant reduction of the infected areas (-49%) was observed, compared to the untreated controls, while in plants sprayed with BTH the reduction was even greater (-76%). In BTH- and CHT-treated plants conidia germination was not inhibited, whereas 4 and 8 days post inoculation (dpi), respectively, the haustorial growth was slowed down by the apposition of phenolic compounds in the epidermal cells neighboring the haustorium, as assessed by cytochemical assays. On the contrary, callose deposition in papillae did not impair fungal penetration in all treated plants, as shown also when its synthesis was impaired by the callose synthesis inhibitor 2-deoxy-D-glucose (DGG). Hydrogen peroxide accumulated in the epidermal cell walls adjacent to mesophyll cells at 0.25 dpi, with a maximum at 1-2 dpi, but no cell death was detected in the same treated tissues. These results, besides confirming the effectiveness of both CHT and BTH as resistance inducers in barley, suggest that the resulting quantitative resistance is a consequence of H sub(2)O sub(2) accumulation and phenolic compound deposition, rather than callose apposition to papillae.