Cytological aspects of the infection process of Pyricularia oryzae on leaves of wheat plants supplied with silicon

Blast, caused by Pyricularia oryzae, has become one of the most important diseases on wheat in Brazil. Knowing the beneficial effects of silicon (Si) in reducing rice blast severity, this study aimed to examine the cytological events occurring during the infectious process of P. oryzae on the leaves of wheat plants non-supplied (-Si) or supplied (+Si) with Si in an experiment arranged in a completely randomized design. The high foliar Si concentration for the +Si plants contributed to decrease the blast symptoms. Hyphae of P. oryzae grew successfully and formed an extensively branched mycelium in the first-invaded epidermal cell and invaded several neighboring cells on the -Si plants, while on the +Si plants fungal hyphae was restricted to the first-invaded epidermal cell. The number of brown adaxial epidermal cells and their intensity of browning were significantly lower for the +Si plants in comparison to the -Si plants. The frequency of appressorial sites showing type B reaction (infection hyphae within the epidermal cell and absence of cytoplasm granulation) was lower for the +Si plants than for the -Si plants from 72 to 96 hours after inoculation because the frequency of appressorial sites showing type A reaction (unsuccessful penetration) was kept higher in comparison to the -Si plants. This study provides novel cytological evidence of the negative effect of Si on the infection process of P. oryzae on leaves of wheat plants contributing, therefore, to reduce the blast symptoms.