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Ultrastructural and Cytochemical Aspects of Silicon-Mediated Rice Blast Resistance

First and fourth authors: University of Florida-IFAS, Department of Plant Pathology, Gainesville 32611; second author: Département de Phytologie, Recherche en Sciences de la Vie et de la Santé, Pavillon C. E. Marchand, Université Laval, Sainte-Foy, Québec, Canada, G1K 7P4; third author: University of Florida-IFAS, Department of Plant Pathology, Everglades Research & Education Center, 3200 E. Palm Beach Rd., Belle Glade 33430; and fifth author: Centre de Recherche en Horticulture, Département de Phytologie, Université Laval, Sainte-Foy, Québec, Canada, G1K 7P4

Although exogenous application of silicon (Si) confers efficient control of rice blast, the probable hypothesis underlying this phenomenon has been confined to that of a mechanical barrier resulting from Si polymerization in planta. However, in this study, we provide the first cytological evidence that Si-mediated resistance to Magnaporthe grisea in rice correlates with specific leaf cell reaction that interfered with the development of the fungus. Accumulation of an amorphous material that stained densely with toluidine blue and reacted positively to osmium tetroxide was a typical feature of cell reaction to infection by M. grisea in samples from Si+ plants. As a result, the extent of fungal colonization was markedly reduced in samples from Si+ plants. In samples from Si- plants, M. grisea grew actively and colonized all leaf tissues. Cytochemi-cal labeling of chitin revealed no difference in the pattern of chitin localization over fungal cell walls of either Si+ or Si- plants at 96 h after inoculation, indicating limited production of chitinases by the rice plant as a mechanism of defense response. On the other hand, the occurrence of empty fungal hyphae, surrounded or trapped in amorphous material, in samples from Si+ plants suggests that phenolic-like compounds or phytoalexins played a primary role in rice defense response against infection by M. grisea. This finding brings new insights into the complex role played by Si in the nature of rice blast resistance.

Additional keywords: gold cytochemistry, host-pathogen interaction, Oryza sativa .