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Resistance mechanisms to Phytophthora nicotianae var. parasitica in mycorrhizal tomato: pathogen development within root tissues and host cell responses

talk session 6
CORDIER CHRISTELLE, VIVIENNE GIANINAZZI-PEARSON & SILVIO GIANINAZZI
Laboratoire de Phytoparasitologie, INRA/CNRS, SGAP, INRA, BV 1540, 21034 Dijon cédex, France
ICOM1 Abstract
Arbuscular mycorrhiza are known to decrease root damage caused by fungal pathogens, but the mechanisms involved are not well understood. Our research is aimed at analysing interactions between Glomus mosseae and the pathogen Phytophthora nicotianae var. parasitica in roots of tomato (Lycopersicon esculentum) at tissue and cellular levels, in order to better understand the bioprotective effects of arbuscular mycorrhiza. The use of antibodies, in the absence of an appropriate stain, to estimate infection intensity of the pathogen in whole root fragments showed that this was considerably lower in mycorrhizal roots as compared to non- mycorrhizal controls. Infection patterns of P. n. v. parasitica in mycorrhizal and non-mycorrhizal root tissues of tomato were analysed by immunocytochemistry to discriminate between the two fungi. P. n. v. parasitica colonizes tomato root tissues inter- or intracellularly, and in non- mycorrhizal roots the presence of the pathogen in the root cortex induces necrosis and phenolics accumulation in cells containing or not the fungus. However, this plant reaction does not prevent pathogen spread through the root tissue to the central cylinder. In pathogen-infected mycorrhizal root systems, hyphae of G. mosseae and P. n. v. parasitica are, in most cases, found in different root regions but they can also develop in the same root tissues. Quantification of P. n. v. parasitica hyphae in root tissues showed that pathogen spread was greatly reduced in both mycorrhizal and non-mycorrhizal portions of mycorrhizal root systems. P. n. v. parasitica never invaded arbuscule-containing cells in mycorrhizal tissues. Moreover, arbuscule-containing cells surrounded by intercellular P. n. v. parasitica hyphae did not necrose and only a weak autofluorescence was associated with the host cells. These observations suggest that bioprotection by G. mosseae against P. n. v. parasitica may be due both to direct interactions between the fungi in the same root tissue as well as long-distance effects of mycorrhizal development on pathogen infection.