Ultrastructural characterisation of pathogen development and host responses during compatible and incompatible interactions between Arabidopsis thaliana and Peronospora parasitica

Compatible and incompatible interactions between the Peronospora parasitica isolate Emoy2 and Arabidopsis thaliana accessions were investigated using transmission electron microscopy. The compatible interaction in the susceptible accession Oystese (Oy-0) was characterized by the rapid spread of intercellular hyphae, a high frequency of haustorium formation within host mesophyll cells and the production of conidia in the absence of host cell necrosis. Resistant Arabidopsis accessions Columbia (Col-0), Landsberg erecta (Ler-0) and Wassilewskija (Ws-0) recognize Emoy2 in a gene-for-gene dependent manner. The RPP4 resistance gene in Col-0, RPP1A,B in Ws-0, and RPP5 and RPP8 in Ler-0 confer resistance to Emoy2. The responses observed in Col-0 indicated only partial incompatibility as the plant reaction was characterized by the occurrence of delayed, light sporulation and formation of necrotic flecks. The deposition of ensheathing material around haustoria followed in some cases by host cell death was identified as the main mechanisms of resistance in this accession. Cotyledons of Ler-0 and Ws-0 developed necrotic fleck or pitting symptoms, respectively, but no sporulation. The main characteristics of the resistant responses were the rapid occurrence of the hypersensitive reaction (HR) and deposition of callose-enriched wall appositions (papillae) in penetrated and adjacent cells. Necrosis extended beyond the zone of restricted colonisation in Ws-O. Location of callose in ensheathing material around haustoria and in papillae was confirmed by immunocytochemistry. The macroscopic phenotypes observed depended on the extent of hyphal development and the number of plant cells affected. (c) 2005 Elsevier Ltd. All rights reserved.