Ultrastructural characterisation of interactions between Arabidopsis thaliana and Albugo candida

Cellular reactions of Arabidopsis accessions to Albugo candida isolated from Arabidopsis thaliana or Capsella bursa-pastoris (Shepherd's purse) are described. On susceptible and resistant accessions, successful penetration was observed through stomatal pores and into mesophyll or epidermal cells. Compatible interactions between susceptible accessions, Weiningen (Wei-1), Wassilewskija (Ws-0) and Ws-eds1, and virulent isolates Acem1 (from Arabidopsis) and Ac3 (from Shepherd's purse) were characterised by the rapid spread of intercellular hyphae, a high frequency of haustorium formation in mesophyll cells and the production of sporangia in the absence of host cell necrosis. The symptoms caused by the isolate Ac3 on accession Keswick (Ksk)-2 indicated only partial compatibility because, although sporulation occurred, pathogen colonisation was significantly restricted. The major host cell response observed in this intermediate interaction was the striking deposition of ensheathing material containing callose around haustoria. Location of callose was confirmed using immunocytochemistry. The interactions Acem1/Ksk-1 or -2 and Ac3/Ksk-1 were highly incompatible with no sporulation observed. Phenotypes were differentiated based on the occurrence of necrotic flecks (FN) as observed with Acem1 in Ksk-2 and Ac3 in Ksk-1, and flecking and yellowing (FYN) as found in the Acem1/Ksk-1 interaction. The severity of the flecking reaction depended on the number of plant cells affected. The FN phenotype was characterised by the restriction of fungal growth after penetration of epidermal cells. In the FYN phenotype, pathogen growth was also restricted close to sites of penetration but extended into mesophyll tissue. The main features of incompatibility, the hypersensitive reaction and deposition of callose at infection sites, were observed soon after the formation of haustoria. Ultrastructural studies with cotyledons undergoing FN or FYN phenotypes revealed cells undergoing the HR prior to the death of Albugo haustoria and intercellular hyphae, suggesting that plant cell death was the cause or closely associated with the cause of restricted pathogen development in the highly resistant interactions. (C) 2004 Elsevier Ltd. All rights reserved.