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    Cellular reactions in Arabidopsis following challenge by strains of Pseudomonas syringae
    (Academic Press Ltd- Elsevier Science Ltd, 2005) Soylu, S; Brown, I; Mansfield, JW
    Reactions to strains of the bean halo-blight pathogen Pseudomonas syringae pv. phaseolicola (Pph) strain 1448A and the Arabidopsis and tomato pathogen P.s. pv. tomato (Pst) strain DC3000 were examined by transmission electron microscopy. Wild-type and hrpL mutant strains of Pph failed to multiply in the accession Columbia (Col)-5, but did not cause a hypersensitive reaction (HR). Symptomless non-host resistance to 1448A and the hrpL mutant was associated with the progressive alteration of the plant cell wall adjacent to bacteria, following the accumulation of membrane bound vesicles within the cytoplasm at reaction sites. Large papillae containing callose accumulated within challenged plant cells. Papillae also formed in the pmr4-1 mutant of Col-0 which lacks an inducible callose synthase but immunocytochemical labelling demonstrated that they contained very little beta-1, 3 glucan. Some papillae formed in Col-5 in response to the virulent pathogen DC3000, but they dispersed during cell collapse and lesion formation. Transconjugants of Pph expressing the avirulence genes avrPpiA and avrPphB matching the RPM1 and RPS5 resistance genes, caused rapid and slow HR development, respectively. Although corpse morphology was observed our observations suggest that in Arabidopsis, plant cell death during the HR is programmed but represents a variant of necrosis rather than apoptosis. Cerium chloride staining revealed the accumulation of H2O2 at reaction sites. The strongest H2O2 response was found during the HR activated by avrPpiA but localised generation of peroxide was also found at sites of papilla deposition next to 1448A or the hrpL mutant. Accumulation of H2O2 during the HR, but not during wall alterations, was strongly suppressed by inhibition of NADPH oxidase. The differential effect of the inhibitor suggests an alternative source of H2O2 to modify the plant wall. Extension of peroxide-driven cross-linking reactions to bacterial cell walls may contribute to the restriction of bacterial multiplication. The lowest level of H2O2 occurred during the compatible reaction to DC3000. Characterisation of the cellular co-ordination of basal (non-host) resistance has revealed several potential targets for bacterial effector proteins. (c) 2005 Elsevier Ltd. All rights reserved.
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    Ultrastructural characterisation of interactions between Arabidopsis thaliana and Albugo candida
    (Academic Press Ltd Elsevier Science Ltd, 2003) Soylu, S; Keshavarzi, M; Brown, I; Mansfield, JW
    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.
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    Ultrastructural characterisation of pathogen development and host responses during compatible and incompatible interactions between Arabidopsis thaliana and Peronospora parasitica
    (Academic Press Ltd Elsevier Science Ltd, 2004) Soylu, EM; Soylu, S; Mansfield, JW
    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.