Induction of differential host responses by Pseudomonas fluorescens in Ri T-DNA-transformed pea roots after challenge with Fusarium oxysporum f. sp. pisi and Pythium ultimum.
1996
Benhamou N. | Belanger R.R. | Paulitz T.C.
In the current study, the influence exerted by the plant growth-promoting bacterium Pseudomonas fluorescens strain 63-28 in stimulating plant defense reactions was investigated at the ultrastructural level by an in vitro system in which Ri T-DNA pea roots were either infected with the pea root rot fungus Fusarium oxysporum f. sp. pisi or with the soil-borne pathogen Pythium ultimum. Scanning electron microscopy (SEM) observations showed that bacteria abundantly colonized the root surface and established close contact with the outermost host cell layers through a thin mucilage. When bacterized pea roots were challenged with either P. ultimum or F. oxysporum f. sp. pisi, strong differences in the extent of fungal damage were observed. Hyphae of P. ultimum were markedly collapsed, as illustrated by their wrinkled appearance, whereas Fusarium cells were apparently undamaged. In line with these SEM observations, cytological investigations of the root surface confirmed that the interaction between Pseudomonas fluorescens and F. oxysporum f. sp. pisi did not result in hyphal disturbances similar to those seen with Pythium hyphae. Restriction of Fusarium growth and development to the epidermis and outer root cortex and a marked decrease in pathogen viability were features observed only in bacterized pea roots. In pretreated roots, striking modifications of the epidermal and cortical cell walls as well as deposition of newly formed barriers were seen in response to Fusarium infection. Deposition onto the inner surface of the cell walls of callose-enriched wall appositions was associated with a lack of fungal ingress toward the vascular stele. Fungal cells in the vicinity of wall appositions frequently were surrounded by an aggregated material containing phenolic compounds, as shown by laccase-gold labeling. The labeling pattern obtained with this probe showed that phenolic compounds were widely distributed in Fusarium-challenged, bacterized roots.
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