Induction of Differential Host Responses byPseudomonas fluorescensin Ri T DNA Transformed Pea Roots After Challenge withFusarium oxysporumf. sp.pisiandPythium ultimum

Abstract

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. In addition to their infiltration at strategic sites of potential penetration, phenolic compounds were detected in the host cell walls. Use of the wheat germ agglutinin/ ovomucoid-gold complex provided evidence that the wall-bound chitin component in Fusarium hyphae colonizing bacterized roots was disrupted in places. However, chitin molecules still occurred over cell walls at a time when hyphae had undergone substantial degradation. The results reported in this paper show that pea root bacterization with Pseudomonas fluorescens strain 63-28 induces a set of plant defense reactions that culminates in the elaboration of physical barriers and the creation of a fungitoxic environment that adversely affects Fusarium growth and development.