Pre-inoculation of Ri T-DNA-transformed pea roots withPseudomonas fluorescensinhibits colonization byPythium ultimumTrow: an ultrastructural and cytochemical study

摘要

The influence exerted byPseudomonas fluorescens, strain 63-28R, in stimulating plant defense reactions was investigated using an in-vitro system in which Ri T-DNA-transformed pea (Pisum sativumL.) roots were subsequently infected withPythium ultimum. Cytological investigations of samples fromP. fluorescens-inoculated roots revealed that the bacteria multiplied abundantly at the root surface and colonized a small number of epidermal and cortical cells. Penetration of the epidermis occurred through the openings made by the disruption of the fibrillar network at the junction of adjacent epidermal cell walls. Direct cell wall penetration was never observed and bacterial ingress into the root tissues proceeded via an intercellular route. Striking differences in the extent of fungal colonization were observed between bacterized and non-bacterized pea roots following inoculation withP. ultimum. In non-bacterized roots, the pathogen multiplied abundantly through most of the tissues while in bacterized roots, pathogen growth was restricted to the epidermis and the outer cortex. At the root surface, the bacteria interacted with the pathogen, in a way similar to that observed in dual culture tests. MostPythiumcells were severely damaged but fungal penetration by the bacteria was never observed. Droplets of the amorphous material formed upon interaction between the bacteria and the host root were frequently found at the fungal cell surface. Incubation of sections with a β-1,4-exoglucanase-gold complex revealed that the cell wall of markedly alteredPythiumhyphae was structurally preserved. Successful penetration of the root epidermis was achieved by the few hyphae ofP. ultimumthat could escape the first defensive line in the rhizosphere. Most hyphae of the pathogen that penetrated the epidermis exhibited considerable changes. The unusual occurrence of polymorphic wall appositions along the host epidermal cells was an indication that the host plant was signalled to defend itself through the elaboration of physical barriers