PRE-INOCULATION OF RI T-DNA-TRANSFORMED PEA ROOTS WITH PSEUDOMONAS-FLUORESCENS INHIBITS COLONIZATION BY PYTHIUM-ULTIMUM TROW - AN ULTRASTRUCTURAL AND CYTOCHEMICAL STUDY

The influence exerted by Pseudomonas fluorescens, strain 63-28R, in st imulating plant defense reactions was investigated using an in-vitro s ystem in which Ri T-DNA-transformed pea (Pisum sativum L.) roots were subsequently infected with Pythium ultimum. Cytological investigations of samples from P. fluorescens-inoculated roots revealed that the bac teria multiplied abundantly at the root surface and colonized a small number of epidermal and cortical cells. Penetration of the epidermis o ccurred through the openings made by the disruption of the fibrillar n etwork at the junction of adjacent epidermal cell walls. Direct cell w all 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 an d non-bacterized pea roots following inoculation with P. ultimum. In n on-bacterized roots, the pathogen multiplied abundantly through most o f the tissues while in bacterized roots, pathogen growth was restricte d to the epidermis and the outer cortex. At the root surface, the bact eria interacted with the pathogen, in a way similar to that observed i n dual culture tests. Most Pythium cells were severely damaged but fun gal penetration by the bacteria was never observed. Droplets of the am orphous material formed upon interaction between the bacteria and the host root were frequently found at the fungal cell surface. Incubation of sections with a beta-1,4-exoglucanase-gold complex revealed that t he cell wall of markedly altered Pythium hyphae was structurally prese rved. Successful penetration of the root epidermis was achieved by the few hyphae of P. ultimum that could escape the first defensive line i n the rhizosphere. Most hyphae of the pathogen that penetrated the epi dermis exhibited considerable changes. The unusual occurrence of polym orphic wall appositions along the host epidermal cells was an indicati on that the host plant was signalled to defend itself through the elab oration of physical barriers.