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Scanning electron microscopy investigations on the relationships between bacteria and plant tissues. II. Investigations on the initial processes of Pseudomonas solanacearum pathogenesis.

Pathogenicity towards particular hosts together with different physiological characteristics permit to individuate in Pseudomonas solanacearum (Smith) Smith, four biotypes (H A Y WARD, 1964) or three races (BUDDENHAGEN and KELMAN, 1964) of the species. Numerous pathogenic processes are involved in the disease (BUDDENHAGEN and KELMAN, 1964) and micromorphological studies help clarify their dynamics. Investigations of this kind were carried out on infected tomato (WALLIS, 1977) and banana (BUDDENHAGEN and TAKANA, 1969) plants, using both optical (OM) and electron microscopy. Sample preparation prior to observation involves treatment and handling which modify the true situation (FOSTER and BOWEN, 1982). Scanning electron microscopy (SEM) together with appropriate techniques can limit these alterations (PETROLINI et al., 1986) and in fact have been used for several years in our research on prokaryotes (LOCCI et al., 1983). The present note reports the results of an investigation on the phenomena which occur between bacteria and plant tissues during the infection process. For this purpose P. solanacearum was chosen as an model for a detailed research. The dynamics of the infection process in xylem vessels of tomato seedlings and tobacco plant leaves were studied. Attention was focused on adherence of bacteria to plant tissue surfaces, which is a fundamental stage in the establishment of the pathological process. Understanding the mechanisms involved is thus essential and in fact many works have tried to elucidate them (GOODMAN, 1982; KEEN and HOLLIDAY, 1982). The aim of the present investigation is the study of the processes which occur at this particular moment of bacterial pathogenesis, but attempting a different approach to the problem. The early phases of the process were examined using SEM together with appropriate techniques for sample preparation.