The objectives of this thesis were to establish the possible role of Bacillus subtilis and its metabolites as an induced resistance agent against Aphis fabae and R. padi. Furthermore, it was our goal to unveil some mechanisms which sustain these induced resistance effects in aphid host plants. The general conclusions derived from this work can be summarized as follows:

Induced resistance to both A. fabae and R. padi was possible when using supernatants (FZB24, FZB37 and FZB38) of B. subtilis in topical treatment.

The supernatants and culture filtrate of B. subtilis (FZB24, FZB37 FZB38) were able to inhibit the development of urediospores of Uromyces appendiculatus in both topical and systemic treatments.

Neither the culture filtrate, the spore suspensions nor the vegetative cells of B. subtilis strains, FZB24, FZB37 and FZB38 were able to significantly influence the growth parameters of A. fabae and R. padi.

B50 treatment was not effective in diminishing A. fabae growth parameters.

Landy-medium treatment also did not affect the growth parameters of A. fabae and R. padi. This suggests that the induced resistance effects obtained with supernatants of B. subtilis strains FZB24, FZB37 and FZB38 cannot at this stage be attributed to the growth medium of the bacteria.

No antibiosis effect against A. fabae and R. padi was observed in the acute toxicity test, where the supernatants of B. subtilis strains FZB24, FZB37 and FZB38 were used. When culture filtrates of the same strains of B. subtilis were added to artificial diet, the relative growth rate of A. fabae was also not significantly affected.

In our acute toxicity and artificial feeding tests, we concluded that the antibiosis effects on A. fabae and R. padi were not due to any contact effect, but mediated only via the plants pre-treated with supernatants of B. subtilis strains, FZB24, FZB 37 and FZB38.

Furthermore, in order to explain the mechanisms of induced resistance, we can conclude that the positive trend of the chlorophyll fluorescence yield in supernatants-induced plants constitutes evidence that change occurred in the plants due to treatments with B. subtilis and its metabolites, which had a further influence on the tested aphids' biomass. The biomass of A. fabae and R. padi feeding on plants treated with B. subtilis and its metabolites was reduced but was not found to be statistically significant, probably due to the small number of replications in the experiment.

B. subtilis spores suspensions introduced into sterile soil had a positive effect on chlorophyll fluorescence. The latter parameter has been found to be higher and statistically significant in Triticum aestivum seedlings after its seeds have been pre-treated with spore suspensions of B. subtilis strain FZB37. The fresh weight and dry weight of the R. padi feeding on such treated host plants were found to be negatively affected compared to the control treatment (water treated plant seed). Chlorophyll fluorescence was found also higher in Vicia faba seedlings treated in the same way as T. aestivum, but was not statistically significant.

The free amino acid in the phloem sap of Vicia faba plants was demonstrated in the investigation to have quantitatively and qualitatively changed.

We could not observe a significant change in the concentration of the so-called 10 essential amino acids in aphids' nutrition in our study.

The total concentration of the amino acid was higher in supernatant and culture filtrate of B. subtilis strain FZB37 treated plants compared to the control (water treatment).

The concentration changed qualitatively in nine of the individual amino acids, cysteine, serine, histidine, glycine, threonine, alanine, tyrosine, leucine and lysine. Of this group, serine was found not to have altered for supernatant treated plants, as opposed to the control and culture filtrate of B. subtilis strain FZB37 treatments.

Serine is presumed to play the key role in the antibiosis effect observed in our research, as this amino acid is understood to influence the growth and development of some insects, including aphids.