RESUMO
Black Shank disease of tobacco, Nicotianae tabacum, caused by Phytophthora nicotianae, is one of the most destructive of soil borne diseases. The present investigation was initiated to study the relative importance of various inducible mechanisms operative in this plant: pathogen interaction. Tobacco cells in culture were treated with heat-released soluble cell-wall elicitors from mycelial walls of the pathogen. The timing and intensity of the induced defence responses of the cells with regard to the synthesis of phytoalexins, lignin and defence hydrolases were evaluated. An elicitor concentration of 40 micrograms ml-1 resulted in the optimal induction of sesquiterpenoid phytoalexin synthesis which was detectable 3-4 hr after elicitation and reached a maximum at 20-28 hr post-elicitation. Increased lignin deposition was detectable 4 hr after elicitation with maximal synthesis between 12 and 96 hr. Induction of extracellular and cellular beta-1,3-glucanases was observed within 4 hr, exhibiting a biphasic response with an initial peak at 8-12 h. Increased cellular and extracellular chitinase activities were detected from 8 and 16 hr onwards, respectively. These results were supported by enzyme staining of electrophoretic separated isoforms. The results obtained indicate that the elicitation of tobacco cells by P. nicotianae derived signal molecules exhibits the properties of a multicomponent dynamic system with different protective mechanisms having complementary roles in the overall expression of the defence response.