A possible role for proteases and deaminases in the development of the symptoms of bacterial speck disease in tomato caused by Pseudomonas syringae pv. tomato

Pseudomonas syringae pv. tomato produces constitutive proteases during the logarithmic phase of growith in culture, which have optimum activity at pH 7·0 and 30°C. The addition of proteina-ceous compounds to the growth medium caused only a slight increase in the specific activity of the enzyme preparation. Proteolytic activity was higher in the diseased tissue of susceptible plants than in resistant plants, reaching maximum levels during the later stages of infection. Total proteolytic activity in diseased plants appeared to be due to four proteases, two originating from the pathogen, one from the host and one which appeared to be produced during the interaction between the pathogen and its host. A correlation between disease severity and proteolytic activity in infected tissue was demonstrated, with activity being greatest around developing necrotic zones. The activity also varied with the age of the leaf at the time of infection. Total nitrogen content and soluble proteins decreased in infected susceptible tissue during disease development, with asparagine and free amino acids accumulating during the first 100 h after inoculation, but then decreasing. The pathogen utilized five amino acids as a source of nitrogen in culture, but only asparagine and glutamine appeared to be utilized in the infected plant. Five different deaminases were detected in culture, but, of the five, only asparaginase and glutaminase were detected in the infected suceptible cultivar. Liberation of gaseous ammonia in the infected susceptible cultivar tissue was first observed 72 h after inoculation. Similar changes also occurred, but on a very limited scale, in inoculated resistant plants. The addition of asparagine and glutamine to infected suceptible plants, increased disease severity and ammonia production.