Interactions between Pseudomonas syringae pv. tabaci and two rhizosphere hosts, Medicago sativa and Avena sativa

The interactions between Pseudomonas syringae pv. tabaci and either nodulating alfalfa (Medicago sativa) or oat (Avena sativa) seedlings were examined to further our understanding of this rhizosphere association. P. syringae pv. tabaci produces and releases a toxin, tabtoxinine-beta -lactam (TOL), that inactivates glutamine synthetase (GS). Sinorhizobium, meliloti grew well in the presence of T betaL in culture and on alfalfa roots. The alfalfa symbiont, S. meliloti, and its bacteroids contained T betaL-sensitive glutamine synthetases and T betaL detoxifying-beta -lactamase. The GS of alfalfa leaves is also sensitive to T betaL, but GS activity was unaffected in infested plants. Toxin production was apparently suppressed in the alfalfa and nitrate-fed oat rhizospheres since these plants survived and retained significant amounts of leaf GS activity. The water-soluble extracts of these rhizospheres inhibited T betaL production in culture and the inhibition was correlated with the amount of reduced nitrogen present. Furthermore, representative mixtures of pure ammonium and amino acids inhibited T betaL production in culture in a concentration dependent manner. Thus, a bi-directional interaction occurs between the nitrogen metabolism of alfalfa and oat and T betaL production by P. syringae pv. tabaci.