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

摘要

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 andreleases a toxin, tabtoxinine-#beta#-lactam (T#beta#L), that inactivates glutamine synthetase (GS). Sinorhizobium meliloti grew well in the presence of T#beta#L in culture and on alfalfa roots. The alfalfa symbiont, S. meliloti, and its bacteroids contained T#beta#L-sensitive glutamine synthetases and T#beta#L detoxifying-#beta#-lactamase. The GS of alfalfa leaves is also sensitive to T#beta#L, 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#beta#L production in culture and the inhibition was correlated with the amountof reduced nitrogen present. Furthermore, representative mixtures of pure ammonium and amino acids inhibited T#beta#L production in culture in a concentration dependent manner. Thus, a bi-directional interaction occurs between the nitrogen metabolism ofalfalfa and oat and T#beta#L production by P. syringae pv. tabaci.