1Department of Natural Sciences and Mathematics, Dominican University of California, San Rafael, CA 94901, USA
2Departrnent of Biological Sciences, University of North Texas, Denton, TX 76203, USA
3Biological Sciences Department, University of Southern Maine, Portland, MA 04103, USA
4Isotope Nuclear Chemistry Division, Mail Stop C346, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
*Corresponding author. E-Mail: sghosh@dorninican.edu
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-β-lactam (TβL), that inactivates glutamine synthetase (GS). Sinorhizobium meliloti grew well in the presence of TβL in culture and on alfalfa roots. The alfalfa symbiont, S. meliloti, and its bacteroids contained TβL-sensitive glutamine synthetases and TβL detoxifying-β-lactamase. The GS of alfalfa leaves is also sensitive to TβL, 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 TβL 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 TβL production in culture in a concentration dependent manner. Thus, a bi-directional interaction occurs between the nitrogen metabolism of alfalfa and oat and TβL production by P. syringae pv. tabaci.
Avena sativa, glutarnine synthetase, Medicago sativa, Pseudornonas syringae pv. tabaci, rhizosphere, Sinorhizobium rneliloti, tabtoxinine-β-lactam
