Oats Tolerant of Pseudomonas syringae pv. tabaci Contain Tabtoxinine-beta-Lactam-Insensitive Leaf Glutamine Synthetases.

Pseudomonas syringae pv. tabaci, a commonly recognized leaf pathogen of tobacco, can infest the rhizosphere of many plants, including oats. Normal oat plants do not survive this infestation as a consequence of the complete and irreversible inactivation of all of their glutamine synthetases by tabtoxinine-beta-lactam (TbetaL), a toxin released by pv. tabaci. We have identified a population of oat (Avena sativa L. var Lodi) plants that are tolerant of pv. tabaci. The tolerant plants had no detectable TbetaL-detoxification mechanisms. Pathogen growth on these plant roots was not inhibited. These plants contain leaf glutamine synthetases (GS(1) and GS(2)) that were less sensitive to inactivation by TbetaL in vitro; these GSs have normal K(m) values for glutamate and ATP when compared with those of GS in control plants. Root glutamine synthetase of the tolerant plants was inactivated in vivo during infestation by the pathogen or by TbetaL in vitro. When growing without pv. tabaci, the tolerant plants contained normal levels of glutamine synthetase in their roots and leaves and normal levels of protein, ammonia, glutamate, and glutamine in their leaves. However, when the tolerant plants' rhizosphere was infested with pv. tabaci, the plant leaves contained elevated levels of glutamine synthetase activity, protein, ammonia, glutamate, and glutamine. No changes in glutamate dehydrogenase activity were detected in leaves and roots of pathogen-infested tolerant plants.