Enhancement of symbiotic dinitrogen fixation by a toxin-releasing plant pathogen.

An approximate doubling in plant growth, total plant nitrogen, nodulation, and overall dinitrogen fixation of alfalfa are the consequences of the action of a toxin delivered by a Pseudomonas infesting the alfalfa rhizosphere. The toxin, tabtoxinine-beta-lactam, inactivates selectively one form of glutamine synthetase in the nodules. Thus, normal glutamine synthetase-catalyzed ammonia assimilation is significantly impaired; yet these plants assimilated about twice the normal amount of nitrogen. How plants regulate dinitrogen fixing symbiotic associations is an important and unresolved question; the current results imply that the glutamine synthetase-catalyzed step in ammonia assimilation, a plant function, strongly influences overall dinitrogen fixation in legumes.