Glutamine Synthetase Inhibition: Possible Mode of Action of Wildfire Toxin from Pseudomonas tabaci

Abstract

TOBACCO wildfire toxin, produced by the phytopathogenic bacterium Pseudomonas tabaci, and methionine sulphoximine (MSO) inhibit the growth of the alga Chlorella vulgaris, and induce chlorosis in leaves of higher plants¹. Because both compounds were considered to be structural analogues of methionine, it was postulated that they acted as methionine antimetabolites. Although L-methionine overcame the growth inhibition effect of both the toxin and MSO in Chlorella, it did not prevent chlorosis in higher plants. It has recently been shown that the inhibition of the growth of oat coleoptiles induced by MSO can be reversed by glutamine, but not by methionine². In animals the wildfire toxin causes convulsions identical to those produced by MSO³; significantly, these two compounds also inhibit cerebral glutamine synthetase (GS)^4,5. In view of the similarities exhibited by the two compounds, both in plants and animals, and the findings implicating glutamine metabolism, we felt that a study of the effect of the wildfire toxin on plant GS might provide additional insight into its mode of action. We report here the results of investigations on the influence of the wildfire toxin on GS obtained from pea⁶, and the effect of L-glutamine in reversing the toxin's effects in intact tobacco leaves and on GS in vitro.