Effect of Glufosinate on Glutamine Synthetase Isozymes from Several Plant Species.

Abstract

Glutamine synthetase (GS) in shoots of cucumber, rice (C3 plants), corn and barnyardgrass seedlings (C4 plants), inhibited by glufosinate (2-amino-4[(hydroxy)(methyl)phosphinoyl]butyric acid), was separated by DEAE-Sephacel column into two fractions (GS1 and GS2). In cucumber and rice seedlings, the GS1 activity was found lower than the GS2, while the GS1 activity was higher than the GS2 in corn seedlings, and the two were almost equal in barnyardgrass seedlings. These results showed that cucumber and rice generally have a great deal of ammonia released from photorespiration and that GS2 is implicated in assimilation of the ammonia. Corn and barnyardgrass, on the contrary, have little ammonia released from that cycle so that GS2 activity was much less than GS1. In root, only a single peak of GS activity was detected and was called GSr.Optimum pH was between 6.0 and 7.0 and optimum temperature was between 40 and 45°C for GS activities of the 4 seedlings. The KCl concentrations eluting GS1 from column were similar to those of GSr, and those of GS2 were higher than GS1 in all the plant species.GS1, GS2 and GSr activities were decreased by increasing concentrations of glufosinate, and the concentrations necessary for inhibition of the activity by 50% (I50) were determined. In all four plant seedlings, I50 values of glufosinate for GS1 and GSr were not greatly different, I50 of GS2 was obtained on the order of 10-6M, while I50 for both GS1 and GSr was on the order of 10-5M. The roles of the GS isozymes in ammonia assimilation and their inhibition by glufosinate in C3 and C4 plants were discussed.