Studies on Enzymology Mechanism of Glyphosate Tolerance in Bean

During the seedling period, EPSP synthase specific activities were different between susceptible and tolerant beans. While the developmental patterns were similar, the two beans EPSP synthase specific activities declined during the first days after germination, then increased to reach a maximum 16 days later, and thereafter began to slowly decrease again. During the same growing stage, enzyme level maximum difference was only 1.1 times between the two beans. After treatment with glyphosate for 24 hours, the two beans enzyme activities increase 15%～20%. Some properties of EPSP synthase purified from susceptible and tolerant beans were examined. The enzyme's specific activity reached more than 6219 nmol·min-1 ·mg-1 protein by a four-step purification. The optimum substrate of susceptible and tolerant bean EPSP synthase Km(PEP) values were 3.5 and 7.1 μmol·L-1 ,respectively. Difference was 2 times between two kinds of beans.Susceptible K i(glyphoste) values were 6.2 and 16.7 μmol·L-1 ,respectively. Difference was 2.7 times between the two kinds beans. It has shown that affinity of tolerant bean EPSP synthase to glyphosate is weaker, to PEP is stronger, while affinity of susceptible bean EPSP synthase to glyphosate is stronger, to PEP is weaker. That's the reason that they are different in glyphosate tolerance