Acetolactate Synthase of Suspension-Cultured Carrot Cells Resistant to Bensulfuron Methyl.

Abstract

Activities of acetolactate synthase (ALS) of 10-8 M bensulfuron methyl (BSM)-resistant carrot cells were compared with those of susceptible (normal) cells.ALS activity, assayed by measuring acetoin formed by decarboxylation of acetolactate, was inhibited in normal cells by low concentrations of BSM. However, nearly or more than half of the total activity was not inhibited by 10-6M BSM at pH lower than 7.5. Acetoin amounts produced in undecarboxylated samples almost coincided with those inhibited by high concentrations of BSM, 10-6 and 10-4M, in normal and resistant cells, respectively. This might indicate the presence of a direct acetoin producing system in ALS assay. Net ALS activity could be obtained by subtracting the undecarboxylated value from the total activity.Optimum pHs were broad, around 8.5 for ALS and 7.0 for direct acetoin producing activity. ALS was saturated at 40mM pyruvate and apparent Km value for ALS was 5-6mM.ALS from resistant cells was inhibited by BSM less than the normal cells. The BSM concentrations inhibiting ALS activity by 50% (I50) were 1×10-8M and 6×10-7M for normal and resistant cells, respectively. As this I50 value of the resistant cells was about 10 times higher than that reported previously (5.2×10-8M), the probability is that insensitivity of ALS to BSM of the resistant cells developed during long-term culture.