METABOLISM OF THE HERBICIDE METRIBUZIN BY AN N-GLUCOSYLTRANSFERASE FROM TOMATO CELL-CULTURES

Callus and cell suspension cultures of tomato (Lycopersicon esculentum L. cv. 'Sheyenne') form a glucose conjugate of the herbicide metribuzin, an asymmetric triazinone that inhibits photosynthesis. The responsible enzyme, a soluble UDP-glucose: metribuzin N-glucosyltransferase (MGT) has been isolated and partially characterized from cells grown in B5 media containing 3 mg 1-1 2,4-dichlorophenoxyacetic acid (2,4-D). The N-glucoside reaction product was identified by co-chromatography and mass spectrometry. Specific activity of MGT (nmol N-glucoside (mg prot.)-1 h-1) ranged from 0.1 to 3.5 in callus and from 0 to 7.5 in cell suspension cultures. In contrast to green plants, MGT activity of cell cultures was not increased by fluorescent light (70-mu-E m-2 s-1). Higher concentrations of 2,4-D had no effect on MGT specific activity, but omission of 2,4-D resulted in decreased specific activity. At 3 days after subculture, suspension cultures with 3 mg 1-1 2,4-D had a maximum MGT specific activity of 4.8, which decreased to 2.3 by 14 days when maximum growth occurred and to zero activity by 23 days. Cell suspensions treated with 3 mg 1-1 of four other auxins or auxinic compounds (dicamba, picloram, indoleacetic acid, and naphthaleneacetic acid) had MGT specific activities ranging from 0.3 to 1.9 with no correlation between growth and enzyme activity. Kinetin had no consistent effect on MGT specific activity in cell cultures.