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Evidence for Compartmentalization of Conjugates of 2,4-Dichlorophenoxyacetic Acid in Soybean Callus Tissue ¹

Pesticide Research Laboratory and Graduate Study Center, Department of Entomology, The Pennsylvania State University, University Park, Pennsylvania 16802, Department of Biology, The Pennsylvania State University, University Park, Pennsylvania 16802

Soybean Glycine max L. Merrill var. Amsoy 71 root callus tissue labeled with [1-¹⁴C]2,4-dichlorophenoxyacetic acid (2,4-D) which was subsequently incubated for 24 hours in the absence of 2,4-D, released considerable amounts of label into the media. These results led to an examination of the efflux of 2,4-D and 2,4-D metabolites during a 6-hour time period. Fifty% of the free 2,4-D was lost in 15 minutes and 99% in 6 hours. After 6 hours, only about 48% of the ether-soluble fraction (mainly the glutamic and aspartic conjugates) and about 33% of the aqueous-soluble fraction (mainly hydroxylated glycosides) effluxed from the tissue. Neutral red efflux from stained callus tissue was enhanced only 5% above the control by treatment with 7.5% dimethylsulfoxide (DMSO) and 50% with 20% DMSO. Similar soybean callus tissue preincubated with [1-¹⁴C]2,4-D and subsequently incubated with H₂O, 7.5% DMSO, and 20% DMSO was examined for efflux of ¹⁴C label. DMSO similarly enhanced the efflux of the ether and aqueous soluble conjugates.

DMSO concentrations of less than 10% did not damage the vacuolar membranes which also has been reported with cultured tobacco cells (Delmer 1979 Plant Physiol 64: 623-629). From these data, it seems that the 2,4-D metabolites are located in a compartment of the cell and presumably the vacuole.