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Metabolism of 2,4-Dichlorophenoxyacetic Acid in 2,4-Dichlorophenoxyacetic Acid-Resistant Soybean Callus Tissue ¹

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

Three 2,4-dichlorophenoxyacetic acid (2,4-D) -resistant root callus tissue lines of Glycine max L. Merrill var. Acme were derived by culturing callus tissue 2 to 6 months on 40 milligrams per liter 2,4-D and designated 40R, 40B, and 40C. Tissue line 40R had a lower level of 2,4-D uptake in 2-week-old tissue which disappeared in 3.5-week-old tissue and less free 2,4-D following incubation for 24 hours with [1-¹⁴C]2,4-D. This tissue line accumulated more hydroxylated glycosides of 2,4-D than did nonresistant tissue. Tissue line 40B showed no difference in uptake of 2,4-D when compared to nonresistant tissue but it did contain less free 2,4-D and more hydroxylated glycosides. The metabolism of 2,4-D in the 40C tissue line did not differ significantly from nonresistant tissue although uptake was less. The 40R line reverted to the same 2,4-D sensitivity as Acme root callus following six transfers on 10 micromolar naphthaleneacetic acid medium. The altered 2,4-D uptake and metabolism characteristic of 40R were also lost. The levels of amino acid conjugates of 2,4-D in the resistant root callus tissue lines were either lower or not significantly different from the Acme tissue lines. Therefore, variations in uptake and metabolism of 2,4-D do not wholly explain the resistance of the derived tissue lines, and perhaps modification of the active site or compartmentation is involved.