Mechanism of Isoxaben Tolerance inAgrostis palustrisvar. Penncross

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Abstract
Previous work has demonstrated that isoxaben tolerant mutants of Arabidopsis thaliana var. Columbia are most likely altered at the site of isoxaben binding. The salient question becomes whether or not species selectivity to this herbicide might also be a result of differential target site binding. Grasses are generally more tolerant to isoxaben than dicots. In this communication we show that Agrostis palustris var. Penncross, a grass, is 83-fold more tolerant in a soil incorporation test and 170-fold more tolerant to inhibition of glucose incorporation into cellulose than is Arabidopsis, a dicot. Cell wall fractionation of Agrostis shows a specific effect on cellulose biosynthesis. At most, 5-fold of the 170-fold tolerance exhibited by Agrostis in terms of cellulose biosynthesis can be attributed to decreased isoxaben uptake under the test conditions. Furthermore, Agrostis is unable to metabolize isoxaben to any significant degree. Therefore, we suggest that the major portion of the tolerance in Agrostis might be due to differences in isoxaben binding.

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