PLANT PHYSIOLOGY , Vol 102, Issue 3 795-802, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Partial Characterization of Glutathione S-Transferase Isozymes Induced by the Herbicide Safener Benoxacor in Maize
E. P. Fuerst, G. P. Irzyk and K. D. Miller
Department of Crop and Soil Sciences, Washington State University, Pullman, Washington 99164-6420
The effects of the dichloroacetamide safener benoxacor on maize (Zea mays
L. var Pioneer 3906) growth and glutathione S-transferase (GST) activity
were evaluated, and GST isozymes induced by benoxacor were partially
separated, characterized, and identified. Protection from metolachlor
injury was closely correlated with GST activity, which was assayed with
metolachlor as a substrate, as benoxacor concentration increased from 0.01
to 1 [mu]M. GST activity continued to increase at higher benoxacor
concentrations (10 and 100 [mu]M), but no further protection was observed.
Total GST activity with metolachlor as a substrate increased 2.6- to
3.8-fold in response to 1 [mu]M benoxacor treatment. Total GST activity
from maize treated with or without 1 [mu]M benoxacor was resolved by fast
protein liquid chromatography anion-exchange chromatography into four major
activities, designated activity peaks A, B, C, and D in their order of
elution. These GST activity peaks were enhanced to varying degrees by
benoxacor. Activity peak B showed the least induction, whereas activity
peak A was absent constitutively and thus highly induced by benoxacor. In
contrast to earlier reports, there appear to be not one, but at least two,
major constitutive isozymes (activity peaks A and D) having activity with
metolachlor as substrate; there were at least three such isozymes in
benoxacor-treated maize (activity peaks A, C, and D). The elution volumes
of activity peaks A, B, C, and D were compared with those of partially
purified maize GST I and GST II; also, the reactivity of polypeptides in
these activity peaks with antisera to GST I or GST I/III (mixture) was
evaluated. Evidence from these experiments indicated that activity peak B
contained GST I, and activity peak C contained GST II and GST III. Activity
peaks A and D contained unique GSTs that may play a major role in
metolachlor metabolism and in the safening activity of benoxacor in maize.
Isozymes present in activity peaks A and D were not detected in earlier
reports because of the very low activity with the artificial substrate
1-chloro-2,4-dinitrobenzene. Immunoblotting experiments also indicated the
presence of numerous unidentified GST subunits, including multiple subunits
in chromatography fractions containing single peaks of GST activity; this
is indicative of the likely complexity and diversity of the maize GST
enzyme family.
