The Identification and Properties of Apoplastic Carboxylesterases from Wheat That Catalyse Deesterification of Herbicides

Formulated herbicides are frequently applied as carboxylesters. The first step in the metabolism of these compounds in plants is enzymatic deesterification, which generally leads to their bioactivation and increases their mobility within the plant. In apoplastic extracts from wheat devoid of cytoplasmic contamination, we have identified a pool of nitrophenyl acetate and naphthyl acetate esterases. These esterases were resolved by isoelectric focusing and activity staining into at least 11 proteins. Furthermore, the apoplastic pool of esterases could be distinguished from soluble esterases in whole-leaf extracts by two-dimensional electrophoresis and their affinity for concanavalin A. The apoplastic fraction deesterified some but not all members of the arylphenoxypropionate family of herbicides tested. Deesterification was inhibited by phenylmethylsulfonyl fluoride. We conclude that there is a pool of soluble carboxylesterases in the leaf apoplast of wheat seedlings that are largely both glycoproteins and serine hydrolases. These apoplastic esterases catalyze the hydrolysis of herbicide esters and are substrate specific. We speculate that the apoplastic esterases contribute to the bioavailability of herbicides in planta