Altered target sites as a mechanism of herbicide resistance

Over 200 distinct herbicide resistant weed biotypes have evolved worldwide. In most of these, resistance is conferred by an altered target site, i.e. a modified target protein with reduced affinity for the herbicide(s) in que stion. This has been documented for herbicides that target most major known sites of action, including those that inhibit photosynthetic electron tran sfer at photosystem II, acetyl-CoA carboxylase, acetolactate synthase, and tubulin polymerization. Patterns of cross-resistance to structurally simila r herbicides and those from Ether chemical classes that target the same sit e vary, depending on the mutation and its effect on protein steric and elec tronic properties. Mechanisms of target site-based herbicide resistance are reviewed, with emphasis on the biochemical and molecular basis for resista nce. (C) 2000 Elsevier Science Ltd. All rights reserved.