Background: Glyphosate (the active ingredient in Roundup®) is the most important herbicide in use. Glyphosate resistant (GR) crops have been introduced in the mid-1990s, which is one of the reasons for the commercial glyphosate boost. Glyphosate is a post emergence, systemic, non-selective herbicide. After its heavy and repeated usage for 20 years, glyphosate resistant weeds have evolved. The resistance mechanisms in the weeds are different from those in the commercial GM crops. The weeds have both evolved altered uptake and allocation patterns, increased production of the target enzyme and increased degradation. All crops have an altered version of the glyphosate target enzyme EPSPS, preventing the inhibitory action of glyphosate.

Besides it pesticidal activity at high doses, glyphosate has been shown to enhance growth when applied at low doses,- a curiosity for a chemical created for the opposite effect. These biphasic dose-response relationships (i.e. an increase in biomass at low concentrations and a decrease at high concentrations compared to control plants) are commonly called hormesis. The physiological mechanism of the phenomenon is not known. If it is related to glyphosate binding to EPSPS, hormesis should not occur in GR crops. However, it should occur, though at higher doses, in those GR weeds, where it is the transport or degradation of glyphosate that is altered. If the hormetic response is due to some effect not related to EPSPS it should occur in all plants, regardless of EPSPS type.

Proposed study: To get closer to understanding the mechanisms behind glyphosate hormesis, the aim of the present study is to test for hormesis in three commercial target site resistant GR crops (maize, cotton and soybean) and three weeds with different resistance mechanisms (Palmer amaranth, Lolium multiflorum and Eleusine indica), and compare the responses with the corresponding susceptible species. The study will include: 1) Germination and if possible plant propagation with seeds of natural occurring GR weeds 2) Construction of dose-response curves via foliar glyphosate application for both types; glyphosate sensitive (GS) and GR plants. 3) Statistical tests for hormesis in all studies and interpretation of data in a mode of action context.

Further information/contact:

Nina Cedergreen (ncf@life.ku.dk)

Lars Schrübbers ()