ORIGINAL ABSTRACT

Herbicide-resistant weeds are a prominent example of rapid global adaptation to new environments, significantly impacting the evolutionary ecology of plants, as well as the technology and economy of modern agriculture. In particular, the resistance of A. hybridus to glyphosate was found to be exceptionally high, and this has been recently attributed to a novel triple mutation in the EPSPS. Nevertheless, in environments lacking glyphosate selection pressure, it is anticipated that resistant plants would exhibit an adaptive cost, manifested as a reduction in ecological fitness relative to susceptible plants. The naturally evolved glyphosate resistance endowing triple TAP-IVS mutation in the EPSPS in the major weed A. hybridus is associated with a plant fitness penalty in environments under plant competitive interactions. This penalty is notably expressed both at the EPSPS enzyme and plant level. Plants with the TAP-IVS mutation express a notable increase in fitness cost in a competitive environment with glyphosate susceptible counterparts (i.e. ecological-based fitness cost), although this cost was absent or moderate under non-competitive conditions. Interestingly, this is also a biochemical-based fitness cost mechanism, as the EPSPS catalytic efficiency of the transformed E. coli TAP-IVS variant from A. hybridus decreased 0.3-fold compared to WT. These findings underscore the importance of considering adaptive costs when assessing the evolutionary trajectory of mutations conferring herbicide resistance.