Resistance Mechanisms to Glyphosate in Lamarckia aurea (L.) Moench Found in Southern Spain

Glyphosate has been used for roadside weed control in southern Spain for over 40 years, and most populations of goldentop (Lamarckia aurea L.) Moench have putatively developed resistance to this active ingredient. The physiological and biochemical basis for glyphosate resistance in this weed has been investigated. Dose–response studies indicated that the resistant biotype (R) was almost 13 times more resistant to glyphosate compared to a known susceptible biotype (S). Studies of foliar glyphosate retention and 14C-glyphosate uptake/translocation showed no significant differences between both L. aurea biotypes. Basal 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) activity (µmol µg−1TSP min−1) showed similar values between R (0.82 ± 0.04) and S (0.75 ± 0.05) biotypes. On the other hand, the resistance factor (I50R/I50S) did not show a difference between the two biotypes. Therefore, it was concluded that target-site (TSR) resistance mechanisms are not involved in glyphosate resistance in this weed species. The metabolism of glyphosate to form the non-toxic metabolites aminomethylphosphonic acid (AMPA), glyoxylate, and sarcosine was greater and faster in the R compared to the S biotype; thus, glyphosate resistance is due to non-target-site resistance (NTSR) mechanisms. This paper is the first report of glyphosate resistance in L. aurea in the world.