From tolerance to resistance: mechanisms governing the differential response to glyphosate in Chloris barbata

BACKGROUND Susceptibility and the mechanism (s) governing tolerance/resistance to glyphosate were characterized in two putative‐glyphosate‐resistant Chloris barbata populations (R1 and R2), collected in Persian lime orchards from Colima State, Mexico, comparing them with one non‐treated population (referred to as S). RESULTS Glyphosate doses required to reduce fresh weight or cause mortality by 50% were 4.2–6.4 times higher in resistant populations than in the S population. The S population accumulated 4.3 and 5.2 times more shikimate than the R2 and R1 populations, respectively. There were no differences in 14C‐glyphosate uptake between R and S populations, but the R plants translocated at least 12% less herbicide to the rest of plant and roots 96 h after treatment. Insignificant amounts of glyphosate were metabolized to aminomethyl phosphonate and glyoxylate in both R and S plants. The 5‐enolpyruvylshikimate‐3‐phosphate synthase gene of the R populations contained the Pro106‐Ser mutation, giving them a resistance 12 (R2) and 14.7 (R1) times greater at target‐site level compared with the S population. CONCLUSION The Pro106‐Ser mutation governs the resistance to glyphosate of the R1 and R2 C barbata populations, but the impaired translocation could contribute to the resistance. These results confirm the first case of glyphosate resistance evolved in this species.