Mechanism(s) of Glyphosate Resistance in a Selected Plantago lanceolata (L.) R Biotype

In 2003, a glyphosate-resistant plantago (Plantago lanceolata L.) population located in the Robertson district of South Africa was subjected to different glyphosate dosages and the highest dosage (7200 g a.e. ha(-1)) gave no acceptable levels of control. Here we reconfirm resistance and investigate the mechanism of glyphosate resistance. Dose-response curves indicated that the glyphosate dosage rate causing 50% survival (LD50) for the resistant (R) biotype is 43 times greater than for the susceptible (S) biotype, i.e., 43-fold resistant to glyphosate. Investigation into the molecular mechanism of plantago showed shikimate accumulation of the R biotype was lower than that of the S biotype. The reported P-31 and C-13 nuclear magnetic resonance (NMR) spectra show rapid glyphosate translocation into the young untreated leaves of the S biotype. No glyphosate translocation was observed in the R biotype. A point mutation in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene, resulting in an amino acid substitution was also observed, indicating two unique glyphosate resistance mechanisms within the R biotype. The rapid evolution of glyphosate-resistant weeds threatens the usage of the world's most important herbicide (glyphosate), which is essential in world food production and further limits grower options for weed control. New weed management strategies will be necessary to combat plantago R biotypes.