Abstract
The evolution of glyphosate resistance (GR) in weeds is an increasing problem. Glyphosate has been used intensively on wild poinsettia (Euphorbia heterophylla L.) populations for at least 20 years in GR crops within South America. We investigated the GR mechanisms in a wild poinsettia population from a soybean field in southern Brazil. The GR population required higher glyphosate doses to achieve 50% control (LD50) and 50% dry mass reduction (MR50) compared to a glyphosate susceptible (GS) population. The ratio between the LD50 and MR50 of GR and GS resulted in resistance factors (RF) of 6.9-fold and 6.1-fold, respectively. Shikimate accumulated 6.7 times more in GS than in GR when leaf-discs were incubated with increasing glyphosate concentrations. No differences were found between GR and GS regarding non-target-site mechanisms. Neither population metabolized glyphosate to significant levels following treatment with 850 g ha-1 glyphosate. Similar levels of 14C-glyphosate uptake and translocation were observed between the two populations. No differences in EPSPS expression were found between GS and GR. Two target site mutations were found in all EPSPS alleles of homozygous resistant plants: Thr102Ile + Pro106Thr (TIPT—mutation). Heterozygous individuals harbored both alleles, wild-type and TIPT. Half of GR individuals were heterozygous, suggesting that resistance is still evolving in the population. A genotyping assay was developed based on the Pro106Thr mutation, demonstrating high efficiency to identify homozygous, heterozygous or wild-type EPSPS sequences across different plants. This is the first report of glyphosate-resistant wild-poinsettia harboring an EPSPS double mutation (TIPT) in the same plant.
Highlights
Glyphosate inhibits 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) in susceptible plants affecting the biosynthesis of aromatic amino acids in the shikimate pathway [1]
The LD50 was 105 g ha-1 for glyphosate susceptible (GS) and 722 g ha-1 for glyphosate resistance (GR) resulting in a resistance factors (RF) of 6.9-fold (Table 2)
Rates as low as 189 g ha-1 (MR90) are required to control GS plants. These results confirm that the GR wild poinsettia population is glyphosate resistant
Summary
Seeds from the putative GR wild poinsettia were collected in a soybean field located at Kalorecity (Parana State, Brazil, 23 ̊51’45” S, 51 ̊39’58” W). Soybean followed by corn had been planted in this field for 20 years under no-till system. Glyphosate had been applied three times a year in this field (once in corn and twice in soybean). Seeds from a glyphosate susceptible (GS) wild poinsettia population were collected in a field with no history of glyphosate applications located at Engenheiro Coelho (São Paulo State, Brazil, 22 ̊05’24” S, 46 ̊34’12” W). Seeds from GR and GS were planted in 200-well flats at 0.5 cm deep. Seeds of the surviving plants were collected as described above to obtain the second generation (G2) of GR, which all experiments were performed with. All herbicide treatments were sprayed at 160 L ha-1 using a chamber track sprayer (Generation 4, DeVries Manufacturing, Hollandale, LN) equipped with an 8002EVS even flat-fan nozzle (TeeJet; Spraying Systems Co., Wheaton, IL)
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