Distribution differences in the EPSPS gene in chromosomes between glyphosate-resistant and glyphosate-susceptible goosegrass (Eleusine indica)

Glyphosate is a popular herbicide used to control goosegrass [Eleusine indica (L.) Gaertn.], one of the most troublesome weeds in cotton (Gossypium hirsutum L.) fields. However, high selection pressure has led to some populations being difficult to control in cotton fields in China. In this study, levels of glyphosate resistance were quantified and 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) amplification was confirmed. In addition, distribution of the EPSPS gene among the chromosomes was also investigated using fluorescence in situ hybridization (FISH) methodology. One population (AHCZ) was confirmed to be glyphosate resistant with a resistance index of 3.4 and significantly reduced shikimate accumulation compared with the susceptible population. All examined AHCZ individuals exhibited no mutations in the EPSPS gene. Expression and copy numbers of EPSPS in the AHCZ population were 5.7 and 15.4 times higher, respectively, than in the susceptible population. A positive correlation was identified between signal intensities of primary anti-EPSPS antibody and copy numbers of the EPSPS protein, as indicated by immunoblot analysis. FISH results revealed that, in mitotic metaphase chromosomes, signals were observed dispersed across two sets of homologous chromosomes in a resistant individual (copy number = 31), while in susceptible individuals, signals were only partially detected in one set of homologous chromosomes. In interphase nuclei, EPSPS signals were brightest in resistant individuals compared with susceptible individuals. In conclusion, one E. indica population from a cotton field in the Anhui Province has evolved resistance to glyphosate, and EPSPS gene amplification was confirmed as the resistance mechanism.