Mechanistic basis for synergism of 2,4-D amine and metribuzin in Avena sterilis

The combination of herbicides with different modes of action has been adopted not only to improve weed control but also to increase the environmental sustainability of plant-protection products. In this study, we showed a synergistic effect of the auxin herbicide 2,4-D amine with the PSII-inhibiting herbicide metribuzin to control the global grass weed wild oat (Avena sterilis) population and investigated the underlying mechanisms. Pretreatment with 2,4-D amine did not change the foliar absorption of metribuzin but did increase metribuzin translocation to the roots and new leaves, although enhancement of the metribuzin metabolism was also observed. Considering that the expression level of the target site psbA gene is significantly higher in leaves than in roots, increased metribuzin translocation to new leaves is likely the major cause of the observed synergism, even though enhanced metribuzin metabolism may offset the metribuzin efficacy. This is the first report on the synergistic mechanism between 2,4-D amine and metribuzin in weed control.