ACCase-inhibitor resistance in bearded sprangletop (Leptochloa fusca spp. fascicularis) associated with target-site mutations in rice monoculture

The presence of weeds is the most serious problem in rice (Oryza sativa L.) cultivation, which is exacerbated by the evolution of herbicide-resistant weed populations. In a whole-plant pot experiment, the possible resistance evolution in a population of bearded sprangletop [Leptochloa fusca spp. fascicularis (Lam.) N.W. Snow], which survived after the application of two ACCase inhibitors (profoxydim and cyhalofop), was investigated. Furthermore, the possible modification of the ACCase gene, as a source of resistance, was evaluated. The putative resistant population exhibited 61-fold resistance to profoxydim, 155-fold resistance to propaquizafop, and 67-fold resistance to cyhalofop + penoxsulam compared to the susceptible population. However, this population was susceptible to cycloxydim, an herbicide registered for the Provisia® technology rice varieties. The molecular genetic analysis of the ACCase gene fragment revealed the presence of a point mutation that resulted in the substitution of tryptophan (Trp) by cysteine (Cys) at position 1999 in the CT domain of the ACCase gene. This is the first record of an ACCase-inhibitor, field-selected, resistant bearded sprangletop population carrying the Trp1999Cys amino acid substitution. The evolution of broad ACCase cross-resistance reduces control options for selective and effective management of bearded sprangletop in infesting rice fields.