Biochemical and physiological mode of action of tiafenacil, a new protoporphyrinogen IX oxidase-inhibiting herbicide.

We conducted biochemical and physiological experiments to investigate the mode of action of tiafenacil (Terrad'or™), a new protoporphyrinogen IX oxidase (PPO)-inhibiting pyrimidinedione herbicide. Analysis of the half-maximal inhibitory concentration (IC50) against recombinant PPO enzymes from various plant species, including amaranth (Amaranthus tuberculatus), soybean (Glycine max), arabidopsis (Arabidopsis thaliana), and rapeseed (Brassica napus), showed that tiafenacil had an IC50 of 22 to 28 nM, similar to the pyrimidinedione herbicides butafenacil and saflufenacil and the N-phenylphthalimide herbicide flumioxazin. By contrast, tiafenacil exhibited 3- to 134-fold lower IC50 values than the diphenyl ether herbicides fomesafen, oxyfluorfen, and acifluorfen. Tiafenacil is non-selective and is herbicidal to both dicots and monocots, such as the weeds velvetleaf (Abutilon theophrasti), amaranth, and barnyardgrass (Echinochloa crus-galli) as well as the crops soybean, rapeseed, rice (Oryza sativa), and maize (Zea mays) at concentrations ranging from 1 to 50 μM. Treatment of plant tissue with tiafenacil in darkness resulted in the accumulation of protoporphyrin IX. Subsequent exposure to light increased the content of malondialdehyde and significantly decreased the Fv/Fm values of chlorophyll fluorescence. The results suggest that tiafenacil is a new PPO-inhibiting pyrimidinedione herbicide.