Role of transporters in paraquat resistance of horseweed Conyza canadensis (L.) Cronq.

Paraquat (Pq) inducible transporters are presumed to play a role in the resistance mechanism of horseweed and to function by carrying paraquat to a metabolically inactive compartment. The uptake and intracellular localisation of paraquat, the effect of transporter inhibitors on resistance, and paraquat-induced gene expression were studied to obtain a better understanding of the mechanism of resistance. Investigations proved that paraquat entered the cells of both resistant and susceptible biotypes, approached the maximum within the first hour in chloroplasts, and then declined in all organelle fractions. In the resistant biotype paraquat was located in the vacuoles a day after treatment. Selective transporter inhibitors blocked the sequestration of paraquat, suggesting the participation of not directly energized transporters. Four EST fragments were identified that were expressed in response to paraquat. Two of them are thought to play a role in the general stress response (Ferr2, Myb). The others exhibit a similarity to transporters (EmrE, CAT) and could conceivably be involved in the intracellular transport of paraquat and the mechanism of resistance.