SEQUESTRATION AND OXYGEN RADICAL DETOXIFICATION AS MECHANISMS OF PARAQUAT RESISTANCE

Evidence in the literature has generally supported either of two paraq uat resistance mechanisms: an increase in activity of oxygen radical-s cavenging enzymes in resistant plants which affords protection from ac tive oxygen species formed by paraquat; and sequestration of paraquat away from its site of action in the chloroplast. Evidence for the firs t model relies primarily on measurement of increased enzyme activity a nd cross-resistance to other oxygen radical-generating stresses in res istant plants. The sequestration model is supported by data showing de creased translocation of paraquat and absence of paraquat injury in pl ant systems that do not have increased levels of protective enzymes. A n alteration in paraquat transport at one of several plant cell membra nes could confer resistance by modifying movement of paraquat into the compartment bounded by that membrane. Properties of the plasmalemma, chloroplast envelope, and tonoplast that may be important to paraquat transport are discussed and data supporting or discounting specific me mbrane alterations in resistant plants are presented. Finally, the pos sibility that both mechanisms may work in concert is addressed.