Practice question

Atrazine belongs to s-triazine class that binds plastoquinone binding niche of photosystem II D1 protein encoded by psbA, interrupting electron flow from QA to QB, generating singlet oxygen and lipid peroxidation causing chlorosis. While psbA Ser264 to Gly mutation confers resistance in weeds, crop tolerance in maize mediated differently via enhanced metabolic detoxification. Glutathione S-transferases especially Tau and Phi classes conjugate triazine chlorine with sulfhydryl group of reduced glutathione producing GS-atrazine conjugate that is transported to vacuole by ABC transporters and further catabolized to lamda mercapturic acid derivatives. Overexpression of maize GST-I in transgenic plants increases atrazine metabolism rate, reducing phytotoxicity. This detoxification pathway reduces herbicide persistence and allows selective application in tolerant crops. GST mediated atrazine resistance contrasts with target-site mutation and illustrates enzymatic detoxification mechanism providing selective weed control strategy exploited in transgenic development and conventional breeding of tolerant varieties. Biochemical pathway of detoxification proceeds via nucleophilic substitution where thiolate anion of glutathione attacks electrophilic chlorine of atrazine displacing chloride forming conjugate. Conjugate recognized by tonoplast ABC transporter MRP1 sequestered into vacuole where further metabolized to cysteine and mercapturic acid derivatives excreted. Overexpression of GSTU1 and GSTF1 isoforms enhanced tolerance in transgenic tobacco. This detoxification illustrates