Changes in the binding and inhibitory properties of urea/triazine-type herbicides upon phospholipid and galactolipid depletion in the outer monolayer of thylakoid membranes

The binding characteristics and the inhibitory power of atrazine and DCMU towards uncoupled electron flow activity were studied in acyl lipid-depleted thylakoid membranes from atrazine-susceptible and-resistant biotypes of Solanum nigrum L. For this purpose, phospholipase A₂ from Vipera russelli and the lipase from Rhizopus arrhizus were used to obtain a selective lipid class (phospholipids or galactolipids) depletion which was restricted to the outer monolayer. Neither phospholipid nor galactolipid removal affected the dissociation constant and the number of binding sites of atrazine. In contrast, the dissociation constant of DCMU was increased in phospholipid-depleted thylakoid membranes but remained unchanged after galactolipid depletion. The number of DCMU binding sites decreased significantly after both lipase treatments, but only in the resistant biotype. The inhibitory effectiveness of the herbicide was either decreased or increased (to different extents) depending on the lipid class which was removed from the membrane and on the biotype considered. These results are discussed with reference to the possible conformational changes of the 32 kDa herbicide-binding polypeptide occurring after lipase treatments.Abbreviations Atrazine 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine - BSA bovine serum albumin - DCMU diuron, 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DGDG digalactosyldiacylglycerol - LRa lipase from Rhizopus arrhizus - MGDG monogalactosyldiacylglycerol - PC phosphatidylcholine - PG phosphatidylglycerol - PLA₂ phospholipase A₂ - R atrazine-resistant - S atrazinesusceptible