Inhibition of trans-membrane hexacyanoferrate III reductase activity and proton secretion of maize (Zea mays L.) roots by thenoyltrifluoroacetone

Intact plants can reduce external oxidants by an appearingly trans-membrane electron transport. In vivo an increase in net medium acidification accomp anies the reduction of the apoplastic substrate. Up to now, several NAD(P)H dehydrogenases, b-type cytochromes, and a phylloquinone have been identifi ed and partially purified from plant plasma membranes. The occurrence of a quinone in the plasma membrane of maize roots supports the hypothetical mod el of a proton-transferring redox system, i.e., an electron transport chain with a quinone as mobile electron and proton carrier. In the present study the trans-membrane electron transport system of intact maize (Zen mays L.) roots was investigated. Flow-through and ionostat systems have been used t o estimate the electron and proton transport activity of this material. App lication of 4,4,4-trifluoro-1-(2-thienyl)-butane-1,3-dione (thenoyltrifluor oacetone) inhibited the reduction of ferricyanide in the incubation solutio n of intact maize roots up to 70%. This inhibition could not be washed off by rinsing the roots with fresh incubation medium. The acidification of the medium induced after ferricyanide application was inhibited to about 62%. The effects of thenoyltrifluoroacetone on proton fluxes in the absence of f erricyanide have been characterized in a pH-stat system. The net medium aci dification by maize roots was inhibited up to 75% by thenoyltrifluoroaceton e in the absence of ferricyanide, while dicumarol inhibited net acidificati on completely. the inhibition of H+-ATPase activity was estimated with plas ma membrane vesicles isolated by phase partitioning and treated with 0.05% (w/v) Brij 58. ATP-dependent proton gradients and Pi release were measured after preincubation with the effectors. The proton pumping activity by thos e plasma membrane vesicles was inhibited by dicumarol (53.6%) and thenoyltr ifluoroacetone (77.8%), while the release of Pi was unaffected by both inhi bitors.