Influence of Trinexapac-Ethyl on Respiration of Isolated Wheat Mitochondria

The plant growth regulators (PGRs) 2,4-D [(2,4-dichlorophenoxy) acetic acid] and malic hydrazide (1,2-dyhydro-3,6-pyridazinedione) have been shown to reduce plant respiration. The effect of other PGRs such as trinexapac-ethyl [(4-cyclopropyl-α-hydroxy-methylene)-3,5-dioxocyclohexanecarboxylic acid methyl ester] on respiration is unknown. Experiments were conducted to evaluate the effects of trinexapac-ethyl and 2-oxoglutarate on the respiration of mitochondria isolated from wheat (Triticum aestivum 'Arapahoe') seedlings. Trinexapac-ethyl applied at increasing concentrations inhibited nicotinamide adenine dinucleotide (NADH)-dependent O uptake, while 2-oxoglutarate had no significant effect. This is different than other reports that trinexapac-ethyl may compete with 2-oxoglutarate for binding sites. Different regions of the mitochondria were tested to determine the site of inhibition caused by trinexapac-ethyl. Nicotinamide adenine dinucleotide dehydrogenase activity, duroquinol-dependent O uptake, and cytochrome bc 1 activity were all reduced by 30% in the presence of 10 mMtrinexapac-ethyl. Succinate-dependent O uptake, alternative oxidase, and cytochrome oxidase were not reduced by any trinexapac-ethyl concentration. This revealed possible interference of trinexapac-ethyl with ubiquinone binding sites. The reduced form of trinexapac-ethyl was observed to inhibit the electron transport chain greater than the oxidized form. Reduction in respiration from trinexapac-ethyl may result in greater stress tolerance in treated plants.