Effects of 3,5-Dihalogenated-4-hydroxybenzonitriles on the Activity of Mitochondria From White Potato Tubers

Unique cellular effect of the herbicide bromoxynil revealed by electrophysiological studies using characean cells

In a previous paper, we proposed that the primary action of the herbicide bromoxynil (BX; 3,5-dibromo-4-hydroxybenzonitrile) is cytosol acidification, based on the fact that bromoxynil induced the inhibition of cytoplasmic streaming and cell death of Chara corallina in acidic external medium (Morimoto and Shimmen in J Plant Res 121:227-233, 2008). In the present study, electrophysiological analysis of the BX effect was carried out in internodal cells of C. corallina. Upon addition of BX, a large and rapid pH-dependent depolarization was induced, supporting our hypothesis. Ioxynil, which belongs to the same group as bromoxynil, also induced a large and rapid membrane depolarization in a pH-dependent manner. On the other hand, four herbicides belonging to other groups of herbicides did not induce such a membrane depolarization. Thus, BX has a unique cellular effect. The decrease in the electro-chemical potential gradient for H(+) across the plasma membrane appears to result in inhibition of cell growth and disturbance of intracellular homeostasis in the presence of BX.

Mode of action of photosynthesis inhibiting 4-hydroxy-benzonitriles containing nitro group

The effects of 3,5-disubstituted 4-hydroxy-benzonitriles - bromoxynil and compounds containing nitro group - on some photosynthetic parameters of spinach and wheat were investigated. There is considerable difference in the effectivity of the compounds even at the level of chloroplasts. On the basis of their effect on the development of CO2 fixation of greening wheat seedlings bromoxynil and 3,5-dinitro-4-hydroxy-derivative were ineffective, while the asymmetric - 3-nitro-5-halogenic-substituted - compounds showed a strong effectivity. The bromine and iodine containing asymmetric compounds reduced the intensity of the short wavelength fluorescence. All asymmetric compounds shifted the long wavelength fluorescence maximum with 5-8 nm to shorter wavelength. These results refer to inhibition of formation of chlorophyll forms absorbing at longer wavelengths and to the change in the conditions of energy migration. The ultrastructure of the chloroplasts in the treated seedlings became similar to the shade adapted chloroplasts.

Intact tissue assay for nitrite reductase in barley aleurone layers

A method has been devised for the detection and measurement of nitrite reductase activity in intact barley (Hordeum vulgare L. cv. Himalaya) aleurone layers. The technique involves feeding aleurone layers nitrite and measuring nitrite disappearance after a given time period. The method also allows simultaneous determination of nitrite uptake by the tissue. Quantitative recovery of nitrite is obtained by rapid heating of tissue in the presence of dimethyl sulfoxide.Using the procedure described, nitrite reductase activity in intact barley aleurone layers was determined. Enzyme activity was increased by prior incubation of the tissue with nitrate, but considerable activity was present in tissue incubated without nitrate. Nitrate-induced activity was inhibited by cycloheximide but not by actinomycin D. Enzyme activity in induced layers was inhibited by 2,4-dinitrophenol, and partially by antimycin A and 2-n-heptyl-4-hydroxyquinoline N-oxide. Activity in noninduced tissue appeared to be less sensitive to these respiratory inhibitors. In contrast, both activities were inhibited more than 90% by anaerobiosis; but nitrate-induced and noninduced aleurone layers were able to reduce nitrite anaerobically when the concentration of substrate in the assay medium was reduced from 250 mum to 25 mum. Nitrite uptake was relatively insensitive to anaerobiosis and to the inhibitors tested.Nitrite depletion from the medium by aleurone layers was rapid at pH 4.5 and negligible at pH 7.5. Nitrite accumulated at pH 4.5 under anaerobic conditions was rapidly released when the tissue was transferred to medium at pH 7.5. Nitrite release at pH 7.5 occurred whether the tissue was maintained under anaerobic or aerobic conditions.

Effects of optically active 1-(alpha-methylbenzyl)-3-(3,4-dichlorophenyl)urea on reactions of mitochondria and chloroplasts

Effects of the R- and S-isomers and racemate of 1-(alpha-methylbenzyl)-3-(3,4-dichlorophenyl)urea (MBPU) were measured on phosphorylation and electron transport in mung bean (Phaseolus aureus L.) mitochondria and spinach (Spinacia oleracea L.) chloroplasts.In chloroplasts, S-MBPU inhibited basal and methylamine-uncoupled electron transport with ferricyanide as the oxidant, both photoreduction and coupled photophosphorylation with water as the electron donor and with ferricyanide and nicotinamide adenine dinucleotide phosphate (NADP) as oxidants, and cyclic photophosphorylation with phenazine methosulfate as the electron mediator under an argon gas phase. With ascorbate 2,6-dichloro-phenolindophenol as the electron donor, phosphorylation coupled to NADP reduction was inhibited, but the reduction of NADP was not inhibited. The R-isomer of MBPU, like the S-isomer, inhibited all of the photophosphorylation reactions studied. However, unlike the S-isomer, the R-isomer either did not inhibit or was a very weak inhibitor of all photoreduction reactions. The effects of the MBPUs on the chloroplast reactions can be explained by action at two different sites: an optically specific site near photosystem II and the oxygen evolution pathway, and a second optically nonspecific site associated with the generation of ATP.In mitochondria, both the R- and S-isomers stimulated state 4 respiration, inhibited state 3 respiration, and released oligomycin-inhibited respiration with malate, succinate, and NADH as substrates. Both enantiomers were equally active in all studies with malate and succinate as substrates. However, with NADH as substrate, R-MBPU was a stronger inhibitor of state 3 respiration and a weaker stimulator of state 4 respiration than S-MBPU.