Hideo Yamasaki

Reversible inhibition of photophosphorylation in chloroplasts by nitric oxide

Nitric oxide (NO) is a bioactive molecule involved in diverse physiological functions in plants. Here we demonstrate that NO is capable of regulating the activity of photophosphorylation in chloroplasts. The electron transport activity in photosystem II determined from chlorophyll a fluorescence was inhibited by NO. NO also inhibited light‐induced ΔpH formation across the thylakoid membrane. High concentrations of nitrite and nitrate did not show such inhibitory effects, suggesting that the inhibition is not due to uncoupling effects of the oxidized products of NO. ATP synthesis activity upon illumination was severely inhibited by NO (IC50=0.7 μM). The inhibition was found to be temporary and the activity was completely recovered by removing NO. Bovine hemoglobin and bicarbonate were effective in preventing NO‐dependent inhibition of photophosphorylation. These results indicate that NO is a reversible inhibitor of photosynthetic ATP synthesis.

Lipid Peroxidation Induced by Phenolics in Conjunction with Aluminum Ions

Using the whole plant and model systems, we demonstrate that the aluminum ions (Al3+) stimulate phenolic-dependent lipid peroxidation. Lipid peroxidation in barley (Hordeum vulgare L. cv. Donor) roots was 30 % higher under AlCl3 treatment than without Al. Major decomposition product of lipid peroxidation was 4-hydroxynonenal (4-HNE) but not thiobarbituric acid reactive substances (TBARS), a widely used markers for lipid peroxidation. Similarly, AlCl3 stimulated lipid peroxidation of soybean liposomes in the presence of chlorogenic acid (CGA) and H2O2/horseradish peroxidase system which can oxidize phenolics. Al3+ was found to enhance lipid peroxidation induced by oxidized CGA. Intermediates of lignin biosynthesis in plants, including p-coumaric acid, ferulic acid, sinapic acid and coniferyl alcohol, also showed similar effects. These results suggest that Al3+ has a potential to induce oxidative stress in plants by stimulating the prooxidant nature of endogenous phenolic compounds.

Involvement of nitric oxide synthase in sucrose-enhanced hydrogen peroxide tolerance of Rhodococcus sp. strain APG1, a plant-colonizing bacterium

Hydrogen peroxide (H2O2) tolerance of Rhodococcus sp. strain APG1, previously isolated from the aquatic fern Azolla pinnata, was examined in relation to nitric oxide (NO) production by cells cultured on a variety of C sources. Cells inoculated onto A. pinnata fronds established a surface-sterilant resistant density of 2-4x10(7) cells g(-1) without causing disease. Compared to cultures containing glucose, fructose, mannitol, or glycerol, those provided only with sucrose displayed, on a per C basis, substantially lower (<10%) growth yields and higher resistance to H2O2. NO, a positive regulator of catalase synthesis in bacteria, was produced in larger amounts in sucrose-grown cells as evidence by eightfold greater per cell accumulations in the medium of nitrite (NO2-), a stable oxidation product of NO. Addition to cells of L-arginine, the substrate for nitric oxide synthase (NOS), stimulated production of NO, detected both by fluorometric reaction with diaminofluorescein-FM diacetate (DAF-FM DA) and by increased levels of NO2- in the culture medium. These results suggest that sucrose may enhance H2O2 tolerance of Rhodococcus APG1 by increasing cellular NO producing capacity. We propose a regulatory role for NOS in promoting tolerance of Rhodococcus APG1 to oxidative stress in the phyllosphere.

Involvement of nitric oxide in the mechanism for stomatal opening in Vicia faba leaves

Nitrite, as well as the nitric oxide (NO) donor S-nitroso-N-acethylpenisilamine (SNAP), was found to increase the aperture of stoma on Vicia faba leaf peels. The results demonstrated here suggest that the nitrite-dependent NO production pathway would be involved in the signal transduction for stomatal movements.

BIODEGRADATION OF DIESEL FUEL BY AN AZOLLA-DERIVED BACTERIAL CONSORTIUM

ABSTRACT The widely distributed water fern Azolla was investigated for use as an amendment in the bioremediation of fuel-contaminated environments. In a field experiment Azolla pinnata as well as Pistia stratiotes and Salvinia molesta were applied to plots containing soil that had been surface-contaminated with diesel fuel (2.4 L m−2) and flooded with water. The plants quickly died and bacterial flocs developed around the dead A. pinnata fronds. After 16 weeks, diesel concentrations (as determined by levels of gas chromatography-detectable hydrocarbons) in the plant-added plots were less than half that of the control plot, and concentrations of xylenes and ethylbenzene were 50–100 times lower. In laboratory experiments, a consortium composed of A. pinnata-derived bacteria displayed dense growth in a 4% diesel-containing mineral salts medium and was found to lower the fluorescence from aromatic compounds by approximately 50% after 19 d. It is concluded that the observed enhancement of diesel degradation in the plant-added plots was due to the release of bacteria (bioaugmentation) and physiochemical improvement of the plot conditions (biostimulation).

Inhibitory effects of flavonoids on alternative respiration of plant mitochondria

Inhibitory effects of flavonoids on plant alternative respiration were investigated using isolated mitochondria of Vigna radiata seedlings. The antioxidant flavonoids quercetin and myricetin effectively inhibited alternative respiration. We suggest that radical scavenging activity is involved in the inhibitory mechanism.