Alternative Oxidase Promotes Biofilm Formation of Candida albicans

This study was designed to analyze the effect of the mitochondrial respiratory pathways of Candida albicans （C. albicans） on the biofilm formation. The 2, 3-bis （2-methoxy- 4-nitro-5-sulfophenyl）-2H-tetrazolium-5-carboxanilide （XTT） reduction assay was used to measure the metabolic activities of biofilms formed by the C. albicans which were cultured in the presence of respiratory pathways inhibitors. The biofilms formed by the wide type （WT）, GOA1-deleted （GOA31）, GOAl-reconstituted （GOA32）, AOXla-deleted （AOX1） and AOX1b- deleted （AOX2） C. albicans strains were examined by the XTT reduction assay and fluorescence microscopy. The expression of adhesion-related genes BCR1, ALS1, ALS3, ECEI and HWP1 in the biofilms formed by the above five C. albicans strains was detected by real time polymerase chain reaction. It was found that the metabolic activity of biofilms formed by C. albicans was decreased in the presence of alternative oxidase inhibitor whereas it was increased in the presence of classical mitochondrial respiratory pathway complex Ⅲ or complex IV inhibitor. AOX1 strain produced scarce biofilms interspersed with few hyphal filaments. Moreover, no significant changes in the expression of BCR1 and ALS3 were observed in the AOX 1 strain, but the expression of ALS1 and ECE1 was down-regulated, and that of HWP1 was up-regulated. These results indicate that both AOX1 and AOX2 can promote the biofilm formation. However, AOX1a primarily plays a regulatory role in biofilm formation in the absence of inducers where the promoting effect is mainly achieved by promoting mycelial formation.

Effects of Enhanced UV-B Radiation on the Activity and Expression of Alternative Oxidase in Red Kidney Bean Leaves

An increase in ultraviolet （UV） B radiation on the earth＇s surface is a feature of current global climate changes. It has been reported that alternative oxidase （AOX） may have a protective role against oxidative stress induced by environmental stresses, such as UV-B. To better understand the characteristic tolerance of plants to UV-B radiation, the effects of enhanced UV-B radiation on the activity and expression of AOX in red kidney bean （Phaseolus vulgaris） leaves were investigated in the present study. The results show that the total respiration rate and AOX activity in red kidney bean leaves increased significantly during treatment with enhanced UV-B. However, cytochrome oxidase （COX） activity did not change significantly. The H2O2 content was also markedly increased and reached a maximum of 4.45 mmol·L^-1·g^-1 DW （dry weight） at 24 h of UV-B treatment, before dropping rapidly. Both alternative pathway content and alternative pathway activity were increased in the presence of exogenous H2O2. Immunoblotting analysis with anti-AOX monoclonal antibody revealed that expression of the AOX protein increased in red kidney bean leaves under enhanced UV-B radiation, reaching a peak at 72 h. In addition, AOX expression in red kidney bean leaves was induced by exogenous H2O2. These data indicate that the increase in AOX activity in red kidney bean leaves under enhanced UV-B radiation was mainly due to H2O2-induced AOX expression.

Distinct roles of alternative oxidase pathway during the greening process of etiolated algae

The vital function of mitochondrial alternative oxidase(AOX) pathway in optimizing photosynthesis during plant de-etiolation has been well recognized. However, whether and how AOX impacts the chloroplast biogenesis in algal cells remains unclear. In the present study, the role of AOX in regulating the reassembly of chloroplast in algal cells was investigated by treating Auxenochlorella protothecoides with salicylhydroxamic acid(SHAM), the specific inhibitor to AOX, in the heterotrophy to autotrophy transition process. Several lines of evidences including delayed chlorophyll accumulation, lagged reorganization of chloroplast structure, altered PSI/PSII stoichiometry, and declined photosynthetic activities in SHAM treated cells indicated that the impairment in AOX activity dramatically hindered the development of functioning chloroplast in algal cells. Besides, the cellular ROS levels and activities of antioxidant enzymes were increased by SHAM treatment, and the perturbation on the balance of NAD+/NADH and NADP+/NADPH ratios was also observed in A. protothecoides lacking AOX activity, indicating that AOX was essential in promoting ROS scavenging and keeping the redox homeostasis for algal chloroplast development during greening. Overall, our study revealed the essentiality of mitochondrial AOX pathway in sustaining algal photosynthetic performance and provided novel insights into the physiological roles of AOX on the biogenesis of photosynthetic organelle in algae.

Contribution of the Alternative Respiratory Pathway to PSII Photoprotection in C3 and C4 Plants

The mechanism by which the mitochondrial alternative oxidase （AOX） pathway contributes to photosystem II （PSII） photoprotection is in dispute. It was generally thought that the AOX pathway protects photosystems by dissipating excess reducing equivalents exported from chloroplasts through the malate/oxaloacetate （Mal/OAA） shuttle and thus preventing the over-reduction of chloroplasts. In this study, using the aoxla Arabidopsis mutant and nine other C3 and C4 plant species, we revealed an additional action model of the AOX pathway in PSII photoprotection. Although the AOX pathway contributes to PSII photoprotection in C3 leaves treated with high light, this contribution was observed to disappear when photorespiration was suppressed. Disruption or inhibition of the AOX pathway significantly decreased the photorespiration in C3 leaves. Moreover, the AOX pathway did not respond to high light and contributed little to PSII photoprotection in C4 leaves possessing a highly active Mal/OAA shuttle but with little photorespiration. These results demonstrate that the AOX pathway contributes to PSII photoprotection in C3 plants by maintaining photo- respiration to detoxify glycolate and via the indirect export of excess reducing equivalents from chloro-plasts by the MaI/OAA shuttle. This new action model explains why the AOX pathway does not contribute to PSII photoprotection in C4 plants.

Effect of Osmotic Stress in Early Stages of Ontogenesis on Root Respiration, Growth, Sugar Content,and Cell Injury in Maize Seedlings Differing in Drought Sensitivity

Cultivars of maize （Zea mays L.） with different sensitivity to drought were exposed to 0.3 mol/L sorbitol （-1.4 MPa water potential） for 24 h. Exposure to water deficiency significantly reduced the growth of both shoots （coleoptile and hypocotyl） and roots. Shoot growth was inhibited more than the growth of roots. Osmotic stress enhanced accumulation of soluble sugars. Electrolyte leakage, a cell injury index, was slightly increased after 0.3 mol/L sorbitoh Respiration was measured in the presence and absence of 2,6-dlchloro-phenol indophenoh 2,6-Dichloro-phenol indophenol did not influence respiration rates, because statistically equal results were observed under both conditions. Total respiration （VT） decreased after osmoticum treatment. There were no significant differences in the VT among the cultlvars analysed. The decrease In VT was caused by a decline In the activities and capacities of both cytochrome （Vcyt, Vcyt） and alternative pathway （Valt, Valt） of respiration. A high residual respiration （Vres） was observed, up to 27% of total uninhibited respiration. The result of uncoupler use clearly indicated that coupling was maintained after 24 h of osmotic stress. The recovery of the respiration rate was comparable with that of non-stressed control rates. According to these observations, no possible mltochondrial damage is expected. Water deficiency did not induce a stimulation of the alternative oxidase, so we assume that the stimulation of the alternative pathway is not related to drought stress resistance; rather, the function of the alternative pathway is to balance carbon metabolism and electron transport in a response to a changing environment.

Mitochondrial Sulfide Detoxification Requires a Functional Isoform O-Acetylserine（thiol）lyase C in Arabidopsis thaliana

In non-cyanogenic species, the main source of cyanide derives from ethylene and camalexin biosyntheses. In mitochondria, cyanide is a potent inhibitor of the cytochrome c oxidase and is metabolized by the β-cyanoalanine synthase CYS-C1, catalyzing the conversion of cysteine and cyanide to hydrogen sulfide and β-cyanoalanine. The hydrogen sulfide released also inhibits the cytochrome c oxidase and needs to be detoxified by the O-acetylserine（thiol）lyase mitochondrial isoform, OAS-C, which catalyzes the incorporation of sulfide to O-acetylserine to produce cysteine, thus generating a cyclic pathway in the mitochondria. The loss of functional OAS-C isoforms causes phenotypic characteristics very similar to the loss of the CYS-C1 enzyme, showing defects in root hair formation. Genetic complementation with the OAS-Cgene rescues the impairment of root hair elongation, restoring the wild-type phenotype. The mitochondria compromise their capacity to properly detoxify cyanide and the resulting sulfide because the latter cannot re-assimilate into cysteine in the oas-c null mutant. Consequently, we observe an accumulation of sulfide and cyanide and of the alternative oxidase, which is unable to prevent the production of reactive oxygen species probably due to the accumulation of both toxic molecules. Our results allow us to suggest that the significance of OAS-C is related to its role in the proper sulfide and cyanide detoxification in mitochondria.

A nuclear-encoded mitochondrial gene AtCIB22 is essential for plant development in Arabidopsis

Complex I （the NADH：ubiquinone oxidoreductase） of the mitochondrial respiratory chain is a complicated, multi-subunit, membrane- bound assembly and contains more than 40 different proteins in higher plants. In this paper, we characterize the Arabidopsis homologue （designated as AtCIB22） of the B22 subunit of eukaryotic mitochondriai Complex I. AtCIB22 is a single-copy gene and is highly con- served throughout eukaryotes. AtCIB22 protein is located in mitochondria and the AtC1B22 gene is widely expressed in different tissues. Mutant Arabidopsis plants with a disrupted AtC1B22 gene display pleiotropic phenotypes including shorter roots, smaller plants and de- layed flowering. Stress analysis indicates that the AtC1B22 mutants＇ seed germination and early seedling growth are severely inhibited by sucrose deprivation stress but more tolerant to ethanol stress. Molecular analysis reveals that in moderate knockdown AtCIB22 mutants, genes including cell redox proteins and stress related proteins are significantly up-regulated, and that in severe knockdown AtCIB22 mu- tants, the alternative respiratory pathways including NDA1, NDB2, AOXla and AtPUMP1 are remarkably elevated. These data demon- strate that AtCIB22 is essential for plant development and mitochondrial electron transport chains in Arabidopsis. Our findings also en- hance our understanding about the physiological role of Complex I in plants.

Relationship Between Stimulated Ethylene Production and Alternative Respiration Pathway in "Royal Gala" Apple Fruit

Endogenous ethylene production and alternative oxidase (AOX) protein expression in 'Royal Gala' apple fruits were investigated after treatments with cold ( 0℃ for 1 week) and heat ( 38℃ for 1 h). A monoclonal antibody to the terminal oxidase of the alternative pathway from Sauromatum guttatum was used to identify the AOX protein in apple fruits. The molecular mass of AOX in 'Royal Gala' apple fruits is approximately 38 kDa, similar to those reported in tobacco and tomato. The cold treatment depressed the release of endogenous ethylene production before the climacteric ethylene production and obviously induced the expression of AOX protein expression. The heat treatment had the opposite effects on the ethylene production and AOX protein expression. In addition, the climax of endogenous ethylene production preceded the maximum AOX expression after the cold temperature treatment. It is therefore proposed that in climacteric fruits the production of induced ethylene is not coordinated with the level of AOX protein.

The Response Difference of Mitochondria in Recalcitrant Antiaris toxicaria Axes and Orthodox Zea mays Embryos to Dehydration Injury

Long-term preservation of recalcitrant seeds is very difficult because the physiological basis on their desiccation sensitivity is poorly understood. Survival of Antiaris toxicaria axes rapidly decreased and that of immature maize embryos very slowly decreased with dehydration. To understand their different responses to dehydration, we examined the changes in mitochondria activity during dehydration. Although activities of cytochrome （Cyt） c oxidase and malate dehydrogenase of the A. toxicaria axis and maize embryo mitochondria decreased with dehydration, the parameters of maize embryo mitochondria were much higher than those of A. toxicaria, showing that the damage was more severe for the A. toxicaria axis mitochondria than for those of maize embryo. The state I and III respiration of the A. toxicaria axis mitochondria were higher than those of maize embryo, the former rapidly decreased, and the latter slowly decreased with dehydration. The proportion of Cyt c pathway to state III respiration for the A. toxicaria axis mitochondria was low and rapidly decreased with dehydration, and the proportion of alternative oxidase pathway was high and slightly increased with dehydration. In contrast, the proportion of Cyt c pathway for maize embryo mitochondria was high, and that of alternative oxidase pathway was low. Both pathways decreased slowly with dehydration.