Regulation of Alternative Oxidase Activity in Six Wild Monocotyledonous Species. An in Vivo Study at the Whole Root Level1

The activity of the alternative pathway is affected by a number of factors, including the level and reduction state of the alternative oxidase (AOX) protein, and the reduction state of the ubiquinone pool. To investigate the significance of these factors for the rate of alternative respiration in vivo, we studied root respiration of six wild monocotyledonous grass species that were grown under identical controlled conditions. The activity of the alternative pathway was determined using the oxygen isotope fractionation technique. In all species, the AOX protein was invariably in its reduced (high activity) state. There was no correlation between AOX activity and AOX protein concentration, ubiquinone (total, reduced, or oxidized) concentration, or the reduction state of the ubiquinone pool. However, when some of these factors are combined in a linear regression model, a good fit to AOX activity is obtained. The function of the AOX is still not fully understood. It is interesting that we found a positive correlation between the activity of the alternative pathway and relative growth rate; a possible explanation for this correlation is discussed. Inhibition of the AOX (with salicylhydroxamic acid) decreases respiration rates less than the activity present before inhibition (i.e. measured with the 18O-fractionation technique)

Impact of mitochondrial alternative oxidase expression on the response of Nicotiana tabacum to cold temperature

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87094/1/j.1399-3054.2011.01471.x.pd

Ecosystem resistance in the face of climate change: a case study from the freshwater marshes of the Florida Everglades

Shaped by the hydrology of the Kissimmee-Okeechobee-Everglades watershed, the Florida Everglades is composed of a conglomerate of wetland ecosystems that have varying capacities to sequester and store carbon. Hydrology, which is a product of the region’s precipitation and temperature patterns combined with water management policy, drives community composition and productivity. As shifts in both precipitation and air temperature are expected over the next 100 years as a consequence of climate change, CO2 dynamics in the greater Everglades are expected to change. To reduce uncertainties associated with climate change and to explore how projected changes in atmospheric CO2 concentration and climate can alter current CO2 exchange rates in Everglades freshwater marsh ecosystems, we simulated fluxes of carbon among the atmosphere, vegetation, and soil using the DAYCENT model. We explored the effects of low, moderate, and high scenarios for atmospheric CO2 (550, 850, and 950 ppm), mean annual air temperature (þ1, þ2.5, and þ4.28C) and precipitation (2, þ7, and þ14%), as predicted by the IPCC for the year 2100 for the region, on CO2 exchange rates in short- and long-hydroperiod wetland ecosystems. Under 100 years of current climate and atmospheric CO2 concentration, Everglades freshwater marsh ecosystems were estimated to be CO2-neutral. As atmospheric CO2 concentration increased and under climate change projections, there were slight shifts in the start and length of the wet season (1 to þ7 days) and a small enhancement in the sink capacity (by 169 to 573 g C m2 century1 ) occurred at both short- and longhydroperiod ecosystems compared to CO2 dynamics under the current climate regime. Over 100 years, rising temperatures increased net CO2 exchange rates (þ1 to 13 g C m2 century1 ) and shifts in precipitation patterns altered cumulative net carbon uptake by þ13 to 46 g C m2 century1 . While changes in ecosystem structure, species composition, and disturbance regimes were beyond the scope of this research, results do indicate that climate change will produce small changes in CO2 dynamics in Everglades freshwater marsh ecosystems and suggest that the hydrologic regime and oligotrophic conditions of Everglades freshwater marshes lowers the ecosystem sensitivity to climate change. Key word

Activation of the plant mitochondrial alternative oxidase: insights from site-directed mutagenesis

AbstractThe homodimeric cyanide-resistant alternative oxidase of plant mitochondria reduces oxygen to water without forming ATP. Arabidopsis thaliana alternative oxidase AOX1a is stimulated by pyruvate or other α-keto acids associating with a regulatory cysteine at position 78, by succinate in a serine-78 mutant, and by site-directed mutation of position 78 to glutamate. The mechanism of activation was explored with additional amino acid substitutions made at Cys-78 in AOX1a, which was functionally expressed in Escherichia coli. Oxidases with positively charged substitutions (Lys and Arg) were insensitive to pyruvate or succinate but were more active than the wild type without pyruvate. Uncharged substitutions (Gln, Leu) produced an inactive enzyme. These results indicate that activation may be due to conformational changes caused by charge repulsion between the dimer subunits and not through a direct role of α-keto acids in catalysis. Oxygen isotope fractionation experiments suggest that the charge of the amino acid at position 78 also affects the entry of oxygen into the active site. Therefore, the N-terminal portion of the protein containing residue 78 can indirectly affect both catalysis at the diiron active site and the path of oxygen to that site. In addition, both positively and negatively substituted alternative oxidases were stimulated by glyoxylate, suggesting the presence of a second activation site, possibly Cys-128

The alternative oxidase in roots of Poa annua after transfer from high-light to low-light conditions

The activity of the alternative pathway can be affected by a number of factors, including the amount and reduction state of the alternative oxidase protein, and the reduction state of the ubiquinone pool. To investigate the importance of these factors in vivo, we manipulated the rate of root respiration by transferring the annual grass Poa annua L. from high-light to low-light conditions, and at the same time from long-day to short-day conditions for four days. As a result of the low-light treatment, the total respiration rate of the roots decreased by 45%, in vitro cytochrome c oxidase capacity decreased by 49%, sugar concentration decreased by 90% and the ubiquinone concentration increased by 31%, relative to control values. The absolute rate of oxygen uptake via the alternative pathway, as determined using the ^(18)O-isotope fraction technique, did not change. Conversely, the cytochrome pathway activity decreased during the low-light treatment; its activity increased upon addition of exogenous sugars to the roots. Interestingly, no change was observed in the concentration of the alternative oxidase protein or in the reduction state of the protein. Also, there was no change in the reduction state of the ubiquine pool. In conclusion, the concentration and activity of the alternative oxidase were not changed, even under serve light deprivation