Regulation of alternative oxidase activity in six wild monocotyledonous species. An in vivo study at the whole root level

The activity of the alternative pathway is affected by a number of factors, including the level and reduction state of the alternative oxidase (AOX) p rotein, and the reduction state of the ubiquinone pool. To investigate the significance of these factors for the rate of alternative respiration in vi vo, 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 t echnique. In all species, the AOX protein was invariably in its reduced (hi gh activity) state. There was no correlation between AOX activity and AOX p rotein concentration, ubiquinone (total, reduced, or oxidized) concentratio n, or the reduction state of the ubiquinone pool. However, when some of the se factors are combined in a linear regression model, a good fit to AOX act ivity is obtained. The function of the AOX is still not fully understood. I t 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 salicylhydr oxamic acid) decreases respiration rates less than the activity present bef ore inhibition (i.e. measured with the O-18-fractionation technique).