Regulation of the activity of the alternative oxidase in callus forming discs from potato tubers

Callus-forming discs from potato tubers lose 80% of their starch during one month of incubation on nutrient medium containing either 0, 3 or 6% (w/v) sucrose. The content of soluble sugar in the discs varies from 5 mg (incubated without sucrose) to 22 mg (on 3% sucrose) and 40 mg (on 6% sucrose) per g fresh weight. The activity of the cytochrome pathway (V_cyt) increases during the first week of incubation on all media. Thereafter V_cyt decreases again on 0% sucrose medium, while it remains constant on 3 and 6% sucrose media. Alternative pathway capacity (V_alt), absent in freshly sliced tissue, shows a sharp increase during the first days of incubation, independent of the sucrose concentration in the medium. This capacity further increases during prolonged incubation on 3 and 6% sucrose but decreases on 0% sucrose. The in vivo activity of the alternative pathway (the participation in uninhibited respiration, ?V_alt) varies with the sucrose concentration and with the culture time. In tissue incubated for 2-3 weeks on 6% sucrose as much as 45% of the electrons are transfered to oxygen via the alternative pathway. In this tissue the factor Q (the part of the alternative pathway capacity that is operative) is about 0.8, while in tissue incubated on 0 and 3% sucrose media p generally does not exceed 0.5. When chloramphenicol, an inhibitor of mitochondrial protein synthesis, is added to the medium together with 3% sucrose, the increase in V_yet does not occur, while the induction of V_alt during the first week of incubation is the same as without chloramphenicol. A greater part of the alternative pathway capacity becomes operative in this tissue, leading to values of Q of almost 1 after prolonged incubation. Apparently, incubation on high sugar medium leads to extra participation in respiration of the energetically inefficient alternative oxidase pathway Excess sugar leads to wasteful respiration suggesting that the alternative oxidase functions as an ‘energy overflow’.