The effect of growth and measurement temperature on the activity of the alternative respiratory pathway

A postulated role of the CN-resistant alternative respiratory pathway in pl ants is the maintenance of mitochondrial electron transport at low temperat ures that would otherwise inhibit the main phosphorylating pathway and prev ent the formation of toxic reactive oxygen species. This role is supported by the observation that alternative oxidase protein levels often increase w hen plants are subjected to growth at low temperatures. We used oxygen isot ope fractionation to measure the distribution of electrons between the main and alternative pathways in mung bean (Vigna radiata) and soybean (Glycine max) following growth at low temperature. The amount of alternative oxidas e protein in mung bean grown at 19 degrees C increased over 2-fold in both hypocotyls and leaves compared with plants grown at 28 degrees C but was un changed in soybean cotyledons grown at 14 degrees C compared with plants gr own at 28 degrees C. When the short-term response of tissue respiration was measured over the temperature range of 35 degrees C to 9 degrees C, decrea ses in the activities of both main and alternative pathway respiration were observed regardless of the growth temperature, and the relative partitioni ng of electrons to the alternative pathway generally decreased as the tempe rature was lowered. However, cold-grown mung bean plants that upregulated t he level of alternative oxidase protein maintained a greater electron parti tioning to the alternative oxidase when measured at temperatures below 19 d egrees C supporting a role for the alternative pathway in response to low t emperatures in mung bean. This response was not observed in soybean cotyled ons, in which high levels of alternative pathway activity were seen at both high and low temperatures.