EXPRESSION AND KINETICS OF THE MITOCHONDRIAL ALTERNATIVE OXIDASE IN NITROGEN-FIXING NODULES OF SOYBEAN ROOTS

Mitochondria from nitrogen-fixing soybean nodules had previously been found to display very low rates of cyanide-insensitive O-2 consumption and to contain little protein immunoreactive with alternative oxidase (AOX) antibodies (A. Kearns et al. 1992, Plant Physiology 99, pp, 712 -717). Following the recent identification of novel regulatory mechani sms for AOX, we have re-investigated AOX activity in nodule mitochondr ia. When AOX is fully activated by adding pyruvate and a reductant, si gnificant AOX activity can be measured in nodule mitochondria when sub strates that cause the ubiquinone (Q) pool to become > 90% reduced are provided, These nodule mitochondria contain approximate to 10% of the immunoreactive protein observed in soybean root or cotyledon mitochon dria. Furthermore, transcripts from all three members of the soybean A OX multigene family were significantly less abundant in total RNA from nodules than from other tissues, The ratio of transcripts from the th ree AOX genes varied between the outer cortex and the inner infected r egion of the nodule, Measurements of Q redox state (Q(r)Q(t)) versus O -2 consumption showed that much higher levels of Q reduction were requ ired for AOX to be active in mitochondria from nodules than in mitocho ndria from roots and cotyledons, and that root and nodule mitochondria were more dependent on added pyruvate for maximum AOX activity, Kinet ic modelling suggests that these differences were simply the result of differences in the relative amount of active AOX protein, rather than differences in Q content or the AOX isoform expressed, in the differe nt tissues.