The alternative oxidase of the inner mitochondrial membrane catalyses cyanide-insensitive respiration in plants and fungi. The molecular biology and regulation of this oxidase have been intensively studied over the past 10 years. Genes have been isolated, expression has been investigated and novel mechanisms for the regulation of activity have been discovered. This paper reviews these recent advances, focusing on the regulation of gene expression and activation by protein modification and organic acids, and possible roles of the alternative oxidase are discussed.

A method for preparing intact peribacteroid units from soybean nodules is described in detail. The polypeptide compositions of the peribacteroid membrane and peribacteroid space contents are described, and the properties of these compartments are discussed In the hght of recent literature. Evidence is presented that the peribacteroid membrane is permeable to succinate and malate but not to sucrose and glutamate. A dicarboxylate transporter on the peribacteroid membrane, which is capable of transporting malate and succinate at rates sufficient to support measured nitrogenase activity, is described The properties of an ATPase found on the peribacteroid membrane are also described and compared to those described in other reports This ATPase is able to catalyse energisation of the peribacteroid membrane in an uncoupler and vanadate-sensitive manner and may play an important role in the regulation of ion movements across the membrane.

The rate of O2 uptake in the dark in bean leaves and leaf slices decreased during development. The use of inhibitors of the cytochrome and alternative pathways and the uncoupier carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) indicated that this decline was largely due to a decline in the activity and capacity of the cytochrome path. However, the capacity of the alternative path remained more or less constant with leaf expansion, and thus percentage cyanide resistance of respiration increased with leaf expansion. The activity of the alternative path was small in bean leaves and leaf slices, and decreased during development. Respiration, mainly the cytochrome pathway, of bean leaf slices was stimulated by FCCP at all ages, the stimulation being more pronounced in young leaves. The rate of overall respiration, the activities of the cytochrome and alternative paths, and the extent to which FCCP stimulated respiration in bean leaf slices were positively correlated with endogenous free sugar levels during aging. Similar trends were observed with mitochondria isolated from pea leaves. Leaf mitochondria from older plants had lower cytochrome path capacity, but the capacity of the alternative path remained much the same as that in leaf mitochondria from seedlings.

Respiratory processes and growth rates of alpine and lowland species of three genera (Ranunculus, Plantago and Luzula) were compared. Relative growth rates were determined for the first 14 weeks of growth at two temperatures (7-10°C and 12-15°C). Generally, the relative growth rates of the alpine species were lower than those of their lowland relatives. Whole-plant respiration rates were measured and leaf slices from each species were used for a detailed analysis of respiratory pathways. Major differences were found between genera, particularly in their alternative oxidase activity, but respiratory patterns (both whole-plant respiration rates and the relative rates of cytochrome and alternative pathways in leaf slices) were maintained within a given genus, independent of the environmental or geographical origin of each species from that genus. The lack of correlation between growth rates and respiration rates suggests that the alpine plants used their respiratory products less efficiently than did the lowland species.

A procedure is described for the preparation of metabolically active mesophyll protoplasts from maize, and of functional, intact chloroplasts from these protoplasts. Intact protoplasts show no oxygen evolution with 3-phosphoglycerate or with oxalacetate plus pyruvate as substrates, even when these substances are provided at high concentrations. On the other hand, protoplast extracts and chloroplasts display rates of oxygen evolution of 2-3 �mol min-1 (mg Chl)-1 with the same substances. Pyruvate stimulates oxalacetate-dependent oxygen evolution substantially, indicating good coupling between non-cyclic electron flow and phosphorylation. Low PI concentrations stimulate 3-phosphoglycerate-dependent oxygen evolution; high PI concentrations, and pyridoxal phosphate, inhibit this activity, suggesting a common carrier for 3-phosphoglycerate and PI.