title: Responses of spinach leaf mitochondria to low N availability.
abstract: Low N availability induces carbohydrate accumulation in leaf cells, which often causes suppression of photosynthesis. Under low N supply, excess carbohydrates would be preferentially respired by the non-phosphorylating pathways, such as the alternative oxidase (AOX) and uncoupling protein (UCP), which would suppress the excessive increase in the ratio of C to N (C/N ratio). In leaves, however, responses of these pathways to the low N stress are still unknown. We examined the mitochondrial respiratory pathways in spinach leaves grown at three different N availabilities to clarify whether the respiratory pathways change depending on the N availabilities. With the decrease in N availability, leaf respiratory rates per leaf area decreased, but the rates on the leaf N basis were comparable. Using fumarase activities of whole leaf extracts and isolated mitochondria, we estimated mitochondrial protein contents per leaf N. The contents increased with the decrease in the N availability, that is, at the low N availability, N was preferentially invested into mitochondria. On the mitochondrial protein basis, capacities of cytochrome pathway (CP) and cytochrome c oxidase (COX) were comparable regardless of the N availabilities, whereas both AOX capacity and the amounts of AOX protein increased with the decrease in the N availability. Some enzymes of tricarboxylic acid (TCA) cycle, especially NAD-dependent malic enzyme (NAD-ME), showed higher capacities under lower N. On the other hand, amounts of UCP did not differ amongst the N availabilities. These results indicated that, under low N stress, AOX will be preferentially up-regulated and will efficiently consume excess carbohydrates, which leads to suppressing the rise in the C/N ratio to a moderate level.
authors.affiliation: Department of Biology, Graduate School of Science, Osaka University, 1-1 Machikaneyama-Cho, Toyonaka, Osaka 560-0043, Japan. knoguchi@bio.sci.osaka-u.ac.jp
authors.forename: Ko
authors.initials: K
authors.lastname: Noguchi
authors.forename: Ichiro
authors.initials: I
authors.lastname: Terashima
mesh_terms: D050260:Carbohydrate Metabolism; D002244:Carbon / Q000378:metabolism; D002952:Citric Acid Cycle / Q000502:physiology; D004579:Electron Transport / Q000502:physiology; D003576:Electron Transport Complex IV / Q000378:metabolism; D005649:Fumarate Hydratase / Q000378:metabolism; D007150:Immunohistochemistry; D007473:Ion Channels / Q000032:analysis / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism* / Q000502:physiology; D024101:Mitochondrial Proteins / Q000032:analysis / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism*; D010088:Oxidoreductases / Q000032:analysis / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000378:metabolism; D010936:Plant Extracts / Q000737:chemistry; D018515:Plant Leaves / Q000033:anatomy & histology / Q000378:metabolism / Q000502:physiology; D010940:Plant Proteins / Q000378:metabolism; D018724:Spinacia oleracea / Q000033:anatomy & histology / Q000378:metabolism* / Q000502:physiology; D000071256:Uncoupling Protein 1; D000099188:Alternative Oxidase
chemical_list: D002244:Carbon; D003576:Electron Transport Complex IV; D005649:Fumarate Hydratase; D007473:Ion Channels; D024101:Mitochondrial Proteins; D009584:Nitrogen; D010088:Oxidoreductases; D045332:Photosystem II Protein Complex; D010936:Plant Extracts; D010940:Plant Proteins; D000071256:Uncoupling Protein 1; D000099188:Alternative Oxidase