When wild-type (wt) tobacco (Nicotiana tabacum cv. Petit Havana SR1) cells are grown under macronutrient (P or N) limitation, they induce large amounts of alternative oxidase (AOX), which constitutes a non-energy-conserving branch of the respiratory electron transport chain. To investigate the significance of AOX induction, wt cells were compared with transgenic (AS8) cells lacking AOX. Under nutrient limitation, growth of wt cell cultures was dramatically reduced and carbon use efficiency (g cell dry weight gain g(-1) sugar consumed) decreased by 42-63%. However, the growth of AS8 was only moderately reduced by the nutrient deficiencies and carbon use efficiency values remained the same as under nutrient-sufficient conditions. As a result, the nutrient limitations more severely compromised the tissue nutrient status (P or N) of AS8 than wt cells. Northern analyses and a comparison of the mitochondrial protein profiles of wt and AS8 cells indicated that the lack of AOX in AS8 under P limitation was associated with increased levels of proteins commonly associated with oxidative stress and/or stress injury. Also, the level of electron transport chain components was consistently reduced in AS8 while tricarboxylic acid cycle enzymes did not show a universal trend in abundance in comparison to the wt. Alternatively, the lack of AOX in AS8 cells under N limitation resulted in enhanced carbohydrate accumulation. It is concluded that AOX respiration provides an important general mechanism by which plant cells can modulate their growth in response to nutrient availability and that AOX also has nutrient-specific roles in maintaining cellular redox and carbon balance.

The alternative oxidase is a non-proton motive 'alternative' to electron transport through the cytochrome pathway. Despite its wasteful nature in terms of energy conservation, the pathway is likely present throughout the plant kingdom and appears to be expressed in most plant tissues. A small alternative oxidase gene family exists, the members of which are differentially expressed in response to environmental, developmental and other cell signals. The alternative oxidase enzyme possesses tight biochemical regulatory properties that determine its ability to compete with the cytochrome pathway for electrons. Studies show that alternative oxidase can be a prominent component of total respiration in important crop species. All these characteristics suggest this pathway plays an important role in metabolism and/or other aspects of cell physiology. This brief review is an introduction to experimental methods and approaches applicable to different areas of alternative oxidase research. We hope it provides a framework for further investigation of this fascinating component of primary plant metabolism.

Photosynthetic organisms undergo photoacclimation in response to changes in environmental conditions to maximize energy production and at the same time protect the light‐sensitive pigments and proteins from excess light. Low temperature and high irradiance both cause the electron transport chain to become more reduced which can result in the production of damaging reactive oxygen species. In the unicellular green alga Chlorella vulgaris Beij., light and temperature regulate light‐harvesting protein accumulation via the redox state of the plastoquinone pool. To investigate temperature‐dependent factor (s) regulating light‐harvesting protein accumulation we measured the abundance of chlorophyll biosynthetic precursors and reactive oxygen species production in C. vulgaris cells acclimated to a series of growth conditions. We observed that Mg‐protoporphyrin accumulates in response to low temperature, but its abundance does not correlate with light‐harvesting protein levels. Reactive oxygen levels measured under the same growth conditions strongly correlated with light‐harvesting protein levels. Therefore, we suggest that reactive oxygen species may act as part of both a temperature‐ and irradiance‐dependent signalling mechanism in the regulation of light‐harvesting protein accumulation in response to growth conditions.