title: Coordinated regulation of photosynthetic and respiratory components is necessary to maintain chloroplast energy balance in varied growth conditions.
abstract: Mitochondria have a non-energy-conserving alternative oxidase (AOX) proposed to support photosynthesis, perhaps by promoting energy balance under varying growth conditions. To investigate this, wild-type (WT) Nicotiana tabacum were compared with AOX knockdown and overexpression lines. In addition, the amount of AOX protein in WT plants was compared with that of chloroplast light-harvesting complex II (LHCB2), whose amount is known to respond to chloroplast energy status. With increased growth irradiance, WT leaves maintained higher rates of respiration in the light (RL), but no differences in RL or photosynthesis were seen between the WT and transgenic lines, suggesting that, under non-stress conditions, AOX was not critical for leaf metabolism, regardless of growth irradiance. However, under drought, the AOX amount became an important determinant of RL, which in turn was an important determinant of chloroplast energy balance (measured as photosystem II excitation pressure, EP), and photosynthetic performance. In the WT, the AOX amount increased and the LHCB2 amount decreased with increased growth irradiance or drought severity. These changes in protein amounts correlated strongly, in opposing ways, with growth EP. This suggests that a signal deriving from the photosynthetic electron transport chain status coordinately controls the amounts of AOX and LHCB2, which then both contribute to maintaining chloroplast energy balance, particularly under stress conditions.
authors.affiliation: Department of Biological Sciences and Department of Cell and Systems Biology, University of Toronto Scarborough, Military Trail, Toronto,
   ON, Canada.
authors.forename: Keshav
authors.initials: K
authors.lastname: Dahal
authors.affiliation: Department of Biological Sciences and Department of Cell and Systems Biology, University of Toronto Scarborough, Military Trail, Toronto,
   ON, Canada.
authors.forename: Greg D
authors.initials: GD
authors.lastname: Martyn
authors.affiliation: Department of Biological Sciences and Department of Cell and Systems Biology, University of Toronto Scarborough, Military Trail, Toronto,
   ON, Canada.
authors.forename: Nicole A
authors.initials: NA
authors.lastname: Alber
authors.affiliation: Department of Biological Sciences and Department of Cell and Systems Biology, University of Toronto Scarborough, Military Trail, Toronto,
   ON, Canada.
authors.forename: Greg C
authors.initials: GC
authors.lastname: Vanlerberghe
mesh_terms: D019069:Cell Respiration; D002736:Chloroplasts / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D055785:Gene Knockdown Techniques; D024101:Mitochondrial Proteins / Q000235:genetics* / Q000378:metabolism; D010088:Oxidoreductases / Q000235:genetics* / Q000378:metabolism; D010788:Photosynthesis; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism; D016082:Solar System; D013312:Stress, Physiological; D014026:Nicotiana / Q000235:genetics / Q000502:physiology*; D000099188:Alternative Oxidase
chemical_list: D024101:Mitochondrial Proteins; D010088:Oxidoreductases; D010940:Plant Proteins; D000099188:Alternative Oxidase
keywords: Chloroplast energy balance; drought stress; excitation pressure; growth irradiance; light-harvesting complex II protein; mitochondrial alternative oxidase; photosynthesis; respiration