Minisymposium 8: Photosynthesis: Carbon

Abs # 18002: Short circuiting photorespiration

Presenter:	Parry, Martin AJ, martin.parry@bbsrc.ac.uk
Authors	Parry, Martin AJ (A)   Madgwick, Pippa J (A)   Keys, Alfred J (A)   Carvalho, Josirley FC (A)   Lea, Peter J (B)   Beale, Mike H (A)   Ward, Jane L (A)
Affiliations:	(A): Crop performance and improvement, Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK (B): Biological Sciences, Lancaster University, Lancaster, LA1 4YQ, UK

Photorespiration is essential to recover carbon skeletons lost through the oxygenase reaction catalysed by Rubisco. The photorespiratory cycle requires substantial amounts of energy. The involvement of amino acids results in production of NH₃ and reassimilating the NH₃ accounts for part of the energy required (1 ATP and 1 NADPH equivalent per NH₃ ). Potentially the photorespiratory pathway is a worthwhile target for attempts to increase productivity of plants by decreasing the number of energy requiring steps. Transgenic tobacco (Nicotiana tabacum) plants have been generated in which the part of the photorespiratory cycle that converts glycine to serine and releases NH₃ has been bypassed. The plants have been transformed with the genes gcl and hyi encoding respectively two enzymes isolated from the bacterium Escherichia coli, glyoxylate carboligase (gcl), which converts glyoxylate to tartronate semialdehyde and CO₂, and hydroxypyruvate isomerase (hyi), which converts tartronate semialdehyde to hydroxypyruvate. The new pathway should decrease the energy requirements of by avoiding the necessity of reassimilating the NH₃ released in the mitochondrial reactions. The transgenic lines 32, 33 and 37 expressing the E. coli gene for gcl modified by the addition of a peroxisome targeting sequence and the lines 79, 84 and 92 bearing both gcl and hyi transgenes appeared to grow normally under elevated CO₂ concentrations. Under bright light and at ambient CO₂ white lesions developed on the leaves adjacent to the veins. Failure to thrive in ambient air but survival in CO₂ -enriched air is a characteristic of many mutant plants with lesions in the photorespiratory metabolism (mutants lacking glycine decarboxylase, glutamine synthetase and glutamine synthase activities). Homozygous lines have been selected and further characterised. Under photorespiratory conditions, less ¹⁴C-glycolate was metabolised to glycine and serine and more to sucrose in one transgenic line than in the wild type plants. The amounts of glutathione in the leaves in gcl and gcl-hyi lines were greater than the wild type particularly in the youngest leaves. The increased glutathione is consistent with redox imbalances. Amino acid quantification revealed that the glutamine:glutamate and the glycine:serine ratios were altered in the gcl and gcl-hyi lines. ¹H NMR has been used to provide metabolic profiles of differences between null and gcl lines growing at high CO₂ and in the glasshouse.