Photorespiration-dependent increases in phosphoenolpyruvate carboxylase, isocitrate dehydrogenase and glutamate dehydrogenase in transformed tobacco plants deficient in ferredoxin-dependent glutamine–α-ketoglutarate aminotransferase

Abstract.

The metabolic cross-talk associated with re-assimilation of photorespiratory NH₄ ⁺ was analysed in transformed tobacco (Nicotiana tabacum L.) plants with low activities of ferredoxin-dependent glutamine–α-ketoglutarate aminotransferase (Fd-GOGAT; EC 1.4.7.1). Amounts of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco; EC 4.1.1.39) protein and Rubisco transcripts were similar in all lines whether photorespiration rates were low (4,000 µl l^–1 CO₂) or high (air). Leaf sucrose, hexose and starch contents were similar in all lines. In contrast, there was evidence that anaplerotic carbon flow was stimulated in the transformed lines with less than 60% Fd-GOGAT, since phosphoenolpyruvate carboxylase (PEPc) activity and (PEPc) protein were increased. A strong positive correlation between leaf PEPc activity and glutamine accumulation was observed, suggesting that the increase in PEPc was related to the accumulation of glutamine. A modest stimulation of total NADP-isocitrate dehydrogenase (ICDH; EC 1.1.1.42) activity was also observed in the transformed lines with less than 60% Fd-GOGAT. This was accompanied by increases in both the cytosolic ICDH and mitochondrial NAD-isocitrate dehydrogenases (IDH; EC 1.1.1.41). IDH protein was also increased in the transformed plants with low Fd-GOGAT, suggesting that both IDH and ICDH are involved in the production of carbon skeletons (and ultimately α-ketoglutarate) necessary for the re-assimilation of NH₄ ⁺. In contrast, PEPc, ICDH and IDH transcripts were similar in all lines. The aminating (but not the de-aminating) activity of NAD(H)-glutamate dehydrogenase (NAD(H)-GDH; EC 1.4.1.2) was greatly increased in plants with less than 60% of Fd-GOGAT after transfer to air. The data confirm that NH₄ ⁺ or glutamine are involved in signalling, leading to modified gene expression and enzyme activity required for enhanced production of the C skeletons, to accommodate increases in the assimilation of photorespiratory NH₄ ⁺. In addition, we provide the first demonstration of a compensatory role for NAD(H)-GDH in stabilising the leaf glutamic acid pool when Fd-GOGAT becomes limiting.