Atypical TCA cycle and replenishment in a non-photosynthetic fully habituated sugarbeet callus overproducing polyamines

In a fully habituated (hormone independent) white sugarbeet callus overconsuming oxygen, and overproducing polyamines from deviated nitrogen metabolism through glutamate, the question of the synthesis of the tricarboxylic acid (TCA) cycle intermediates (between (α-cetoglutarate and citrate) was raised. The present paper first completes earlier results indicating an anaplerotic pathway involving non-photosynthetic CO2-fixation with higher activities of phosphoenolpyruvate carboxylase and malate dehydrogenase. Additional original results indicate a second TCA replenishing pathway through a γ-aminobutyric acid (GABA)-shunt from polyamines to succinate, with higher activities of the intermediary enzymes of the habituated callus than in the normal counterpart. Such a GABA-shunt shown to occur frequently in tissues under stress is well in agreement with the formerly proposed permanent stress of the habituated callus. A tentative scheme of this atypical TCA cycle, which also illustrates the interrelationships between the deviated sugar and nitrogen metabolisms in this callus, is presented.