In plants, excess cellular lysine (Lys) is catabolized into glutamic acid and acetyl-coenzyme A; yet, it is still not clear whether this pathway has other functions in addition to balancing Lys levels. To address this issue, we examined the effects of stress-related hormones, abscisic acid (ABA), and jasmonate, as well as various metabolic signals on the production of the mRNA and polypeptide of the bifunctional Lys-ketoglutarate reductase (LKR)/saccharopine dehydrogenase (SDH) enzyme, which contains the first two linked enzymes of Lys catabolism. The level of LKR/SDH was strongly enhanced by ABA, jasmonate, and sugar starvation, whereas excess sugars and nitrogen starvation reduced its level; thus this pathway appears to fulfill multiple functions in stress-related and carbon/nitrogen metabolism. Treatments with combination of hormones and/or metabolites, as well as use of ABA mutants in conjunction with the tester sugars mannose and 3- O -methyl-glucose further supported the idea that the hormonal and metabolic signals apparently operate through different signal transduction cascades. The stimulation of LKR/SDH protein expression by ABA is regulated by a signal transduction cascade that contains the ABI1-1 and ABI2-1 protein phosphatases. By contrast, the stimulation of LKR/SDH protein expression by sugar starvation is regulated by the hexokinase-signaling cascade in a similar manner to the repression of many photosynthetic genes by sugars. These findings suggest a metabolic and mechanistic link between Lys catabolism and photosynthesis-related metabolism in the regulation of carbon/nitrogen partitioning.
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