Synthesis of the Arabidopsis bifunctional lysine-ketoglutarate reductase/saccharopine dehydrogenase enzyme of lysine catabolism is concertedly regulated by metabolic and stress-associated signals.
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.
Department of Plant Sciences, The Weizmann Institute of Science, Rehovot 76100 Israel.
Asya
A
Stepansky
Gad
G
Galili
D000040:Abscisic Acid / Q000494:pharmacology; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000378:metabolism; D002241:Carbohydrates / Q000494:pharmacology; D003517:Cyclopentanes / Q000494:pharmacology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D006593:Hexokinase / Q000378:metabolism; D008239:Lysine / Q000378:metabolism; D009584:Nitrogen / Q000494:pharmacology; D054883:Oxylipins; D010710:Phosphates / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D012443:Saccharopine Dehydrogenases / Q000096:biosynthesis / Q000235:genetics*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology
D029681:Arabidopsis Proteins; D002241:Carbohydrates; D003517:Cyclopentanes; D054883:Oxylipins; D010710:Phosphates; C011006:jasmonic acid; D000040:Abscisic Acid; D012443:Saccharopine Dehydrogenases; C029496:saccharopine dehydrogenase (NADP+, lysine-forming); D006593:Hexokinase; C087434:ABI1 protein, Arabidopsis; C106443:ABI2 protein, Arabidopsis; D010749:Phosphoprotein Phosphatases; D008239:Lysine; D009584:Nitrogen