Cysteine catabolism and glucosinolate turnover in Arabidopsis thaliana

Abstract

Sulfur is an important chemical element in plants. It is taken up in form of sulfate and assimilation takes place in the cytosol, mitochondria and chloroplasts. The result of this process is, among others, the amino acid cysteine and the secondary metabolite glucosinolate. Under abiotic stress conditions such as low light or extended darkness, the plant has to catabolize amino acids and other metabolites to gain nutrients and energy. Cysteine can be catabolized via three pathways to pyruvate and persulfide. The first reaction step of the mitochondrial pathway, in which glutathione persulfide is oxidized to sulfite by the enzyme ETHE1, was yet unknown. Within this thesis, this reaction step was identified by using isolated mitochondria and recombinantly expressed enzymes (Chapter 4.1). This step requires a mitochondrial aminotransferase, which transaminates L-cysteine to yield 3-mercaptopyruvate. The pathway continues including three further steps, catalyzed by the enzymes Str1 and ETHE1, to produce pyruvate and thiosulfate as products. An early senescent phenotype and an altered amino acid profile in leaves under light limiting conditions have been shown before in the ethe1-1 mutant and were confirmed, in the course of this thesis, for the ethe1- 1/str1-1 double mutant. Furthermore, the delay in seed development was increased under extended darkness and mitochondrial structure as well as the amino acid metabolism were altered in the single mutant (Chapter 4.2). Due to these alterations, the breakdown of cysteine via ETHE1 is proposed to play an important role during light- and carbon limiting conditions to gain energy. Extended darkness conditions also influence glucosinolate metabolism in Arabidopsis thaliana: The leaf glucosinolate content is decreased and the activities and abundances of the enzymes myrosinase and nitrilase are increased (Chapter 4.3). These findings show that glucosinolates are turned over under carbon starvation conditions and can possibly act as alternative respiratory substrates, like shown for cysteine. This is remarkable, because the breakdown products of glucosinolates are referred to be important active substances. In addition, new insights into the regulation of glucosinolate metabolism in Arabidospis thaliana were gained in the frame of this thesis. This includes developmental changes in leaf myrosinase activity under different growth conditions, developmentally altered protein abundance of the myrosinases TGG1 and TGG2 as well as a circadian rhythm for leaf myrosinase activity in two week old plants.