Characterization of the extracellular -glutamyl transpeptidases, GGT1 and GGT2, in Arabidopsis
-Glutamyl transpeptidase (GGT) is the only enzyme known that can cleave the -peptide bond between glutamate and cysteine in glutathione, and is therefore a key step in glutathione degradation. There are three functional GGT genes in Arabidopsis, two of which are considered here. GGT1 and GGT2 are apoplastic, associated with the plasma membrane and/or cell wall. RNA blots and analysis of enzyme activity in knockout mutants suggest that GGT1 is expressed most strongly in leaves but is found throughout the plant. A GGT1::GUS fusion construct showed expression only in vascular tissue, specifically the phloem of the mid-rib and minor veins of leaves, roots and flowers. This localization was confirmed in leaves by laser microdissection. GGT2 expression is limited to embryo, endosperm, outer integument, and a small portion of the funiculus in developing siliques. The ggt2 mutants had no detectable phenotype, while the ggt1 knockouts were smaller and flowered sooner than wild-type. In ggt1 plants, the cotyledons and older leaves yellowed early, and GSSG, the oxidized form of glutathione, accumulated in the apoplastic space. These observations suggest that GGT1 is important in preventing oxidative stress by metabolizing extracellular GSSG, while GGT2 might be important in transporting glutathione into developing seeds.
Document Type: Research Article
Affiliations: 1: Department of Genetics, Development and Cell Biology, Iowa State University, Ames, IA 50011, USA, 2: School of Life Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China, and 3: Faculty of Science, Tanta University, Tanta, Egypt
Publication date: March 1, 2007