Multiforn of γ-Glutamyltransfemse in Higher Plants and Cysteine Recycle

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1 Occurrence of two forms of radish γ-glutamyltransferases (GGT; EC.2.3.2.2), soluble and cell wall bound ones, were demonstrated in many plants including radish (Raphanus sativus).2 We cloned three cDNAs (rsggt1, AB098475; rsggt2, AB102676;rsggt3, unregistered) predicted to encode radish soluble GGTs, sequenced them and analyzed their properties. These cDNA seems to encode a heterodimeric type of GGTs homologous to mammalians GGTs.3 These three genes were transcribed by organ and tissue specific manner: rsggt1 was expressed characteristically in immature seeds and cotyledons; rsggt2 in cotyledons, leaves and roots; and rsggt 3 in leaves, roots and immature seeds.4 We transformed tobacco plants with rsggt1 and rsggt2. Transformed tobacco plants grew normally. They developed higher level of GGT activity, although wild type tobacco plants exhibited a very low GGT activity. Expressed GGT activity was found in the bound fraction, although cDNA used encoded soluble radish GGT. To understand this discrepancy remains to be solved in future.5 We purified bound GGTs from radish cotyledons. The bound GGT was a monomeric enzyme unlike soluble GGTs. cDNA was presumed to be highly homologous to Arabidopsis cDNA (AK118068) encoding putative β-1,3-glucanase. Tobacco plants transformed with putative β-1,3-glucanase cDNA exhibited elevated level of GGT activity, but not β-1,3-glucanase.6 We measured thiol compounds in apoplastic space of leaves of transgenic and wild type tobacco plants. Decreased level of glutathione (GSH) and elevated level of cysteinylglycine, a product of GGT reaction, were found in transgenic plants. These findings suggest that cell wall bound GGTs participate in GSH catabolism in apoplastic space.